Wednesday, July 26, 2017

Weather Update: Regional Precipitation Tallies Through July 26th and This Friday's Impending Potentially Heavy Rainfall Event

Sunday evening deluge in Georgetown, Washington, D.C., 6:37PM July 23, 2017.

Of note, virtually none of the regular-sized Georgetown row houses have any sort of overhang. The above picture was taken in the 3200 block of Prospect St NW -- and the house at 3275 had a small overhang above the front entrance and under which I took temporary shelter.

I was going to use this picture as the lead image for my planned entry featuring my Sunday walk-through-Georgetown pictures, but it seems more relevant here.


Downtown Washington, D.C., deluge; undated photo by Robert Miller, The Washington Post.

This image is featured in the below-linked CWG entry. I assume it was taken this past weekend (near McPherson Square) but I couldn't determine for sure.


This is a weather-related entry to post both the current monthly and yearly precipitation tallies for the four regional climate stations and info on the possibly heavy rain fall event for Friday and Saturday.

The maps shown below are taken from the NWS and for the GFS and European model outputs, as prettied up by and Tropical Tidbits.

Below are the precipitation tallies for both month-to-date (MTD) and year-to-date (YTD) through July 26, 2017 with plus/minus departures and normal-to-date amounts given in parentheses (based upon the 1980 - 2010 averages, except in the case of KMDH, which only has a dataset back to Apr. 1998).

MTD: 5.37" +2.22" (3.15")
YTD: 21.29" -1.60" (22.89")

Of note, the Saturday-Sunday two-day rainfall total at KDCA was 2.37 inches -- making it the wettest weekend of the year.

MTD: 3.17" -0.23" (3.40")
YTD: 21.70" -2.19" (23.89")

MTD: 7.11" +4.05" (3.06")
YTD: 27.48" +3.62" (23.86")

MTD: 2.19" -1.73" (3.92")
YTD: 17.84" -5.44" (23.28")

Note the 9.64" YTD and 4.92" MTD differences between KIAD and KDMH.

NWS/NCEP/WPC cumulative QPF for days 1 - 3 valid through 0Z 30 July 2017.


Turning to the weather forecast ...

Tonight is a partly cloudy one with a southeasterly breeze as a maritime tropical (mT) airmass moves in from the Atlantic across the mid-Atlantic coastal plain and piedmont. At KDCA, the dew point has risen from about 60F at noon to 71F at the 11PM hour. It is a bit lower at KIAD at 65F and 67F at both KBWI and KDMH.

Even at this late hour, the clouds are puffy with a hint of convection.

The 0Z 7/26/2017 European model's precipitation total through hour 102 for the Mid-Atlantic region, as prettied up by

The legend (in inches) is on the side.


Looking ahead, there is the potential for a heavy rainfall event starting late Thursday and through Friday and into Saturday morning across the extended Baltimore / Washington region with rainfall totals anywhere from 1 to 6+ inches -- depending on how the synoptic and mesoscale situation unfolds.

The 12Z 7/26/20167 GFS model's precipitation total through hour 90 for the Mid-Atlantic region, as prettied up by

Again, the legend (in inches) is on the right side. Also, both images are from this CWG entry.


Below is part of the Sterling (LWX) area forecast discussion (AFD) updated at 9:32PM tonight interspersed with some relevant images. I'm going to end my part of the entry here.


As the mid-west disturbances approach from the Ohio Valley Thursday morning, the wind field is expected to strengthen as do the lapse rates. There is a model spread for the amt of available energy by Thursday afternoon but 1000 J/Kg or more is expected by Thu afternoon. Southerly surface flow will continue during the day with about 30 kt of 0-6km shear. Isolated strong to severe storms are expected Thurs afternoon into the evening.

Tonight's 0Z 7/27/2017 GFS model precipitation total through hour 90 for the Mid-Atlantic region, as prettied up by Tropical


The showers and thunderstorms will likely move to the south and a brief break in activity is likely Thu night. The cold front will be approaching the Appalachians Thu night while a closed low passes across the Great Lakes.

Cyclogenesis will be occurring across the Mid-Atlantic region Friday and depending on the placement of the sfc low and front hazardous weather is possible. The main threat at this time is heavy rain due to the increased PWAT air and overrunning Fri morning into the afternoon.

The NWS/WPC National Digital Forecast Database (NDFD) weather map forecast for the Lower 48 valid at 18Z July 27, 2017 issued earlier Wednesday.


Thunderstorms are possible mainly in the warm sector and across the southern CWA. Deepening of the sfc low will continue off the Mid-Atlantic coast resulting in northerly flow on the backside of the low Fri night. The wind field will strengthen aloft as the upper level low approaches the Mid-Atlantic region. Bands of heavy rain may continue Friday night into Saturday.

There will likely be multiple rounds of heavy rain Friday morning into Saturday. Uncertainty exists in the location of the heaviest bands of rainfall but all interested parties across the Mid-Atlantic region should stay tuned to the rainfall threat during this time.

The NWS/WPC NDFD weather map forecast for the Lower 48 valid at 0Z July 28, 2017 issued earlier Wednesday.


Still some uncertainty in the forecast for Saturday. Some guidance holds back the upper level trough longer during the day while the new 12Z GFS/06Z GEFS push moisture out quicker.

Fairly confident though that lingering impacts from the departing surface low will continue into Saturday morning and maybe into the afternoon east of the Blue Ridge. Clouds and showers are a good bet all areas Saturday morning.

The NWS/WPC NDFD weather map forecast for the Lower 48 valid at 12Z July 28, 2017 issued earlier Wednesday.


Then by Saturday afternoon drying will begin in earnest across western areas with clearing skies during the afternoon while in the east...clouds linger longer into afternoon with even some spotty showers activity. By Saturday night...most areas will be drying as a large area of high pressure over the Great Lakes builds into the region.

The NWS/WPC NDFD weather map forecast for the Lower 48 valid at 0Z July 29, 2017 issued earlier Wednesday.


Sunday through Tuesday will be influenced by a large area of high pressure centered over the eastern U.S., and overhead on Tuesday. Sunday will be a great day featuring dry weather with sunny skies and low humidities and well below normal temperatures for late July.

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