000
FOUS30 KWBC 120057
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Thu Apr 11 2024

Day 1
Valid 01Z Fri Apr 12 2024 - 12Z Fri Apr 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PART OF THE
OHIO VALLEY EASTWARD INTO PORTIONS OF THE CENTRAL APPALACHIANS,
MID-ATLANTIC, AND THE NORTHEAST...

...01Z Update...
Adjustments made reflect current observation trends and recent runs
of the hi-res guidance.

While some of the Slight Risk was removed from the Ohio Valley, ongoing
moderate to heavy rain and flash flooding will remain a concern
through the evening from West Virginia northward into western
Pennsylvania. Deep, moist southerly flow ahead of a mid-level wave
will continue to support south-north training of storms across
this region for at least the next few hours. PWs around 1.3 inches,
along with modest instability, continue to support rainfall rates
of 0.25-1 inch/hr across this region, with only a slow eastward
progression of the ongoing band. Consensus of the CAMs guidance
shows additional amounts of 0.5-1 inch for most areas, however
given the already saturated conditions, any additional rainfall may
raise additional runoff concerns. Refer to WPC MPD #0153 (valid
through ~0540Z) for more detailed information concerning near-term
heavy rainfall and flash flooding concerns in this area.

Further east, maintained a Slight Risk extending through
northeastern Pennsylvania into the Catskills. Guidance shows
multiple rounds of showers and storms lifting north across the
region. While these storms are expected to be much more
progressive, HREF guidance shows some potential for combined total
amounts of 1-3 inches across this area overnight, especially in the
Poconos-Catskills region. These may may cause localized runoff
concerns as well, given the relatively moist antecedent conditions
reflected by low flash flood guidance values.

Pereira

Day 2
Valid 12Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHERN NEW ENGLAND...

...20Z Update...

There were no changes made to the SLGT and MRGL risk areas
inherited for D2 across the Northeastern US. Both global and hi-res
deterministic maintain a QPF maximum in-of of the White Mountains
in NH up through the northern Appalachians in western ME due to
primed upslope enhancement from a strong 85H LLJ aligned well with
the south to southeast facing terrain. Modest SWE located over
Northern New England coupled with locally heavy rainfall will cause
some flood concerns for the interior where snowpack has held firm.
Heaviest precip will fall between 12-20z Friday with a high
probability of at least 1" of total precip over the aforementioned
areas with EAS signals between 40-50% in NH with a focal point
within the peaks of the White`s. Signal is a bit stronger into
west-central ME where probabilities peak near 70-80%, the same
locations with the deepest remaining snowpack. Given the trends
remaining favorable, and with local WFO headlines highlighting the
flood risk from the tandem rainfall/snowmelt, maintained continuity
from previous forecast.

Kleebauer


...Previous Discussion...

Made only minor modifications to the previously-issued Slight and
Marginal Risk area. There was a bit of an eastward expansion based
on the latest WPC deterministic QPF and the NBM. But the northern
and western periphery remained in place without any real changes
where the risk of flooding was due to a combination of rainfall and
snow melt. There has been some run to run variation but the overall
idea of strong moisture transport of deep moisture and higher
dewpoints at low levels from the Atlantic that gets directed
normally to the terrain has remained fairly consistent. Thus saw
little or no reason to make any meaningful removal from the
Slight/Marginal Risk areas due to the low-end excessive rainfall
threat across northern New England.

Bann

Day 3
Valid 12Z Sat Apr 13 2024 - 12Z Sun Apr 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG A PORTION
OF THE CALIFORNIA COAST...

...20Z Update...

Very little change in regards to the overall synoptic forecast for
the incoming upper-low off the Pacific. Fairly consistent
presentation within all global deterministic and ensembles with
only some minor shifts of the total rainfall depictions that are
within tolerance to avoid any necessary risk upgrades. NBM
probabilities are fairly bullish within the zones encompassing Big
Sur and the Transverse Range with probabilities around 40-50% for
at least 2" in the vicinity of both zones. A general 1-2" is
expected for the area just south of San Jose, down the coast into
the north side of Los Angeles. The highest totals are within the
coastal areas with emphasis to the terrain focused areas just
inland of the coastline. This is fairly typical with these setups
and the flood threat remains marginal in the grand scheme with
rates being capped closer to the 0.25-0.5" marker with perhaps a
few convective cells overachieving late-Saturday into Sunday
morning as the upper-low moves ashore and the height falls help
steepen lapse rates and promote a burst of heavier rainfall over
the central CA coast. Maintained continuity from the previous
forecast issuance with a MRGL risk over portions of the CA coast
and terrain just inland.

Kleebauer


...Previous Discussion...

Generally light to locally moderate rainfall should gradually be
working its way southward from southwest Oregon into northern and
central California by the time the Day 3 period begins on Saturday
morning as low pressure over the eastern North Pacific eases
southward. On-shore flow ahead of the low and associated upper
level trough will support some one quarter- to isolated on half-
inch per 6 hours along the coastal ranges. Thinking is that the
on-shore flow will be shunted southward with time, but there could
be an up-tick in rainfall rates as cold mid-level temperatures near
the center of the upper low results due to steepening low level
lapse rates at time of maximum daytime heating.

Bann


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