088 
FOUS30 KWBC 050139
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
939 PM EDT Tue Aug 04 2020

Day 1
Valid 0132Z Wed Aug 05 2020 - 12Z Wed Aug 05 2020

...A MARGINAL RISK OF EXCESSIVE RAINFALL REMAINS ACROSS INTERIOR
NEW ENGLAND, THE CENTRAL HIGH PLAINS, AND SOUTHERN PLAINS...

...Interior New England...
Showers and thunderstorms continue to move northward from NH into
western ME which led to the maintenance of the Marginal Risk area,
while the heavy rain north of the center of Isaias across
northeast NY and northern VT has ejected into Quebec.   The
decline in the coverage and intensity of the heavy rainfall led to
the elimination of the Slight Risk area here.  Heavy rains across
western ME should move northward while fading overnight as Isaias
moves into eastern Canada. 


...Plains...
Convection has attempted to organize over the High Plains of CO
and has been moving southeastward.  There remain indications that
convection could grow upscale/organize overnight over TX/KS/OK
while remaining generally progressive.  The 12z Hi-Res CAMs are
presenting a greater potential further south into S CO moving into
the OK/TX panhandles and lesser influence further north, which is
also in line with the 12z GFS.  There is some indication in the
guidance that activity could hang up across the TX Panhandle
generally in the 04-08z time frame, though this area has had below
average two week precipitation.  There doesn't appear to be a
mid-level capping inversion to argue for the more northerly 12z
ECMWF and 12z Canadian solutions, so they were deemed lower
probability alternate scenarios.  The rates, totals and still
lingering uncertainty still falls below criteria for introduction
of a Slight Risk, so kept the risk level Marginal.

Roth/Gallina/Chenard


Day 2
Valid 12Z Wed Aug 05 2020 - 12Z Thu Aug 06 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE CAROLINAS AND INTO THE MID-ATLANTIC...

...Southern Appalachians eastward to the coast and north to
Maryland...
A slow moving cold front will serve as the impetus for
thunderstorms on Wednesday as it drifts eastward across the
region. As it moves east, forcing for ascent will be provided by
low-level convergence, but also within a modest RRQ of a departing
upper jet streak, and weak vorticity maxima rotating northeastward
embedded within the mid-level flow. Instability will climb through
the day due to steepened lapse rates beneath an upper trough and
surface heating, with MUCape reaching 2000 J/kg across the eastern
Carolinas/VA, decreasing to 500-1000 J/kg into the mountains and
north into Maryland/Delaware. At the same time, modest 850mb moist
flow will drive PWs above 1.5", approaching 2" near the coast,
which is 0.5 to 1 standard deviations above the climo mean. In
this environment, thunderstorms are likely to develop and become
scattered to widespread during the aftn/eve. While storm motions
may be quick at 15 kts to the northeast, training along the front
due to boundary parallel flow, and storm interactions further
east, could lead to prolonged periods of heavy rainfall exceeding
1"/hr at times. Much of this region has seen anomalous rainfall in
the past 7 days, mostly from recently departed Tropical Storm
Isaias, and soils are fully saturated. This has led to extremely
compromised FFG which is as low as 0.5"/1hr or 1-1.5"/3hrs across
parts of the region. The inherited MRGL risk was expanded to
include a larger area where rainfall rates on top of saturated
soils could lead to isolated instances of flash flooding.

Note that some guidance produces an additional 2-3+" of rainfall
in a short period across parts of VA/MD. This is not out of the
question as the weak impulse aloft tapping the favorable
thermodynamics and potential for training could lead to prolonged
or repeated rounds of heavy rainfall. If this should occur, it may
necessitate an upgrade to SLGT risk especially over areas which
just received excessive rainfall from Isaias. However, there is
enough uncertainty in placement and coverage at this time to keep
the risk at MRGL, but should be re-evaluated with the next model
cycle and with the addition of the high-res guidance.

Weiss


Day 3
Valid 12Z Thu Aug 06 2020 - 12Z Fri Aug 07 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR NORTH
CAROLINA NORTHWARD TO NEW YORK CITY, AND PARTS OF THE CENTRAL
PLAINS...

...North Carolina northward to New York City...
Another day of scattered to widespread thunderstorms is predicted
as a slow moving cold front remains in the vicinity beneath a
mid-level trough. Although jet-dynamics will be pulling off to the
northeast during day 3, instability which may reach 2000 J/kg,
focused in the eastern parts of the risk area, will combine with
PWs of 1.5-2", 1-1.5 standard deviations above the climo mean, to
produce an environment favorable for heavy rain rates. Although
there remains some uncertainty into the coverage of convection on
D3, rainfall rates of 1-1.5"/hr anywhere across the region could
lead to flash flooding as soils are saturated from 7-day rainfall
that is 300-600% of normal. Additionally, note a widespread signal
for NWM streamflow anomalies ranging from much above normal, to
high, flow across the region, indicating that any heavy rainfall
could lead to stream rises and flash flooding. FFG across the area
is generally less than 1.5"/3hrs, slightly higher in NC, so these
rain rates could produce additional flash flooding Thursday
afternoon into Thursday night.


...Southeast Kansas, Northeast Oklahoma, Southwest Missouri,
Northwest Arkansas...
Continued a small MRGL risk area but with slightly lower QPF than
inherited as an MCS is likely to dive southeastward Thursday night
into Friday morning across this area. A shortwave cresting the
ridge will dive into a region characterized by extreme instability
of >3000 J/kg and PWs exceeding 1.25", about 1 standard deviation
above the climo mean. At the same time, an 850mb LLJ will be
emerging from the the S/SW transporting moisture and resupplying
instability, likely leading to a more southward diving convective
cluster than shown by some of the guidance which progresses the
feature too far to the east. This is echoed by a local 700mb omega
max beneath a region of enhanced divergence. ECENS and GEFS
probabilities show low-end chances for 2" of rainfall, and much of
this region has seen 7-day rainfall of more than 300% of normal.
Any heavy rain could quickly lead to runoff and flash flooding,
and with some training potential along the W/SW side of the MCS,
flash flooding will be possible.


Weiss



Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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