More severe weather possible Sunday afternoon through Monday morning

/ April 18, 2020 at 5:26 PM

Our active spring severe weather season looks to continue Sunday night into Monday morning, as a potent storm system traverses a ripe, well-sheared airmass in the Southeast with the potential for tornadoes and damaging winds.

Severe weather outlook

The Storm Prediction Center once again has the Lowcountry in the “enhanced risk” area on Sunday, primarily for Sunday night into early Monday morning. The “Enhanced” risk generally means that there’s a 30% chance of a severe thunderstorm within 25 miles of a point; it’s smack-dab in the middle of a 1-5 scale. For reference, Monday morning’s risk was also Enhanced. Further to the west in Alabama, Mississippi, and Louisiana, a more widespread severe weather event appears likely, with the Storm Prediction Center placing a moderate risk in parts of those states.

A rough timeline

Model guidance is hinting at two rounds of severe storms near us Sunday into Monday. Right now, the best risk for severe weather in the Charleston metro area looks timed for overnight Sunday into early Monday morning.

Sunday afternoon and evening: Warm front is key

The weather feature we will be watching on Sunday is a warm front lifting north. You’ll be familiar with this front; this is the one that will knock humidity down overnight tonight as a cold front. Said front is going to stall out near the Florida/Georgia border and then lift back northward as the storm system takes shape to our west.

Where the warm front ultimately sets up is going to be key to the severe weather threat during Sunday afternoon and evening as an upper disturbance traverses the area. If it tracks a little further north, the Tri-County area gets into the warm sector, and with low-level wind shear being enhanced near the front, a few strong to severe storms would certainly be possible in the area with the potential for damaging wind gusts and tornadoes.

Right now, NWS favors a solution that keeps the warm front south of the area. Here’s an excerpt from this afternoon’s forecast discussion:

It is unclear exactly how far north the warm front will progress with diabatic processes associated with rain falling north of the strongest thermal gradient acting to slow its northward progression. This will have an impact on the severe weather potential as the bulk of the strongest instability should remain along and south of the sharpening warm front. At this time, the slower GFS solution is favored which matches the latest HREF trends. Although surface winds look to turn southerly across the entire area by afternoon, the strongest low-level baroclinicity looks to remain south the I-16 corridor suggesting that is where the main warm front will reside. This is where the best severe risk should be centered Sunday afternoon, highest near the Altamaha River.

National Weather Service Charleston, SC forecast discussion

Essentially, rain falling to the north of the front is expected to reinforce the relatively cool airmass there, making it harder for the warm front to translate northward across the area. We’ll see how this plays out during the afternoon. Thus, while the severe risk during Sunday afternoon and evening is low, it isn’t zero, especially if the warm front can nudge a little more northward.

Sunday night into Monday morning: Another squall line with damaging winds, strong tornadoes possible

As we get into overnight Sunday, the warm front will clear the Tri-County area in the wake of the upper disturbance producing rains earlier in the day, putting the area back into the warm sector and setting the stage for more strong to severe storms.

It appears the main severe weather event for the Lowcountry will once again be timed for overnight Sunday into Monday morning as a squall line organizes to our west and subsequently marches eastward to the coast. (Sound familiar?) Damaging wind gusts and embedded tornadoes — a few perhaps turning strong — will once again be possible.

The Storm Prediction Center has outlined the Tri-County area and points west in a 10% risk area for significant tornadoes. It is worth noting that while low-level shear will certainly favor the potential for embedded supercells and strong spin-ups on the leading edge of the squall line, the wind profile in the atmosphere is not as favorable for longer-track storms as it was this past Monday. Still, every event is different, and a poorly-placed tornado doesn’t have to be on the ground for a long time to still radically disrupt your life.

While tornadoes admittedly grab the headlines, the more widespread risk will be from damaging straight-line wind gusts. The wind doesn’t have to be spinning to be dangerous, after all.

While the storms will be moving through during a time that is usually a climatological minimum for severe weather, once again wind shear will be more than adequate to help aid along more limited instability. And while the synoptic-scale setup is not nearly as scary as April 13th’s, it’s still plenty good enough to support severe storms. Nocturnal severe storms are particularly dangerous; I have more on how to prepare for that later in this post.

Confidence in exact timing and intensity of the squall line is low at this point; check back for forecast refinements.

Heavy rainfall and flooding threat

In addition to the potential for severe weather, these storms will also pack quite a bit of rainfall with plenty of Gulf moisture in place. There is the potential for some flooding to develop where it rains the hardest. Generally, expect 1-3″ across the Tri-County area with locally heavier amounts. Areas inland of 17-A are likely to see the heaviest rainfall amounts at this point, but a poorly-placed downpour over Downtown Charleston during high tide could be enough to cause significant disruptions. Thus, the Weather Prediction Center has outlined the area for a slight risk of excessive rainfall.

Be prepared

Now is the time to once again review your severe weather plan. Last Monday likely put it through its paces, and it appears we will get another shot to give it a go this upcoming Monday. Now’s the time to examine your response to last week’s storms and tweak a few things if you need to. Remember, in a tornado warning, the safest place to be is on the lowest floor of a site-built structure, in an interior room or closet. If you live in a mobile home, you might want to seek out more substantial structure for Sunday night and Monday morning, like these folks in Hampton County:

The most important thing to do in advance of this event is to ensure that your weather alerts will be able to wake you, as it is conceivable that these storms could be an overnight affair especially the further west you go. Before you go to bed Sunday night, keep your phone off Do Not Disturb in case your weather app or a Wireless Emergency Alert needs to sound off. Make sure your NOAA Weather Radio is tuned to the right frequency, too. Use this website to find the closest transmitter.

Finally, advice around sheltering in the time of COVID-19 remains the same: Seek shelter first from the most imminent threat to your safety, the tornado, and then use physical distancing and other methods (such as masks) to help prevent the spread of the novel coronavirus.

Please stay aware of changes to the weather forecast throughout the day on Sunday as the event unfolds.

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