A strengthening cold front sweeps through our area overnight bringing the threat of thunderstorms. The ingredients and setup we need to get strong storms during the winter is a little different than what we look for in the summer, so this post will focus more on the why behind this storm threat. If you're just looking for timing and AdvanceTrak images, scroll to the bottom of this post :)
The Storm Prediction Center has placed or southwestern communities in a Slight risk of severe storms tonight. That particular risk level means isolated severe storms will be possible, but the threat is not widespread. The main threat is strong wind gusts, but localized flooding and isolated quick spin-up tornadoes cannot be ruled out.
We don't have a lot of CAPE. In full disclosure I picked the model that shows the most CAPE (or instability) to put in this post. This is what we call a "low CAPE, high shear" environment where wind drives the action. But any instability at all this time of year can add fuel to the storms, so while this is a low amount of CAPE, it may be enough to help strengthen our storms.
As mentioned above strong wind gusts at the surface are the main concern with this line of storms, and wind is the driving force behind the threat of strong storms. At the surface we have already seen wind gusts in the 20-30 mph range and those will continue tonight. When storms roll through later, gusts to 40-50 mph will be possible.
The wind speed and direction higher up in the atmosphere are important parts of this setup. The low level jet (shown below) is a little less than a mile above our heads. Overnight the wind really strengthens at this level which is why we could see stronger wind gusts at the surface.
Higher up at the jet stream level (where airplanes fly), you can see a trough digging in (a.k.a. amplifying) over our area. That places us in the right entrance region of this jet streak which increases vertical motion in the atmosphere. Plus the wind speed inside the jet streak is really picking up as the trough deepens over our region.
It's also worth mentioning temperatures climbed into the upper 60s and some low 70s in our area this afternoon and dewpoints are climbing into the lower 60s. That's unusually warm and moist for this time of year and certainly helps our storm chances tonight. Temperatures won't drop very much until the cold front passes through and the moisture will keep climbing until that time.
All of these factors combine to tell us wind is the main threat tonight, but an isolated tornado, while much less likely, cannot be ruled out.
Around 10-11 PM the activity will start to ramp up with the stronger storms most likely between midnight and daybreak Monday morning.
Tune into WDRB News tonight at 10 to see where the storms are and the new data about timing and threats overnight.