Thursday Severe Weather Threat

The Storm Prediction Center has been outlining the potential for severe weather across the Mid-Atlantic for several days now. It appears the greatest threat is along and east of I-95 mainly in Virginia and North Carolina, with our area being on the northern fringe of the worst. However, as we’ve gotten closer to the event, the models have been bringing better instability further north, and as of the mid-day runs on Wednesday, have sufficient instability into southern Maryland. If this continues, I’d expect the SPC to bring the enhanced risk farther north.

If you live in southern Maryland, you have the greatest threat for severe weather tomorrow. This will include the potential for damaging straight-line winds, tornadoes, large hail, and very heavy rainfall. The farther north you live, the lower the threat, and in particular the tornado threat. Heavy rain and some gusty showers/storms are likely across northern Maryland, but the threat for severe weather will be much lower.

Damaging winds are the primary threat, as usual, with large hail and isolated tornadoes also possible.

 

Inside the forecast…

There will be possibly two rounds of storms tomorrow. One between 2AM and 8AM, which will be mostly showers and thunderstorms. It looks more scattered during this time frame, and not everyone may see storms.

During round one, the severe threat should remain limited, but a few stronger storms are possible with gusty winds. Then we’ll get a break, before another round moves in 11AM – 2PM. Round 2 is the one with the greatest severe weather potential.

 

At this point, nothing is set in stone, and we’ll have to watch how the environment responds tomorrow morning to determine if the severe threat will materialize. Temperatures are expected to jump during the morning, with southern areas reaching the upper 60s to low 70s, with dew points rising into the 60s, providing some warm, moist air for the next round to feed off of.

 

One of the things that has been arguing against severe weather is the early morning cloud cover and rain, in addition to, an easterly component in the wind across much of the area. This combination would likely result in a stable layer of air over the region, and prevent any thunderstorms from forming.

However, the wind field aloft is more than favorable for severe weather, and if any instability can build in, a nasty severe weather event could unfold. The following map shows the winds at the 500 millibar level (500mb is a pressure level, about 20,000 ft above the surface and is roughly the midpoint of the lowest layer of the atmosphere). The winds at this level are 60 – 100+ knots (70 – 115+ MPH) and this maximum of winds is called a jet streak. A couple things are happening on this map. First, as the air flows around that upper low on the east side, the air begins to spread out (notice the solid black lines spread out going north from our area into New England). Being technical, this is called diffluence, but what it does is provide a mechanism for air to rise – and that is a key ingredient in any storm forecast. The other thing, notice how that jet streak ends right around our region (where the reds and yellows end). As air leaves a jet streak, it also tends to support rising air. It goes without questioning, that we have more than enough uplift for storms tomorrow.

Now if we look a little lower in the atmosphere, about 5000ft above the ground, we still have a very strong wind field. Winds at this level are over 60 MPH, and if any storms can become strong enough, they can tap some of this wind energy and transfer it to the surface.

There is also sufficient turning of the winds, and because of this, the tornado threat is not zero. The winds at the surface will be out of the south-east, but at 5000 ft they are out of the south. Fortunately, the winds do not continue turning more southwest the higher we go in the atmosphere. It’s enough turning of the winds for an isolated tornado threat, but not enough for a tornado outbreak.

Here’s just a couple more maps to show the potential threat. Remember, this is still not set in stone, and if the stable layer decides to be extra stubborn, the severe threat won’t materialize across the area.

Storm Energy, or called Convective Available Potential Energy (CAPE), and is a measure of instability. Notice how the model brings some higher instability further north into Southern Maryland. A day or two ago, the instability failed to make it north of the Potomac.

The Supercell Composite Parameter (SCP) is a method of seeing whether supercells are likely, where higher values indicate a better chance of seeing a supercell thunderstorm (possible rotating updraft and large hail). It does NOT guarantee that a supercell thunderstorm will form. If you draw a horizontal line through DC, anyone south of there has a good chance of severe weather, per this particular Parameter. Again, this does not guarantee anything. The values of the SCP are not incredibly strong, but are high enough to warrant a severe weather risk.

The Significant Tornado Parameter (STP), is a method of seeing whether tornadoes are likely, where higher values indicate a better chance of seeing a tornado, and possibly increases in strength. Again, it does NOT guarantee a tornado will occur. Again, areas south of DC look to be in the most favorable position for seeing severe weather. The values of the STP are not incredibly strong, but are high enough to warrant an isolated tornado risk.

 

Stay tuned to the forecast and be sure, especially if you live across southern areas, that you have a method to receive any warnings that come out.

-Andrew