784
FOUS30 KWBC 100058
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Thu May 9 2024

Day 1
Valid 01Z Fri May 10 2024 - 12Z Fri May 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS
THROUGH THE SOUTHEAST U.S...

...Southern U.S....

Isolated to scattered flash flooding is expected across a wide
swath of the Southern U.S. tonight...stretching from eastern TX
into southern GA. MLCAPE values ranging from 3000-5000k j/kg and
PWs around 1.75" supports deep convection capable of intense
rainfall rates. As we go through the evening hours convective
coverage should increase as mid/upper forcing ramps up and low
level convergence increases as a cold front drifts southward.
Multiple areas of convection are likely across this swath through
the overnight hours. Eventually we may see a more dominant squall
line, but before that we are likely to see numerous convective
clusters. Individual cell motions will generally be fairly quick,
however some areas are likely to see multiple rounds of convection.
Also, as activity grows upscale overnight a west to east training
axis is a possibility somewhere from LA into AL.

Not seeing a strong or consistent enough signal for a MDT risk
upgrade, especially since a lot of this region has relatively high
FFG that will need to be overcome. However this is likely a higher
end Slight risk event and scattered flash flooding is expected
overnight across this entire corridor. The robust instability and
moisture will support hourly rainfall as high as ~3". So even
storms that are moving along at a decent clip will have the
potential to result in urban flash flood concerns.

East central TX into west central LA is the corridor that has the
most saturated conditions and highest streamflows. There is
growing concern for a locally significant flash flood threat in
this vicinity through the evening hours. Observational trends
suggest a period of colliding and training convection is possible
over a narrow corridor during the next several hours...with the
00z experimental WoFS indicating a 40-70% chance of exceeding 5" of
rain, and a 90th percentile QPF of ~8". Any rainfall swath of this
magnitude will be pretty narrow, but the potential is there for a
localized significant flash flood given the antecedent conditions
over this area.

...Mid Atlantic...

MPD #253 describes the localized flash flood risk over portions of
WV/PA and far western MD this evening. Brief convective training
segments could locally drop 1-2" of rain within an hour, resulting
in an isolated flash flood risk through the evening hours.


Chenard


Day 2
Valid 12Z Fri May 10 2024 - 12Z Sat May 11 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST U.S....

...20Z Update...

Very little change in the previous MRGL risk across southeast AL,
southern GA, and northern FL as the expected synoptic and
mesoscale evolution have deviated little since the last forecast.
The primary target will be across southwest GA through northern FL
between Tallahassee and Jacksonville. Propagating MCS across the
Southeast U.S will continue its progression into GA by the morning
with locally heavy rainfall between 1-2"/hr possible before the
primary outflow exits off the eastern seaboard. Secondary
outflow(s) from associated convection could very well contribute to
deviated motion away from the mean flow with some guidance
introducing secondary and tertiary maxes within the FL Panhandle.
The latest HREF probabilistic output for 1" and 2"/hr rates
indicates that opportunity with small areas of 20-40% probs for the
1"/hr rates and 10-20% for the 2"/hr within the Panhandle east of
Panama City over towards Jacksonville. This allowed for a minor
southern nudge of the risk area to account for the chance that the
core of the precip ends up further south than the mean QPF
forecast. Otherwise, areas within GA are expected to see another
wave of precip with a footprint of 25-35% neighborhood probabilities
for at least 3" within the zone of southwest GA, an area that has
seen significant rainfall and slightly lower FFGs compared to the
surrounding locales. Overall, the threat is low to medium end of
the MRGL risk considering the convective nature of the precip and
expected overlap from the D1 rainfall associated with another MCS
dropping southeast through GA currently. Thus, continuity was
maintained in the grand scheme with just those minor adjustments
based on the probability fields and blended mean QPF.

Kleebauer


...Previous Discussion...

Only subtle changes were made to the Marginal Risk outlook area
from yesterday`s Day 3 ERO. The advancing frontal boundary across
the Southeast U.S. will be the primary focus for deep convection
Friday into Friday afternoon and will be capable of producing rain
rates in excess of 1-1.5"/hr at times. Showers and thunderstorms
are expected to be ongoing at the start of the period, lingering
from the deep, organized convective line from the previous day.
This activity will be in the presence of a sufficiently high
moisture pool, characterized by dewpoints well into the 60s and low
70s and a PW axis of 1.5-1.75". A subtle shortwave trough noted in
the flow will be the main large scale forcing mechanism to drive
the convection that looks to track through southeast Alabama,
southern Georgia, northern Florida and then eventually off the
Southeast coast. Some localized heavier rain totals and rain rates
will be possible that could pose an isolated flash flood risk.

Hurley/Taylor


Day 3
Valid 12Z Sat May 11 2024 - 12Z Sun May 12 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF WEST TEXAS...

Despite a marginal airmass in wake of a cold front to begin the
weekend, a low-end MRGL risk for heavy rainfall will exist within
the confines of the Concho Valley and adjacent areas of the Permian
Basin across west TX. A stout upper low across the Great Basin
will pivot eastward to the Four Corners with a prevailing diffluent
signature focused across west TX and NM allowing for multiple mid-
level perturbations to eject northeast out of the mean flow.
Theta-E ridge located over southwest TX in conjunction with the
passing vorticity maxima will help initiate area convection with
focal points generally tied to the topography near the Davis and
Glass Mountains to the west of the Pecos River. Efficient mid-level
shear will aid in preservation of convection as it motions to the
northeast with enough large scale ascent to continue it`s
progression well to the northeast of the initiation point(s).
Environment will be conducive for locally heavy rainfall within a
general spread of 1.1-1.5" PWATs, a solid 1-2 standard deviations
above normal based on the latest NAEFS standardized anomalies. The
second mid-level vort max is being depicted as the most prolific
with best prospects for more widespread convective coverage,
specially for areas east of the Pecos River. Ensemble bias
corrected QPF output is generally between 0.75-1" with some
embedded maxes around 1.25-1.5" which is fairly aggressive at these
leads without any input from relevant CAMs suite. ML output from
both the GFS/ECMWF Graphcast indicates a relative max across the
area outlined with even some potential for the risk area to be
expanded west, pending the convective spread for areas within the
zone of better theta-E`s. Have introduced a Marginal Risk for the
aforementioned locations with future forecast iterations
potentially expanding in any given direction pending favor
dynamically or thermodynamically.

Kleebauer


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