Thunderstorms and Snow to End the Weekend?

In True March Fashion, It's Coming In Like a Lion

“In like a lion, out like a lamb” is a phrase you may have heard once or twice; commonly used to describe the way March starts and ends. Part of what I’ve dubbed a “transition season”, consisting of March and April, March usually starts off cold and unpleasant and ends warm and more pleasant as the jet stream begins to settle more north for Summer. March is certainly going to start off “like a lion” as a strong low pressure system is forecasted to move through the KDLT forecast area to end the weekend.

Starting Friday, southerly winds on the backside of an exiting high pressure system are going to prime the atmosphere and help strengthen an incoming low. Forecasted to be between 15 and 25 miles per hour sustained, these southerly winds will be quickly moving warm air northwards from the Southern Plains to the Northern Plains; a processes known as warm air advection (WAA). Below are MSLP images taken from the Global Forecasting System model, otherwise known as the GFS, showing the rapidly intensifying low pressure system starting Friday morning (first image) into Saturday evening (second image).

The reason that this low is able to intensify so quickly is because of the ongoing warm air advection (WAA). WAA at 850 millibars, about a mile and a half above the surface, creates a bubble, if you will, in the atmosphere. If you were to look at the atmosphere from the side, that “bubble” of warm air at 850mb causes the pressure below the “bubble” to fall and rise above the “bubble”; creating an area of lower pressure below the “bubble” and an area of higher pressure above the “bubble”. Cold air advection (CAA) does the opposite. Stronger WAA or CAA will, and does, influence the pressure throughout the atmosphere. Below is an image of the temperature advection at 850mb Friday night, during the same time the low undergoes intensification based on the MSLP images above. Also below is a cross-section drawing of what the atmosphere looks like when warm air is being transported at 850mb explaining the “bubble”.

850mb Temperature Advection Friday night, 3/3

 

Cross-section of WAA occurring at 850mb

In addition to helping intensify the low, southerly winds will also bring up a little bit of CAPE into southern parts of the KDLT forecast area. Otherwise known as Convective Available Potential Energy, as the name suggests, CAPE provides energy for storms to use. Indicating the amount of instability, measured in joules per kilograms, in the atmosphere, the KDLT forecast area and the rest of the Northern Plains usually sees the CAPE values increase towards the end of Spring into the start of Summer as the jet stream settles farther north (coinciding with the start of our severe weather system).
Though the values are going to be small, there will be a small window where CAPE values between 0 – 100 j/kg will be present along the South Dakota/Nebraska border with higher values found in central Nebraska. If conditions are right, those values may be enough to get a thunderstorm or two to develop in the mid/late afternoon… especially if a more robust updraft can develop. The threat for a stray thunderstorm would be very isolated and the life of the cell would be short lived as the environment is still not conducive for thunderstorm, severe or not, activity.

CAPE Values Sunday Late Afternoon

While rain and the potential for a thunderstorm or two will be present across the south/southeast, the threat for snow will be found the farther northwest you live. Northwesterly winds on the backside of the low pressure system will begin to bring down cooler air as early as Saturday evening and continue to bring down cooler air as the low moves east. The cold air will interact with the moisture in the atmosphere and the chance for snow will begin to fall as early as Sunday morning across northwestern portions of South Dakota. As cold air makes its way eastward, precipitation will gradually turn from rain to snow.

Futurecast Showing Possible Scenario Saturday through Next Tuesday

Unfortunately it’s still too early to determine possible snow/rain accumulations however, the GFS is picking up on heavier precipitation rates throughout afternoon on Sunday into the overnight hours and carrying into the day on Monday. We will be carefully watching where these heavier bands of precipitation ultimately set up as their placement will determine who’ll see higher amounts of rain/snow. There’s still time for the path of this low to change however, recent trends have the center of this low moving through parts of Nebraska and Iowa Monday morning which would increase precipitation chances across the KDLT forecast area.

 

 

Blaise Keller
KDLT News Morning Meteorologist
Twitter – @blaisekellerr

Categories: Weather, Weather Blog

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