Flash Flood Guidance
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AWUS01 KWNH 200654

Mesoscale Precipitation Discussion 0147
NWS Weather Prediction Center College Park MD
253 AM EDT Sat Apr 20 2019

Areas affected...Northern Maine, Northern New Hampshire, Northern

Concerning...Heavy rainfall...Flash flooding possible

Valid 200650Z - 201245Z

Summary...A northward surge of atmospheric moisture will produce
widespread rain this morning in northern New England. The
combination of rain with melting snow may produce some areas of
flooding. Six-hour rainfall amounts could locally approach 1 inch
by 9 AM EDT.

Discussion...RAP 6-hour difference fields at 06Z show a PW
increase maximum of 0.2+ inches centered on New Hampshire. Strong
northward moisture flux continues, and the northern edge of an
enhanced ribbon of deep moisture is beginning to reach the
mountains of interior New England. CIRA blended TPW product shows
values of 1.5 to 1.6 inches well up the Hudson Valley and into
southern Vermont at the same time, supported by GPS-PW
observations in the area. This is near record values for Albany in
mid-April, and similarly will be for the Gray sounding location as
well. The increasing, anomalously high PWs and strong low-mid
level jet should generate widespread rainfall in the 06-12Z time
frame with some orographic enhancement. Radar trends have
indicated an area of rain filling in over the past couple hours.
Rain rates will not be exceptionally high, and six-hour rainfall
amounts may only peak as high as 1 inch. Nevertheless, flash flood
guidance is exceptionally low (around 0.5 inches for a six-hour
period) in parts of northern Maine. This is likely due to a still
significant snow pack in the area with over 2 inches of liquid
equivalent contained within the snow over much of interior Maine
and the higher elevations of Vermont and New Hampshire. Although
all of this water is unlikely to be released immediately, gradual
runoff from snow melt should combine with persistent rain to lead
to some areas of flooding. Some river gauges in northern Vermont
were already showing rises from relatively modest rainfall amounts
of 0.3 to 0.5 inches over the prior 6 hours. Therefore, the amount
of runoff should be high compared to what would normally be
expected from the rainfall across the region this morning.


...Please see www.wpc.ncep.noaa.gov for graphic product...



LAT...LON   47596858 47126767 45676734 45136932 44537064
            44227188 44177300 45047316 45247200 45777082

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