Clifton, Virginia Weather

596
FOUS30 KWBC 181556
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sat May 18 2024

Day 1
Valid 16Z Sat May 18 2024 - 12Z Sun May 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST/CENTRAL GULF COAST...

...16Z Update...

Maintained continuity through a majority of the previously covered
areas within the SLGT and MRGL risk areas. The prospects for flash
flooding remain highest over the Southeast U.S with convergence
along the cold front slowly progressing eastward through the
period. The primary period will be through the 00z hour before the
threat wanes as the best ascent moves away from the region and we
see more isolated to scattered rainfall coverage with less
prominence in the hourly rates. Further north over AL, a remnant
MCV will move east-southeast into northern MS/AL by this afternoon
providing a focal point for convective initiation during peak
instability. The PWAT anomalies are fairly tame for a higher end
threat, but some isolated spots could see 2-3" over the course of
the afternoon with a risk of flash flooding while cells move
overhead. Given the probabilities for 1"/hr running between 40-60%
and 2"/hr around 10-15% at peak, the threat is certainly capped for
any prospects for widespread flooding, so the MRGl risk in place
will suffice for the setup. Across NC, antecedent conditions favor
a better opportunity for flash flooding concerns with streamflow
anomalies relatively high after the recent active pattern across
the region. A backdoor front will progress to the south during the
afternoon and evening with a convergent low-level ascent pattern
developing from western NC through east-central portions of the
state. Instability and PWAT anomalies are generally modest with
scattered convection likely to form within the next few hours
across the terrain west of the Triad. Some isolated totals will
likely hit the 2-3" marker with perhaps a max around 4" based on
the latest HREFpmm and higher-end deterministic, so the threat is
certainly within reason, but the isolated nature of the potential
leans this more on the cusp of a true SLGT risk and a MRGL.

Considering the downward trend of some of the probability fields
from the previous iteration of the ensemble, as well as
collaboration with the affected WFO`s, decided to maintain
continuity to allow for consistent messaging and the threat being
truly isolated. There is a potential for a short term upgrade, but
not at this time.

Kleebauer


...Previous Discussion...

...Southern Alabama/Georgia into the Florida Panhandle...

Mid-level trough/weak closed low over the Lower MS Valley/Mid-
South this morning will continue eastward across southern TN today
and into the southern Appalachians by early Sunday. On its
southeast side, a stream of mid- level impulses will move across
the FL Panhandle and southern AL/GA today ahead of a cold front
that will finally clear through the area by tomorrow. SW flow out
of the Gulf will maintain relatively high precipitable water values
(~1.75" or +1.5-2 sigma) into the region ahead of the trough and
surface front, with more impressive 850-700mb moisture flux
anomalies into the region (+3 to +4 sigma). With ongoing convection
over southern LA moving eastward, expect this to continue past 12Z
along the I-10 corridor from southern MS eastward through early
afternoon. Thereafter, models suggest a loss of mid-level vorticity
dynamics to sustain heavier rainfall but this may be underdone
given the approach of the cold front with still sufficient moisture
available. Slight Risk outline was generally constrained to the
higher probabilities of >2" QPF in the 12-18Z period this morning.
Broader Marginal Risk covers much of the rest of the Southeast into
the Carolinas where isolated heavier rain rates are possible.
Trimmed away the northern/northeastern side of the Marginal Risk
where instability is low (or non-existent) on easterly flow.

Fracasso


Day 2
Valid 12Z Sun May 19 2024 - 12Z Mon May 20 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS/CORN BELT...

Broad SW flow over the central Plains across the Corn Belt will
bring in increasing moisture to the region on Sunday with a surface
frontal boundary hung up over Iowa. Mid-level vorticity will exit
the central Rockies and moves across the central Plains as
precipitable water values rise to around 1.50" (+1.5-2 sigma) with
some instability in the afternoon/early evening (500-2000J/kg) that
could yield some 1"/hr rates (per 00Z HREF thru 00Z Mon) which are
near FFG values over Iowa and nearby areas. The region has been
fairly dry the past week (most of the region has had about 50% of
the normal rainfall) so much of the rain will be beneficial, but
given 1-hr FFG values near 1"/hr (between MCI-OMA-DSM), maintained
the Marginal Risk outline for the region. Higher probabilities
exist farther southwest into central Kansas (to where the Marginal
was extended), but FFG values are higher as well (2"/hr). 12Z CAM
guidance through 12Z Mon may shed a little more light on the higher
rainfall potential over the entire 24 hours.


Fracasso


Day 3
Valid 12Z Mon May 20 2024 - 12Z Tue May 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS/CORN BELT...

Upper pattern remains fairly similar to D2 across the Plains, with
broad SW flow over the region. One wave will exit through the
Upper Midwest as troughing over the West deepens via northern and
incoming southern stream vorticity. Upper diffluence east of the
Rockies will support widespread showers/storms over the central
Plains Monday into Monday night along the stalled surface boundary,
with sufficient moisture in place and sfc CAPE 500-1500J/kg.
Heaviest rainfall per the ensembles lie over western Nebraska (Sand
Hills) where FFG values are highest, so focused the area to the
southeast and east into Iowa where rainfall on D2 may lower FFG
values a bit (from already somewhat lower numbers).

Fracasso


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