Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Wed Mar 5 2025

Day 1
Valid 12Z Wed Mar 05 2025 - 12Z Thu Mar 06 2025

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
SOUTHERN CALIFORNIA, NORTHEAST, AND MID-ATLANTIC...

...16z update...

...Southern California...
No significant changes were made out in California with the
Marginal Risk; lighter than average onshore/upslope flow is
counter-balanced with slightly higher than average 1.25"+ total
PWat values will support 300 kg/m/s IVT flux should mitigate
intense rain-rates but slow eastward progressions may allow for
solid, hopefully more beneficial rainfall totals. A Marginal Risk
still seems appropriate.

...Mid-Atlantic to New England...
South of the Mason-Dixon line, the risk for excessive rainfall
will be associated with slowly strengthening squall/convective line
that is starting to cross into the Piedmont. Low level flow will
continue to strengthen with 65-70kt southerly 850mb flow, rapidly
increasing deeper layer moisture along/ahead of the cold front,
this will bring total PWats over 1.5" nearing the NC/VA boarder
over the next few hours (this combination results in 1200-1400
kg/m/s IVT stream). The higher moisture/WAA along with steepening
lapse rates aloft may allow for some weak instability 250 J/kg may
reach 500 J/kg MUCAPE by 18z into NC/S VA to help fuel greater
vertical depth and expansion of the line northward with time. Rates
should steadily come up to 1"/hr but given flux and convergence
along the line, may allow for scattered cores within the line to
produce higher rates and streaks of 1-2" and may reach as far north
as DC/S Central MD. These rates/totals are at the limit of FFG
value mainly north of the VA/NC line with greatest potential for
flooding in urban locations, but given progressive nature of the
line a Marginal Risk seems appropriate for coverage.

North of the Mason Dixon line, the moisture flux will support solid
over-running stratiform rainfall across frozen/snow covered ground
allowing for increased run-off potential. While not truly
exceeding FFG values, the risk of flooding from the combination of
rain on snow is covered by the Marginal Risk. A few convective
cells within the cold-air aloft across the Upper Ohio River Valley
may induce highly localized showers/hail and pose some similar
risk, so have adjusted the the Marginal Risk a bit further west and
north to account for this potential.

Gallina


---Prior Discussions---

...Mid-Atlantic States through New England...

Potent mid-latitude cyclone over the Midwest will progress further
into the Great Lakes during the period with a strong southerly
moisture advection regime located over the Eastern CONUS. Fairly
textbook meridional push of warm, unstable air will press northward
amidst a potent 850-700mb layer of winds between 60-80kts with PWAT
anomalies pushing the 95-99th percentiles extending from the
Carolinas all the way into Southern New England. Sharp cold front
pivoting underneath the primary cyclone will press eastward through
the Southeastern U.S with the fronts northern latitudinal placement
shifting through the Mid-Atlantic and Northeast. The combination of
deep layer moisture, modest theta_E advection, and strong
surface/low-level convergence will create a relatively progressive
line of convection later this morning over Appalachia, moving
eastward through the Piedmont creating a period of heavy rain
potential within the urban corridors extending from Philadelphia to
the Triad of North Carolina. Area FFGs are most prone to flood
prospects within the DC/Balt, Richmond, and Philadelphia metros
with the Raleigh-Durham corridor signaling a much higher threshold
which should limit the potential despite the convective signature
present.

00z HREF neighborhood probs for >1" are very high (80%) over much
of the Mid-Atlantic and Northeast, but probs for >2" are much more
muted outside some spots within the Blue Ridge of VA up through PA.
The urbanization factors will create the greatest prospects for
localized flash flood concerns, especially given the 1-2"/hr rates
anticipated as the primary axis of convection blows through during
the afternoon. Southeastern VA across the Hampton Roads area will
be another area of concern with much of the latest CAMs output
fairly aggressive for the local 1-2" totals over a very short span
of time. Considering the heightened risk for this area due to the
urbanization factors, the previous MRGL risk was extended south and
southeast to include those places like Norfolk/VA Beach within the
Tidewater, as well as the Richmond metro where hi-res depictions
for heavy rainfall within a corridor of greater instability is
increasingly likely. Further north into New England, heavy rainfall
coupled with snow melt will allow for greater flood concerns, along
with ice jam concerns coupled with the rain over portions of
Downeast ME. The previous MRGL issuance was relatively unchanged in
those areas with the biggest addition focused over VA and Southern
MD.

...Southern California...

Troughing over the Eastern PAC will make headway into the West
coast with sights on CA as we move through period. A formidable
moisture advection pattern is forecast across Central and Southern
CA with the strongest anomalies (95-99th percentiles) focused over
the Southern CA terrain thanks to prevailing southwesterly flow
providing a but of an orthogonal component to the wind field.
PWATs will be generally between 0.75-1" with locally higher within
the confines of the coast. This is generally an output that puts
the coastal terrain within the LA and San Diego domains at risk for
localized flash flood concerns normally due to the complexity of
the terrain. We add in the positioning of several major burn scars
lingering over the Transverse and Peninsular Ranges and you create
a better opportunity for those flood concerns, especially
considering the threshold is hovering ~0.5"/hr at best. Thankfully
the rates will be mainly relegated to the 0.5-1"/hr at peak
intensity during the height of impacts this evening. This was
enough to warrant a continuation of the previous MRGL in place with
an extension to the Southern Sierra Foothills where heavy precip
totals between 2-3" are forecast within complex terrain just east
of the Valley.

Kleebauer

Day 2
Valid 12Z Thu Mar 06 2025 - 12Z Fri Mar 07 2025

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

...21Z Outlook Update...
The previous forecast is on track, with lingering heavy rain
potential continuing from D1/Wed especially in terrain-favored
areas of the Transverse and Peninsular Ranges of southern
California. Latest guidance has trended wetter in the latter half
of the forecast period (00Z-12Z Fri) especially across San Diego
County. Isolated instances of flash flooding remain possible as
areas of 0.75-1.5 inch rain amounts fall over/near burn scars
within the Marginal Risk area.

See the previous discussion for more information.

Cook

...Previous Discussion...

Persistent in-flow trailing the mean trough propagating into the
Western CONUS will allow for continued moderate precip in-of the
Southern California terrain, mainly the Transverse and Peninsular
ranges within the Los Angeles and San Diego WFO domains. Areal
moisture flux will be trending down in terms of anomalies with the
highest PWATs centered further inland. Despite the evolution in
question, this is mainly a continuation from the period prior
leading to ongoing flash flood potential within the terrain where
several burn scars are present. Fortunately, the setup degrades
further by the second half of the period, leading to mainly
scattered shower instances as we move into the overnight period
Thursday through early Friday morning. Majority of guidance has
precip between 0.5-1" within the mountains outside LA and San
Diego, enough to continue any flood prospects that occurs on D1.
The previous MRGL was relatively unchanged as guidance maintains
run-to-run continuity.

Kleebauer

Day 3
Valid 12Z Fri Mar 07 2025 - 12Z Sat Mar 08 2025

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

Cook/Kleebauer

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