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FOUS30 KWBC 171547
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1147 AM EDT Fri May 17 2024

Day 1
Valid 16Z Fri May 17 2024 - 12Z Sat May 18 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST/CENTRAL GULF COAST...

...16z Update...
No wholesale categorical changes were made for this update, but
the inherited Moderate Risk was expanded a bit (based on
observational and hi-res model trends). The most significant
adjustment was was to the eastern portion of the Moderate Risk,
which was expanded to include portions of southeast AL and
southwest GA. Elevated convection this morning has already managed
to train from west-to-east across portions of southern MS/AL (see
MPD #281 for more info), resulting in localized totals of 3-6" (per
MRMS estimates) over the past 3-6 hours. This activity may
continue to grow upscale through early afternoon while becoming
more surface-based, increasing the risk for flash flooding for
points south and east of the ongoing flooding in south-central MS.
Uncertainty is highest on the eastern and western periphery of the
Moderate Risk, as questions remain with regard to how far east the
training convective axis makes it this afternoon and evening (with
increasing confidence that impacts will be felt into southwest GA)
and whether a secondary MCS late tonight is able to impact already
rain-soaked portions of southern LA (or possibly remain too weak or
too far south along the Gulf coast). Closer to the center of the
expanded Moderate Risk (portions of southeast MS into south-central
AL), there is high confidence for locally significant flash floods
(with additional localized totals of 3-6" possible, per the latest
12z HREF PMM output).

Elsewhere, the forecast remains on track with no changes to the
inherited outlook.

Churchill


...Previous Discussion...

...Lower MS Valley into Alabama...

Overnight convection will move through much of the area at the
start of the period, associated with the leading shortwave in the
SW flow aloft and coincident with a strong surge in precipitable
water values to ~2 inches (+2 sigma). Through the afternoon, models
have backed off on additional rainfall, perhaps due to limited
surface heating and abundant cloud cover. However, with the trough
axis approaching northeastern TX this evening, additional broad and
the approach of mid-level vorticity may be enough to spark
convection anew around/after 00Z over the northern Gulf into the
I-10 corridor. PWs remain high with sfc Tds into the low 70s with
the quasi-stationary front likely advancing northward as a warm
front. Thus, maintained the Moderate Risk outline especially given
the recent wet period and rain this morning. But, will need to re-
evaluate later this morning for convective trends. HREF
neighborhood probs for >3"/24-hr are still >40% (and up to ~70%)
but likely driven by the lagged model cycles. 06Z HRRR does show
two rounds of convection within/near the Moderate outline with 4-5"
totals so the potential certainly is there.

...Mid-South/MO Bootheel/western KY...

Jet streak passing through this morning will move out of the
region after 15Z, though mid-level vorticity will continue to move
through today. Maintained a Slight Risk for this area based on
ongoing rainfall over saturated soils with the continued chance of
~1"/hr rates this morning just after 12Z and likely more isolated
convection this afternoon.

...Western Virginia...
Added a small/targeted Slight Risk area for the Potomac Highlands
and Shenandoah Valley due to weak flow in the lower half of the
atmosphere (<20kts sfc-500mb) as convection should fire this
afternoon with the advancement of the jet streak across the central
Appalachians. Some of the CAM guidance shows 1"/hr rates as PWs
rise to +1 sigma (1.50"), and it has been a bit wetter than normal
lately.

Fracasso



Day 2
Valid 12Z Sat May 18 2024 - 12Z Sun May 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST...

...Southern Alabama/Georgia into the Florida Panhandle...

Trough over the Lower MS Valley Saturday morning will continue
eastward through the day into the Southeast, with downstream mid-
level impulses traversing the region. SW flow out of the Gulf will
maintain relatively high precipitable water values (~1.75" or
+1.5-2 sigma) into the region ahead of the trough and surface
front, with more impressive 850-700mb moisture flux anomalies into
the region (+3 to +4 sigma). Models have trended farther south and
a bit less organized with convection/QPF closer to the better
inflow (I-10 corridor), and have consequently focused the inherited
Slight Risk area farther south as well. The QPF into D2 will be
dependent on the late D1 evolution across Louisiana (yet
uncertain), which could even force the higher QPF over the Gulf
(e.g., RRFS-A).

Broader Marginal Risk covers much of the rest of the Southeast
into the Carolinas where additional heavier rain rates are
possible, though FFG values are not much lower than over GA as
recent rainfall focused farther to the west.

Fracasso



Day 3
Valid 12Z Sun May 19 2024 - 12Z Mon May 20 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CORN BELT...

Surface frontal boundary will hang up over Iowa Sunday morning as
mid-level vorticity exits the central Rockies and moves across the
central Plains. Precipitable water values will rise ahead of the
approaching cold front from the west to around 1.50" (+1.5 sigma)
with some instability in the afternoon/early evening (500-2000J/kg)
that could yield some 1"/hr rates which are near FFG values over
Iowa and nearby areas. Maintained the Marginal Risk outline from
the inherited D4.


Fracasso


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