201
FOUS30 KWBC 162007
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 PM EDT Thu May 16 2024

Day 1
Valid 16Z Thu May 16 2024 - 12Z Fri May 17 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
PINEY WOODS REGION OF EASTERN TEXAS AND WESTERN LOUISIANA...

...16Z Update...

The premise of the previous forecast remains the same with a
Dangerous and Life-Threatening flash flood event forecast across
portions of east TX through western LA. The recent trends in hi-res
guidance have allowed for a further consensus on where the heaviest
precip will occur, and the magnitude of the potential with the 12z
HREF probability fields very much outlining the upper end prospects
for not only flash flooding, but significantly impactful flooding
given the anticipated totals, rates, and antecedent soil moisture
for the area(s) of impact.

A couple of the probability fields that stand out include; 12z
HREF EAS probability for at least 3" is now upwards of 50-80%
within the confines of east TX from Trinity county to just across
the LA line. Probabilities above 60% for the EAS are statistically
significant due to how that probability is calculated and reserved
for higher end consensus amongst the hi-res deterministic and
time- lagged suite within the ensemble. Neighborhood probability
for at least 2"/hr rates have grown in areal coverage through the
period with a general 20-40% located from central TX down through
the Piney Woods area north of Houston. A bullseye of 50-60% is
located across east TX during the late-afternoon to early evening
hours across the above area as well, signaling the potential for
prolific rainfall within the expected High Risk zone. The last
statistically significant probability is the zone of >50%
probability for at least 5" based on the 12z HREF neighborhood
probability over east TX. 50-70% probabilities encompass the entire
zone within the forecasted High Risk, a positive correlation for
the potential and current forecast.

Precip totals within the latest HREF blended mean are now between
5-8" within the outlined High Risk as well, so the pattern is
favorable and becoming inherently agreed upon by the short term
guidance. This is a high-end flash flood potential for many across
TX through portions of the Lower Mississippi Valley as the complex
is expected to translate eastward into the Deep South by the end of
the period.

A SLGT risk was also added to portions of the Mid-Mississippi
Valley. More on that risk below...

...Mid-Mississippi Valley...

Increasing ascent within the right-entrance region of a
strengthening jet streak will provide sufficient forcing across
portions of northern AR up through southern MO and adjacent bounds
of KY/TN/IL. Surface cold front will lie to the north of the
aforementioned area with a defined warm sector positioned south of
the front. Weak shortwave pulse ejecting out from the southwest
will cross overhead after sunset with a percolating convective
field beginning over AR. As the impulse moves northeast, a steady
initiation of scattered thunderstorms will transpire over the above
region with rainfall rates capable of exceeding 1-2"/hr based on
the latest HREF probabilities for the respective hourly rates
(1"/hr exceedance probability of 50-70% across northern AR into
southeast MO). FFG indices are much lower over the AR/MO border
extending to the northeast due to multiple events that have allowed
for soil priming the past few weeks. With rainfall totals generally
between 1-3" with locally higher among the latest hi-res suite, and
HREF EAS probabilities for at least 1" now upwards of 60-70% with
2" around 20%, the prospects for localized flash flooding have
increased enough to warrant an upgrade to a SLGT risk across the
aforementioned portion of the Mid-Mississippi Valley.

Kleebauer


...Previous Discussion...

...DANGEROUS AND LIFE-THREATENING FLASH FLOODING LIKELY...

In coordination with HGX/Houston, TX; SHV/Shreveport, LA; and
LCH/Lake Charles, LA forecast offices, a High Risk upgrade was
introduced with this update. Dangerous and life threatening flash
flooding is likely in the High Risk area of the Piney Woods.

A vigorous shortwave trough leading a 110 kt zonal jet will move
across Texas today into tonight. Meanwhile, a moisture laden air
mass from the Gulf will advect north into eastern Texas today. The
air mass is characterized by PWATs that may exceed 2.25 inches,
which is above the 95th percentile for PWATs for this time of year,
and is over 3 sigma above normal. In portions of Louisiana this
exceeds the 99th percentile. All this to say, there will be plenty
of moisture for storms to work with as they move east. Instability
advecting northward with the moisture from the Gulf will approach
4,000 J/kg south of the line, but that level of instability will
further support very deep convection. The result is the expected
storms will be capable of extreme rainfall rates that may exceed 3
inches per hour with the strongest storms, but even with much of
the convection, 1-2 inch per hour rainfall rates will be common.

An impressive theta-e gradient over south-central Texas will push
northward on the low-level jet this morning. Some convection has
already fired across the Moderate and High Risk areas already this
morning, but it will take the arrival of the deep Gulf moisture
characterized by surface dewpoints will into the 70s to get the
storms going with daytime heating late this morning. An MCS is
likely to develop across north Texas late this morning with
embedded storms producing extreme rainfall rates. The line will
push southeastward rather quickly, but more convection developing
out ahead of the line, as well as training segments will make for a
rather small-width, but extended length line of convection that
could produce rainfall totals of 3-5 inches with local amounts as
high as 8 inches.

As the storms move into the High Risk area, a push of even deeper
moisture will greet the storms and allow them to further intensify
as they push east across the area. There has been better than
normal agreement in the overall guidance suite for multiple inches
of rain to occur as the storms move through, highlighting the Piney
Woods region for multiple days. With the full CAMs suite now
actualizing the event in the same place, confidence has increased
for the issuance of the High Risk.

AHPS data shows that the High Risk area has seen over 600% of their
normal rainfall for the past 2 weeks alone. Moderate to Major river
flooding is already ongoing in this area. Soils are not expected
to retain any of the rainfall expected today. Rates of 2 inches
per hour with locally higher rates will very quickly re-raise
levels in smaller creeks and streams, while the major rivers
already in flood stage will also see rises. Widespread flash
flooding with locally significant and life-threatening flash
flooding is likely as a result.

A couple cons still adding a bit of uncertainty to the forecast:
1) The cells will be fast-moving so any one area may only see the
extreme rainfall rates for a fraction of an hour, but training of
multiple storms capable of the high rates is expected over the High
Risk area. 2) Rain will only fall for 3-6 hours over the high risk
area as the strong storms move through. Thus, the 3-5 inches of
rain expected will fall over that much shorter time period. This
certainly raises the chances for flash flooding, but with no rain
expected following the storms, this does limit the severity of the
flooding expected somewhat.

As the storms push east across Louisiana tonight, they will weaken
a bit due to the loss of daytime heating and associated
instability, so amounts will drop off as the storms approach
Mississippi. This area has been hard hit with recent rains and
nearly saturated soils, but not as hard-hit as portions of Texas.
Thus, with decreasing rates, the risk areas trail off to the east.
However, there is considerable uncertainty as to how fast that will
happen and certainly where the storms will train towards the east
across Louisiana. Thus, the eastern boundaries of the Moderate and
Slight risk areas are more uncertain.

...North Central Texas...

Better agreement in the CAMs for an axis of heavy rain developing
along the aforementioned theta-e gradient this morning will allow
convection capable of very heavy rains to develop in the vicinity
of the Dallas-Fort Worth Metroplex. Most of the guidance suggests
the strongest convection will be on the west/Fort Worth side of the
Metroplex, so the threat is a little bit lower for Dallas. Nonetheless,
low FFGs characteristic of large cities will allow flash flooding
to develop much more quickly and with less rain than surrounding
areas. In coordination with FWD/Fort Worth, TX forecast office, a
Moderate Risk upgrade was introduced with this update. The
strongest storms will quickly shift south of the Metroplex this
afternoon, but lingering light to moderate rain may continue into
the evening, lengthening the duration of any flash flooding.

Wegman

Day 2
Valid 12Z Fri May 17 2024 - 12Z Sat May 18 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY INTO SOUTHERN AND CENTRAL ALABAMA...

...20Z Update...

Recent trends in guidance have shifted the heaviest axis of precip
for the D2 period a bit further south with the corridor extending
from I-55 in LA to the northeast through southern MS/AL into central
AL as the primary focus for heavy precip and flash flooding
potential Friday into Saturday. The area encompassed by the new MDT
orientation is the region most likely to be impacted by both the
remnant complex from the west that will move through the early
portion of the period, as well as the shortwave perturbation
ejecting out of the western Gulf into the Gulf Coast the second
half of the forecast. The latter is the period of interest with
regards to highest impacts with the current moisture flux anomalies
based within the latest NAEFS Climatological data set indicating
the low to mid-level moisture field reaching the 99th percentile
relative to climo across southeastern LA through parts of the Deep
South. With a strong mid-level vorticity maxima added to the
synoptic picture, this historically is favorable for widespread
convective initiation with a heavy QPF footprint given the
anomalous deep layer moisture field.

Probabilistically, the trends are slowly becoming more favorable
for the aforementioned area with the most daunting signal coming
from the 12z HREF neighborhood probability for at least 5" of
rainfall running between 35-45%, now centered over southern MS with
some probs of 20% dotted over parts of southeast LA through
southern AL. The 12z HREF EAS probabilities paint another picture
with very high probs for at least 1" of precip across much of the
Deep South, but a precipitous drop off once you move to 2" and 3"
with values more in-line around 10-20% which relays some of the
uncertainty in the eventual evolution of the pattern and the
expected heavy rain output. The environment will remain very
favorable for locally heavy rainfall with the region fairly
saturated from previous events, leading to lower than normal FFG
indices to exceed. This is one of the main reasons for the MDT
potential, and the rest becomes correlated to the expected multi-
round threat of convection during the beginning of Friday and
eventual impacts Friday night into early Saturday. After assessing
the mean QPF output and probability fields, as well as coordination
with the offices involved across the Southeast, we were able to
tighten the bounds of the Moderate Risk area and surrounding SLGT
as the pattern likely focuses in-of where the best instability
aligns with the flow running parallel to the frontal boundary
across the southeast. Mesoscale evolution pertaining to remnant
outflows from early convection will likely have a say on the,
"Where" the heaviest precip will occur as convection focuses along
those convergent zones. That will be contingent on short term
assessment, so there could be further shifts in the risk areas with
climo-based trends likely leaning towards a more southerly
adjustment in the higher-end potential. As of now, the MDT risk
extended from portions of southeast LA up through southern MS/AL to
south of Birmingham. SLGT risk extends back through Houston and
points northeast into west-central GA.

The second SLGT risk across portions of the Ohio Valley was
maintained given continuity in recent guidance for some scattered
heavy thunderstorms promoting local totals of 1-2" with locally
higher through the morning and afternoon hours Friday.

Kleebauer


...Previous Discussion...

By the start of the period 12Z Friday, the ongoing showers and
thunderstorms from the Day 1 period will be moving across the Day 2
Moderate risk area already. This first batch will quickly move off
to the north and east into central Alabama and Georgia through the
day, with occasional showers and storms following behind it.

The front that moved the LLJ advecting the impressive Gulf moisture
into the southern US will stall out across Louisiana, Mississippi,
and Alabama by Friday morning. With the LLJ no longer moving, the
low level southwesterly flow will align with the 90 kt upper level
jet streak. This will increase the potential for training storms,
if fast-moving, across the Moderate and surrounding Slight Risk
areas. Once the first batch of storms moves through in the morning,
there will be a relative break for much of the afternoon. However,
weak shortwave impulses tracking parallel to the jet streak may
still allow for occasional waves of storms to track across the
Moderate Risk area through the day.

Late Friday afternoon into the evening, the approach of a final and
strongest shortwave will reach the Slight and Moderate risk areas,
resulting in a "blossoming" of convection into the region. With
continued very high PWATs and instability, the storms will remain
capable of producing rates as high as 2 inches per hour, locally
higher with the strongest storms. Multiple rounds of storms are
likely through Friday night, which will result in storm total
rainfall through early Saturday morning of 2-4 inches with local
totals to 6 inches. The strongest and most prolific rain-makers are
expected during the predawn hours Saturday as the aforementioned
strongest upper level shortwave moves northeast across the Slight
and Moderate Risk areas.

Soils in this area are also quite saturated from recent heavy
rainfall, so it`s expected that most of the rainfall through
Saturday morning will convert to runoff, resulting in scattered
instances of flash flooding with locally considerable flash
flooding.

In coordination with JAN/Jackson, MS; BMX/Birmingham, AL; and
MOB/Mobile, AL forecast offices, the Moderate Risk was expanded
northward into Birmingham and southward into Mobile, AL with this
update.

...Mississippi and Ohio Confluence Region...

Little has changed in the forecast for this region. A secondary
shortwave trough on the northern side of the jet stream will take
some of the Gulf moisture streaming in from the south and support
convection across the area. This convection will likely be ongoing
at the start of the period Friday morning, with any resultant flash
flooding starting soon thereafter. The storms will be capable of 1
inch per hour rainfall rates. Recent heavy rainfall along the Ohio
and Mississippi Rivers in this region have locally saturated the
soils, and additional rainfall amounts of 1 to locally 2 inches of
rain from morning convection may result in widely scattered
instances of flash flooding.

Wegman

Day 3
Valid 12Z Sat May 18 2024 - 12Z Sun May 19 2024

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

...20Z Update...

Little change was necessary in the previous SLGT risk area due to
continuity amongst deterministic and subsequent ensemble output
with locally heavy rainfall possible across portions of the
Southeast extending from the FL Panhandle and coastal AL up through
much of AL/GA. Guidance remains privy to periods of scattered
convection over the Southeast due to a passing shortwave ejecting
out of the Gulf. Moisture flux from the disturbance is forecast to
hold within the upper bounds of climatological norms with the
recent NAEFS output signaling 850-700mb moisture anomalies between
95-99th percentile, a significant ingredient in the potential for
heavy rainfall within any convective schemes. Model output is
generally within that 1-2" zone with some higher QPF maxes embedded
in the QPF fields of deterministic. The potential is a bit capped
compared to the days prior given the expected instability forecast
and less organized in the convective evolution. There is a chance
the Gulf coast areas could be more favored early on in the period
pending the convective evolution upstream from the end of D2. The
contingency of prior periods is too much for any significant shift
in the risk potential, so decided to maintain what was inherited
from prior forecast package.

Further north, a secondary QPF max is plausible across portions of
the Mid-Atlantic with the best opportunity around southeastern VA.
Guidance is still all over the place when it comes to the threat
as, once again, the risk is contingent on previous convection and
the evolution into D3. The MRGL risk was maintained given the
uncertainty.

Kleebauer


...Previous Discussion...

The shortwave tracking across the Southeast causing the heaviest
rains of the Day 2/Friday period Friday night are now expected to
continue into the Saturday period, though from 12Z Saturday through
the morning, the convection should be on a weakening trend.
Nonetheless, given the expected rainfall the previous day,
continued abundance of moisture the thunderstorms can feed on, and
the influence of a strong positively tilted shortwave trough, a
Slight risk upgrade was introduced with this update, and the areas
have been shifted west due to slowing of the forward progression of
the upper level shortwave from previous runs.

By Saturday afternoon however, diurnal heating may allow for a
renewed round of strong thunderstorms as far west as Birmingham,
but more likely further south and east into southeast Alabama and
Georgia. These storms will be capable of 2 inch per hour rates, but
uncertainty with the convective evolution suggests a broad Slight
over the area is the best course of action until the convection
going on out west is better resolved. By Saturday evening, the
convection should be clear north and east of the Slight risk area.

Locally heavy rainfall is possible across southern Virginia
Saturday night. As with areas further south, given high uncertainty
by this point as to the convective evolution due to reliance on
that evolution for convective placement by this point, a Marginal
Risk was maintained for this area. Instability will also be limited
in as well. With better resolution and potential for higher
instability a Slight Risk may need to be considered in this area
with future updates.

Wegman


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