FOUS30 KWBC 270800
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Fri Dec 27 2024

Day 1   Valid 12Z Fri Dec 27 2024 - 12Z Sat Dec 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PACIFIC NORTHWEST AND PORTIONS OF THE MID-SOUTH...

...Northwest...

After what should be a relative lull in rainfall for most of the
day, there will be another round of stronger IVT/atmospheric river
activity arriving this evening and continuing overnight across the
Pacific Northwest with an emphasis on southwest OR and northwest 
CA. This will be driven by the arrival of another Pacific shortwave
trough and attendant surface low that will be yielding an eastward
advance of strong low to mid-level flow. The latest GFS/ECMWF 
solutions suggest IVT values here surging upwards of 750+ kg/m/s in
the 00Z to 12Z time frame tonight across especially southwest OR 
and far northwest CA. A combination of strong warm air advection, 
enhanced moisture transport and orographic ascent over the coastal 
ranges will support 0.25" to 0.50"/hour rainfall rates with these 
rates likely exceeding a 0.50"/hour at least occasionally across
areas of Coos, Curry and Del Norte Counties. More broadly across 
the region, the additional rainfall amounts should reach 1 to 2 
inches by early Saturday morning, but the latest HREF guidance 
suggests some of the favored windward slopes seeing as much as 2 to
4 inches where the rainfall rates will be highest in conjunction 
with the stronger IVT values. Given the continuation of the very 
wet pattern across the region and with streamflows already running 
high, these additional rainfall totals may pose concerns for 
increasing runoff problems and flooding. Thus, a Marginal Risk for 
excessive rainfall remains in place for this period.

...Mid-South...

A Marginal Risk remains depicted across portions of MS, AL, TN and
has been tweaked to include a small part of northwest GA for this
update. A few broken bands of convection will be crossing through
central and northern MS this morning and gradually advancing
downstream across areas of central and northern AL, middle TN and
far northwest GA going through the afternoon and early evening
hours. The activity will be associated with the northeast ejection
of a strong mid-level shortwave trough out of the Lower MS Valley
early this morning which will cross the Mid-South and lift into the
OH Valley. Convection should remain focused in close proximity to 
a trailing front, but instability along the boundary should be 
quite modest and this coupled with the stronger forcing lifting 
away off to the northeast should favor the activity being quite 
disorganized overall with fairly modest rainfall rates. A low- 
end, very modest threat for some runoff problems will exist if the 
broken pockets of convection near the front can train over the same area.

Orrison

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...

...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...

...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
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