FOUS30 KWBC 281525
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1123 AM EDT Thu Oct 28 2021

Day 1
Valid 16Z Thu Oct 28 2021 - 12Z Fri Oct 29 2021

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
WESTERN WASHINGTON...

16Z Update...

...Pacific Northwest...
Precipitation will continue to fall over the region as a strong AR
event unfolds. Saturated soils only exacerbate the situation even
more. Latest Hi-Res indicates 2 to 3 inches with pockets of 4 to 5
inches possible for the higher elevations in the Cascades and
Olympics. Left the Slight Risk and Marginal Risk footprints as is
with radar trends and model guidance aligning well with previous
thoughts.

...Blue Ridge Mountains of NC/VA...
Amounts through the mountain range for the period are still
relatively low, but given upslope flow and overall flooding
potential for the region, have opted to keep the Marginal Risk
area as is.

...Florida Panhandle...
The frontal boundary continues its progression eastward this
morning with locally heavy rains. The Marginal footprint was
adjusted further eastward and northward to account for latest
radar trends and model guidance.

Chiari

Previous Discussion...

...Pacific Northwest...
A strong atmospheric river will impact western WA today bringing
some impressive rainfall totals to the Olympics and northern
Cascades. Not much in the way of instability, but the strong
onshore flow and anomalous PWs will still support some periodic
higher rainfall rates. Just about all the higher res guidance
indicate hourly totals locally exceeding 0.5" at times through
00z. The higher res model consensus is for ~3-5" of rain over the
Olympics and as much as 4-6" over the northern Cascades. For the
Olympics, amounts of this magnitude are not atypical...however
soil conditions are quite saturated, and streamflows are already
running above average before this rain falls. Thus some flooding
issues can be expected. Over the northern Cascades, the forecast
rainfall totals are a bit more rare...with our 24hr forecast QPF
exceeding the 5 year ARI over a rather broad swath, and even
locally exceeding the 50 year. Combine this with the high snow
levels (meaning most of the QPF will fall as rain), and this
expected rain is certainly less common for these areas. That
combined with streamflows already running above average should
result in some hydrologic impacts across the region.

...Blue Ridge Mountains of NC/VA... 
Strong upslope flow into the terrain will result in moderate
rainfall amounts this evening into tonight across the area.
Generally looking at 1-2", although localized amounts over 3" seem
possible along the most favored terrain within a southeast wind
regime. Instability is forecast to be minimal, and thus most high
res guidance peak hourly rainfall at around 0.75". Given these
moderate rainfall amounts, and antecedent streamflows only around
normal, nothing really jumps out indicating a pronounced flood
risk. However, 850mb easterly wind anomalies are close to max
values for the time of year, suggesting the likelihood of strong
orographics in play. So even if some weak instability is able to
produce some embedded convective elements into the terrain, then
some localized flood issues could develop. Autumn leaf fall could
cause clogging drainage as well making some ponding of water a bit
more common. Thus while a relatively low risk, thought the prudent
thing was to maintain the Marginal risk that has been in place.

...Florida Panhandle...
Added in a Marginal risk across portions of the FL panhandle into
northwest FL. Locally heavy convection will continue to push
eastward across the region through the morning hours. Some of
these areas could see multiple rounds of convection within a broad
swath of convective activity ahead of the front. This should be
enough to push some areas over 3"...and antecedent soil and
streamflow conditions are above average over this corridor. Thus
can not rule out some localized flash flood issues through the
morning hours until the cold front clears the area.

Chenard


Day 2
Valid 12Z Fri Oct 29 2021 - 12Z Sat Oct 30 2021

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE MID-ATLANTIC STATES...

...Mid-Atlantic States and Southwest New England...
No changes were made to the risk areas with this issuance. A
Slight risk was maintained from northern VA into eastern WV, south
central PA and much of MD. As the deep layered closed low ejects
eastward guidance indicates an uptick in low level convergence
associated with the cyclone centered over this region. While still
rather low, there is also a subtle increase in CAPE forecast.
These factors should result in an uptick in rainfall amounts over
the region, with a broad 1-2" currently forecast...and localized
amounts over 3". For the most part, antecedent soil conditions and
streamflows are around average (or even a bit below average) over
this region. Thus in general, given the progressive nature of the
system and limited instability, the forecast rainfall magnitudes
and rates are probably not enough to result in any widespread
flooding concerns.

With that said, the strengthening low level convergence and uptick
in marginal CAPE values does suggest at least some potential for a
couple localized linear training convective segments that could
bump hourly totals into the 1-2" range. This could locally exceed
FFG across the DC/Baltimore urban corridor where recent rainfall
has lowered FFG. Across the terrain of northern VA/WV/western
MD/south central PA...anomalous easterly flow should result in
pretty strong orographic enhancement to the rainfall, which
combined with any embedded convective elements, could push
rainfall magnitudes up over 3" in spots...potentially resulting in
some flooding concerns. It is for these reasons that the Slight
risk will be maintained from the urban corridor and points west.
Leaf fall could also hamper drainage in spots resulting in more
ponding of water than we would typically see with this amount of
rainfall.

The Marginal risk extends north into PA/NJ and southern NY/CT.
Rainfall totals here are forecast to be lower...and little to no
instability should hamper rainfall rates. However the northern
1/3rd or so of this Marginal risk has saturated soil conditions
and well above average streamflows...thus even the 1-2" forecast
warrants a Marginal risk.

Chenard

Day 3
Valid 12Z Sat Oct 30 2021 - 12Z Sun Oct 31 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
EASTERN NEW ENGLAND...

As the closed deep layered cyclone continues to push eastward,
there are some indications it may try to take on a bit more of a
negative tilt on Saturday, which would result in an uptick in
moisture transport and forcing ahead of the low over portions of
coastal New England. Thus we may end up with another localized
maximum in rainfall with the system during this time frame. At the
moment the model consensus is for this max to occur over portions
of southeast NH into southern ME...where localized rainfall of
2-4" is within reason. Instability is weak...but potentially
non-zero as southerly flow increases off the Atlantic. Thus some
1"+ hourly magnitudes seem plausible over eastern New England. At
the moment there is a bit of a displacement between the more
susceptible areas and the greatest risk of heaviest rainfall
totals. Portions of southern new England have the wetter soils and
above average streamflows, while southern NH/ME (where the
heaviest rain is currently forecast) have been drier. For this
reason think the risk level is best characterized by a Marginal
risk at the moment. But will need to continue to watch trends, as
can not rule out a small scale Slight risk on future
updates...especially if guidance trends wetter and/or there is a
southward shift into the more susceptible areas.

Chenard


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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