By: Peter Forister, Denver Murray
This is a fairly typical late fall setup for severe weather in the Mid-Atlantic, but there are a few details worth exploring that stand out. This won’t be a “record” severe event, but because of the timing with Halloween festivities in the evening, please pay careful attention and be weather aware
This event is being driven by a very strong low pressure over the Great Lakes. While that low pressure continues to strengthen, it will pull warm air up from the Gulf of Mexico all the way into PA. Additionally, record cold air has been present over the West and Midwest over the past few days, bringing snowstorms to places like Colorado. The cold front that will result from these two air masses interacting will produce one of the strongest fronts that we’ve seen so far this year. It will move fast! The forward speed of storms connected to the front will be 50-70mph. Key takeaway: the storms will move fast and the temperature will drop fast.
ENHANCED RISK (LEVEL 3/5) FOR MUCH OF THE MID-ATLANTIC
The Storm Prediction Center has an ENHANCED RISK across much of the Mid-Atlantic. Even if you are not in the Enhanced Risk, severe storms are still a concern across the Slight and Marginal Risk areas as well. All modes of severe weather will be possible this afternoon and evening, including damaging wind gusts, flooding, and the potential for a few tornadoes.
STRONG, DAMAGING WINDS POSSIBLE
The National Weather Service has issued Wind Advisories for Southwest Virginia and the Shenandoah Valley for this afternoon into tonight. Wind gusts could reach 50 mph. A High Wind Watch is in effect for the Roanoke Valley, where wind gusts could exceed 60 mph. Strong winds will continue well after the frontal passage this afternoon and evening.
Winds will increase throughout the afternoon, ahead of the front. Wind gusts could be as high as 50 mph across many locations. The strongest winds will arrive along the squall line and after the front, with a wind direction change from the south/southwest to northwest. Wind damage is possible, including down trees, power lines, and structural damage to homes and property.
STRONG WIND SHEAR: INGREDIENT FOR TORNADOES
Shear is very strong! Upper level winds and mid level winds will be CRANKING with a deep trough. The jet stream will peak at 120kts in the afternoon. Winds much lower will also be strong, around 70kts at 850mb (a few thousand feet up). With any thunderstorms, this wind could be pulled down to the surface pretty effectively, so there will probably be plenty of severe thunderstorm warnings tomorrow with storm winds ranging from 50 to 70 mph at the surface. In addition, wind will probably “veer” with height, allowing for some rotation in the stronger storm cells.
Our TORNADO INDEX is “medium” across the Shenandoah Valley into Southern Virginia and the North Carolina Piedmont. This means a 40 to 50 percent (or 4 or 5 out of 10 chance) of seeing a tornado within 25 miles of a location.
Instability could be higher than normal (but there is a lot of uncertainty). CAPE values of around 500 j/kg could provide enough fuel for a few strong storms, but if it gets much higher than that (i.e. around 1000-1500 j/kg) the storms will be much stronger. This will be one to carefully watch through the afternoon, as CAPE is extremely hard to forecast for events like this.
There will be lots of “SRH” ( Storm-Relative Helicity) available tomorrow, which is a measure of how much rotational shear/energy is available for storms to use. However, you have to have more than just SRH for supercells to form: The storms also have to become isolated, or “break up” in a line. If this doesn’t occur, any rotation will be deeply embedded in rain and probably not very strong. If the line breaks up a bit, then the storms will be more likely to have a tornado risk. Again, this will be a “real time” thing to watch on radar.
We except a squall line to produce strong to severe thunderstorms, including strong to straight-line damaging winds, very heavy rain, and a few tornadoes. This is coming AT THE WORST TIME — just in time for Trick-Or-Treating.
IT IS STRONGLY ADVISED THAT YOU DO NOT GO OUT, EVEN AFTER THE SEVERE WEATHER IS OVER, due to wind damage and possible flooding.
Our HIGH WIND THREAT INDEX includes strong winds (50-60+ mph) during any time this afternoon and evening — including the strong winds behind the front this evening into the overnight.
HEAVY RAIN THREAT & FLOODING
Precipitable Water table shows a very moist environment ahead of the cold front. Strong south/southwesterly winds will help to pump in moisture throughout the day. Once these two air masses “clash” together, it’s like a sponge wringing out all the available moisture. The atmosphere is extremely moist and the strong front will help to produce very heavy rainfall. There could also be several hours of rain on the back end of the front.
FLASH FLOODING IS POSSIBLE ALONG THE BLUE RIDGE AND ALLEGHENY HIGHLANDS. Across the higher terrain, rainfall amounts could range 3 to 5 inches, with 1 to 3 inches of rain across lower elevations of Central and Western Virginia. We are currently in a drought, and unfortunately, most of this rain will be runoff (not sinking into the ground.) All of this water has to go somewhere and areas just east of the mountains should monitor local creeks, streams, and rivers, and move to higher ground if flooding begins to occur in your area.
IN CONCLUSION: The storms will be strong, quick moving, and potentially rotating through the area this afternoon and evening across the Mid-Atlantic. Stay tuned to the latest forecasts with us, NOVA Storm Chasing, and your local trusted media. But ESPECIALLY pay attention to the NWS forecast and warnings through the day tomorrow for your specific location. This won’t be a “record” severe event, but because of the timing with Halloween festivities in the evening, please pay careful attention and be weather aware.