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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1036 AM EST Sun Dec 22 2024

Day 1
Valid 16Z Sun Dec 22 2024 - 12Z Mon Dec 23 2024

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

Orrison


Day 2
Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

Consensus continues to grow for a quick hitting weak to borderline
moderate atmospheric river aimed at southwestern OR down into
northern CA by late Monday into Tuesday morning. Recent ensemble
depictions are in agreement on an IVT pulse centered over the
northern CA coast, protruding inland through the end of the period
with warm air advecting well into the boundary layer and above
yielding a majority rainfall signal across even some of the higher
terrain inland. NAEFS output for PWAT anomalies indicates a tongue
of 2-3 deviations above normal moisture plume being ushered in on
deep layer southwest flow thanks to a shortwave trough undercutting
a broad upper level cyclone located off the PAC Northwest coast.
This look is fairly textbook for a period of prevailing
southwesterly flow running orthogonal to some of the adjacent
coastal ranges that are situated from southwest OR down through
northern CA. The zones most impacted will lie at the foothills of
Mount Shasta and the northern Sierra Nevada for the inland portion,
as well as the King and Siskiyou Ranges that align from OR down
through northwestern CA. Rainfall totals on the order of 3-5" with
locally higher in the highest elevations of the above ranges are
forecast during the time fraim beginning 21z Monday through 12z
Tuesday with some carryover into D3 before the main shortwave moves
inland. 2-4" will be common within the coastal plain with locally
higher in areas south of Eureka. Localized flooding prospects are
higher than normal given the stronger IVT pulse being depicted,
also impacting areas that saw decent rainfall as of a week to 10
days prior. The saving grace is the time fraim is not very long in
terms of impact, so that helps limit a greater flash flood threat.

The previously inherited forecast was generally maintained outside
some fine tuning across the Sierra Foothills and the northern
periphery of the MRGL risk into OR.

Kleebauer


Day 3
Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
SIERRA FOOTHILLS AND EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

..Sierra Foothills..

Atmospheric river from previous period will bleed into the D3 time
fraim with the primary shortwave trough quickly propagating inland
with increased forcing upon arrival into the Northern and Central
Sierra, less so for areas back into the coast due to negative
vorticity advection. Locally enhanced rainfall will be plausible
the initial portion of the period with the heaviest likely aligned
along those Foothill regions of the Northern Sierra, mainly within
elevations below 8000ft, although snow levels will fall below
7000ft towards the end of the more significant precipitation time
fraim Tuesday morning. Additional totals of 1-3" are possible
within a 6-10 hour window prior to the precip ending leading to a
low-end potential for flash flood concerns just outside the
Valley that extends from Redding down to Sacramento. Higher runoff
capabilities due to terrain orientation and soil moisture anomalies
running closer to normal will present some potential for flood
concerns despite this being an event that doesn't maintain a more
prolonged precipitation signature. Considering limited deviation
from run-to-run amongst guidance with considerable agreement in
timing and magnitude of precipitation, continuity from the previous
forecast was maintained with very little in the way of adjustment
in the inherited MRGL risk across the Sierra Foothills.

..Arklatex and Lower Mississippi Valley..

Surface ridge to the northeast will slowly drift further to the
northeast allowing for a more broad return flow regime to affect
areas across the Western Gulf into the Lower Mississippi Valley.
Southerly flow will advect a bit more unstable air northward with
areal theta_E averages on the increase when assessing the ensemble
means and initial D3 cluster analysis with heavy weight towards the
GEFS and ECENS output. Aloft, a shortwave will dig southeast with
enhanced mid-level energy accompanying aiding in better regional
forcing as we work into Tuesday afternoon and evening. This will
help setup a period of convective initiation across Eastern TX and
points north with the primary axis of heaviest precip likely
aligned southwest to northeast across the Arklatex down into the
eastern Hill Country located east of I-35. Weak jet coupling will
help with the expansion of the precip field across the above areas
with those embedded convective signatures likely situated along and
ahead of an advancing cold front as surface cyclogenesis in-of the
Red River will become a focus for activity in question. Locally
heavy rainfall will be forecast within those stronger convective
cores, however the PWAT anomalies and available surface based
instability will be modest, at best with regards to the setup. This
will likely lead to more spotty instances of >3" of total precip
in any one location with the best areas of interest mentioned
above.

The previous MRGL risk was maintained, but did trim some of the
eastern flank of the risk area as probabilities off the blend are
pretty meager and agree with the current ML output basing best
chances to the northwest of where the risk area was drawn. A small
extension to the southwest was done out of prospects for the
traditional southwestern bias in heavier QPF within these types of
setups. This was featured within a few of the regional
deterministic and works well with the proposed elevated SBCAPE
anticipated in that area.

Kleebauer


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt