Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Fri May 17 2024
Day 1
Valid 01Z Sat May 18 2024 - 12Z Sat May 18 2024

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

...01Z Update...
No wholesale changes were made with this update either, including
the Moderate Risk area along the Gulf coast despite a relative lull
in activity this evening. Expectation is that additional rainfall
will develop and fall on areas made hydrologically sensitive areas
in the past 24 hours. Latest runs of the high-resolution CAMS
suggests that the axis of heaviest rainfall will be south of the
stripe of excessive rainfall earlier this morning. Even so,
thunderstorms this afternoon have made the area hydrologically
sensitive along the border of the northern Florida panhandle with
Alabama. There is some disagreement among the high-resolution CAMS
in how far north/inland instability reaches overnight from
southeast Louisiana to the Florida panhandle although the signal is
consistent that instability builds overnight with diminishing CIN.
That could result in sufficiently high rainfall rates to result in
additional flooding.

Elsewhere...changes tended to be fairly minor and largely based on
short-term radar/satellite imagery with the expectation that loss
of daytime heating should result in a decrease in the flash
flooding threat through late evening,

Bann



...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