Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 AM EST Sat Dec 09 2023

Day 1
Valid 12Z Sat Dec 09 2023 - 12Z Sun Dec 10 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE PACIFIC NORTHWEST...

...Pacific Northwest...
An Atmospheric River will impact the Pacific Northwest from 18z
Saturday into the overnight hours. Forecast IVT from the 00Z
guidance supports peak values of 500-700 kg/m/s. With flow
relatively orthogonal to the coast, some heavier rainfall totals
are expected along the Coastal Ranges into the Cascades. Model
guidance generally remains in good agreement, focusing the highest
rainfall totals across northwest OR into western WA. A lack of
instability should generally keep rainfall rates below 0.5"/hr,
however HREF guidance indicates up to a couple hundred j/kg of
CAPE offshore to right along the coast later today, which may
allow for a few areas of embedded higher rates to make it onshore.
HREF neighborhood probabilities of exceeding 0.5"/hr do get up
towards 50%, suggesting that localized rates exceeding that
threshold are probable. Most of the region will pick up 2-3" of
rainfall, although localized 3-5" totals are likely in the more
favored west to southwest facing slopes. Flooding impacts will be
increased within the Slight Risk area given the saturated soil
conditions and above average streamflows over this region from the
atmospheric river event from earlier in the week. 

...OH/TN Valley into the Southeast...
A Marginal risk was maintained for a broad swath from the central
Gulf Coast northward into portions of the TN and OH valley.
Convection ongoing at 12z this morning may pose a localized flash
flood risk over portions of central AR with storm motions parallel
to the convergence axis supporting some training activity. However
by mid morning convection should align more southwest to
northeast, generally allowing for less training and more of an
eastward progression.

By late morning into the afternoon hours a fairly substantial
instability plume for the time of year will stretch from the Gulf
Coast all the way into KY...with values exceeding 1000 j/kg with
northern extent, and over 2000 j/kg further southwest. This will
be plenty of instability to support convective development along
the eastward moving cold front from LA into KY. In general this
activity should stay progressive enough off to the east to limit
the overall flash flood risk. HREF guidance shows high
probabilities of rainfall exceeding 0.5"/hr, with lower
probabilities of 1"/hr. This seems reasonable, with the
instability supporting briefly intense rates, but the eastward
motions limiting the duration of these higher rates. Most areas
should see total rainfall of 1.5" or less as this system
progresses off to the east...although do suspect that the 00z HREF
is probably underdoing max QPF values to some extent over portions
of MS/AL/TN where more robust instability should support localized
2"+ totals. Nonetheless even with these locally higher totals any
flash flood risk should remain isolated in nature.

One area to keep a closer eye is in the vicinity of the southern
Appalachians...generally from northern GA into southeast TN and
southwest NC. The 00z HREF shows 3"+ storm total neighborhood
probabilities exceeding 40% across this area, with showery
activity this morning and afternoon (with some upslope component
in play as well), giving way to what should be a more intense
convective line by tonight where hourly rainfall may push 1". This
type of rain may begin to push us close to Slight risk
levels...but with soil saturation and streamflows well below
average, it seems like this rain probably won't be enough to cause
anything more than isolated flood issues. Thus will keep the risk
level at Marginal, but continue to monitor.

Chenard


Day 2
Valid 12Z Sun Dec 10 2023 - 12Z Mon Dec 11 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHEAST...

...Eastern U.S...
A very dynamic system will impact the East Coast Sunday into
Sunday night, continuing to advance downstream from the Midwest
and central Gulf Coast from Saturday. A cold front is forecast to
be progressive across the southern U.S. as the base of the upper
trough becomes negatively tilted as it swings from the Lower
Mississippi Valley to the East Coast through the period, with a
deepening surface low forecast to track from the Mid Atlantic into
the Northeast.

A Slight risk was maintained across portions of the
Northeast...generally including northern NJ, into southeast NY,
CT, central/western MA and southern VT/NH. Rainfall amounts of
2-3" are likely over a good portion of the Slight risk area, with
most of the deterministic guidance indicating localized swaths as
high as 3-4". Limited instability will keep rainfall rates from
getting too high, but the negative tilt of the mid level trough
and strong low level convergence along/near the low track should
be enough to result in embedded heavier convective elements within
the broader rain shield...with pockets of greater than 0.5"/hr
probable. The increasingly negative tilt of the trough and
associated surface low deepening will prolong the duration of
rainfall over this area...which is responsible for the localized
3"+ totals that are anticipated. This is still a borderline areal
vs flash flood threat given the sub 1"/hr rates that are expected.
However soil information from NASA SPoRT and CREST suggests that
after some initial infiltration a good chunk of this rainfall will
become runoff. Thus would expect to see the potential increase for
some flooding of streams and other low lying or flood prone
locations...including poor drainage and urban areas. If we end up
with 0.5"/hr plus rainfall rates and total rainfall over 3" over
an urban or other flood prone area then even flash flooding is
certainly a possibility. There remains some spread in the axis
most favored for these higher rates and totals...but overall think
the Slight risk area encompasses the most likely location at the
moment. A big wild card continues to be snow melt over northern
New England. The 50+ dewpoint air gets quite close to areas of
deeper snow pack and higher SWE over central/southern VT into
central NH. At this point it appears like there will be some SWE
loss adding to the flood potential over VT and possibly NH, but
probably not a long enough overlap of 50+ dewpoints and higher SWE
for there to be a significant contribution. Nonetheless something
to continue to monitor.

Over the Mid Atlantic generally dry soil conditions and below
average streamflows should be more of a limiting factor for
flooding compared to areas of the Northeast described above.
However still expecting a swath of 2-3" of rain, and potentially a
more narrow swath of 3-4" across this region as well...with some
hourly rates getting over 0.5"/hr. In general exceedance of the
current 1,3, or 6hr FFG appears unlikely...however would still
anticipate there to be some excess runoff concerns that could
develop...especially where total rainfall begins to exceed 3". The
threat of true flash flooding remains low given the the only
modest rainfall rates...but some urban and areal flood impacts are
certainly a possibility. With the flash flood risk lower than over
the Northeast, a Marginal risk should still suffice. Will need to
keep an eye on portions of NC into southeast VA, where there may
be some overlap in higher instability and longer duration rainfall.

Farther south, limited instability and the progressive nature of
the front continues to warrant the gap in the Marginal Risk across
portions of Georgia and South Carolina. However we will maintain a
risk across the eastern Florida Panhandle into southern Georgia
where guidance continues to indicate some potential for training,
which could yield localized flash flooding.

...Northwest...
Little change made to the existing Marginal Risk over western
Oregon given the atmospheric river event will still be ongoing at
12Z Sunday. While IVT values are expected to wane through the
period, localized high rainfall rates (near 0.5 in/hr peak values)
will continue for the first 6-12 hours of the period prior to the
moisture plume weakening and the Pacific front moving through the
region. At this time it appears that the heaviest rainfall Sunday
will be south of the areas with the most rain on Saturday as the
AR shifts southward. Given that, and the overall weakening of the
IVT plume with time, we think a Marginal risk should still
suffice...with a general 1-3" of rainfall expected.

Chenard

Day 3
Valid 12Z Mon Dec 11 2023 - 12Z Tue Dec 12 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF MAINE...

The event described in more detail in the day 2 discussion will be
ongoing Monday morning across portions of ME. More uncertainty
exists with the forecast by this time...both with regards to ptype
and additional QPF magnitudes. The model consensus favors mainly
rain over eastern ME Monday, but a transition to snow is certainly
possible over northern ME. The GFS is most aggressive with the mid
level energy and surface low...and thus drops an impressive 4-7"
of rain over ME for a 48 hour total. On the other hand the ECMWF
and UKMET are more in the 2-4" range, with the official WPC
forecast also 2-4". The PQPF 75th percentile 3-4" and the 90th
percentile is more in the 3-5" range. Thus it becomes apparent
that the 00z GFS is an outlier and lower probability outcome. We
can not completely rule it out, and a solution like the GFS would
likely pose a more significant flood risk over ME, but for the
moment it appears like a lower probability outcome. The more
probable rainfall of 2-4" would still pose a flood threat, but
likely more at the Marginal risk level. Snow melt is a factor to
consider and keep an eye on, although the more substantial SWE is
located over northern ME...and at the moment not expecting the 50+
dewpoint air to make it that far north. So while snow melt may
play some factor in the flood risk over ME, at the moment not
expecting it to have a significant contribution to any flash flood
risk.

Chenard


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