FOUS30 KWBC 041932
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
232 PM EST Sat Jan 4 2025

Day 1
Valid 16Z Sat Jan 04 2025 - 12Z Sun Jan 05 2025

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

Gallina


Day 2
Valid 12Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

...20z update...
The overall setup and environmental conditions remain on track as
noted in discussion below and this update will focus mainly on
guidance trends and further assessment of the Hi-Res CAM/HREF suite
as the entire D2 period falls within the model run times. The
environmental conditions are very favorable for strong
thunderstorms and high moisture flux...as the CSU-ERO first guess
fields are highlighting a potential well into the Slight Risk Range
for but given forward progress/speed of convection... overall=20
totals barely reach 1.5-2.5" totals which barely reach 10-15%=20
coverage exceedance of 1-2 year ARI. So contingency for flash=20
flooding conditions will be more likely due to storm mode and sub-
hourly rates and totals of 1-2", favored by increased localized=20
moisture flux with any rotating updrafts embedded along the quick=20
moving boundary. This will likely occur earlier than later in the=20
development cycle across E TX into central AR between 21-03z.=20
Still, HREF probability of 2"/3hr remains maximized at 30-40% which
is no particularly impressive given the environmental setup.=20
Hydrologically, the earlier and further west, would also rise the=20
risk given recent 7-10 precipitation anomalies and soil saturation
values are more at risk points west of the Mississippi River=20
Valley. Additionally, closer to stronger forcing/pre-frontal=20
moisture flux convergence and naturally lower FFG values across the
TN River Valley will see reduced rainfall rates/totals but still=20
may nose to those FFG to continue a justification of the Marginal=20
across the area into the Southern Appalachians, but did try to pull
back the risk area through the southern Cumberland Plateau.

The only expansion was to include portions of SE LA into coastal MS
and into the western portions of Mobile, AL; this is where
convergence along the western edge of the synoptic sub-tropical
ridge over FL/Eastern Gulf and along the moisture/higher theta-E
gradient will provide an axis of enhanced thunderstorm activity in
the morning into early afternoon. In fact, these cells will have
the greatest potential for some training cells and totals over 3".
Both the ARW (00z and 12z) and HRRR solutions remain most
aggressive and persistent with timing/placement...other Hi-Res CAMs
are not as confidence. Combine this potential with recent heavy
rainfall and increased soil saturation, and there was sufficient
signal to press the eastern portion of the Marginal Risk to cover
it.=20

Gallina

---Prior Discussion---

An impressive deep layer cyclone will develop over the central US
into Day 2, originating from a potent shortwave trough currently
digging into the Four Corners region. The trough is expected to
cut-off into an upper-low as it emerges into the central/southern
Plains with strong DPVA interacts with the right-entrance region
of a broad phased jet structure (~130 kt jet streak @ 250 mb over
the Mid-Atlantic, and a smaller ~100 kt jet streak near the Ozarks).
A substantial low-level jet (850 mb of 40-60 kts) is expected to
form as a result of deep cyclogenesis, ushering in highly
anomalous tropospheric moisture (for this time of year) with
precipitable water values nearing 1.5" (near the 90th percentile to
max moving average, per JAN/BNA/BMX sounding climatology). While
downscaled global guidance (GFS/ECMWF) continues to suggest only
localized 1-2" totals, the impressive dynamics of this system
(along with the anomalous moisture) suggest the potential for this
guidance to locally be exceeded (as the 00z FV3 suggests, but
additional CAMs should better capture the even with the upcoming
12z runs). That said, both the progressive nature of the system and
the limited window of time in the afternoon to take advantage of
destabilization (with SB CAPE from east TX and southern LA to the
Mid-South reaches 500-2000 J/kg) are expected limit localized
totals to the 2-3" range (with as much as 1-2" in an hour with the
main line of convection). Have maintained the inherited Marginal
risk area for much of the Southeast (with some additional expansion
towards the southwest, where instability will be most abundant)
generally encompassing the expected warm sector of the system
(while excluding areas farther north where more impressive broad
QPF exists, as this precipitation will mostly occur in the form of
ice and snow).

Churchill


Day 3
Valid 12Z Mon Jan 06 2025 - 12Z Tue Jan 07 2025

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

Gallina


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!9quVOMdH5Q6N8_DG5vnO86Z99H-PJhLeFOeBEQyAzSke=
F52jVspDuo1AsRBr6i5LPqF49U2mR2qjiFXHLsOSYrhzeq4$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!9quVOMdH5Q6N8_DG5vnO86Z99H-PJhLeFOeBEQyAzSke=
F52jVspDuo1AsRBr6i5LPqF49U2mR2qjiFXHLsOSpQP8dyI$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!9quVOMdH5Q6N8_DG5vnO86Z99H-PJhLeFOeBEQyAzSke=
F52jVspDuo1AsRBr6i5LPqF49U2mR2qjiFXHLsOSdxIDznY$=20

=3D =3D =3D
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