Saturday, July 21, 2018

Saturday Evening Post for July 21st, 2018: High Summer Monsoon Edition

**This entry was posted July 21, 2018.**

Random picture of a deluge that I found on the internet; no idea when or where it was taken.



That's the weather word for today for the Metro D.C. area and, to a somewhat lesser extent, the Metro Baltimore area as an out-of-season coastal storm located over the Norfolk area and strong tropical moisture advection on courtesy a low-level jet have acted as a focus for hours of steady, heavy rainfall.

Rainfall totals in the D.C. have been, in general, in the 3 to 6 inch range with a daily record rainfall at KDCA and a wettest July day record at KIAD while the Baltimore area has had 2 to 5 inches, although the axis of the heaviest precipitation has shifted northward, so it will pick up more after midnight.

(UPDATED) Final daily rainfall amounts were:

KIAD: 5.02"
KBWI: 4.79"
KDCA: 4.00"
KDMH: 2.77"

There are flood warnings around the D.C. area including the District proper and flash flood warnings for much of the Virginia D.C. suburbs.

The weather warnings flashed on Me-TV keep interrupting / shutting off the volume for minutes at a time of the shows I'm watching (see below) and replaced by a stupid tick-tick-ticking.

NWS Baltimore/Washington (LWX) county warning area (CWA) weather advisories at 5:03 p.m. EDT July 21, 2018.


It is raining heavily -- slashing against the window -- as I write this (9:17 p.m.). At this rate, we could pick up nearly 5 inches of rainfall today -- ranking it among at least the top 20 wettest days of all time officially in D.C.

Here is info from a tweet by the Capital Weather Gang (CWG)'s Ian Livingston:

Thru 10p, 3.79 inches of rain today in DC.

There are 53,894 days in the DC weather record including today. This one is currently (and rising) wetter than 99.95% of all days in the record. That's a tie for 27th wettest day since 1871.

Source here.

And here is some additional information from a tweet by a guy named Chris Laudicina

Wettest July day on record @Reagan_Airport is July 9, 1970 (4.69”); last time #DCA saw 4”/+ of daily rainfall: September 30, 2010 (4.66”); #DCA also saw 3.85” on October 29, 2012 (#Sandy)…

To be clear, he is talking about the National Airport-only portion of the full D.C. weather record back to Jan. 1871.

The KDCA portion began in or around Aug. 1945, although I've never been sure of the exact date since "unofficial" weather obs possibly began there as early as late 1941.

NWS LWX CWA weather advisories updated 6:51 p.m. EDT July 21, 2018.


My own view is that the NWS should have touted his weather event a little more than it did. For starters, it is a significant out-of-season coastal storm with hybrid tropical characteristics. I mean, the NHC has named systems as subtropical storms for a lot less -- blobs of thunderstorms off South Carolina, ghost circulations with only stratocumulus clouds off the Outer Banks, and even occluding lows east of Nova Scotia.

NWS Wakefield (AKQ) CWA weather advisories updated 5:27 p.m. EDT July 21, 2018,


Note: This entry is being updated, and it is possibly that the entry title might change as well, but the URL won't.

NWS Mt. Holly / Philadelphia (PHI) CWA weather advisories updated 6:57 p.m. EDT July 21, 2018.


As this coastal low drifts northward, an upper level low will remain anchored over the Ohio River valley while a large blocking Bermuda high stays fixed over the western Atlantic. As I noted in my previous entry (linking to a CWG entry), this will keep the pattern quite humid and wet through the work week

NWS LWX CWA weather advisories updated 8:10 p.m. EDT July 21, 2018.

The dark red areas are flash flood warnings. The lime green are flood warnings.


Below is the Sterling (LWX) area forecast discussion updated at 9:41 p.m. EDT. I've interspersed the text with a surface weather map image and a number of still and animated radar images to break up the text.

NWS LWX CWA weather advisories updated 9:49 p.m. EDT July 21, 2018.


An area of low pressure will move north across the Delmarva Peninsula. Meanwhile, a broad and slow moving low pressure system will drop from the Upper Midwest into the southern Appalachians by the end of the weekend. This low will continue to influence the Mid Atlantic during the early part of the coming week.

NWS high-resolution surface weather map for a portion of the eastern U.S. as of 21Z July 21, 2018.


Low pressure will continue moving northward through the Delmarva this evening and tonight, reaching northeastern PA by Sunday morning. Widespread heavy rain has occurred and will continue to occur over the next several hours, before rain moves northward and out of the region after midnight. Widespread amounts of 2-5" have already fallen along and east of a line from Spotsylvania - Fauquier - Jefferson counties eastward, with localized amounts of 5-7" in Fairfax, Prince William, Stafford, Spotsylvania, and Charles counties.

NWS LWX radar, enhanced base mode reflectivity as of 5:01 p.m. EDT July 21, 2018.

I can't download the animated enhanced radar image loops -- only the standard mode ones.


This has led to numerous reports of flooding and flash flooding. Please refer to warning products for additional information. As the low moves northward, the axis of additional heavy rain will shift northward into central and northern Maryland and will need to monitor for additional flooding concerns. Flash Flood Watch will remain in effect until 2 AM.

NWS LWX radar in enhanced composite mode at 4:54 p.m. EDT July 21, 2018.


After main area of rain exits, off and on light rain or drizzle could continue in convergent surface flow. Light fog and low clouds will also be likely much of the night.

NWS LWX radar in standard base mode reflectivity
looped 4:20 p.m. - 5:08 p.m. July 21, 2018.


For Sunday and Monday, upper low will be drifting south to our west. This mean copious moisture will continue to be drawn into the region with nearly constant chances of showers and thunderstorms. While there will be some diurnal component, especially to convective elements, forcing and moisture will support chances of rain through the nights as well.

NWS LWX radar in standard composite mode
looped 4:14 p.m. - 5:01 p.m. July 21, 2018.


Details regarding timing and locations of heaviest rain will be ironed out as finer scale features are identified. Unidirectional flow and/or southeast low level flow into the terrain could enhance the threat for locally heavy rain at times. High temperatures will be below normal in the 80s, while lows remain in the 60s to lower 70s due to the high dew points.

NWS LWX radar in standard base mode reflectivity
looped 6:04 p.m. - 6:52 p.m. July 21, 2018.


The upper low will remain anchored to our southwest through Thursday, thanks to a blocking upper-level ridge over the northern Atlantic. Multiple fluxes of moisture from the southeast and south, along with the meandering trough of low pressure will induce several rounds of showers and thunderstorms through Thursday night.

NWS LWX radar in standard composite mode
looped 5:58 p.m. - 6:45 p.m. July 21, 2018.


Expecting the shower and thunderstorm activity to peak during the warmest part of the afternoon each day. Although we are currently not anticipating widespread flooding, PWAT values approaching and exceeding 2 inches will provide plenty of moisture for heavy rainfall with any showers or storms.

Regional composite radar for a portion of the eastern U.S. looped 6:15 p.m. - 7:00 p.m. July 21, 2018, as prettied up by Weather Underground.


This could potentially lead to some flooding concerns over localized areas through this period, if heavy rain sits over one area long enough. With constant cloud cover throughout this period, expect high temperatures in the mid 80s, with lows only in the low to mid 70s.

NECONUS composite radar mosaic looped 1918 - 2028 UTC July 21, 2018.


The Upper level ridge over the northern Atlantic should begin to break down and move east late Thursday into Friday. This will allow for that stubborn upper-level trough to move out of the area. However, the chance of showers and a few thunderstorms still linger Friday and Friday night because of the added lift of a front. High temperatures will be closer to normal as we could see a southwest wind develop along a weak boundary as it moves east.

NECONUS composite radar mosaic looped 2058 - 2208 UTC July 21, 2018.


And one last radar image:

Regional composite radar for a portion of the eastern U.S. looped 8:30 p.m. - 9:15 p.m. July 21, 2018, as prettied up by Weather Underground.


As a brief update, I'm home tonight even though I'd like to go out in the rain and find a bar. The trouble is, I got way too drunk last night and I'm still recovering. Indeed, I spent most of the day sleeping and didn't even make it to the gym, which kinda sucks. I also need to stick to a budget this pay period.

Last night, I stayed at work late and then stopped at Round Robin in the Willard InterContinental Hotel and then MXDC before heading to Trade and No. 9. Yes, way too much.

The episode of Wonder Woman was "Wonder Woman in Hollywood" (and it guest starred Debra Winger as her younger sister Drusilla, a.k.a., Wonder Girl).

The Svengoolie-hosted monster movie was the truly awful (in a car wreck sort of way) 1978 movie Barracuda -- one of a series of 1970s ocean creature horror movies including most famously (of course) Jaws.

After two 30-min episodes of Batman, the Star Trek: TOS episode at 11 p.m. is "Elaan of Troyius" and the episode of Battlestar Galactica at 12 a.m. is the second part of the pilot episode "Saga Of A Star World (a.k.a. Battlestar Galactica): Part 2."

Kolchak: The Night Stalker is airing the episode "Spanish Moss Murders" at 1 a.m.

Rounding out the lineup is Lost in Space at 2 a.m. ("The Space Trader"), Voyage to the Bottom of the Sea at 3 a.m. ("The Exile"), and Land of the Giants at 4 a.m. ("Deadly Pawn").

I'm going to end this entry. OK, that's all for now. My next planned entry will be late Monday or Tuesday. Jukebox Saturday Night edition to follow momentarily.


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