Flood watch: How much rain, where, and when?
Snow, thunderstorms, end of drought?
As we mentioned in our forecast this morning, the National Weather Service has issued a Flood Watch for the metro region Wednesday night through Thursday night. The NWS is calling for up to 3 inches of rain - enough to send creeks and streams over their banks. At the moment, our forecast is for 1.5-2.5 inches of rain, with locally heavier amounts possible. Of course, this is on top of the 1-3 inches of rain that fell Sunday. So the ground is fairly saturated.
When will the rain start?
Light rain showers may begin as early Wednesday afternoon or even late morning in the western suburbs (where a little sleet could mix in if the precipitation starts early enough).
When will the heaviest rain fall?
Right now, models indicate Thursday mid-morning through late afternoon will be prime time for the heavy rain - although this timing could shift. The afternoon rush hour Thursday could be most impacted. At this point, lighter rains seem likely for Thursday morning's rush hour, but stay tuned.
Where will the heaviest rain fall?
That will depend on the exact track of the area of low pressure riding along the front moving through the region. The heaviest rain will tend to fall just west and northwest of the low. The models show a range of tracks, most favoring heavier rain to set up west of I-95 like Sunday's event with just one, the GFS, showing the precipitation bullseye farther east. At this point, there is the potential for 1-3" or so pretty much everywhere in the region. A few spots could even see a little more than that, particularly if the axis of heaviest rain sets up over the mountains, where rainfall is enhanced due to lifting (orographic effects). We'll try to pin down where the axis of the heaviest rain will fall tomorrow, but am leaning toward west of town at this point.
How much rain do the various weather models simulate?
The NAM indicates 1.5-2", with heaviest amounts along and west of I-95
The GFS indicates about 1.5-2", with heaviest amounts along and east of I-95.
The European model indicates 1.5-2.5" of rain, with the heaviest amounts west of I-95.
The SREF model (mean) indicates 1.5-2" of rain, with heaviest precipitation west of I-95.
Meteorologist Howard Bernstein at WUSA-TV9 indicates a proprietary high resolution model called the RPM simulates 4-6" of rain along the I-81 corridor in western Virginia.
What's causing the heavy rain?
A slow moving but strong cold front coupled with a deep feed of moisture from the south are the key ingredients for the heavy rain. This is a very similar setup to Sunday.
Could there be sleet or snow with this storm?
It's not completely out of the question, but most likely in the high elevations of western Virginia, Maryland and eastern West Virginia. It could actually be an issue on both the front and back end of the storm.
If the precipitation moves in early enough tomorrow (before noon, 30% chance), temperatures in the colder suburbs to the west and northwest may be sufficiently cold for a little sleet at the onset. Accumulation is not expected.
As the storm pulls away Thursday night, the rain could briefly mix with or change to sleet/snow east of the mountains, but odds are just around 20% in colder north and west suburbs, and around 10% inside the beltway. The push of cold air is a little weaker than last Sunday night's so little or no accumulation appears likely (except possibly light amounts in the mountains).
Could there be severe thunderstorms?
The risk of severe thunderstorms will probably be confined to locations southeast of Fredericksburg as cool, stable air should limit thunderstorm intensity in the metro region. Nonetheless, some thunder and gusty winds are certainly possible during Thursday's heavy rains when the front passes through.
How severe is the flooding threat?
This storm would seem to represent a low to moderate flood risk relative to the kind of flood events we've experienced historically. Except in the mountains, it doesn't have high potential to drop 5-10 inches of rain like we sometimes see during June flood events and from tropical systems in the fall. On the other hand, the rain will be falling on top of 1-3" from Sunday, so five-day totals could exceed five inches in some locations east of the mountains.
Will the rain close the rainfall deficit?
As a result of Sunday's rain, the precipitation deficit for the year is now under 1" at Reagan National Airport. However, we were in the "red" for precipitation during November and December, so if you factor those months, the 4-month rainfall deficit is about 3". This rain event should go a long ways to filling that gap.
How should I prepare for the prospect of flooding?
Follow this link for flood safety and preparedness. The main thing to remember is never to attempt to drive through a flooded roadway.
Other rainfall assessments:
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