Quantitative Precipitation Forecast Issued by NWS
000
FOUS30 KWBC 110102
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
902 PM EDT Wed Apr 10 2024
Day 1
Valid 01Z Thu Apr 11 2024 - 12Z Thu Apr 11 2024
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE
FLORIDA PANHANDLE, SOUTHERN ALABAMA, AND SOUTHERN GEORGIA...
...01Z Update...
Changes to the Day 1 ERO reflect recent observation trends, as well
as guidance from the latest CAMs.
The convective system that is now moving across southeastern
Alabama, southwestern Georgia, and the Florida Panhandle continues
to produce some heavy rainfall rates on the order of 1-2 in/hr
across portions of the Florida Panhandle northward to the southern
Alabama-Georgia border. While recent MRMS data shows rates have
been coming down over the past hour as the system interacts with a
more stable environment, renewed convective development, supported
by increasing low level inflow and upper level divergence ahead of
an amplified trough, is expected to carry the threat for heavy
rainfall and flash flooding into the overnight hours. A low level
jet increasing to 50-60 kts will support deepening moisture (PWs
1.75-2 inches) and instability across the Florida Panhandle into
southern Georgia and southeastern Alabama along with the threat for
increasing rainfall rates. Consensus of the CAMs shows widespread
2-4 inches, with locally heavier amounts overnight within the area
highlighted by the Slight Risk. The heaviest amounts are more
likely to center with the deepest moisture and greater instability
over the Florida Panhandle into the neighboring counties of
southwestern Georgia, and southeastern Alabama, where a Moderate
Risk was maintained.
Further west, a Slight Risk was maintained over Arkansas where
strong synoptic forcing and modest instability are supporting some
locally heavier rainfall rates. This is expected to continue
through the evening, with the CAMs guidance showing some potential
for locally heavy amounts of 1-3 inches. Fortunately, guidance
suggests most of this will fall outside of the areas hit with the
heaviest rainfall earlier today. While a broad Marginal Risk was
maintained further to the northeast into the Ohio Valley, a lack of
instability is expected to limit the threat for widespread heavy
rainfall rates.
Pereira
Day 2
Valid 12Z Thu Apr 11 2024 - 12Z Fri Apr 12 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
CENTRAL APPALACHIANS AND MID ATLANTIC, ALONG WITH PORTIONS OF THE
OHIO VALLEY...
...Summary...
Trimmed backed the eastern fringes of the previous Slight Risk
across NJ and NY state. Extended the SLGT risk through eastern OH,
western PA, and WV given the increasing threat of convection
within an area of lower FFGs.
...20Z Update...
Recent trends in the synoptic evolution of our broad mid-latitude
cyclone have allowed for some additions to the area SLGT risk
from the previous forecast period. Large scale ascent on the
eastern flank of the strengthening SLP over the central Midwest
will increase drastically during peak diurnal instability across
portions of OH/PA/WV with scattered to widespread thunderstorms
anticipated after 18z Thursday. 12z HREF probability fields
indicate a decent threat for rates to reach upwards of 1"/hr
(25-40%) with a low-end potential for 2"/3-hrs (5-10%) which would
meet the current FFG exceedance intervals located over eastern OH
and western PA, west of the Laurel Highlands. Surface low will be
pretty robust with very efficient moisture transport all the way
into the Ohio Valley and Northeastern US with regional environment
solidly within a bound of 1-2 deviations above normal for PWATs.
This is attributed to the strong low-level moisture advection
flanking into the eastern side of the low within the warm-conveyor
belt. This is not a textbook flash flood threat, but one that is
coincident with stronger surface cyclones (<990mb). HREF EAS
probabilities for at least 1" in precip are between 45-65% over the
outlined area in eastern OH with a precipitous drop in
probabilities for at least 2". This caps the higher end outcomes
necessary for anything higher than a SLGT risk, but was warranted
given the setup and lower FFG intervals in place after last weeks
heavy rain episode.
Further east, very low FFG intervals for all 1/3/6 hr periods will
lead to a heightened risk of flash flooding anywhere there a decent
convective coverage. The urban corridor from NoVA up through
DC/Balt into the eastern half of PA is the most susceptible to
convective impacts. Guidance has somewhat trended away from the
"worst case scenarios" as the primary 85H LLJ screams into the
area, but quickly vacates to the northeast with best moisture flux
aimed more at the adjacent Delmarva and shores of NJ up into LI/CT.
Those areas have higher FFG intervals given the sandier soils, thus
the threat for flash flooding is low enough to maintain just a MRGL
risk for the area. As a result for the above, maintained mostly
continuity from the previous forecast cycle with the largest shift
being the addition of a SLGT out across OH, western PA, and
northern WV.
Kleebauer
...Previous Discussion...
...Synopsis...
As the mid-upper level trough becomes more negatively tilted,
large-scale forcing and deep-layer moisture transport will remain
favorable up the Eastern Seaboard. The main differences however
include the increased forward speed of the upper trough and
surface low/associated fronts, along with weaker deep-layer
instability the atmosphere will have to work with.
...Parts of the Central Appalachians-Mid Atlantic...
Per collaboration with WFOs RNK and AKQ, have removed the Slight
Risk across much of central and southern VA. Rather, have confined
the Slight Risk where the guidance means indicate greater coverage
of 1-3 inches of rainfall in areas of decidedly lower FFG values
(widespread 1hr FFGs of inch or less, 3hr FFGs between 1-1.5").
...Elsewhere across the Eastern CONUS...
Maintained the Marginal Risk area outside of the areas with lower
FFGs, as 1-3" of rainfall within 2-3 hours would more likely lead
to localized/isolated areas of flash flooding.
Day 3
Valid 12Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024
...THERE IS A SLIGHT RISK OVER PORTIONS OF NORTHERN NEW ENGLAND...
...Summary...
Based on the latest guidance trends and collaboration with the Gray
WFO, have expanded the Slight Risk area northwest across western
ME and northern NH. This matches well with the Flood headlines in
place due to the incoming heavier rain and snowmelt combination
that will result in localized flooding.
...20Z Update...
In coordination with the Gray/Portland WFO in Maine, have expanded
the SLGT risk from the previous forecast to the Canadian border to
align with current Flood headlines in preparation for the incoming
moderate to heavy rain threat that will couple with rapid snowmelt
along the higher terrain of NH and western ME. Current forecast
remains steadfast from previous runs within the ensemble suite with
deterministic only showing minor shifts in the precipitation
maximum located over the White Mountains into western ME. A solid
SWE of 1-3" is positioned within the southern and southeast slopes
of the northern Appalachians which is subject to rapid snowmelt due
to the flux of higher dew points and expected heavier rains from
our sprawling surface cyclone to the southwest. Forecast precip of
0.75-1.5" when combined with the snowmelt will generate local flood
concerns within the terrain as stream flows will take in a large
quantity of water over the 24 hour period. The MRGL risk area was
unchanged from previous forecast as the threat of locally heavy
rain from the setup will allow for a low-end flash flood threat
over New England.
Kleebauer
...Previous Discussion...
...Northeast...
Anticipated mainly (at most) localized instances of flash flooding
as the low lifts northward across western Quebec and the cold and
occluded surface fronts push through. A big factor to the excessive
rainfall outlook areas is the aforementioned wet antecedent soils,
exacerbated somewhat by the ongoing snowmelt. Maintained the Slight
Risk across eastern parts of NH into ME, given the heavier
rainfall footprint (1-2") per the ECMWF and GEM.
Hurley
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