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Yellow began creeping back into southern Virginia on the U.S. Drought Monitor maps last week. And yellow is painting much of Virginia on the Storm Prediction Center maps for today and Thursday.
Those yellow tones mean very different things, and in theory, they could mean opposite things, though that’s not entirely settled this particular week.
On the Drought Monitor maps, yellow means “abnormally dry.” Copious rains in December and January washed all the different colors that had stained a fiery fall off the Drought Monitor in Southwest and Southside Virginia by the start of February. But then it started getting a little dry again, and last week, yellow for the “pre-drought” stage of dryness again covered much of Southside northwest to Roanoke and most of the New River Valley, plus about half of Lee County at the southwest tip of Virginia.
On the Storm Prediction Center map, yellow represents a “slight risk” of severe thunderstorms, or level 2 of 5 on a rating scale for the likelihood of a severe storm — 58 mph wind gusts, 1-inch diameter hail or a tornado — occurring within 25 miles of a given location.
“Slight” can sound deceiving — it doesn’t mean there is a slight chance of severe storms occurring within the entire zone covered. Rather, it means severe storms are likely to develop but their coverage will be scattered enough that it’s unlikely to occur right at your house, but possible. Or, similarly, many storms will develop and only some of them will be severe, (We covered what the Storm Prediction Center risk ratings mean in much more detail in this space last spring, linked here.)
The Southwest corner of our state has actually been bumped into an orange shade called “Enhanced Risk” which is level 3 of 5 on the severe storms risk scale. It is possible more of our region could be raised to “Enhanced Risk” before the storms arrive. Also, weather service offices in Morristown, Tennessee, and Charleston, West Virginia, had placed seven Southwest corner counties under a flash flood watch for the potential of locally heavy downpours.
What is happening in the big picture is that a trough of low pressure that has been bringing round after round of severe thunderstorm and tornadoes to the central U.S. is shifting eastward.
On Tuesday, the focus of severe storms moved to the Ohio Valley and Great Lakes region, repeating the pattern that has been with us since winter, where the strongest dynamics of storm systems track to our north and west. That has spared us winter storms and has also spared us having a lot of severe storms and kept us out of widespread rain, thus far.
In this case, the advance of a cold front with some reasonably strong winds aloft and decent lapse rates — cool, dry air overtopping warm, moist air near the surface — will create the potential for strong to severe storms across our region overnight on this Wednesday and then developing anew on Thursday. Locally damaging winds and some isolated large hail are the most probable severe storm effects, but there is a chance of tornadoes, also. (We discussed here a couple weeks ago why tornadoes aren’t frequent, but sometimes occur, in our region.)
The severe risk will be greater to the west over Southwest Virginia on this Wednesday evening and overnight, as storms that develop in the Tennessee Valley move in from the west, and then greater along and east of the Blue Ridge on Thursday with new development in warmth and humidity.
UPDATE 5/9/2024, 1PM: The Storm Prediction Center has reduced the severe storm risk to “marginal,” or level 1 of 5, for all of Virginia on this Thursday, excluding a few counties in the southwest tip that are now out of the severe risk altogether. The greatest instability and wind dynamics have pushed into the Deep South, and the combination of instability-reducing debris clouds from overnight storms to our west and southwest and downsloping westerly to southwesterly winds drying out as they cross the Appalachians will greatly limit storm development in the commonwealth. A few storms may still develop with the advance of a cold front, with locally damaging wind gusts a possibility, but they will likely be isolated to widely scattered. END UPDATE
The cold front will eventually lead to a cooler, drier period entering next week, a break from recent warm, sticky, showery weather. At this time, it does not appear the cool air will be deep enough for a widespread frost/freeze situation, mostly 40s lows. Warmth will recover next week.
Dryness-ending rains?
So if we’re having all these thunderstorms, that should take care of the dryness, right?
Well, not necessarily.
Thunderstorms, even when they’re fairly numerous, tend to bring rain in a streaky fashion. Some locations get a ton of rain under the tracks of the heaviest storms, an inch or two, sometimes more. Other locations between the heavier downpours get less, sometimes nothing.
We saw this happen on Sunday and Monday, when rain bands and storms were unevenly distributed across our region. Some places got more than inch, others got a quarter-inch or less.
Danville, which just had its fourth driest April in over a century of records with only 0.82 inch total for the entire 30 days, quickly collected nearly 2 inches Saturday through Monday and was getting yet more heavy rain with a severe-warned thunderstorm as I type this on Tuesday evening.
Blacksburg, which experienced its fourth driest March-April period (3.76 inches) and fifth driest April (1.41 inches) on record, managed a little more than a half-inch early this week in a couple of lighter rain periods.
So this week’s yellow-level storms may not entirely wipe out the developing yellow-level dryness.
Journalist Kevin Myatt has been writing about weather for 20 years. His weekly column, appearing on Wednesdays, is sponsored by Oakey’s, a family-run, locally owned funeral home with locations throughout the Roanoke Valley.
