Friday’s severe weather threat coming into focus

/ April 18, 2019 at 12:31 AM
GOES-East mid-level water vapor illustrating the vigorous trough of low pressure which is kicking off severe weather in Texas and Oklahoma this evening.

Friday is an important day to watch in the weather department as there is good model agreement that a storm system will be affecting the area during the day, driving a squall line into the area by afternoon. Gusty winds, hail, coastal flooding, heavy rain, and perhaps even a tornado will be possible. Let’s take a look at some of the issues we may encounter.

Winds & coastal flooding

Time-height graph from Bufkit indicates the possibility for 40-50 knot gusts to mix down to the surface (yellow line is the mixing height; when the surface warms up during the day, the height of this layer of air nearest the surface called the boundary layer will tend to

Regardless of what thunderstorms do, winds will be turning downright strong on Friday as a surface cold front approaches the area. At the same time, some impressive upper-level winds will be overspreading the area. These winds look to mix down to the surface in the form of strong wind gusts over 40 MPH, even outside of thunderstorms. This will make driving over the bridges difficult, and could even bring a weak tree or two down if the gusts become even stronger.

These strong winds will also be coming in from the ocean. This combined with a full moon will contribute to some potential coastal flooding issues starting with Thursday evening’s high tide. High tides on Friday may also produce minor coastal flooding, though it appears right now that the heaviest rain will miss this timing by a few hours (thankfully!)

Squall line & severe thunderstorms

Severe weather outlook for Friday from the Storm Prediction Center, updated this morning, depicts an enhanced severe weather risk over much of the Carolinas.

Updated 7:30am on 4/18 to depict the enhanced risk area, tweak wording to lower hail risk, and reflect overnight thinking from NWS Charleston and the Storm Prediction Center.

A risk for severe thunderstorms will be evident Friday into Friday evening as a squall line approaches the area during the day. Wind profiles are going to be quite impressive as a 50-60 knot low-level jet develops during the afternoon. It may not take much in the way of instability to pull these damaging wind gusts to the surface in showers and thunderstorms.

How widespread the severe weather threat gets ultimately does revolve around the question of how much instability develops during the day. An earlier squall line passage generally will tap the instability early on, limiting the severe threat. If the squall line arrives after sunset, some stabilization may occur, also limiting the severe threat. Right now, the most likely timing is in the early afternoon to evening hours (I’d call it 2-5 PM, but don’t hold me to that exact number just yet). This should give the atmosphere enough time to charge up to allow the squall line to maintain itself well to the coast. If this can happen, thunderstorms within the line can tap into some incredibly strong winds aloft (shown earlier) and bring those down to the surface as damaging wind gusts.

There is, as always, the issue of the cooler coastal waters. Despite the fact that they are slowly warming, sampling upper-air profiles of the environment directly offshore suggests that there will be some at least slightly more stable air coming ashore with the ridiculously gusty southerly flow. If this comes through for us again, then at a minimum we may see the storms become a little more elevated, reducing (but not eliminating) the damaging wind threat as they approach the Charleston area. However, the dynamics aloft are such that this may not matter so much.

Tornadoes won’t be the main threat for our neck of the woods, but they will still be a possibility as they often can spin up (and spin down) quickly on the leading edge of squall lines. If the line moves a little more slowly, a couple discrete thunderstorms could possibly get going ahead of the line, increasing the tornado threat. This doesn’t appear to be a likely scenario for Charleston right now, though; it is more likely to occur in northeast South Carolina and eastern North Carolina.

Some hail will be possible with these storms as well, but it right now does not appear that this will pose a considerable threat.

Bottom line on severe weather

What to do between now and Friday

Here are a few good things to do between now and Friday:

I’m targeting sometime during lunchtime on Thursday with an update on the severe threat as more data and updated outlooks come in from the Storm Prediction Center. Stay tuned…

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