Thursday, April 30, 2020

Arriving Next on the Atmospheric River Track, An End-of-April Scheduled Rain Train: Capping Off This Month With a Deluge

Constitution Gardens view, Washington, D.C., 3:28 p.m., April 29, 2020

On Wednesday, I walked into my little L'Enfant Plaza office. The weather was about 74°F warm, dew point around 55°F, and a humid-feeling easterly breeze carrying its maritime ocean. My route was a bit more southerly and I intersected the Mall area at the northeastern edge of Constitution Gardens.

Another view of Constitution Gardens, Washington, D.C., 3:28 p.m., April 29, 2020

Seemingly missing was the usual infestation of aggressive, shit-filled-and-dropping Canada geese, although I'm sure they were around by the edge of the pond. God, they're horrid creatures. That makes them perfectly adapted to an human urban American environment.

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This is a weather-themed entry because, yes, interesting and noteworthy weather can and still does happen even during a global viral quasi-pandemic (about that, once again, the COVID-19 and political-themed posting is delayed).

NWS 24-hour quantitative precipitation forecast (QPF) for the U.S. Lower 48 and adjoining regions (in inches)
valid 0Z 30 April 2020 to 0Z 01 May 2020

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The information and images in this entry are taken from both the National Weather Service and the following Capital Weather Gang entry (link embedded): Heavy rain to drench Washington on Thursday, with flooding possible.

NWS 72-hour QPF for the U.S. Lower 48 and adjoining regions valid 0Z 30 April 2020 to 0Z 03 May 2020

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A heavy rainfall event is progged for the Eastern Seaboard to include the "DMV" -- Washington, D.C., much of Maryland, and Northern Virginia -- over the next 6 to 18 hours, as reckoned from midnight Eastern time.

Baltimore / Washington NWS ("Sterling" (LWX)) county warning area (CWA) 36-hour rainfall forecast map
through 8 a.m. EDT May 1, 2020

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As this CWG entry notes, quoted / adapted:

The setup for heavy rain Thursday features a combination of potent moisture surging north from the Gulf of Mexico and strong dynamics in the upper atmosphere that will promote vigorous and sustained uplift of air.

NECONUS mosaic base reflectivity radar looped
0228 UTC - 0338 UTC 30 April 2020

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First, the moisture piece. A narrow conduit or plume of high humidity air, termed an “atmospheric river,” will race north on southerly winds. As shown below, the moisture-sodden winds are part of the broad flow of air drawn into a low-pressure system over the Great Lakes.

NWS high-resolution surface weather map for a portion of the eastern United States valid at 0Z 30 April 2020
(8 p.m. EDT April 29, 2020)

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Concerning the upper level dynamics, the jet stream will be surging from the south, curling counterclockwise around a massive low-pressure region ambling across the eastern United States.

SECONUS mosaic base reflectivity radar looped
0228 UTC - 0338 UTC 30 April 2020

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The flow configuration at all levels will be from the south, building an impressive corridor of deep tropical moisture. This will be a slow-moving system, which will help rain totals pile up east of the Blue Ridge.

Severe weather is not expected because the surge of air from the south will bring more moisture than low-level warmth this time around, limiting atmospheric destabilization.

NWS/WPC/NDFD U.S. surface weather map forecast looped 06Z 30 April to 06Z 02 May 2020 showing fronts, precipitation type, and likelihood

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While embedded convective cells will develop in the plume, with perhaps a rumble of thunder, we do not expect damaging thunderstorms. With low-level winds paralleling the cold front (slowly approaching from the west), this is a classic setup for “training” of rain cells, i.e., a "rain train." This refers to the tendency for cells to repeatedly pass over the same regions, from south to north.

3-km NAM 0Z 30 April 2020 simulated radar and mean sea level pressure (MSLP) for the northeastern quadrant of the U.S. Lower 48 looped between hours 12 and 19

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Rainfall amounts for the DMV from the panoply of models (NAM, high-res NAM, HRRR, GFS, European / ECMWF, Canadian, high-res Canadian, and UK Met) range from 1 to 2 inches, although the NWS actually has amounts progged a bit higher in spots such as the Blue Ridge and into northeastern Maryland.

Top 10 wettest Aprils in Washington, D.C., base period 01/01/1871 - 04/27/2020

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KDCA is presently up to 5.20" for the month of April (more on that below). As the above chart indicates, if at least 0.77" of rain occurs at National Airport (KDCA) tomorrow, it will push D.C. into at least tied for the 10th wettest April on record. If 1.69 inches fell before May 1st, it would bump up this April to 2nd wettest. (They're very tightly spaced.)

NWS weather advisories for the United States with legend updated 0457 UTC 30 April 2020

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The axis of heavy rainfall will shift toward the Atlantic coast with the upper low diving into West Virginia and then over the Virginia/North Carolina boarder on Friday. The result will be cloudy, and daytime cool conditions (55 to 65F range) with a chance of showers.

GFS 12Z 04/29/2020 300-mb heights (in dekameters) and wind (in knots) valid 18Z 4/30/2020 as prettied up by TwisterData.com

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Tonight's updated Sterling (LWX) area forecast discussion indicates that as a warm front crosses the DMV ahead of a highly north-south elongated cold front while the meridional elongation of the upper level low will cause upper level winds to back from the southwest to due southerly, resulting in a tropical plume of moisture with PWAT values reaching 1 to 1.5 inches and surface dew points around 60F. Southerly winds could gust in the 30 to 40 mph range early Thursday.

Precipitable water (PWAT) values for the northeastern quadrant of the Lower 48 as forecasted by the GFS 06Z 20 April 2020 valid 2 p.m. EDT April 30, 2020, as prettied up by Pivotal Weather.

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General flood watches are in effect across much of the DMV and into Pennsylvania, New Jersey, and southern New York, as well as the lower Tidewater area.

Sterling (LWX) county warning area (CWA) advisories updated 12:55 a.m. EDT April 30, 2020

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Of note, much of the PHI CWA -- as shown in the map image directly below -- have both a wind advisory and a flood watch in effect with the exception being the Jersey shore counties (Monmouth, Ocean, coastal Burlington, Atlantic, and Cape May but you can't tell that fact on the map since the main map can only show one color.

Philadelphia - Mount Holly NWS (PHI) CWA advisories updated 12:55 a.m. EDT April 30, 2020

In order to tell if there is more than one hazard product in effect, you have to click on the point grid forecast since on the map, only the higher priority hazard color shows up. There are 129 products on the NWS Hazards List. In this case, a wind advisory (ranked #90) supersedes a flood watch (ranks #108).

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On Friday-Saturday, a coastal low will take over with upper level energy from the west reinforcing the system and giving the DMV a chance of wraparound moisture before things dry out by Sunday with a warming trend early next week.

For the record, the current monthly (April 1st - 29th), spring seasonal (March 1st - April 29th), and year-to-date (Jan 1st - April 29th) precipitation totals along with departures from the 1981 - 2010 base period averages are given below for the usual set of regional NWS ASOS climate stations including the three main airports.

KDCA
April 1-29: 5.20" or +2.24" (2.96")
Season: 7.51" or +1.07" (6.44")
Year: 13.51" or +1.64" (11.87")

KBWI
April 1-29: 4.28" or +1.19" (3.09")
Season: 7.33" or +0.34" (6.99")
Year: 13.42" or +0.48" (12.94")

KIAD
April 1-29: 4.18" or +0.82" (3.36")
Season: 6.17" or -0.57" (6.74")
Year: 12.74" or +0.58" (12.16")

KDMH
April 1-29: 4.54" or +1.42" (3.12")
Season: 7.60" or +0.62" (6.98")
Year: 13.53" or +1.03" (12.50")

KDMH's first first full year period begins in 1999.

KNAK
April 1-29: 3.75" or +0.19" (3.56")
Season: 5.78" or N/A
Year: 9.35" or N/A

Note: The 2020 season-to-date and year-to-date averages and departures given on the LWX climate webpage are incorrect / incomplete.

OK, that concludes this entry, nor am I posting a second entry tonight.

--Regulus

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