FOUS30 KWBC 280815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Sat Dec 28 2024

Day 1
Valid 12Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA ALONG WITH PORTIONS OF EASTERN
TEXAS THROUGH THE LOWER MISSISSIPPI VALLEY AND MID-SOUTH...

...Northwest...

A front will be in the process of gradually stalling out in a west
to east fashion this morning across areas of southwest OR and 
northwest CA as the deeper layer Pacific flow becomes parallel to
the boundary. This will maintain atmospheric river conditions today
across the coastal ranges which will be rather modest at least for
this morning. However, the last of a series of offshore shortwaves
embedded within the deeper layer Pacific flow should approach 
later today and cross the region tonight which will yield a strong 
resurgence of IVT magnitudes that should approach and locally 
exceed 750 kg/m/s. This will be driven by shortwave amplification 
and the development of a new area of surface low pressure which 
will move into the Pacific Northwest by early Sunday morning. 
Rainfall rates are expected to increase once again as the strong 
warm air advection pattern ahead of the shortwave combine with the 
stronger low to mid-level moisture transport into the terrain. The 
00Z HREF guidance supports rainfall rates rising to locally over a 
0.50"/hour, but they will tend to peak along and just ahead of the 
arrival of the offshore cold late tonight. Some stronger convective
elements/heavy showers are suggested in the HREF guidance with 
passage of the cold front closer to 12Z/Sunday and this may foster 
some spotty rainfall rates peaking into the 0.75" to 1.0"/hour range.

This latest atmospheric river will then weaken as the front moves 
inland, but additional rainfall amounts of as much as 3 to 6+ 
inches are expected, with the heaviest amounts likely tending to be
in vicinity of Coos, Curry and Del Norte Counties involving the 
favored coastal ranges of southwest OR and northwest CA. Lesser 
amounts are expected to advance farther south down the coast, with 
some locally heavy rains likely making it as far south as the north
side of the Bay Area Sunday morning along with the northern Sierra
Nevada. Overall, given the wet antecedent conditions and high 
streamflows, these additional rains are likely to increase the 
flooding threat on area streams and tributaries and thus the Slight
Risk area is maintained for the coastal ranges. The Marginal Risk 
area for this update was tweaked and adjusted a bit farther south 
to account for some heavy rainfall getting closer to the 
aforementioned Bay Area.

...Eastern Texas into the Lower Mississippi Valley/Mid-South and 
Ohio Valley...

A well-defined and organized outbreak of strong convection is
expected to begin this morning across portions of central and
especially eastern TX as a strong shortwave trough amplifies 
across the southern Plains and drives cyclogenesis from west to
east while interacting with an increasingly strong southerly low- 
level jet. A substantial pool of warm-sector instability with 
MLCAPE values of 1500 to 2500 J/kg has already pooled across south-
central to southeast TX and stretching over into southern LA. This
will be lifting north through midday as a warm front lifts north
ahead of the approaching surface low. Strong convection with
widespread concerns for supercell thunderstorms will evolve by
midday and through this afternoon across eastern TX with impacts
overspreading the Arklatex and broader Lower MS Valley by this
evening. The latest HREF model consensus supports the low-level jet
increasing to as much as 40 to 60+ kts across far eastern TX, LA,
southern AR and into western and northern MS by 00Z. As this
unfolds, the aforementioned warm front attempting to lift north 
ahead of the surface low should become a focus for widespread 
convection with potentially multiple rounds of very heavy rainfall 
impacting the same area. Given the level of moisture and 
instability transport coupled with strong shear, the stronger and 
more organized convective cells will be capable of producing 
rainfall rates of 1.5 to 2.5 inches/hour. Concerns for cell-mergers
and cell-training will exist and by this evening, some regional
swaths of 2 to 4 inches of rain with isolated 5+ inch totals will be possible.

Going into the overnight hours, the strong shortwave energy should
take on a bit of a negative tilt as it approaches the Mid-South and
aims for the OH Valley. This will drive the strong low-level jet 
up through large areas of MS, AL, TN and nosing up toward the OH 
River by early Sunday morning. Strong bands of heavy showers and
thunderstorms with a large degree organization should impact the
region, with a QLCS evolution expected to generally unfold across
much of the Mid-South going through the overnight hours given
enhanced deep layer forcing/shear and robust moisture and
instability transport. Localized swaths of 2 to 4 inches of rain
will also be possible across the Mid-South where any cell-training
can take place.

Given the wet antecedent conditions and elevated streamflows 
across much of eastern TX and the Lower MS Valley, scattered to 
locally numerous areas of flash flooding are expected to evolve, 
and some of the flash flooding may be locally significant. The 
flash flood threat will be a bit more conditional downstream across
the Mid-South and especially areas farther north into the OH 
Valley, but given the heavy rainfall potential, the threat of flash
flooding will certainly exist. Given the wetter trend in the 00Z 
hires model CAMS and broader convective outbreak that is expected, 
a much more expansive Marginal Risk has been accommodated along 
with a large Slight Risk area involving eastern TX, the Lower MS 
Valley and basically all of the Mid-South. It cannot be ruled out 
that a need for a targeted Moderate Risk upgrade will exist with 
the next cycle update across some portions of the Slight Risk area.

Orrison

Day 2
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 1/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, the Slight Risk area was 
continued with this update with few changes for the southwest corner 
of Oregon and the northwest corner of California. 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 isolated instances of flash flooding, especially should 
any training of heavier rain occur over flood-sensitive and urban areas.

Wegman

Day 3
Valid 12Z Mon Dec 30 2024 - 12Z Tue Dec 31 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
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