WPC Met Watch |
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Mesoscale Precipitation Discussion: #0652 (2021) |
(Issued at 1036 PM EDT Tue Jul 27 2021
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MPD Selection |
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Mesoscale Precipitation Discussion 0652
NWS Weather Prediction Center College Park MD
1036 PM EDT Tue Jul 27 2021
Areas affected...Southeast Georgia, Southeast South Carolina
Concerning...Heavy rainfall...Flash flooding possible
Valid 280234Z - 280830Z
Summary...Slow moving thunderstorms in the vicinity of a trough of
low pressure will expand in coverage tonight. Rainfall rates of
2-3"/hr at times could produce locally in excess of 3" of rain.
Flash flooding is possible.
Discussion...The regional radar mosaic this evening indicates
scattered showers and thunderstorms expanding in coverage and
intensifying across Southeast Georgia. These storms are developing
around a wave of low pressure that is weakening into a surface
trough, accompanied by intense thermodynamics. Recent 00Z U/A
soundings from KCHS and KJAX indicated PWs of 2.23" and 2.34",
respectively, with 850mb dewpoints of 16-17C, all above the 90th
percentile for the date. MLCapes from the RAP/SPC Mesoanalysis
were still 1500-2000 J/kg, highest along the coastal plain where
modest onshore flow was transporting the higher instability
onshore. Within this environment, recent rainfall rate estimates
from KCLX had reached 3"/hr, and 6 hour rainfall has been measured
as high as 6" at a mesonet near Tarboro, GA.
Despite the weakening of the surface wave, the residual trough is
progged to lift slowly northward during the next few hours. At the
same time, a shortwave noted in GOES-E WV imagery and analyzed by
the RAP will lift northward above it, while a jet streak to the
north drops slowly southward to produce at least modest enhanced
lift within its RRQ. With favorable thermodynamics persisting into
the night, this region of enhanced ascent should lead to an
expansion in convective coverage the next several hours with rain
rates reaching 2-3"/hr at times as shown by the HREF
probabilities, fueled by modest inflow of these anomalous 850mb
dewpoint temperatures.
Although 40cm soil moisture across the region is near normal due
to relatively normal 14-day rainfall, a low-end flash flood risk
will persist. The mean 850-300mb winds in the vicinity of this low
are quite light, only around 5 kts, with similarly slow
propagation vectors. In a region of weak shear, this implies that
storm motions will be slow, chaotic, and at times dependent on
storm mergers or boundary collisions. This suggests rainfall could
reach more than 3" in a short period of time, and while the
high-res is scattered in its depiction, the recent HRRR runs have
shown a gradual increase in coverage and amounts, and the HREF
12-hr probabilities feature local chances for 5" or more. Should
this rainfall occur atop any urban areas, especially near the
coast where moisture transport is maximized and FFG is slightly
lower, scattered instances of flash flooding would be possible.
Otherwise, an isolated risk exists across much of the MPD area.
Weiss
ATTN...WFO...CAE...CHS...FFC...JAX...
ATTN...RFC...SERFC...NWC...
LAT...LON 33308020 33257958 32957955 32528005 31938059
31268098 30838127 30738175 30888249 31408271
32528242 33148177 33258096
Last Updated: 1036 PM EDT Tue Jul 27 2021
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