FOUS30 KWBC 280109 CCA
QPFERD

Excessive Rainfall Discussion...CORRECTED
NWS Weather Prediction Center College Park MD
809 PM EST Fri Dec 27 2024

Day 1
Valid 01Z Sat Dec 28 2024 - 12Z Sat Dec 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PACIFIC NORTHWEST AND PORTIONS OF MISSISSIPPI/ALABAMA...

Corrected for removal of extra header

...Northwest...
GOES West water vapor imagery showed a shortwave rounding the top
of a ridge axis 400-500 miles west of the WA coast with an
elongated W-E axis of mid-upper level moisture to its south, moving
across OR. A longwave upper trough axis was noted back to the west
near 160W with Blended TPW imagery showing the next round of
moisture poised to reach the West Coast later tonight, containing
PWAT values of 1.0 to 1.4 inches a couple hundred miles west of OR.

As the shortwave and longwave troughs track eastward/downstream, a
surface cyclone and accompanying warm front will allow for a surge
in IVT values from northern CA into southern OR (peaking near 800
kg/m/s through ~05Z). With this initial surge, rainfall rates are
likely to peak near 0.5 in/hr from the central/southern OR coast
into the northern CA coast. Beyond 05Z, IVT values are forecast by
the latest model consensus to come down a bit but remain moderate
in intensity (~500 kg/m/s) as the moisture transport axis shifts
southward, primarily into northern CA. Low level flow oriented
nearly orthogonal to the coastline will still maintain the threat
for higher rain rates near 0.5 in/hr. Peak 12 hour rainfall=20
totals, ending at 12Z Saturday, are expected to range from 2 to 4
inches (locally higher) from the favored west-facing terrain of
southern OR into northern CA. Additional rainfall atop wet=20
antecedent conditions and high water levels in area streams/rivers=20
may result in localized flooding.

...Mississippi, Alabama and far western Georgia...
As a negatively tilted shortwave trough axis over the=20
MS Valley continues to lift north and upper level jet forcing=20
begins to weaken, the threat for flash flooding across the central=20
Gulf Coast into inland areas is expected to decrease in coverage=20
and become more localized, likely focused near the greatest=20
instability (500 to 1000+ J/kg MLCAPE) near the coast. PWATs near=20
or just over 1.5 inches and potential for SW to NE training will=20
continue to support the potential for 1-3 in/hr rainfall rates=20
across southern MS/AL into far western FL. Farther north into=20
interior portions of AL and far western GA, while MLCAPE reduces to
near zero, weak elevated instability will maintain a lower end=20
flash flood threat through the night.

Otto


Day 2
Valid 12Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

20Z Update...

Forecast remains largely unchanged from overnight. Over the
Southeast, 12Z models still show enough spatial spread to preclude
a focused Slight Risk outline, and the higher QPF may lie over
areas with higher FFG (even after today's rain).

Fracasso

...Northwest...

A stalling front and IVT plume will allow for a prolonged period of
moderate to locally heavy rain Saturday into Saturday night across
portions of northwest CA into southwest OR. The magnitude of the
event should peak Saturday night as IVT values likely exceed 750
J/kg/m^2 and large scale forcing increases ahead of an approaching
mid level trough. Not expecting there to be any instability to work
with, but the increasing IVT and forcing should allow for rainfall
rates around a half inch per hour, and the stalling of the front/IVT
axis will allow these higher rates to persist. There are some model
solutions that even suggest 5-7" of rain is a possibility.

Even though this area is used to seeing rainfall of this magnitude
without significant impacts, in this case this rain will be falling
after what has already been a prolonged wet period. So saturated
soil conditions and elevated stream and river levels will likely
increase the susceptibility to flooding. For this reason think we
will see a notable increase in the flood risk over this area by
Saturday night, justifying the Slight risk. The inland extent of the
heavier rain could also end up greater than normal, with 1-3"
possible even in inland areas of southwest and central Oregon.

Given that setup, in coordination with MFR/Medford, OR forecast
office, the inherited Slight Risk was expanded northeastward to
include much of Coos, Douglas, and Jackson Counties in Oregon. An
internal higher end Slight was introduced to include Coos & Curry
Counties in OR and Del Norte County in CA. The lion's share of the
rainfall expected in this Day 2 period will be from the evening
through much of the overnight. Since the AR will remain relatively
stationary through this period, expect widespread 4-6 inch rainfall
amounts. This will be added to the rainfall expected today into
tonight and well-above-average river levels already established in
this area.


...Lower Mississippi Valley into the Southeast...

A swath of heavy rain is likely across portions of the Southeast
into the TN valley on Saturday into Saturday night. A strong
shortwave at the base of the longwave trough will eject eastward
into the area, accompanied by a strong upper jet. These two features
support robust divergence aloft and support a widespread convective
threat. The system as a whole should remain progressive, however as
low level moisture transport increases there does appear to be a
window for some training/backbuilding of convection. Overall this
looks to be a widespread 1-2" event, however localized swaths of
heavier amounts are likely. Given the magnitude of mid/upper forcing
in place, combined with PWs increasing over the 90th percentile for
late December, and sufficient upstream instability forecast...it
seems plausible that we could see localized swaths of 3-4".

As yesterday, it appears with some instability and potential for
training convection that there remains a reasonable likelihood that
somewhere in the Marginal Risk area that Slight Risk impacts will
materialize. Unfortunately, guidance remains all over the map as to
where that potential will be realized, from the mountains of the
western Carolinas to central Louisiana. Given last evening's
rainfall and several Flash Flood Warnings, it seemed prudent to
expand the inherited Marginal south and west to cover much of
Louisiana. While the heaviest totals remain most likely over
northern MS/AL and TN, the dry antecedent soil conditions should
preclude more than isolated Slight Risk impacts...so the large
Marginal Risk remains for this update.

Wegman/Chenard


Day 3
Valid 12Z Sun Dec 29 2024 - 12Z Mon Dec 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

20Z Update... Very little change to the forecast from overnight.
Rain rates into southwestern OR may still exceed 0.5/hr across 12Z
thus supporting the Slight Risk. Trimmed a bit from the
northeastern side of the Marginal outline into the Northeast and
expanded a bit on the southwest side to account for just a tad
slower motion of the system in the east, but otherwise left a
somewhat broader Marginal in place that may focus more to the
northwest of I-95 per current trends.

Fracasso


...Northwest...

The A.R. moving into the Northwest will weaken through Sunday
morning. Thus, rainfall rates should diminish with time. Since the
bulk of the Day 2/Saturday A.R. will occur overnight Saturday night,
much of the impacts of the A.R. will be felt with the diminishing
rainfall on Sunday morning. Thus, for consistency both in messaging
and the timing of the worst impacts, a small Slight Risk area was
introduced with this update for the southwest corner of Oregon and
the northwest corner of California. This area will be the hardest
hit area through Saturday night, so the continued rainfall into
Sunday morning will have the greatest impacts in this area.
Otherwise, expect continued rainfall all up and down the Pacific
Northwest, with high elevation snow.

...Mid-Atlantic...

A mature low over the Midwest will track north up the Mitten of
Michigan and into Ontario by Monday morning. Ahead of the low's cold
front, a plume of subtropical moisture will stretch up the entire
Eastern Seaboard. The combination of the moisture and much warmer
temperatures will support widespread light to moderate rain over
much of the Mid-Atlantic. While much of the Mid-Atlantic has been
dry, especially south of the Mason Dixon line, some upslope may help
wring out the moisture along the I-81 corridor through New York and
the I-95 corridor from Richmond through NYC. Any localized training
may cause widely scattered instances of flash flooding.

Wegman


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7QE-m-ghyFnltON9ArMcMvomumlGJpjY8KUQc5zlwtho= y4yv4As39waqKg2POrU3heQkrwc4pUeTk5w6l0M0LkTQPQg$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7QE-m-ghyFnltON9ArMcMvomumlGJpjY8KUQc5zlwtho= y4yv4As39waqKg2POrU3heQkrwc4pUeTk5w6l0M0PbY5L_Y$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7QE-m-ghyFnltON9ArMcMvomumlGJpjY8KUQc5zlwtho= y4yv4As39waqKg2POrU3heQkrwc4pUeTk5w6l0M0-gbDL44$=20

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FOUS30 KWBC 301701 CCA
QPFERD

Excessive Rainfall Discussion...CORRECTED
NWS Weather Prediction Center College Park MD
1201 PM EST Thu Jan 30 2025

Corrected for addition of Slight Risk for Iowa and vicinity.

Day 1 Valid 16Z Thu Jan 30 2025 - 12Z Fri Jan 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX
INTO THE LOWER OHIO VALLEY AND FROM MISSOURI/IOWA INTO NORTHERN=20
ILLINOIS...

...1645Z update...

After coordination with WFO LOT, a targeted Slight Risk was added
from the Missouri/Iowa border into northern Illinois to account
for sensitive hydrologic conditions. The recent stretch of cold
temperatures during much of January has resulted in a deep frost
layer beneath the surface which will likely contribute to
additional runoff of rainfall than what would otherwise occur.
Warm advection rain followed by a developing deformation axis
later today into tonight should be accompanied by limited=20
rainfall rates (remaining mostly below 0.5 in/hr) given a lack of
instability but steady rain of near 1 inch is expected to fall in
a roughly 6 hour window which will likely result in some excess=20
runoff across portions of the Missouri/Iowa border into portions=20
of northern Illinois. While this outlook ends at 12Z Friday, the=20
event will be ongoing at that time with a continuation of runoff=20
concerns extending into the Day 2 ERO period beginning 12Z Friday
into northern Illinois.

Otto

...16Z update...

Adjustments for this update were minor and based on the latest
radar trends out of northeastern TX and the 12Z HREF suite of
guidance. The 12Z members of the HREF appear to be largely in line
with their 00Z counterparts with perhaps a subtle northward shift
with the highest QPF axis. In general, the HREF looks to be
handling the event well so far. Unidirectional southwesterly flow
will continue to allow for areas of training moderate to heavy
rainfall from the ArkLaTex into and across the lower/middle
Mississippi Valley through the day today. Instability is forecast=20
by a vast majority of the latest models to remain below ~500 J/kg=20
across much of the Slight Risk area which will limit hourly=20
rainfall rates to around an inch per hour, but repeating rounds of
0.25 to 0.75 in/hr will meet or exceed area flash flood guidance
values with areas of heavy rain translating eastward into the Ohio
Valley for early Friday morning.

Otto

...previous discussion follows...


Not much change to the inherited Slight risk stretching from
northeast TX, across central AR and into portions of TN and KY.
Both PWs and IVT are approaching climatological maximum values for
late January across this corridor, so a very dynamic system with
plenty of moisture to work with. Instability will be a limiting
factor, keeping rainfall rates and the overall flash flood
potential lower than it otherwise could be. The highest rates will
probably be early in the period over the southwestern portion of
the Slight risk (northeast TX into southwest AR), where hourly rain
could approach 1.5". Further downstream over AR into KY and TN
rainfall rates off the 06z HRRR and 00z HREF peak closer to
0.5-0.75"/hr.

Much of the Slight risk area is expected to see at least 2" of
areal averaged rainfall today into this evening, with the areas
from northeast AR into far western TN/KY having the highest
probabilities of 3"+ amounts. There is good overlap between the
higher 3" EAS probabilities from the HREF and the 06z HRRR max QPF axis...adding confidence in the potential for a 3-5" rainfall
swath from northeast AR into far western KY/TN. Rainfall rates here
will not be all that high to start out, with these higher totals
driven more by a persistent moderate rain. However by 00z guidance
indicates these areas could move into the warm sector and see more
robust convection with higher rate potential. If this occurs then a
greater coverage of flash flooding could evolve.

Stronger convection should develop over portions of southern AR
into MS by this evening. However this activity will be progressive
enough to limit the flash flood risk.

A Marginal risk was maintained in the developing comma head from
eastern KS into northern MO, southern IA and west central IL. Low
topped convection could locally increase rainfall rates, with a
broad 1-3" of rain forecast (highest probs of approaching 3" are in
far northeast KS into northwest MO).

Chenard


Day 2
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WEST VIRGINIA...

...West Virginia...
A Slight risk was issued for portions of central and eastern
WV...primarily for areas where significant snowpack still exists.
The latest NOHRSC analysis indicates upwards of 2-4" of snow water
equivalent over the terrain of eastern WV and the latest model
consensus is for 1-3" of rainfall over this area. With
temperatures and dewpoints forecast to at least get into the 40s
(maybe 50s in spots) the rain on snow should trigger a fairly
efficient melting of the snowpack. This is likely to result in
numerous instances of areal flooding and at least some river
flooding.

Rainfall rates are generally forecast to stay in the 0.25" to 0.5"
an hour range, which is probably not enough to generate flooding
of a more flash variety. However there are indications in both the
06z HRRR and 00z HREF that embedded convective elements Friday
could locally push hourly rainfall a bit over 0.5". Given the
amounts of runoff that will be occurring from the combined snow
melt and rain, any of these higher rates could locally result in a
flash flood threat as well.


...Northern Illinois and Indiana...
A Marginal risk was maintained across this area as comma head
rainfall drops up to 1.5" of rain in a 6hr period Friday. Typically
this degree of rain would not result in impacts, however frozen
ground will likely enhance runoff potential. Given the extra
runoff this will generate, amounts around 1.5" in 6 hrs could be
enough to trigger at least some flood concerns across this
corridor.


...West Coast...
An atmospheric river will bring heavy rainfall from western WA
southward into northern CA Friday into Friday night. The IVT plume
is generally progressive and weakening through the period, so not
expecting major rainfall totals or impacts. Generally forecasting a
1-3" rainfall (locally higher in the favored terrain), which should
mainly drive an isolated minor flood threat.

The IVT axis does start to stall as it shifts into northern CA, and
so could be looking at totals more in the 2-4" range across the
terrain here. Will need to monitor for a potential Slight risk
upgrade across portions of the northern CA coast, but for now will
stick with the Marginal. Leaning towards this day 2 rain mainly
helping saturate conditions leading towards a greater flood risk
on later days. However should things trend quicker and/or wetter
an upgrade may need to be considered.

Chenard


Day 3
Valid 12Z Sat Feb 01 2025 - 12Z Sun Feb 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...

A pretty significant model change has been noted over the past
couple cycles across the eastern Pacific into the west coast. The
earlier model consensus was for a stalling of a moderate to strong
AR over northern CA and southwest OR resulting in significant
rainfall amounts. However recent model runs have unanimously
shifted further south with this AR, now stalling it more over
central to north central CA. There has also been a weakening trend
in the AR, so QPF values are not as high as earlier forecast.
However, with the southward shift the max rainfall axis is now over
areas that tend to be more sensitive to heavy rainfall and related
flood impacts. Thus even with the drop in QPF, think a southward
shift of the Slight risk is warranted. The Slight risk now
stretches from Mendocino county south into the San Francisco area
and as far south as Santa Cruz county. It also extends inland
across the Sacramento Valley and into the Sierra Nevada.

Rainfall along the coast will start on Friday and continue into
the day Saturday. A gradual saturating of soils are expected to
lead to potential greater impacts on Saturday, warranting the
Slight risk. Two day rainfall totals are currently forecast in the
2-4" range. Two day rainfall amounts of 1-3" are expected within
Sacramento valley locations, with upwards of 4-6" within the
Sierra Nevada (most of which will be rain given high snow levels).

Chenard


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7OaPx0J68M44d-ArU2mleNiDjmo5d_i-AE5HwruQYIt2= YxX2nXYtwoKdhHYHnsJ9UEdgVY2x6Z5N_kEQ0bAtU7x0EcM$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7OaPx0J68M44d-ArU2mleNiDjmo5d_i-AE5HwruQYIt2= YxX2nXYtwoKdhHYHnsJ9UEdgVY2x6Z5N_kEQ0bAtTBM5nzo$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7OaPx0J68M44d-ArU2mleNiDjmo5d_i-AE5HwruQYIt2= YxX2nXYtwoKdhHYHnsJ9UEdgVY2x6Z5N_kEQ0bAtB6QMQfs$=20

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