FOUS30 KWBC 040748
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
248 AM EST Tue Mar 4 2025

Day 1
Valid 12Z Tue Mar 04 2025 - 12Z Wed Mar 05 2025

..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF 
THE CORN BELT INTO THE ARKLATEX...

A deep layer cyclone progresses across the middle portion of the 
country, drawing in moisture and instability from the south. QPF 
maxima stretch between two general regions, in and around the
ArkLaTex (primarily with a squall line which could have a LEWP/QLCS
character at times and preceding isolated to scattered convection 
which could merge into the line) 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 (a lower end Slight risk probability) with relatively 
dry antecedent conditions and a rather progressive storm system 
motion, the dynamic nature of the system is likely to bring periods
of strong convection with hourly rain totals to 2" and localized 
storm totals nearing 3". 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 3" total will also be relatively long duration, as rates 
will be limited to 0.5"/hr with a relative lack of instability for 
northern areas of the Slight Risk. The main change this update was 
to link together the Slight Risk areas.

Over the Southeast, the bigger problem spot appears the be the
southern Appalachians as Atlantic and Gulf fetches maintain
moisture/stronger low-level inflow longer than other areas of the
region. The 00z HREF probabilities of 3"+ are above 50%, with non-
zero probabilities of 5"+ amounts. The area has been dry as of 
late, so antecedent conditions suggest that some degree of this 
rainfall would be beneficial. Capped the risk as a Marginal for
now, after coordination with GSP/the Greer SC forecast office, 
though it is a high end Marginal Risk for the southern Appalachians.

Roth/Churchill


Day 2
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. The strength of the low-level inflow 
has weakened from what was seen in the guidance from 24 hours ago, 
except in eastern sections of the Transverse Ranges, the 
Peninsular Ranges, and upslope areas of the Sierra Nevada. 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 was pared back a bit across the western
Transverse Ranges and coastal ranges north of Lompoc from continuity.

...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


Day 3
Valid 12Z Thu Mar 06 2025 - 12Z Fri Mar 07 2025

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

...Southern California...
Sufficient low-level inflow from the Pacific, as high as 40 kts or
so, combined with small amounts of instability and precipitable
water values of 0.5-0.75" appear sufficient for hourly rain totals
to rise towards 0.5" an hour, so left the Marginal Risk from 
continuity reasonably intact. It is expected to be the relatively 
cold atmosphere, so heavy snow is possible within the risk area in 
the higher peaks. The main flash flood risk would be to areas with
burn scars.

Roth

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