FOUS30 KWBC 290055
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
755 PM EST Sat Dec 28 2024

Day 1
Valid 01Z Sun Dec 29 2024 - 12Z Sun Dec 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA ALONG WITH PORTIONS OF THE LOWER=20
MISSISSIPPI VALLEY AND MID-SOUTH...

...01Z update...

...Lower Mississippi Valley/Mid-South and Lower Ohio Valley...=20
Changes made to the Day 1 ERO here were to trim the back edges of=20
both the Slight and Marginal Risk areas based on the latest=20
observational trends (radar and satellite especially), along with=20
the more recent (18Z) HREF 40km neighborhood exceedance=20
probabilities.=20

...Northwest...
No significant changes were made to the Marginal and Slight Risks
in place across the West Coast with similar thinking in the=20
previous discussion still holding true.

Hurley

...Previous discussion follows...


...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...

Daytime update...
In the East, trimmed back the northern extent of the Marginal Risk
out of the northern Mid Atlantic and Northeast -- confining it
mainly to the southern and central Appalachians. Apart from a few
localized areas in the Pocono-Catskill Mountain region, the general
consensus of the 12Z guidance shows 24-hour amounts remaining
under an inch across much of the Northeast and the northern Mid-
Atlantic. This along with little instability, suggests any threat
for heavy rates and amounts will be limited. Further to the south,
the area centered along the southern to central Appalachians,
remains much the same.

The outlook area in the Northwest remains close to the same as
well, including the Slight Risk centered over far southwestern
Oregon into northwestern California.

Previous Discussion...

...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.

...Southern/Central Appalachians/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 southern to central Appalachians.
Any localized training may cause isolated instances of flash
flooding, especially should any training of heavier rain occur over
flood- sensitive and urban areas.

Pereira/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.

Pereira


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!5-fglq2W5xQmNHrYfnBrcbGAm3JE6asQTc-fl9mzVNhf=
T-nPeTdf4n3ryiFvO9lum4rjR-V3kNpD8TY-zTl50Zg9phM$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!5-fglq2W5xQmNHrYfnBrcbGAm3JE6asQTc-fl9mzVNhf=
T-nPeTdf4n3ryiFvO9lum4rjR-V3kNpD8TY-zTl5e2ZE4ZM$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!5-fglq2W5xQmNHrYfnBrcbGAm3JE6asQTc-fl9mzVNhf=
T-nPeTdf4n3ryiFvO9lum4rjR-V3kNpD8TY-zTl5DR2Bamo$=20

=3D =3D =3D
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