Friday, July 31, 2020

Weather Update at July's End: A Mid-Summer Night Quasi-Stationary Boundary's Impulses, Odd Clouds, and Briefly Stormy Evening -- Recap

Stormy - chaotic sky above the National Mall, Washington, D.C., 7:42 p.m., July 30, 2020

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The weather this Thursday was somewhat unusual for High Summer and worth noting in a non-shrieking media panic porn Covid-dick-teen and non-WOKE Cultural Revolution-themed entry.

NWS high resolution surface weather map for a portion of the eastern and central CONUS, 15Z 30 July 2020

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For starters, there was an elongated east-west quasi-stationary boundary earlier this afternoon extending from the Metro Baltimore and Washington areas all the way back to Kansas City and impulses were riding along it, at least one of which was originally associated with Tropical Storm Hannah last week. The boundary itself basically represents the periphery of an elongated lobe of the summertime semi-permanent Bermuda - Azores subtropical high.

NWS high resolution surface weather map for a portion of the eastern and central CONUS, 03Z 31 July 2020

Note that the boundary -- 12 hours later -- is still in place except for some back-and-forth undulation.

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What was interesting is that there was a very strong moisture (precipitable water) gradient across the region with a big slug of moisture over the Ohio River Valley and central Appalachians. Rainfall totals across those areas was in the 0.75 to 2.5 inch range.

Chaotic stormy sky as seen from Independence Ave near 12th St SW, Washington, D.C., 7:38 p.m., July 30, 2020

I walked from my office at L'Enfant Plaza this evening across the Mall and over to Joe's Restaurant, where I was able to get a high-top table by the bar. I had a very time there -- my first visit there since all the Covid crap started. I then walked home by quarter to ten tonight.

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Stormy evening as seen from the corner of the National Museum of African American History and Culture ("NMAAH&C"), Washington, D.C., 7:48 p.m., July 30, 2020

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This precipitation came careening into the D.C. area in the 7 to 8 p.m. timeframe. It turned what had been a hot, moderately humid, but "dry" day with intense end-of-July sunshine into a windy, rain squally evening. There wasn't much lightning / thunder, though.

In this area, rainfall totals weren't especially impressive and included the following through the end of the day (i.e., midnight Friday, July 31st):

KDCA: 0.15"
KBWI: 0.08"
KIAD: 0.20"
KNAK: Missing
KDMH: Missing / offline

However, the NWS put in a flash flood watch through 6 p.m. EDT Friday for additional possible heavy rainfall.

Sterling (LWX) county warning area (CWA) map with weather advisories, updated 3:00 a.m. EDT, July 31, 2020

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What was interesting, or so it seemed to me, was that the west-to-east trajectory of the weather system and rapidly varying atmospheric moisture content created unusual cloud formations and convection that by U.S. East Coast mid-summer standards was high base.

Nighttime intersection, New Hampshire Ave and N Street NW, Washington, D.C., 9:35 p.m., July 30, 2020

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Below is part of the Sterling (LWX) area forecast discussion from earlier tonight (10:27 p.m. EDT) interspersed with some weather-related imagery.

A weak boundary will remain stalled over the area through Friday. Waves of low pressure will track along the boundary passing through our area during this time. The boundary will shift to the north for Saturday and tropical moisture may impact the area for the early portion of next week. Tropical Storm Isaias has formed in the Atlantic Ocean. Refer to the National Hurricane Center forecasts for the latest information regarding Isaias.

Great Lakes Sector base reflectivity radar mosaic, looped 1738 - 1848 UTC 30 July 2020

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.NEAR TERM /THROUGH 6 AM/... A large, strung out area of middle- level spin (vorticity) and an upper-level jet streak associated with an approaching shortwave trough riding along a stalled surface front continue igniting showers and thunderstorms over the area as of mid evening. The first wave has produced a local minimum in instability along the crest of the Allegheny Front eastward to near I-95. Instability will increase overnight, however, given strong moisture advection in the lower levels.

NWS Charleston (RLX) NWS radar in standard composite mode, looped 2:24 p.m. - 3:04 p.m. EDT, July 30, 2020

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Enough guidance is on-board for scattered areas of heavy rainfall across the CWA to suggest a flash flood watch is necessary. Thus, have spread one eastward from the RLX watch to include the metro. It is noted that some areas are pretty dry at present, so a heavy rainfall may be more absorbed than run off.

NWS NDFD surface weather map forecast with p-type and likelihood for Lower 48 CONUS looped in six hour steps, 00Z 31 July to 00Z 02 August 2020

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However, dry vegetation and hard ground could actually increase run-off in some locales, so its not necessarily off the table in these areas, either. Given uncertainty and the fact the event is pretty much on top of us now, felt it was necessary. Confidence is not as high as preferred, however.

NWS 72-hour quantitative precipitation forecast (QPF) through 00Z 03 August 2020

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The overall pattern will remain largely unchanged through the first half of the weekend. Multiple rounds of heavy showers and thunderstorms will likely result, though specific timing and placement remains somewhat uncertain. Severe weather from gusty winds seems a little less likely Friday due to cloud cover and the boundary likely nudging southward (keeping the best instability just outside of our area), but any subtle shift given the stronger mid/upper level flow could result in some strong to severe storms.

Great Lakes Sector base reflectivity radar mosaic, looped 0438 - 0548 UTC 31 July 2020

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As the front returns slowly northward Saturday, instability will increase, which may lend to a higher threat for at least isolated strong storms. The flooding threat will also gradually increase at least in a general sense as multiple rounds of showers and thunderstorms saturate the region.

NE CONUS base reflectivity radar mosaic, looped
0448 - 0558 UTC 31 July 2020

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Guidance continues to show an active pattern through the first half of next week. A H5 trough will continue to deepen over the Mississippi Valley as it becomes wedged by high pressure over the southwestern CONUS and a strengthening Bermuda High offshore.

Sterling (LWX) NWS radar in standard composite mode looped 11:12 p.m. - 11:52 p.m. EDT July 30, 2020

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This AFD doesn't get into any potential impacts of Tropical Storm or Hurricane Isaias. It seems as though the system will move up / just off the Southeastern U.S. coast, clipping the OBX and Mid-Atlantic shore. Depending on how the situation evolves, I might pay a lot of attention to this situation -- or just forget about it.

NHC information and 5-day forecast track with uncertainty cone for Tropical Storm Isaias, Advisory 11, issued 11 p.m. EDT 30 July 2020

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OK, I'm going to wrap up this entry. Just fyi, I composed most of this in the 1 a.m. - 3 a.m. timeframe, but did not complete and post it until early afternoon Friday. However, I am putting a 3:30 a.m. timestamp on it.

Shrubbery illuminated by lights, exterior, The Fairfax at Embassy Row hotel, Washington, D.C.,
9:41 p.m., July 30, 2020

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OK, that's all for now. My intention is to post a new entry on Friday night -- and, again, my intention is to have a non-shrieking media panic porn Covid-dick-teen and non-WOKE Cultural Revolution-themed entry.

--Regulus

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