Tuesday, August 19, 2014

Singin' In the Rain: An Overview of Last Week's Multiple Extreme Rainfall Events



So before we get too far removed from the weather events of last week, I would like to post a recap of four extreme rainfall events that happened in the U.S. including three on the Eastern Seaboard. These included rainfall events in Detroit (August 11th), the Baltimore / Washington region (August 12th), Long Island (August 13th), and southeastern Maine (August 13th - 14th).

Among the figures of note, Islip, New York officially recorded (at Long Island MacArthur Airport) an astounding 13.51 inches for the day and for the 24-hour period between 11PM Aug. 12th and 11PM Aug. 13th, 13.57 inches -- making for a new 24-hour New York State record rainfall event, while a day earlier (and closer to home for me), Baltimore, Md. (as measured at BWI Airport) recorded officially its second wettest calendar day ever at 6.30 inches.

This entry focuses on the Baltimore / Washington and Long Island ones (in that order). However, I have links to information on the extreme rainfall events in Detroit area and southeastern coastal Maine.

The Detroit rainfall event witnessed 4.57 inches at Detroit Metropolitan Wayne County Airport (KDTW) on the 11th in what was officially the 2nd greatest calendar-day rainfall ever (behind 4.74 on July 31, 1925) and a 2-day total of 4.85 inches.

Map of southern Maine and New Hampshire rainfall totals for Aug. 13 - 14, 2014.

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The coastal Maine rainfall event was centered around Portland where 6.44 inches at Portland International Jetport (KPWM) on the 13th and 14th (virtually all of it fell on the 13th -- 6.43 inches, and which blew away the previous daily record of 2.38 inches). I couldn't find the all-time heaviest rainfall for a calendar day in Portland. (Also, I think Portland's airport must be called "Jetport" to distinguish it from Portland International Airport in Portland, Oregon.)

The CWG has a series of blog posts on these events.

For an overview of why the Baltimore / Washington area event happened, see this entry. For information on the Long Island event, see this entry. (The entry gives a preliminary calendar-day figure of 13.26 inches, this amount subsequently increased to 13.51 inches.)

Some of the images in this entry are taken from those CWG entries as well as this comprehensive Wunderground blog entry on these events. For more information on the Detroit event, see also this NWS write-up.

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A word on nomenclature: I typically use the term "climate station" to refer to the National Weather Service weather stations because it is at those spots that the "climate" for a spot is determined over a sufficiently long time (30 years).

The Baltimore / Washington August 12th, 2014 extreme rainfall event ...

Doppler radar-estimated rainfall totals for August 12, 2014 for the Baltimore area.

Click on image for larger version with more readable legend.

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As I noted in this blog entry, the Baltimore/Washington Metro areas had an extreme rainfall event on Aug. 12th, 2014. As noted, the Baltimore climate station at BWI Airport (KBWI) had 6.30 inches. But areas just east of it centered on Glen Burnie (GLEN BURNIE!) and Pasadena had radar estimated totals ranging as high as 11 inches. The highest recorded figure was in Green Haven in Pasadena with 10.32 inches recorded in a rain gauge.

The Sterling (LWX) county warning area (CWA) advisories map updated at 4:00PM August 12, 2014.

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There was also widespread 2 to 5 inches of rain in the D.C. area, although the two climate stations -- National Airport (KDCA) and Dulles Airport (KIAD) -- were at the edge and didn't get nearly as much. KDCA recorded 1.60 inches and KIAD logged 1.13 inches.

So to recap ... (and these figures aren't "official" until NCDC blesses them) ...

BWI Airport (KBWI): 6.30 inches

This was, of course, a daily record -- and it blew past the previous daily record that was quite high, 4.91 inches set in 1955. It was also the 2nd highest ever officially for Baltimore behind only the pre-airport record of 7.62 inches on Aug. 23, 1933. Prior to June 1950, records were kept at the U.S. Custom House in Baltimore.

The full Baltimore official climate record -- like the full Washington, D.C., record goes back to 1871. (In June 1950, the record-keeping was moved to then-Friendship Airport, now Thurgood Marshall BWI Airport.)

Deluge on the tarmac at BWI Airport, August 12, 2014.

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Through Aug. 18th, BWI's 2014 yearly total is 37.50 inches -- or +10.97 inches and not too far from the current 30-year baseline annual average of 41.88 inches.

The Sterling LWX climate discussion on August 13th discussed the rarity of such an extreme event. In short, the observed 1-hour total of 2.70 inches between 1654 UTC and 1754 UTC is a once-in-a-50-year event and the 2-hour burst of 4.19 inches from 1554 UTC to 1754 UTC is estimated as a once-in-a-200 year event.

Additionally, the slightly-greater-than-1-hour monsoonal deluge of 3.91 inches from 1629 UTC to 1732 UTC is considered a once-in-a-500 year to once-in-a-1,000 year event.

I'm not sure I quite believe that: 500 to 1,000 years is a VERY long time.

CWG had an entry about that as well.

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National Airport (KDCA): 1.60 inches

This wasn't even close to the daily record of 5.44 inches set on the same very wet day in 1955. Official record-keeping at KDCA goes back to about 1945. The airport itself opened in 1941. Prior to that, DC's weather records were kept at various locations including M and 24th NW.

As I've complained about so many times, the KDCA climate station is located at at the edge of the airport (toward the south end / not quite to Hunter Point) at the edge of an expansive tidal river about 2 miles south of downtown.

The KDCA Automated Surface Observation System (ASOS) spot -- that is, climate station -- in an aerial view with the Potomac River visible.

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KDCA is a very difficult to record extreme snowfall events, extreme high temperatures, and extreme low temperatures, and it's not surprising that, in fact, none of these have been recorded in Washington, D.C., since the records have been kept at National Airport. In short, it's just a lousy spot.

As case in point on the hot temperature side, and as I noted in this entry, back on July 7, 2012, we would have tied the all-time record temp of 106F but it "only" hit 105F at KDCA. In point of fact, it actually reached 106F at KDCA but for just 1 minute and the rule is 3 minutes. More to the point, though, that same 105F reading at Hunter Point was certainly 106F or even 107F at 24th and M Streets NW, but we'll never know.)

By contrast, KBWI is a very good / representative spot for central Maryland (unlike, say, the old U.S. Custom House spot and, to a lesser extent, the Maryland Science Center spot).

The BWI Airport ASOS, which is to say, the KBWI climate station.

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Dulles Airport (KIAD): 1.13 inches

There was a tight gradient between KDCA and KIAD with radar estimates showing 4+ inches in parts of Fairfax County but dropping off rapidly to Sterling, Va., and Dulles Airport.

Dulles records go back to 1960 when the airport opened.

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The Sterling (LWX) NWS radar in composite mode at 1040AM EDT August 12, 2014.

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Maryland Science Center (KDMH): 5.42 inches

KDMH does not quite have a full 30-year weather record yet. I think it started around 1987. However, this 5.42 inches was certainly in the top 10 events for the spot. (The Sterling LWX site doesn't have comprehensive info for KDMH.)

As of Aug. 18th, KDMH's yearly total is at 39.42 inches of precipitation, which is within 2 inches of its typical annual average. (Again, KDMH doesn't really have a set annual average although Sterling uses "normal" for it based on its partial record. Based on that, KDMH is running 13.50 inches above normal.)

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The Long Island August 13th, 2014 extreme rainfall event ...

Doppler radar-estimated rainfall totals for Long Island for August 13, 2014.

Again, click on the image for a larger version with more readable legend. The pinkish coloring is 12 to 14 inches of rainfall.

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There was also an extreme event also in the form of a record-setting deluge in New York on Long Island on Aug. 13th, in particular, in parts of Nassau and Suffolk Counties, but with wide variations. Long Island MacArthur Airport ("LIMA") in the Town of Islip in Suffolk County, N.Y., achieved a new all-time New York State 24-hour precipitation record.

Flooded cars on the Sunrise Highway on Long Island in the most torrential of the torrential rainfall on the morning of August 13, 2014.

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The LIMA climate station-- denoted KISP -- had a preliminary total of 13.51 inches of rain on Aug. 13th, 2014 which I assume is an all-time extreme event for that location. More noteworthy, though, for the 24-hour period between 11PM Aug. 12th and 11PM Aug. 13th, KISP received 13.57 inches, which is a new 24-hour precipitation record for the entire State of New York, surpassing the 11.60 inches recorded at Tannersville, N.Y., on Aug. 27 - 28, 2011 during Hurricane / Tropical Storm Irene.

A flooded portion of the Sunrise Highway on Long Island after the August 13, 2014 deluge.

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The 13.57 inches was also KISP's 2-day total since 0.06 fell on August 12th.

A chart of the KISP 1-minute interval cumulative rainfall total between 12AM and 12PM August 13, 2014.

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Other 2-day totals for Aug. 12th and 13th ranged from 0.59 inches at Newark International Airport (KEWR) to 0.72 inches at Central Park (KNYC) to 0.92 inches at LaGuardia International Airport (KLGA) to 3.26 inches at JFK International Airport (KJFK). JFK Airport was at the edge of the really heavy rainfall.

Selected rainfall totals for different locations as shown on channel 7 WABC in New York during a weather segment, August 13, 2014. The Jersey shore area got a 3 to 4 inch dousing as well (as shown by the Asbury Park High School total).

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As noted (and shown in the above radar estimated precipitation totals), there was as tight precipitation gradient across and Nassau County with amounts ranging (as of a 129PM Aug. 14th report that I can't find any longer) from 0.98 inches at Malverne to 8.20 inches in Massapequa. There was also a gradient across Suffolk County from the aforementioned 13.57 inches at KISP to less than 2 inches in Upton and Stony Brook to a scant 0.14 inches near Montauk Point.

Observed U.S. trends in heavy precipitation events from the 1900s to the early 2010s. This appeared in the 2014 National Climate Assessment.

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Oh, and as a concluding thought, yes, the frequency of heavy rainfall events is increasing in the U.S. as a result of human-induced climate change. Share the above bar chart with (to borrow a phrase from the late blogger Bartcop) ya' nearest Teabagger.

OK, that's all for now.  

--Regulus

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