Seems March Has A lot to Get Off Her Chest...High Amplitude "Big Boy" Trough to Kick Off 2025 Severe Weather Season?

March Has Something To Say...

It would appear that March has some pent up energy as she looks to unleash her fury in the form of a "mega" trough to kick of severe weather season in 2025. The Storm Prediction Center (SPC) has highlighted the South and the Mid-Atlantic region for the possibility of some widespread severe weather on Tuesday and Wednesday next week. For us here in Virginia, we will be watching closely during the day on Wednesday for the passage of the strong cold front and the possibility of some damaging wind gusts. While still several days out and subject to change, it sure looks like a very large, powerful and widespread storm system will be swinging across the US on Tuesday and Wednesday. So what is a "mega" trough and why are models hinting at a major storm impacting a large part of the U.S.? 

SPC Day 4 Categorical Risk. An "Enhanced" 30% Risk Area was introduced for portions of AR, LA, MS, and TN

SPC Day 5 Categorical Risk. A 15% "Slight" Risk area was denoted for most of the Mid-Atlantic and Southern Atlantic coast

When the Subtropical Jet and the Polar Jet Decide to Have A Party...You Know It's Going To Get Wild:

Credit: NOAA
        One of the main reasons such a big storm system is on tap for next week stems partially from a subtle interaction between a Upper Level Subtropical Jet and the Polar Jet. Broadly speaking, the Subtropical Jet is a belt of strong winds high in the atmosphere that occurs near the 30 degree latitude mark. This jet stream migrates north or south during the year due to the sun's positioning through the year and the northward migration of warmer air. The Polar Jet, on the other hand, originates near the 50-60 degree latitude region and demarcates areas of very cold, dry air. Similar to the Subtropical Jet, the Polar Jet migrates north and south during the year due to changes in the sun's positioning and the migration of cold air southward. 

        When these two pieces of "energy" interact or "phase", you end up with a highly amplified trough over a large area. Models, including the GFS have been hinting at this interaction with this coming week's storm. As seen below, Monday starts off with an "impulse" provided by a trough in the Subtropical Jet, but notice how the Polar Jet winds are located well to the north, creating this "split" flow. However, as Tuesday rolls into Wednesday, these two jet streams begin to interact with a noticeable "dip" of the Polar Jet teaming up with the strengthening Subtropical trough. Eventually by late Tuesday into Wednesday, these systems have more or less "phased" to produce a very large "mega" trough. The highly amplified nature of the upper level impulse also leads to a few other things... 
GFS Modeled 300 MLB (Upper Level) Forecast. A "split" flow pattern is noticeable to start with our main trough originating from the Subtropical Jet (red line) and the Polar Jet well to the North (blue line)

GFS Modeled 300 MLB (Upper Level) Forecast. As we move into Tuesday, things get interesting with a noticeable "dip" in the Polar Jet (blue line) beginning to interact with the Subtropical trough (red line). 

GFS Modeled 300 MLB (Upper Level) Forecast. By Tuesday night into Wednesday, that southern "dip" provided by the Polar Jet begins to give an added "push" to the original impulse, leading to somewhat of a "phasing" and an amplification of the original trough.

Talking About Divergence...No Not The Movie:   

         The creation of such a large, powerful trough leads to some incredibly fast winds aloft. This creates "divergence" in the upper atmosphere, where air is being evacuated at a rapid pace. To stabilize this loss of air and create a new equilibrium, the atmosphere has to "replace" this lost air by drawing from air at the surface. As divergence aloft intensifies, more air is needed to fill in this gap and air begins to rush into this area of lower pressure. What has just been described is the development of a surface low pressure. 

        As described, a surface low pressure strengthens as divergence or the winds aloft intensify. As a surface low intensifies, it creates more upward motion, drawing warmer air in and creating "lift." Therefore, we would expect a robust trough like this to create significant divergence with the development of a very intense surface low on the right entrance region of the fast winds aloft. Indeed, this is what happens as the GFS forecasts the development of a very strong, sub 980 mlb surface low to form. This strong of a low would promote significant warm air advection, strong surface winds, and the creation of a strong cold front. All these ingredients add up to the potential for a widespread severe weather event. 


Will We See Severe Weather Here in Virginia?

        Considering we are about 4-5 days out from this event, significant uncertainty remains for the severe weather potential in the Mid-Atlantic region. While uncertainty at this point is high, one thing is for sure: the SPC has had the confidence to outline much of Virginia in a "Slight" Risk Category for Day 5. A high degree of uncertainty remains especially over how much instability or "juice" there is for robust thunderstorm development. The GFS has hinted that there could be just enough "juice" for robust thunderstorm development along the cold front, which would have the potential to bring down some of those ferocious winds aloft. As is often the case for Mid-Atlantic severe weather events during this time of year, we are looking at a classic case of a "High Shear, Low CAPE" event in which the winds aloft are more than supportive...but the atmosphere is held back by a lack of "oomph" to get tall thunderstorms going. If enough instability can build and thunderstorms can become robust enough to tap into those dangerous winds aloft, we could see damaging wind gusts that could knock down trees and powerlines.  

Surface-Based Convective Available Potential Energy (SBCAPE) GFS Forecast. This measure provides an estimation of the potential updraft energy originating from the surface. While the forecast doesn't call for much, there are subtle hints of just enough "juice" to get vigorous thunderstorms going.  

   




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