FOUS30 KWBC 281950
QPFERD

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

Day 1   Valid 16Z 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...

...16Z update...

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

Complex and messy evolution of convection is underway across
northeastern TX into the Lower MS Valley this morning. Increasing
low level winds ahead of an approaching potent mid-level shortwave
trough will allow for increasing instability from the TX/LA border
into and across the Lower MS Valley during the day. The
orientation of a warm front from northeastern TX into northern MS
(SW to NE) matches mean steering flow from the SW which will
promote areas of training heavy rain. Forcing for ascent will
continue to increase across these same areas ahead of the shortwave
trough and with increasing upper level jet divergence/diffluence.
Storm scale evolution remains a bit uncertain with the latest 12Z
HREF in disagreement with the exact placement of a stripe of 3 to
6+ inches of rain from northeastern TX into northern LA and
northern MS, which precludes an upgrade to Moderate but within that
zone of expected training, Moderate-type impacts could be
observed, especially given overlap with potentially sensitive
grounds due to recent rainfall and/or urban environments.

...Northwest...
No significant changes were made to the Marginal and Slight Risks
in place across the West Coast with similar thinking in the
previous discussion still holding true. Ongoing steady rain into
western OR and northwestern CA will continue during the day with an
expected increase in precipitation intensity near 00Z ahead of an
approaching Pacific cold front. The main area of concern is across
the northern CA/southern OR coast where heavy rain has been
observed over the past week or so and adding an additional 3 to 5
inches through 12Z Sunday (locally higher possible), where what is
essentially a higher end Slight Risk is in place.

Otto

...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
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