Saturday, April 20, 2019

General Weather For and Regional Rainfall Update Through April 20th, 2019

**This entry was posted April 20th, 2019.**

Nice evening as seen from the 1500 block of V Street NW, Washington, D.C., 5:57 p.m. April 20, 2019

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This is a weather update that I've separated out from my main entry -- otherwise that entry would just run on excessively long.

Today was a variably cloudy, mild, and breezy day with a gusty southerly wind and temps reaching 74F (at both KDCA and KBWI while KIAD climbed to 72F).

NWS high-resolution surface weather map for the eastern United States, valid 12Z 19 April 2019

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A cold front swept through last night with heavy rain showers. In fact, there were squally showers and some severe weather that produced an EF0 tornado in Reston that downed some large trees and caused structural damage to a few houses. The weather was part of a large storm system that brought severe / tornadic storms to parts of the Southeast.

Some radar imagery from yesterday …

Wakefield (AKQ) NWS radar in standard composite mode looped 11:19 a.m. - 11:47 a.m. EDT April 19, 2019

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SECONUS quadrant base reflectivity mosaic looped 1358 - 1508 UTC April 19, 2019

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Sterling (LWX) NWS radar in standard composite mode looped 6:55 p.m. - 7:45 p.m. April 19, 2019

This radar loop shows a few tornado warnings that were in effect -- including for the storm that brought aforementioned minor tornado to parts of the D.C. Virginia suburbs.

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We actually had a flash flood watch and a tornado watch in effect at some point yesterday but the rainfall amounts in the immediate area weren't that great. It was mostly just a warm, windy, showery day.

Some weather advisory maps from yesterday …

NWS weather advisories as of 1538UTC (11:38 a.m. EDT)
19 April 2019

This doesn't include the legend but the sea green colors refers to flash flood watches and flood watches and yellow are tornado watches. It's noteworthy that there were such extensive flash flood watches across the eastern United States.

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Some NWS Forecast Office advisories …

Philadelphia / Mt. Holly (PHI) county warning area (CWA) weather advisories updated
11:33 a.m. EDT April 19, 2019

The entire CWA and adjacent CWAs on this map were under a flash flood watch. Again, you don't normally see that. 

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Sterling (LWX) CWA weather advisories updated
11:38 a.m. EDT April 19, 2019

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New York / Upton (OKX) CWA weather advisories updated 11:40 a.m. EDT April 19, 2019

This entire CWA was likewise under a flood watch or flash flood watch. Actually, the two colors are the same -- and I'm not sure which was which (you'd have to click on the actual point grid forecast to see the advisory). 

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Sterling (LWX) CWA weather advisories updated 
1:16 p.m. EDT April 19, 2019

The flash flood watch was still in effect but the tornado watch is considered higher priority and takes precedence. The full list of weather products and their rankings are available here.

The highest priority is a tsunami warning (#1) followed by a tornado warning (#2).

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Rainfall amounts yesterday were as follows:

KDCA: 0.31 inches
KBWI: 0.36 inches
KIAD: 1.27 inches

Yes, Dulles had quite a bit more rainfall. Both KDCA and KBWI are actually below normal for the month.

The month- and season-to-date totals and departures are as follows (where "spring season" started March 1st):

KDCA: 1.62" -0.42" (2.04") / 5.62" -0.10" (5.52")
KBWI: 0.97" -1.18" (2.15") / 5.11" -0.94" (6.05")
KIAD: 2.24" -0.07" (2.31") / 6.79" +1.10" (5.69")

The year-to-date totals are as follows:

KDCA: 12.44" +1.49" (10.95")
KBWI: 11.90" -0.10" (12.00")
KIAD: 14.25" +3.14" (11.11")

2018 was a record wet one for all three airports, we are receding from a record wet 365-day period. I think we got quite close in March at some point -- and maybe one of the airports actually did reach that tally.

Returning to the weather, the reason the winds were southerly despite the cold frontal passage because this front crossed the region in more of a southwest to northeast fashion even while the main upper level low hangs back over the southern Appalachians.

NWS/WPC/NDFD U.S. surface weather map forecast looped, as shown, from 18Z 19 April - 12Z 21 April 2019

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Tonight's Sterling (LWX) area forecast discussion includes the following …

The upper low will track overhead on Sunday, aiding in the development of scattered shower activity during the midday and afternoon hours on Sunday. Not expected a total washout, and any showers will be transitory and generally light in nature. Will be a touch more cloud cover over the area as a result, and with the wind field turning more west northwesterly, temperatures will hold in the low to middle 60s.

NWS/WPC/NDFD U.S. surface weather map forecast looped, as shown, from 6Z 21 April - 0Z 23 April 2019

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The upper low will be slow to track to our east Sunday night into Monday morning, so lingering showers will remain in the forecast. High pressure building to our south will finally win out the second half of Monday and into Monday night with drier conditions expected. Temperatures will be moderating, running around 5 to 10 degrees above normal

NWS high-resolution surface weather map for a portion of the eastern United States, valid 0Z 20 April 2019

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OK, that wraps up the weather entry. I'm going to post another update entry shortly.

--Regulus

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