Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
734 PM EST Sun Dec 22 2024
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...


Update...

The 12Z guidance did not suggest any significant changes were
required, therefore made only minor adjustments to the previous
outlook areas.

Previous Discussion...

..Sierra Foothills..

Atmospheric river from previous period will
bleed into the D3 time frame 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 frame 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.

Pereira/Kleebauer

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt