FOUS30 KWBC 052013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
313 PM EST Sun Jan 5 2025

Day 1
Valid 16Z 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...

...16z update...
Small adjustments were made to the broad Marginal Risk area based
on guidance trends and 12z Hi-Res/HREF probability output. 12z HREF
continues to show a solid high probability axis with the convective
line for 1"/hr or 1"/3hr (over 90%). However, the forward speed of
cells continues to quickly diminish the potential for high overall
totals, as 2"/3hr values only peak at 30% in west-central MS,
eventually expanding into northern MS and mainly in the 18-00z time frames.

Based on the evolution from the Hi-Res CAMs, the highest potential
is across this axis, with some lower signals further south and west
into central LA and far SE TX though higher FFG values in that
region would have lower potential of exceedance anyway.
Observational trends and rapid refresh guidance from the HRRR/RAP
denote the core of surface to 850mb moisture and flux is generally
parallel to the approaching front and about 25-50 miles downstream.
Increasing insolation and steepening lapse rates will allow for the
instability axis to align with the moisture, that pre-cursory
convection is likely to develop in the 15-18z time frame across SE
TX into central LA, ahead of the developing convection noted in W
AR/NE TX attm. Eventually, the gap between closes and
streams/ascent merge for the potential for some very short-term
enhancement of totals to 1-2" ranges. This axis does align with an
area of recent dryness compared to locations further west over the
last 7-10 days per AHPS...further diminishing the need for a Slight
Risk area; though central/northeast LA into northern MS will be
the area of greatest potential for any incidents of localized flash
flooding concerns.

Elsewhere, the warm advection across the mid-MS Valley has trended
a bit northward and some of these cells may reach SE MO/W KY where
grounds may have some ice in place. As such, the Marginal Risk was
lifted a bit northward with this update. Equally, trends with
convection across central LA into S MS and slightly slower cold
front pressing eastward after 06z; has warranted a bit further
trimming of the eastern side of the Marginal Risk in AL.

Gallina

---Prior Discussion---

An impressive deep layer cyclone is taking shape over the central
US early this morning, originating from a potent shortwave trough
digging through the Central Rockies over the past couple of days.
The trough is in the process of cutting off into an upper-low as
it emerges from the Rockies into the central Plains. Strong DPVA
in association with the trough will interact with the right-
entrance region of a broad phased jet structure (~140 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,
stretching from the Central Gulf Coast to the Mid-South, ushering
in highly anomalous tropospheric moisture (for this time of year)
with precipitable water values nearing 1.5" (between the 90th
percentile and max moving average, per JAN/BNA/BMX sounding
climatology). Most hi-res CAMs continue to suggest localized 1-2"
totals in association with combined totals from scattered
convection in the open warm sector and the following primary squall
line in association with the approaching cold front, though a
couple of solutions (FV3 and CMCreg) suggest localized streaks of
2-3" totals (from northeast LA through north/central MS into
northwest AL). While much of these totals may occur over a
relatively short period (with any flash flooding conditions more
likely due to storm mode and sub-hourly rates and totals of 1-2",
00z HREF 40-km neighborhood probabilities for 2" exceedance over 3
hours barely peak at 20-30%. Therefore the progressiveness of this
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 reaching 500-2000 J/kg) should largely
preclude any organized flash flood threat. Have maintained the
inherited Marginal risk area once again, 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 2
Valid 12Z Mon Jan 06 2025 - 12Z Tue Jan 07 2025

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

Bann


Day 3
Valid 12Z Tue Jan 07 2025 - 12Z Wed Jan 08 2025

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

Bann

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