A strong area of low pressure developing over the Ohio Valley tonight doesn't only spell tribble for the winter fatigued from Chicago to Maine expecting as much as two feet of snow, but a powerful cold front encroaching on a warm, moist airmass over the Mid-Atlantic will help touch off a nasty line of thunderstorms that could be severe, including the potential for a weak tornado or two.
The colorful map above shows the predicted surface temperatures (color gradients), surface pressure in millibars (black lines), and wind speeds (barbs) from this afternoon's run of the GFS model, valid around 6PM Eastern tomorrow afternoon.
A cold front that extends off the low pressure center will result in a very impressive drop in temperatures, causing readings to plummet 30 to 40 degrees Fahrenheit in just a of a couple of hours when it passes through, as shown by the dark blue shading on map below.
This powerful cold front will move into relatively warm, moist air and serve as a focus for the development of a strong line of thunderstorms.
The main threat for severe weather exists within the area shaded in yellow on this map from the Storm Prediction Center (SPC). The yellow area means that there is a "slight risk" for severe weather (damaging winds >60 MPH, quarter size hail or larger, or a tornado), while the areas shaded in green could see general thunderstorms that aren't bad enough to be considered "severe." A slight risk indicates that the chance exists for severe weather but it's not going to be a massive outbreak.
For the weather geeks among us, here's the exact wording from the SPC's discussion this afternoon:
STRONG CONVECTIVE MOMENTUM TRANSPORT WILL SUPPORT THE POTENTIAL FOR DAMAGING WIND GUSTS...WITH THIS POTENTIAL BOLSTERED BY THE STRONG CROSS-FRONTAL...SYNOPTIC PRESSURE RISE-FALL COUPLET. STRONG LOW-LEVEL SHEAR — E.G. AROUND 30-40 KT OF 0-1-KM BULK SHEAR — WILL SUPPORT SOME POTENTIAL FOR LINE-EMBEDDED MESOVORTICES TO PRODUCE TORNADOES.
The primary concern with these thunderstorms will be damaging winds and the potential for one or two weak (but still dangerous) tornadoes.
Here's a look at the 700 millibar (between 9,000-10,000 feet) winds and heights from this afternoon's run of the GFS model, showing winds at this level of the atmosphere raging at 60-70 knots, or 70-80 mph, over the area the SPC outlined as most at risk tomorrow.
Given that these strong mid-level winds are almost aligned with the forward motion of the expected line of thunderstorms, it won't take much for the downdrafts (sinking air) in the storms to drag these severe winds to ground level and produce damage.
The risk for tornadoes tomorrow is small but it isn't null, as low-level wind shear (wind changing speed and direction with height) will allow for some thunderstorms in the squall line to begin rotating and spin off a weak tornado or two. Even though the risk is small and the tornadoes will be weak, even a "weak" tornado is dangerous if it hits your house or your car. Take the threat seriously if it plays out as forecast.
Stay on top of the situation tomorrow by following updates from your local National Weather Service office, as well as local news affiliates. As spring draws closer and the bulk of severe weather season approaches, it wouldn't be a bad idea to invest in a weather radio.