Tuesday September 21 2021
A Flash Flood Watch has been issued for western Maryland from Frederick to Garrett County, and the mountains of Virginia and WV. I want to first point out that the time frame is from Wednesday to Thursday morning.
Below we will see when that rain will shift east, and how much it may bring to metro areas that are concerning.
Flash Flood Watch
Rainfall Potential: ECMWF
The European Model forecast rain is mapped here to Friday morning. Notice the storm total (after the current ‘Flood Watch’) brings over 1 inch to many areas in central Maryland, southern PA, and Delmarva.
This model was on the higher end of guidance with TS Ida, and ended up being closest to what verified.
Afternoon Surface Map
The reason for the heavy rain is two-fold. A slow moving cold front located near Chicago and St. Louis will take until Friday to reach the coast.
Low Pressure with a tropical moisture plume from Florida and the Carolinas is pumping rain into the mountains.
This will all combine to the event tracked below.
Rain Timeline Animation
Here the European Model shows the developing showers on Wednesday, with most of the steady rain in the mountains.
That line will swing through central Maryland on Thursday afternoon and evening. Thunderstorms and downpours can be expected.
Rainfall Potential: GFS Model
This is another forecast plot, showing a little less rain in the mountains. However it will still shows over 1 inch in central Maryland and Delmarva.
This may be enough for some flooding with that cold front.
This is why I think the Flash Flood Watch may get expanded farther east for Thursday… But that would be updated as the time gets closer.
14 Local Maryland Pages (and York PA)
We have made a page for Maryland Weather which gives you the current conditions for 14 present area locations.
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