Monday, July 8, 2019

My One-Day Belated, CWG-Reliant Review of the Epic Monday Monsoonal Metro D.C. Deluge: (Officially) 3.30 Inches of Rain In One Hour and ~3 Billion Gallons of Water District-Wide

Flooded intersection at 15th St and Constitution Ave NW with the Washington Monument in the background,
July 8, 2019

The images in this entry are taken primarily from the Capital Weather Gang (CWG) entries linked below and from the Sterling NWS website.

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Car water rescue on the Clara Barton Parkway near the Capital Beltway in the morning deluge of July 8, 2019

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This is my belated entry about Monday morning's epic monsoonal deluge across the Metro D.C. area that dropped 3.44 inches of rainfall officially at National Airport (KDCA) to include 3.30 inches in a single hour and unloaded an estimated 3 billion gallons of water on the District of Columbia proper.

The deluge triggered extensive flash flooding and the need for multiple vehicle rescues. Fortunately, it doesn't appear that there were any fatalities.

Flooded intersection of 15th St and Constitution Ave NW, Washington, D.C., July 8, 2019

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The 3.30 inches that fell between 8:52 a.m. and 9:52 a.m. at KDCA was at least the heaviest hourly rainfall total since 1936. Full weather records extend back to 1871, but the hourly rainfall tabulation could only be quantitatively assessed back to 1936 (which still includes part of the pre-National Airport record). (Based on a 1934 Post article -- see below -- the evidence suggests this is the heaviest total in the full record.)

Caption: Very impressive: according to data from the Iowa Mesonet the 3.30" recorded between 8:52-9:52 AM yesterday was Washington DC's highest hourly precip report in records dating back to 1936.

Of note, KDCA hourly observations are between 52 minutes after the hour and 52 minutes after the next hour.

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Below is a particularly dramatic image of the flash flooding and inundated vehicles on the aptly-named Canal Road in the Palisades section of D.C.

Vehicles inundated by flood waters force morning motorists to climb atop their cars on Canal Road, Washington, D.C., July 8, 2019

These guys look relatively able-bodied -- which helped in the rescues. But it is always amazing to me how the ordinary can so quickly become the extraordinary -- even life and death.

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The CWG has had multiple entries about this event (links embedded):

How and why the D.C. area was deluged by a month's worth of rain in an hour Monday

Updated forecast: After historic deluge, rain to ease this afternoon, but expect residual areas of flooding

These titles might change but at this point are probably set.


Below are a series of NWS radar images from yesterday morning to include standard composite and standard base reflectivity modes (the former showing activity within the clouds):

NECONUS base reflectivity radar mosaic looped 1128 - 1238UTC (7:28 a.m. - 8:38 a.m. EDT) July 8, 2019

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NWS Sterling (LWX) in standard composite mode looped 7:51 a.m. - 8:28 a.m. EDT July 8, 2019

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NWS Sterling (LWX) in standard composite mode looped 8:09 a.m. - 8:42 a.m. EDT July 8, 2019

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NWS Sterling (LWX) in standard base reflectivity looped 8:24 a.m. - 8:52 a.m. EDT July 8, 2019

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NWS Sterling (LWX) in standard composite mode looped 10:52 a.m. - 11:28 a.m. EDT July 8, 2019

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The KDCA rainfall daily total of 3.44 inches was easily a daily record (surpassing the 2.16 inches set in 1958) and ranked among the top 25 rainiest days in the climo summer months (June/July/August).

Of note, KIAD picked up 1.05 inches and KBWI recorded 0.75 inches of rainfall. (KDMH received 0.77 inches.)

Here is an update on the precipitation tallies through July 8, 2019 for these four locations including departures and normal values for the 1981 - 2010 base period (all numbers in inches):

Abbreviations:
MTD: Month-to-date
STD: Season-to-date (for climo summer starting June 1st)
YTD: Year-to-date

KDCA
Day: 3.44R (previous record: 2.16 - 1958)
MTD: 4.82 +3.83 (0.99)
STD: 9.09 +4.32 (4.77)
YTD: 27.12 +6.39 (20.73)
Record period: Jan 1, 1871 - July 8, 2019 (including pre-airport period)

Of note, last year at this point, KDCA was at 25.18 inches -- the record wetness of the second half of the year yet to kick in.

KBWI
Day: 0.75
(record: 5.85 - 1952)
MTD: 2.38 +0.97 (1.41)
STD: 5.33 +0.90 (4.43)
YTD: 23.23 +1.77 (21.46)
Record period: Jan 1, 1871 - July 8, 2019 (including pre-airport period)

KIAD
Day: 1.05
(record: 1.96 - 2005)
MTD: 1.28 +0.32 (0.96)
STD: 3.72 -1.22 (4.94)
YTD: 23.60 +1.84 (21.76)

Record period: April 1, 1960 - July 8, 2019 (however, continuous record-keeping began Jan 1, 1964)

KDMH
Day: 0.77
MTD: 1.95 +0.84 (1.11)
STD: 4.69 +0.31 (4.38)
YTD: 24.02 +3.55 (20.47)
Record period: April 30, 1998 - July 8, 2019 (i.e., not a full 30-year base period)

Image of flash flooding in the D.C. area, July 8, 2019

I'm unsure of exact location as this was from an unspecified twitter posting in one of the CWG entries from above.

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The New York Times had a piece on how the deluge exposed the twin problems of climate change -- that is, anthropogenic global warming that is making these extreme events more commonplace -- and antiquated infrastructure, in this case, the District's nearly century-old drainage and sewer system. The system was simply overwhelmed by the inrushing volume of water.

Screenshot of flash flooding from the Waycroft / Woodlawn section of Arlington, Va., July 8, 2019

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Excerpt:

When almost a month's worth of rain deluged this city on Monday morning, turning streets into rivers and basements into wading pools, it showed just how vulnerable cities with aging water systems can be in the era of climate change.

The rainfall overwhelmed the capital's storm-water system, much of it built almost a century ago to handle a smaller population, far less pavement and not nearly as much water.

"We're still approaching this 21st-century problem with 20th-century infrastructure, and it's completely inadequate," said Constantine Samaras, a professor of civil and environmental engineering at Carnegie Mellon University. "And it's only going to get worse."

Updating that infrastructure will be enormously expensive, experts warn, not just in Washington but around the country. That's not only because upgrades are required. In many cases, cities are facing huge backlogs in general maintenance.

Jason Berry ponders from his bicycle flooded Baker Park, Frederick, Md., July 8, 2019

Areas around Frederick, Maryland picked up 4 to 6+ inches of rain.

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Flash flooding near Frederick High School, Frederick, Md., July 8, 2019

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As for forecasting this event, I realize it is so easy to figure out in retrospect what happened (and why it should have been obvious), and so I really don't want to beat up on the Sterling LWX NWSFO, but it is worth noting that the 3:59 a.m. Monday morning discussion -- that is, just a couple of hours before the event started to take shape -- featured a definite downplaying.

Surface weather map for the "DMV" featuring precipitation, July 8, 2019

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Excerpt:

A Flash Flood Watch remains in effect for central and northern zones, as well as much of the metro areas through 6 am this morning. As we enter into the heart of this morning, much of heavier showers and any thunderstorms should diminish and lead to a stray shower or drizzle. Dry air finally advances enough to end precipitation. Highs today near 80.

A stray shower or drizzle.

Sterling (LWX) county warning area (CWA) weather advisories updated 11:25 a.m. EDT July 8, 2019

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Of note, they got the high temp forecast spot on (80F at KDCA). In a subsequent Sterling tweet featuring a radar loop between 3 a.m. and 2 p.m., which, unfortunately, I can't capture as an image, it wrote:

Yesterday's high-precipitation supercell as seen by NWS radar between 3am and 2pm Monday traversing MD/DC/and northern VA. As shown on previous tweets, the highest rain totals of 3-6" were in a band only about 15 miles wide, but extended in a path more than 100 miles long.

Radar-estimated rainfall totals between 11 a.m. Sunday and 11 a.m. Tuesday. (National Weather Service Mid-Atlantic River Forecast Center)

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I would like to note that the Wakefield (AKQ) NWS radar the night before had an interesting signature of the way the convective precipitation appeared to be back-building over the Virginia Northern Neck. What's more, the Sterling radar showed convective precipitation elements stalled over Frederick and Washington Counties, Md. Together, these suggested a convergence zone (as shown in the surface weather map above).

Wakefield (AKQ) NWS radar in standard composite mode, looped 5:49 p.m. - 6:29 p.m. EDT July 7, 2019

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Finally, there is this CWG entry (link embedded): Monday morning's downpour was the heaviest on record in Washington. I'm reposting several excerpts.

KDCA observations 7 a.m. and 4 p.m. EDT July 8, 2019

This shows the 3.30 inch hourly rainfall total and overall storm rainfall total of 3.44 inches.

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Excerpt:

Exploring Washington weather records dating back farther, to 1871, there appears to be no heavier one-hour rainfall. A weather system on Sept. 13, 1934 dispensed 3.25 inches in a single hour which, at the time, was more "than any previous 60-minute period in the 63-year-old records of the Weather Bureau" according to the Sept. 14, 1934 edition of The Washington Post.

KIAD observations 7 a.m. and 4 p.m. EDT July 8, 2019

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At one point Monday, it was raining at a rate of 5.04 inches per hour. The severity of the precipitation over such a quick window of time had about a 0.5 percent chance of happening in any given year.

But just how much water came down? We crunched the numbers, and the answer is: 3 billion gallons. And that's only within the perimeter of Washington proper.

(If you include all of Virginia and Maryland in this calculation, the volume balloons to nearly half a trillion gallons, according to meteorologist Ryan Maue, who ran a similar analysis. Meteorologist Ryan Miller estimated over a billion gallons fell in Arlington alone.)

KBWI observations 7 a.m. and 4 p.m. EDT July 8, 2019

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That amount of water would be enough to fill 27 million whirlpool bathtubs. It could also fill 1.1 million hot air balloons (would that make them water balloons?). If the rainfall was distributed equally across everyone living within the District, each person would receive 35,000 pounds of it. That’s probably nine or 10 times the weight of your car! It’s also roughly the same as a residential swimming pool.

Consider enough water fell for everyone to fill their own swimming pool, and it’s easy to see why Monday’s rain was so disastrous.

Today, the weather was dry -- well, summery warm and humid -- but no rain. OK, that concludes this entry.

--Regulus

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