Wednesday, July 18, 2018

Overview of July 17th,2018 D.C. Area Deluge With Photos; KDCA Daily Record Rainfall Set; Plus, Regional Precip Summaries and Stats

Cumulus humulus cloud above the Shaw neighborhood of Washington, D.C., 2:19 p.m. July 18, 2018.

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This entry is an overview of the deluge that occurred yesterday -- July 17th, 2018 -- that the ended the 19-day dry spell here in and around the D.C. area in rather spectacular fashion with Reagan Washington National Airport (KDCA) officially logging 2.79 inches of rain falling in a one-hour period including 2.63 inches in a 42-minute period.

Flooded George Washington Parkway near National Airport, Arlington, Va., July 17, 2018; image taken from CWG entry linked below.

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Baltimore, Md., as measured at Thurgood Marshall BWI Airport (KBWI), had even more with 3.35 inches. Downtown Baltimore at the Maryland Science Center (KDMH) picked up just a scant 0.35 inches as most of the storms missed it.

Frame grab of video of rainwater pouring into the Capitol South Station, Washington, D.C., July 17, 2018; image from CWG entry linked below.

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Washington Dulles International Airport (KIAD) recorded 1.11 inches including 1.05 inches during the main event.


The totals at all three airport stations were enough for daily rainfall records -- in the case of KIAD, due mostly to the fact that the recording period only goes back to April 1960 versus Jan 1871 for KDCA and KBWI to include their pre-airport periods.

For KDCA, the previous daily record was 2.05 inches set in 1945 (right around the time that KDCA became the official weather station).

It was also the most rainfall in a single day since July 28, 2017 and second most for any day in July since 1975. (I'm not sure why 1975 was chosen.) .

Some other factoids:

* More rain fell from the storm in an hour than had been observed in the previous 38 days at KDCA.

* During the deluge, 0.95 inches of rain fell in just nine minutes and 1.22 inches fell in 15 minutes. Such rainfall intensities are expected only once every 10 to 50 years on average.

*In the 2016 Ellicott City flood (not to be confused with the 2018 Ellicott City flood), 3.2 inches of rain fell in 30 minutes (as measured in a rain gauge at a Howard County gov't building).

The previous KBWI daily record of 2.25 inches was set in 1947 (a pre-airport record). The previous KIAD record was just 0.60 inches set in 1988.

Below are monthly, seasonal, and yearly totals to date (through July 17th, 2018) for KDCA, KBWI, KIAD, and KDMH compared to the NWS 1981 - 2010 base period (given in parentheses). (In the case of KDHM, it does not yet have a full 30-year average base period.) "Seasonal" refers to climatological summer starting June 1st.

KDCA
Month-to-date: 2.79" +0.69" (2.10")
Season-to-date: 8.00" +2.12" (5.88")
Year-to-date: 27.97" +6.13" (21.84")

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KBWI
Month-to-date: 4.26" +2.07" (2.19")
Season-to-date: 9.03" +3.38" (5.65")
Year-to-date: 28.95" +6.27" (22.68")

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KIAD
Month-to-date: 1.99" -0.02" (2.01")
Season-to-date: 6.16" +0.17" (5.99")
Year-to-date: 26.81" +4.00" (22.81")

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KDMH
Month-to-date: 1.09" -1.46" (2.55")
Season-to-date: 4.29" -1.53" (5.82")
Year-to-date: 26.68" +4.77" (21.91")

KDMH records are based on a record stretching back only to 1998.

One other item -- in the CWG entry, it quotes a National Airport weather observer  as saying: "I've been here 40 years. I've never had that."

I can believe that.

Flooded Four Mile Run, Arlington, Va., July 17, 2018; frame grab image from above-linked CWG entry.

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I would also note that between him and the guy who oversaw the FAA contractor crew of airport weather observers for at least 30 years (a fact I knew from a certain ex-friend of mine who knew him)  -- there is fully 70 years of KDCA observations.

That's 70 years staring at the same damn tarmac and same vapid, polluted river. That's a WHOLE LOTTA "partly cloudy and mild" and in winter, the inevitable "snow quickly changing to rain with little or no accumulation."

Speaking of that other guy (and I don't particularly want to mention his name), he also has provided the official snowfall measurements for KDCA for at least a 30-year period possibly ending in 2009 (unsure of his current status) and coincidentally or not, that damn 1979 Presidents Day storm snow total has never been broken. It is the 4th largest Washington, D.C., total, but the top two are pre-KDCA records.

Here is a chart of the top 10 biggest snowstorms for Washington, D.C. (KDCA and pre-KDCA), Baltimore, Md. (KBWI and pre-KBWI), and Washington Dulles Int'l Airport (KIAD):

Click on image for larger version; text quality isn't that good; source here.

Instead of a distinct snowstorm, the measurement period is "3-days" since that is -- invariably -- the duration of a standalone (distinct) major snowstorm in our region. Occasionally, there is a follow-on minor event on day 4/5 (as happened in Jan. 1996).

Note the strange way the next six all cluster. Having been here in (or very close) to D.C. for five of those six, I just buy it, but it's not for me to decide. Source here.

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Reagan Washington National Airport in the foreground and Washington, D.C., in the distance.

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As for yesterday's deluge, on a personal note, it didn't seem abnormally heavy or prolonged to me.

It resembled any number of typical, short-lived drenching summer downpours, and I figured maybe an inch fell. I wasn't too far from National Airport at L'Enfant Plaza about 2.4 miles away directly from the ASOS spot. I mention this because almost always in these events, what falls officially at KDCA is significantly less than what I perceive.

I recall this episode back in Aug. 2011 when I went to Friday night baseball game at Nationals Park, and there occurred an extended monsoon-like deluge that dropped about 3 inches of rainfall over the course of 90 minutes.

Split image of flooded Martha Custis Drive, Alexandria, Va., July 17, 2018; images by twitter user Amelia Draper and reposted in above-linked CWG entry.

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In the end, though, KDCA ASOS spot -- located just about 2.1 miles away from the stadium and with the general area visible across the river -- picked up just 1.53 inches. I mention the view because it appeared to be deluging at National Airport itself. And, no, I don't think I'm always overestimating precipitation. Rather, I think in thunderstorm events, a non-trivial fraction simply doesn't fall into the gauge.

Building exterior, 1400 block Rhode Island Ave NW, Washington, D.C., 7:54 p.m. July 15, 2018.

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By the way, in that entry, you will see photos of me, Keep in mind that I was quite doughy-pudgy back then, not to mention friends with forever ex-friend GASY the Hangry Chipmungorilla. Thank GOD that ended.

Yellow flowers (daisies, I guess) in a driveway, Georgetown, Washington, D.C., 3:38 p.m. July 15, 2018.

This was near Dumbarton and 31st St.

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Anyway, in this weather event, clearly this always-less-rainfall-at-National Airport was not the case -- and, indeed, as the some of the pictures from Arlington and Alexandria show, areas right around the airport got the rainfall jackpot and resulting flooding. However, there were no reported injuries or anything like that.

A pink flower growing in a small yard, 2100 block 13th St NW, Washington, D.C., 2:08 p.m. July 17, 2018.

Noted 12:15 a.m. 7/28/2018: I had to correct the image file name and caption -- this was taken in the 2100 block, not the 2000 block of 13th St NW.

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OK, I'm going to wrap up this entry. I'm home tonight just resting and watching my old lineup on MeTV in my tiny, dimly lamp-lit, air conditioned apartment. Today was a day that I just wanted to get home from work and cocoon. Tomorrow night is a gym night for me. I'll try to post an entry thereafter.

--Regulus

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