Category: General Weather

Severe Weather Outbreak Across the Southern Plains on Tuesday

While the attention has been on heavy rain and flooding across portions of southern and eastern Texas recently, areas to the north of there will be in the spotlight early next week.

Severe Weather Outlook for Tuesday from the Storm Prediction Center
Severe Weather Outlook for Tuesday from the Storm Prediction Center

All of the ingredients appear to be coming together for a severe weather outbreak from Kansas into northern Texas on Tuesday. Low pressure, both at the surface and aloft will move out of the Rockies and across the Central Plains on Tuesday. Ahead of the storm, warm, moist air will flow northward from the Gulf of Mexico into Texas and the Southern Plains. A cold front trailing the system will provide the lift needed to initiate thunderstorm development during the afternoon hours across the region.

Forecasted CAPE (Convectice Available Potential Energy) for Tuesday evening. Image courtest of WeatherBell.
Expected CAPE (Convective Available Potential Energy) for Tuesday evening. Image courtesy of WeatherBell.

As the storms move into south-central Kansas, central Oklahoma, and north-central Texas, they will encounter a very unstable airmass. The above map shows the forecast of Convective Available Potential Energy (CAPE) across the Southern Plains for Tuesday evening. CAPE is a measure of instability through the depth of the atmosphere, and is related to updraft strength in thunderstorms and is measured in Joules of energy per kilogram of air. A CAPE value of 1000 or less is considered “weak” instability, a value of 1000-2500 is “moderate” instability, a value of 2500-4000 is “strong” instability, and over 4000 is “extreme” instability. Areas shaded in purple on the map have values over 4000 Joules/kg, with some of the lighter purple shading in excess of 4500 Joules/kg. With this much instability in place, it won’t take much for storms to quickly become severe, with strong winds, large hail, torrential downpours, and tornadoes all possible.

Late April and early May are a time when severe outbreaks can be common across the Central and Southern Plains. If this outbreak does pan out, it will fall on the 25th anniversary of another outbreak across parts of the same area. On April 26, 1991, a total of 54 tornadoes were observed from Iowa and eastern Nebraska into portions of Missouri, Kansas, Oklahoma, and northeastern Texas, resulting in 21 deaths.

Plot of all 55 tornadoes that were reported on April 26, 1991 across the Central and Southern Plains. Image courtesy of the Storm Prediction Center
Plot of all 54 tornadoes that were reported on April 26, 1991 across the Central and Southern Plains. Image courtesy of the Storm Prediction Center

 

There was one F5 tornado reported that day – the one that moved across portions of Wichita, Kansas, including McConnell Air Force Base, before devastating the town of Andover, Kansas.

Residents of this region should pay attention to the forecast over the next few days and keep an eye to the sky if they have outdoor plans for Tuesday.

Flooding Possible in the Southern Plains

Heavy rain will move across the Southern and Central Plains through Monday night as a result of an upper level low pressure system west of the region. What is a dry line and pre frontal trough of instability Saturday into the overnight hours will turn into a cold front early Sunday. This will provide a center for showers and thunderstorms with the threat of heavy rain as warm/moist flow comes in from the south. Parameters for this event indicate heavy rain with precipitable waters (a measure of the amount of moisture in a column of the atmosphere that could fall as rain) above normal by 2 to 3 standard deviations and low level jet stream winds reaching 1 to 2 standard deviations above normal from the south indicating strong inflow of warm/moist air. This is also collocated with a strong area of large scale ascent indicated by upper level divergence of winds that also translate east through the time period.

Departures from normal of Precipitable Water forecast by the NAEFS ensemble system for Sunday morning.

Departures from normal of Precipitable Water forecast by the NAEFS ensemble system for Sunday morning.

2 week departure from normal of precipitation over the Southern and Central Plains. Notice the higher amounts in Texas and Oklahoma.
2 week departure from normal of precipitation over the Southern and Central Plains. Notice the higher amounts in Texas and Oklahoma.

 

Placement of midlevel theta-e (a measure of warmth and moisture) and midlevel vertical velocity indicate the placement of heavy rain will move from a line between western Nebraska to western Texas later Saturday to a line between eastern Nebraska and eastern Texas by daybreak Monday along the aforementioned eventual cold front. Many times, overnight convection and thunderstorms die off as instability fades, but a self-sustaining area of heavy rainfall (called a mesoscale convective system, MCS) results. A similar process looks to form an MCS at the southern end of the cold front in southwestern/south-central Texas. This will move east slowly toward southeastern Texas by daybreak Tuesday.

Moderate to Severe drought conditions have formed in portions of the Central and Southern Plains.
Moderate to Severe drought conditions have formed in portions of the Central and Southern Plains.
Forecast precipitation over the Plains region. Notice the 4 to 8+ inches of rainfall forecast.
Forecast precipitation over the Plains region. Notice the 4 to 8+ inches of rainfall forecast.

 

Flooding is very likely with this event especially in Oklahoma and Kansas where areas of Moderate to Severe drought conditions have developed. Remember that after a long dry spell and drought conditions, drier ground is harder to penetrate with water and so it remains standing on the surface allowing for flooding quickly. In addition, above normal rainfall has fallen over the past 2 weeks especially around Abilene and San Angelo, Texas and into Southwestern Oklahoma, which would allow for already swollen bodies of water to overflow quickly. In addition to the influence of the current ground conditions, the shear amount of moisture that is forecast to fall further exacerbates the flooding situation. 4 to 8 inches of rainfall is forecast to fall over portions of the Southern and Central Plains through Tuesday morning with the potential for higher amounts in thunderstorms. As a result, the National Weather Service has issued flash flood watches on a line from southwestern Nebraska to southwestern Texas ahead of the convection and heavy rain. This is very likely to translate east along with and ahead of the line of convection that is forecast to form.

Severe Weather Possible in the Plains This Week

The Storm Prediction Center has highlighted the Southern Plains with the potential for severe weather on Monday and Tuesday with Monday’s main hazard being marginally severe hail and Tuesday containing all of the main hazards including wind, hail and tornadoes.

Storm Prediction Center forecast of severe weather risk for Monday (left) and Tuesday (right). Hazards for Monday: marginally severe hail. Hazards for Tuesday: all hazard types including tornadoes are possible.
Storm Prediction Center forecast of severe weather risk for Monday (left) and Tuesday (right). Hazards for Monday: marginally severe hail. Hazards for Tuesday: all hazard types including tornadoes are possible.
CIPS analogs based on the NAM (left) for Monday and GFS (right) for Tuesday. Notice that the analogs for Monday are contoured for 5 reports in an area and analogs for Tuesday are contoured for 1 report in an area.
CIPS analogs based on the NAM (left) for Monday and GFS (right) for Tuesday. Notice that the analogs for Monday are contoured for 5 reports in an area and analogs for Tuesday are contoured for 1 report in an area.

 

This is a result of a strong low level jet flowing in from the Gulf of Mexico bringing in plenty of warm and moist air. Air with a dew point of 60F or greater moves into eastern Texas toward the end of the day Monday and dew points into the upper 50s exist in parts of the Central Plains. Plenty of turning in the wind, shear, exists in the atmosphere along with plenty of upward motion, CAPE. An area of very dry air exists in the mid levels of the atmosphere, which will act to inhibit weaker storm growth, but in areas where the upward motion is strong, the storms may have the chance to create hail, especially in any supercells that form. Analogs based on the Saturday night run of the short range forecast model, the NAM, have an area from the Panhandle of Texas into central Kansas where at least 5 previous severe weather reports occurred, which is close, but not the same as the SPC forecast severe weather risk, which has its severe area from southern Texas into south-central Kansas. Short range model simulations from the NAM as well as the NCAR ensemble forecast show convection firing in central Texas and eastern Oklahoma and moving into eastern Oklahoma and eastern Kansas as the day turns to night.

Height anomalies showing the general high and low pressure placements. Low anomalies (blue) indicate low pressure.
Height anomalies showing the general high and low pressure placements. Low anomalies (blue) indicate low pressure and vice versa. Via PivotalWeather.

 

For Tuesday, the strong energy that helped to force along the heavy flooding rain along the West Coast will move south into Mexico. This energy will help to drive more moisture and warm air into Southern Plains on Tuesday. Dew points will be into the 60s in eastern Texas and plenty of CAPE and shear will exist in that region. Plenty more energy around the region will help to spark more storms. With the copious amounts of CAPE and shear, all types of severe hazards will be in play including tornadoes. Towards the end of the day, when the sun’s heating turns off, storms tend to form into a convective line with strong winds or into a large complex with heavy rain and thunderstorms, called an MCS. The Storm Prediction Center hints that an MCS may form towards the end of the day in southern and Eastern Texas. Analogs based on the Saturday night run of the long range forecast, the GFS, have an area from central into northeast Texas where at least 1 previous severe weather report occurred. This is a smaller area of than the Monday setup, perhaps indicating a lesser event or because of the general bias of the shorter range NAM model to having more moisture to enhance storms. The SPC has a slight risk, which is the same severe risk category given for Monday covering eastern Texas and portions of the States bordering Texas. The SPC risk includes the analog risk area. Shorter range model projections have convection in Oklahoma and Missouri.

The severe weather risk continues into the middle and possibly the end of the work week in Southern United States.

Sunday’s Pacific Northwest System to Affect Northeast By Midweek

A strong surge of warm air will move east from the Northern and Central Plains into the Mid to Upper Mississippi Valley, Great Lakes, and Ohio River Valley on Sunday. Surface temperatures were forecast to be 15 to 30 degrees above normal for Saturday in the Plains and will be 10 to 20 degrees above normal in the Mid to Upper Mississippi Valley, Great Lakes and Ohio River Valley areas on Sunday. The normal setup for warm air into a region is a pattern of high pressure to the south and low pressure to the north. This allows warm air to come in on the back of high pressure and ahead of a cold front from the low pressure system. Also, with warm air moving into a region, the distance from the ground to a layer with a pressure of 500 mb increases to an above normal height. Above normal heights are forecast for the Mid to Upper Mississippi Valley, Great Lakes and Ohio River Valley areas on Sunday.

Above normal temperatures over the Plains and Pacific Northwest. Notice the below temperature anomalies in the Cadcades mountain range in Northwest Washington.
Above normal temperatures over the Plains and Pacific Northwest. Notice the below temperature anomalies in the Cadcades mountain range in Northwest Washington.
Above normal heights indicating warm air is nearby.
Above normal heights in the west and east indicating warm air is nearby.

 

Farther west, a cold front moves ashore in the Pacific Northwest as a low pressure system moves ashore in Western Canada. The aforementioned warmer temperatures were in the Pacific Northwest 24 hours ago, so many places will receive rain from this front, but higher elevations will receive snowfall. Northern portions of the Cascades may receive near 1 foot of snowfall from this frontal system. In addition, the northern portion of the Rockies within Western Montana and Western Wyoming are forecast to receive 2 to 4 inches of snowfall from this system.

Forecast snowfall over the Cascades and Northern Rockies via WPC.
Forecast snowfall over the Cascades and Northern Rockies via WPC.
European model depiction of a more northerly track and less colder air for snow or freezing rain.
European model depiction of a more northerly track and less colder air for less snow or freezing rain.
GFS model depiction of overnight snowfall or freezing rain.
GFS model depiction of overnight snowfall or freezing rain.
Current snow pack as of Friday night.
Current snow pack as of Friday night. Via the National Operational Hydrological Remote Sensing Center.

 

As the energy for the frontal system in the Pacific Northwest moves ashore, an upper level low pressure system in Central Canada will help to steer the energy towards the Northeast. The energy will eject from the Rockies dive south into the Ohio River Valley region. The track of the system after its movement toward the Ohio Valley region is key for the Northeast. Longer range models initialized Friday night show two possible track options for the storm after the track through the Ohio River Valley. A movement across Southern New England (the American GFS model) will allow more cold air to flow into the region. With more cold air and some moisture around the region, an onset of snow or freezing rain is possible along a cold front that would be across portions of New England at that time, especially north of Route 2 in Massachusetts and into Central and portions of Northern New England. However, a shift to more of a northern track (the European model) would allow for less snow, if any, at the onset overnight Tuesday night as the cooler air wouldn’t be able to come into the region as well as the more southern track. With the current snowpack only in the Berkshires and all the way into Central to Northern New England, a push northward of the warm front overnight, as the European model is hinting at may be possible, but temperatures on Tuesday and Tuesday night will have to be assessed to see how far south the below-freezing air will be able to come into Southern New England. If the colder air is able to come far enough south, the front will probably struggle to move northward and some snow or freezing rain would be possible Tuesday night before changing to rain during the day Wednesday and as the low pressure system approaches from the west.

Strong Storm to Impact the Midwest and Southeast Early Next Week

Two pieces of energy will be phasing or combining this weekend off the West Coast. This will allow for a low pressure system to develop and intensify as the energy digs across southwestern United States. The storm is developing early enough that the storm should take a general northeast track after the storm ejects off the Rocky Mountains.

 

The two pieces of energy offshore before phasing over the weekend.
The two pieces of energy offshore before phasing over the weekend.
After the pieces of energy have phased and storm begins intensifying and turning northeast.
After the pieces of energy have phased and storm begins intensifying and turning northeast early Monday.

 

It will have all of the precipitation types that winter normally features with snow north of the low pressure system’s track as well as a mixed bag of snow/sleet/freezing rain near the warm front as the warm and moist air runs into the cold air aloft. This cold air begins settling south from Canada early Tuesday, which is when the snow should fall north of the track. It’s way too early to looking at snowfall accumulation maps so outlooks are best at this point for the system. Strong winds are often associated with developing low pressure systems. This system is no different with forecast analogs based on the GFS forecast (America long range forecast model) showing a possibility of blizzard conditions over Iowa. Remember, blizzard conditions are more about wind and visibility than snowfall so that even just light blowing snow could still be designation as a blizzard as long as visibility is reduced.

 

Appreciable snowfall/sleet highlighted on day 4. Via the Weather Prediction Center.
Appreciable snowfall/sleet highlighted on day 4. Via the Weather Prediction Center.

 

In addition to the wintry precipitation types, this system will also produce some severe weather. Severe weather is forecast to begin occurring Monday Night across the Southern Plains and continue moving eastward as the cold front and warm sector (area where warm air resides) move east. The limiting factor in this case will be the instability, but there may be enough shear (turning of the wind with height or change in speed with height) to overcome any shortcomings that instability might have. With that in mind, a look at forecast analogs based on the GFS show the potential for severe weather to occur Tuesday in the Lower Mississippi Valley. The winds are strong aloft so wind damage and a few tornadoes are possible from this event as well as some hail. Flash flooding may also be possible under any thunderstorm.

Snapshot of the cold air(north and west of the low) and warm sector (south and east of the low). Notice the stronger winds (barbs with triangles indicating 50+ knot winds) in the area within the packed contours.
Snapshot of the cold air(north and west of the low) and warm sector (south and east of the low). Notice the stronger winds (barbs with triangles indicating 50+ knot winds) in the area within the packed contours just southeast of the pressure system indicating stronger shear. This is at a level where air is at a pressure of 700mb. Via the College of DuPage.

 

 

Analog guidance based on the GFS models highlighting severe weather in the Lower Mississippi Valley for Tuesday.
Analog guidance based on the GFS models highlighting the placement of severe weather in the Lower Mississippi Valley for Tuesday. Via the Cooperative Institute for Precipitation Systems.
Tuesday's probability of severe weather over the Lower Mississippi Valley. This seems to correlate well with the CIPS analog guidance's placement of severe weather. Via the Storm Prediction Center.
Tuesday’s probability of severe weather over the Lower Mississippi Valley. This seems to correlate well with the CIPS analog guidance’s placement of severe weather. Via the Storm Prediction Center.

 

Blizzard of 2016 Recap

This post will go back over some of the highlights of the Blizzard of 2016 with a brief summary of why it happened, some of the reasons why plenty of snow fell where it did and some of the cool visuals that came out of the storm.

Snow totals along the Eastern US.
Snow totals along the Eastern US.

 

The storm had many interesting features that helped it become what it was. It started out over the Pacific Ocean and came onto the United States. As the energy energy ejected out of the Rockies, another low pressure system was entering the West Coast and causing a wave break. This would cause the high pressure ridge to develop behind the storm allowing plenty of cold air to wrap in behind the storm and rapidly develop the storm into a cutoff low even before it exited the East Coast.

Energy (reds and oranges) over the Eastern Pacific before entering the Western US.
Energy (reds and oranges) over the Eastern Pacific before entering the Western US.
The ridge building as a result of a wave break from the storm entering the Western US. The eventual Blizzard of 2016 was in the Eastern US at this point.
The ridge building (green arrow) as a result of a wave break from the storm entering the Western US. The eventual Blizzard of 2016 was in the Eastern US at this point.
Storm position as it affected the Northeast. The blue colors indicate the bent back warm front and frontogensis signature.
Storm position as it affected the Northeast. The blue colors indicate the instability within the bent back warm front and frontogensis signature.

 

Well forecasted position of the coastal front. The front would have heavier snow along it as a result of air colliding along it.
Well forecasted position of the coastal front. The front would have heavier snow along it as a result of air colliding along it.

 

In fact, there was plenty of “wait for the energy to come on shore” phrases being dropped in the meteorological community because the models had the storm on the map for such a long time. However, the snow maps being generated by the models didn’t really give meteorologists much confidence along the way because there were plenty of differences in how far north the snow would come into the Northeast. It was a given that the storm would drop plenty of snow in the Mid-Atlantic since one of the most similar analogs or similar storms to this storm was the January 1996 Blizzard. This storm had areas of 20+ inches in the Mid-Atlantic, but had way too much snowfall in the Northeast. Meteorologists needed to pin point where the banding would set up. This occurred in a coastal front and another band system called a bent back warm front or TROWAL signature. It had plenty of moisture within it and as the storm was cycling and the moisture was wrapping into the storm, the bent back warm front would slowly pivot from a west to east signature to a northeast to southwest signature and then pivot out to sea as the storm passed by New England to the southeast. One wrinkle leading up to the storm had the model that is easy to disregard, the NAM, adjusted moisture way up for the New York region and many final snow maps didn’t include it because it was such an outlier. In fact, many areas over performed as a result of the bent back warm front band along the South Coast of Southern New England and the Coastal front over Southeastern Southern New England. So taking the NAM into consideration in some way may have actually helped.

Blizzard of 1996 is similar for the DC area in snowfall output, but not so similar to the Northeast.
Blizzard of 1996 is similar for the DC area in snowfall output, but not so similar to the Northeast.

 

Coastal flooding would be a huge problem along the East Coast:

 

Some pictures and interesting photos:

Winterized radar image showing snow in the Mid-Atlantic and portions of the Northeast. Image courtesy of Intellicast.com

 

Astronaut Scott Kelly’s photograph from space of thundersnow.

It was a case of the haves and have-nots as relatively small distances separated high and low snowfall totals.

After a while, you’ve gotta have fun with it.

 

The morning after’s visible satellite imagery distinctly showing the snowline.

 

Alberta Clipper Approaches the Northeast Later this Week

After the weekend, the next storm that the Northeast will be watching comes in the form of an Alberta Clipper late Tuesday into Wednesday. These storms are called Alberta Clippers because they come southeast from Alberta, Canada to the Northeastern United States. This means they have quite the cold punch with them. They also have a strong high pressure system closely trailing the low pressure system so that the cold air usually comes rushing in with strong winds. However, the other part of the name, clipper, gives reference to how the system generally doesn’t drop much precipitation as a result of the area of low pressure being quite dry and moving quite fast through an area.

wx image 4

GFS forecast model showing the clipper and trailing high pressure (added by author). Notice the strong jet stream from north to south, which could help to push the clipper farther south and bring cooler air into the United States.
GFS forecast model showing the clipper and trailing high pressure (added by author). Notice the strong jet stream from north to south in Western Canada, which could help to push the clipper farther south and should bring cooler air into the United States.

 

In many cases however, the Alberta Clipper can drop plenty of snow in a given region. The Alberta Clipper is known for containing plenty of cold air and an appreciable amount of energy, but a lack of precipitation. So if moisture from a low pressure system to the south is able to phase, or combine into the system, which usually occurs in the Eastern United States, the clipper can redevelop off the East Coast into a strong nor’easter. This occurred January 25-28, 2015 and resulted in snowfall over 30 inches in portions of eastern and central Massachusetts. The other way Alberta Clippers can pack a punch, which is usually weaker than when clippers phase to strengthen the system, is to have the trough that the low pressure system resides in turn negative. This means that the direction from the northern part of the trough to southern part of the trough goes from northwest to southeast. This causes a strong draw of moisture into the storm on favorable winds and can also cause a redevelopment low into a strong low pressure system in the Northeast, usually east of Massachusetts. The proximity of the redevelopment low pressure system to land can determine if the resultant storm skirts by the region with minimal snowfall or a closer track leading to higher snowfall. One other part of forecasting an Alberta Clipper is to forecast its track from Northwestern Canada to Northeastern United States. There have been plenty of times where the models have the system way to far north and end up trending it farther south over time. This can trend any negatively tilted troughs further west and have a new, stronger low pressure system develop closer to land than out to sea.

 

The storm total snowfall via the NESIS scale from January 25 -28, 2015.
The storm total snowfall via the NESIS scale from January 25 -28, 2015.
GEFS forecast showing the negative trough (red line), possible evolution of the trough farther south with the blue colors south of the red line. Also the blue colors north of the line indicate a possible strong storm north of the red line already at this hour. This indicates the uncertainty with this forecast.
GEFS forecast showing the negative trough (red line added by author) and possible evolution of the trough farther south with the blue colors south of the red line. Also the blue colors north of the line indicate a possible strong storm north of the red line already at this hour. This indicates the uncertainty with this forecast.

 

The Alberta Clipper system taking aim at the Northeast does have a negatively tilted trough and also has quite a bit of instability in the low levels of the atmosphere. Snow squalls could occur as a result of the low level instability and a stronger low pressure system does look to develop somewhere east of Massachusetts. Temperatures may be just borderline enough that precipitation could start as rain instead of snow as well. While, it is NOT anticipated that this system will become one like January 25-28, 2015 and produce high amounts of snow, it is possible that current models aren’t depicting the whole picture of what this system could become especially with possible trends south with the system and possible adjustments with the model solutions after the weekend rainstorm moves through. Currently, snow totals are expected to be light.

How does Lake/Ocean-effect Snowfall Happen?

Have you ever wondered how lake effect snow develops? It works as a result of buoyancy and wind direction and is easier for it to take place in the earlier part of winter. One of the biggest pieces of the puzzle, aside from a totally below freezing atmospheric column, is the fact that lakes need to be warmer than the cooler air coming in across the lakes. In fact, it needs to be 13 degrees colder at the surface than at around 1.5 kilometers into the atmosphere. This is because there needs to be enough lift to generate the storm itself.

 

Visual depiction of how lake effect snow forms. Via The Vane and Dennis Mersereau.
Visual depiction of how lake effect snow forms. Via The Vane and Dennis Mersereau.

 

It is easier for this setup to take place in the transition from fall to winter because the area’s average air temperatures are falling while the lake’s average sea surface temperatures are falling slower, or if not, remaining the same. This is as a result of a few of water’s properties (heat capacity and thermal conductivity) which allow it to change from warm to cold or cold to warm temperatures slower than the air. Also later in the winter, the lake has a tendency to have a frozen surface, which shuts off the possibility of lake effect snow. One of the most common ways this setup works is to have a cold front come in from Canada with a vigorous piece of energy associated with it and move right over the lake. This often happens as a strong low pressure system is leaving the Northeast and cold air streams into the Northeast behind it.

 

Snowfall totals from the November 17-19, 2014 lake effect even. Notice the large gradient near Buffalo, New York.
Snowfall totals from the November 17-19, 2014 lake effect event. Notice the large gradient near Buffalo, New York. Via NWS Buffalo.

 

So, in our setup, the lake is warm, the air is cold, now buoyancy comes into play because the cooler air aloft is more dense and falls while the warmer air rises, condenses and creates our snow storm. The other piece that is necessary with lake effect snowfall is to have a favorable wind. This is very important because often, if the wind direction stays constant for a long period of time, the snowfall amounts can pile up within the favorable area for snowfall while the area just outside the favorable area can have minimal amounts. For example, from November 17-19 of 2014, around 5 feet of snow fell in Cheektowaga, New York off of Lake Erie while just a few miles away only received a few inches. So wind direction is important because, in some cases it can mean the difference between a few inches and many feet.

GFS Forecast depiction. Note the pink and yellow colorations along the temperature (red) line indicates the dendritic growth layer. It is also close to the moisture (green) line indicating some moisture. Note the 1.5 m temperature at 850 millibars.
GFS Forecast depiction for Hyannis, MA. Note the pink and yellow colorations along the temperature (red) line indicates the dendritic growth layer. It is also close to the moisture (green) line indicating some moisture. Note the 1.5 km temperature at 850 millibars.

 

This can also work with the ocean in the same way that it does with the lake. A warm ocean compared to very cold temperatures 1.5 kilometers into the atmosphere can lead to snowfall along the eastern side of the Northeast and especially Cape Cod. Such an event is forecast to take place on Monday as a strong arctic cold front approaches from the northwest. Some of the coldest air forecast to come into the region with around -10 to -12 Celsius temperatures at 1.5 km while the sea surface temperatures just east of Massachusetts are still around 9 Celsius. That’s a temperature difference of around 20 degrees Celsius around 1PM on Monday! Of course, enough lift has to be in place and moisture within the area where growth of dendrites, the type of snowflake that allows for the highest accumulations, occurs in the atmosphere. Some dendritic growth zone moisture is available at the time of ocean effect snowfall, but lift is really lacking. So snowfall accumulations should be minimal. Sometimes Boston gets into the snowfall as well, but the wind needs to have a slight easterly component. If it does, then some snowfall could occur.

First Winter Storm of the Year for the Northeast?

Much of the talk over the past few days has been about the record breaking temperatures around Christmas time. And rightly so, because many temperatures reached the mid to upper 60s in the East and Northeastern US breaking old records by as much as 5 to 10 degrees while many hoped for a White Christmas. This all happened at the same time as intense storms ripped through the Midwest and Southern US, leaving damage behind. However, a  quick cold shot will bring the Northeast quickly right back to reality come Monday as a piece of energy dives into the Northeast that can trace its origins to Northwestern Canada and will bring with it, very cold temperatures. Some of the coldest temperatures that the Northeast has had all winter are expected. Monday, as high pressure sits in Southeastern Canada, cold air will be able to drain south from Canada on northerly winds. Temperatures may not get out of the 30s in many spots on Monday and dew point temperatures (a measure of moisture) will be below freezing, indicating the strength of the cold and dry air.

Record temperatures around Christmas time. Via coolwx.com
Record temperatures around Christmas time. Via coolwx.com

 

Dew point temperatures below freezing indicate a very cold and dry airmass before Tuesday's precipitation.
Dew point temperatures below freezing indicate a very cold and dry airmass before Tuesday’s precipitation. Via Pivotalweather.com

The strength of this cold and dry air is very important because right after this cold shot passes through, moisture will be coming up from the south in a very common setup, for what is called overrunning.  What happens is that warm, less dense, moisture from the south rides over cooler, denser air at the surface and results in precipitation. It is common for this setup to have precipitation hold off as the moisture aloft fights with the drier air at the surface to be able to reach the surface. As the moist air evaporates before it reaches the ground, it actually acts to cool the air, thus keeping temperatures near or just above the freezing mark during the day, much like sweating keeps us cool during the summertime. With surface temperatures remaining around freezing, the initial precipitation that does fall will have an opportunity to fall as wintry precipitation. Forecast atmospheric profiles, known as soundings, show that temperatures at the surface and aloft will remain below freezing as the warm moist air enters the region so the precipitation type should start as snow. However, as the warm, moist air continues to come in, especially since it will be helped along with any additional daytime warmth, midlevel temperatures will be able to warm and the atmospheric profile will begin to look like one that will produce sleet and freezing rain before eventually changing to rain by days end. The strength of the winds aloft behind the warm front (signaling the entrance of the warm, moist air) is important because this will tell how quickly the warm air mass can come in from the south and change everything to sleet, freezing rain and rain and it will tell how far inland the warm air is able to come from the ocean as the winds come in from the southeast in the form of a coastal front. If this coastal front is able to set up, snow fall amounts could be increased on the western cold side of the coastal front. In addition snowfall amounts may be bumped up just to the north of the warm front because it will have decent mid to lower level energy along it, but at the same time, most of the energy appears to be outside of the zone for best snowfall growth so total snowfall won’t be as high as it could be. The most likely time frame for the snow to occur would be the morning into the midday hours ahead of a possible change over to sleet and freezing rain, which would occur from the midday hours on and a changeover to all rain could occur in the afternoon to evening hours. Of course, if any snow accumulation does occur, rain on top of that would spell trouble for driving as slush may form on the street, further exacerbated by any sleet and freezing rain that could fall on Tuesday. Of course, there’s always the wrinkle of if a coastal low pressure system does form because this would also act to keep cooler air around longer extending the longevity that the wintry precipitation resides in New England on Tuesday instead of rainfall. The area of low pressure should then push northeast and be out of the area overnight Tuesday night.

Analogs indicate the chance for freezing rain.
Analogs indicate the chance for freezing rain. Via CIPS.
Analogs indicate the chance for snowfall in New England.
Analogs indicate the chance for snowfall in New England. Via CIPS.

Snow, Rain and Severe Weather in the Central US, Record Warmth in the East

A low pressure system that formed on Saturday and will intensify into Monday as it moves from the Southern Plains into the Great Lakes and eventually into the Northeast by Monday night. Saturday night will see showers and thunderstorms in the Southern Plains ahead of, and especially along, a cold front extending southward from the system. That cold front will move from the Southern Plains and into the Tennessee River Valley and parts of the Southeast with strong winds and possibly a tornado being the main threats, along with flooding rainfall. As much as 2 to 5 inches of rainfall is forecast to fall by Monday morning in Northeast Texas and Eastern Oklahoma. The main limiting factor for the thunderstorms in the Southern Plains will be instability, especially with the morning cloud cover expected to remain around the region through the day. For the most part, this storm is expected to be mostly rainfall, but the outermost edges to the north and west of the storm may have a chance to change to snow, from Northern New Mexico and southeastern Colorado northeastward into Northern Minnesota. Some of the higher elevations in Colorado could see a foot or more of snow while the lower elevations could see 2 to 6 inches, especially in southern Colorado and northeastern New Mexico.

48 hour rainfall forecast. Note the higher amounts in northeast, Texas and western Oklahoma.Via WPC.
48 hour rainfall forecast. Note the higher amounts in northeast, Texas and western Oklahoma. Map provided by the National Weather Service’s Weather Prediction Center
Snowfall forecast. note the higher amounts in Colorado and New Mexico. Via WPC.
Snowfall forecast. note the higher amounts in Colorado and New Mexico. Map provided by the National Weather Service’s Weather Prediction Center.

 

As the low pressure system intensifies, it will have quite a pull of warm and moist air from the south into the eastern half of the United States. This could result in dozens of record high temperatures and record high minimum temperatures being set across the region. In many places, the forecasts would exceed the current records by 5 to 10 degrees or more.

Record high minimum temperatures on Saturday night denoted by white circles.
Record high minimum temperatures on Saturday night denoted by white circles. Map provided by WeatherBell.
Record high temperatures circled in white especially over the eastern half of the US.
Record high temperatures expected for Sunday circled in white, especially over the eastern half of the US. Map provided by WeatherBell.
Record high minimum temperatures on Sunday night denoted by white circles.
Record high minimum temperatures on Sunday night denoted by white circles. Map provided by WeatherBell.

 

This pattern is almost the complete opposite of what we had at this time last year when anomalously warm temperatures were draped across most of western and central United States with below normal temperatures residing over most of Eastern United States.

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