FOUS30 KWBC 031545
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1045 AM EST Mon Mar 3 2025

Day 1
Valid 12Z Mon Mar 03 2025 - 12Z Tue Mar 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN PLAINS...

...16z update...

12z Hi-Res CAMs continue to depict run to run consistency with
small adjustments. Timing of initialization still appears to
be peaking in the 03-06z time window and from north (KS) to south
northern TX. CIRA LPW sfc-850mb moisture shows the dry slot from
yesterday's low level cyclone has started to erode with return
moisture coming through the Concho Valley with the dry line
orienting more WSW to ENE lifting north with Tds increasing from
10-20F into 50-low 60s. This orientation is a bit flatter to the
deeper steering flow once cells do develop/mature after 06z and may
allow for a tad higher potential for repeating/increased duration
of those intense rainfall rates. Still forward speeds will limit
overall totals and flash flooding is more likely to be limited to
urban/typical poor drainage areas that could be very quickly
inundated. Still overall expected totals are not likely to induce
large areas of flooding conditions and a Marginal Risk continues to
be appropriate risk level.

Gallina

---Prior Discussion---
Robust convective development is expected tonight across portions
of north central TX into central OK along a cold front. As
impressive forcing (strong mid-level shortwave and upper-level
divergence) overruns this front convection should quickly develop
by 06z. Activity will likely organize into a line pretty quickly
and then track eastward across north TX and OK. The expected quick
eastward motion of the cold front will likely limit the extent of
the flash flood risk. However with a 50-60kt south southwesterly
low-level jet developing and deep layer flow largely parallel to
the front, there should be some opportunity for brief convective
training. While the duration of training will not be very long, it
may locally be enough to exceed 2" in an hour of rainfall. Setups
like this tend to overachieve rainfall wise, and that will likely
be the case this time as well. Thus we still tend to think the new
00z HREF guidance is too dry (with the PMM suggesting localized
totals of ~1.5", and there may still be an uptick in model QPF in
future runs (with totals in vicinity of one of the wetter models,
the 00z GEM-Reg, still quite possible). Overall, not anticipating
a significant flash flood threat, but localized, mainly urban,
flash flooding is a possibility.

Churchill/Chenard


Day 2
Valid 12Z Tue Mar 04 2025 - 12Z Wed Mar 05 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MIDWEST AND PORTIONS OF THE MID-SOUTH INTO THE SOUTHEAST...

A pattern change will bring widespread precipitation to much of the
Central and Southeast CONUS, as a cut-off low/upper-level trough
and associated surface cyclone progress across the middle portion
of the country. QPF maxima continue to be expected across two
general regions, in and around the Mid-South (primarily in
associated with precipitation from the broad open warm sector and
subsequent cold frontal passage) and much of IA into northern MO
(mainly from the pivoting low pressure center as it occludes).
While flash flood impact coverage should remain somewhat limited in
both areas (low-end Slight risk probabilities) with relatively dry
antecedent conditions and a rather progressive storm system
motion, the very dynamic nature of the system is still likely to
bring periods of strong convection with localized totals nearing
2". While a portion of the QPF over IA is likely to be in the form
of snow on the back-end of the low, the expected 2" total will also
be relatively long duration, as rates will be limited to 0.5"/hr
with a relative lack of instability. Over the Mid-South and
Southeast, any localized 2" totals could occur over a much shorter
period of time (as little as an hour) as instability which be much
more plentiful. Barring any major changes in the forecast strength
and progression of the storm system, impacts are likely to be
capped at the Slight threshold with continued refinement to the
outlook with the addition of more CAM data over the next day or so.

Churchill


Day 3
Valid 12Z Wed Mar 05 2025 - 12Z Thu Mar 06 2025

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
SOUTHERN CALIFORNIA AND THE NORTHEAST...

...Southern California...
A cyclone moving towards the region at the base of an amplifying
and progressive West Coast upper level trough brings precipitable
water values near the coastal ranges above 1", with 0.75"+
extending into the deserts. While the strength of the low-level
inflow is modest -- 25+ kts -- the system has some instability to
work with which could allow for hourly rain totals to reach 0.5",
which would be most problematic in burn scars. Overall totals
appear to be capped near 3". The inherited Marginal Risk appears
reasonable, so changes to the area depicted were minimal.

...Northern Mid-Atlantic States/Southern New England...
A progressive deep layer cyclone moves through the Great Lakes into
Southeast Canada, bringing of shot of 60 kt inflow at 850 hPa into
the region with precipitable water values rising to 1-1.25".
Portions of PA, NY, and the southern coast of New England appear
to have some instability to work with, as MU CAPE of 250-500 J/kg
is forecast to be available. Along with low- to mid-level
frontogenesis, hourly rain totals to 1.25" should be possible.
While most of this area has had below average precipitation this
past week, a good portion of New England north of CT/RI/southeast
MA have 1-2" of snow water equivalent on the ground, so melting
snow should be occurring as well considering the strength of the
warm air advection which could raise both temperatures and dew
points well into the 40s. The inherited Marginal Risk both fits
the most recent QPF guidance and the ideas above, so changes to the
area depicted were minimal.

Roth

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