Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Wed May 25 2022
Day 1
Valid 01Z Thu May 26 2022 - 12Z Thu May 26 2022
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS THE
CENTRAL GULF COAST...
01Z Update: The HREF suite of ensemble guidance still shows some
potential for pockets of 2- to 3-inch per hour rainfall rates
along the central Gulf Coast region into the overnight hours.
Maintained the Moderate Risk near the central Gulf Coast given the
localized amounts of 3 to 8 inches in the past 24 to 36 hours and
due to the convergent low-level flow still upstream from the area.
Additional convection had started to redevelop over portions of
Arkansas ahead of the next cold front and at least some rain will
track over areas with lowered flash flood guidance. Given a very
sharp gradient in the precipitable water field over eastern
Alabama and western Georgia...saw little reason to make more than
tweaks to the eastern periphery of the Marginal or Slight Risk
areas. Moisture transport into Louisiana...Mississippi and
Alabama remained strong given deep low-level southerly flow 25 to
35 kts as shown by 00Z soundings...which did lend some credence to
the 18Z HREF guidance that still maintained 20 percent
probabilities of 5 inch rainfall amounts in the 12 hour period
ending at 12Z over coastal sections of Louisiana and
Mississippi...which was embedded within a broader 40 percent area
of 2 inch amounts during the same period from eastern Louisiana
into southern Mississippi and Alabama. The remainder of the area
was largely unchanged from the earlier outlooks.
Bann
16Z Update: The potential remains for pockets of 2-3" hourly
totals along the central Gulf Coast the rest of today into tonight
as a few lines of deep convection producing heavy rainfall moves
through. Ongoing bands of rain - one over western FL and another
across southeast LA - may begin to become more intense with peak
heating this afternoon especially where clearing pockets
destabilize more. The cold front - still positioned back west over
eastern TX will slowly approach the region through tonight. Some
12Z hi-res guidance suggests another round of heavy rainfall over
southern/southeast LA is likely and this could spread along the
MS/AL/western FL coast through 12Z Thursday. Given potential rates
and accumulations (24-hr probabilities of 5"+ is 50-60 percent)
along sensitive and increasingly wetted areas - the Moderate Risk
was expanded along much of the central Gulf Coast. Further north,
despite unidirectional flow and higher moisture streaming
northward, the limited instability will likely keep rain rates
suppressed enough to limit more widespread flash flooding issues,
as a result the Moderate Risk over western TN was dropped in
coordination with WFO MEG.
Taylor
Central Plains, Lower MS River Valley to Gulf Coast...
A closed mid to upper level level low slowly tracks eastward
through the period across the Plains & Midwest. There is a
growing
signal in the guidance that the upper cold low itself could
instigate convection as moisture returns via the poleward end of
the warm conveyor belt circulation, and if sunny enough, develops
under the upper center itself where storm motions would be slow.
The Marginal Risk area across in and near eastern KS and southeast
NE has been maintained. Local amounts of 3-4" remain advertised
by some of the guidance in this area.
Return flow drawn northward ahead of the feature will bring
anomalously high PWs reaching 1.75-2.0", particularly along and
ahead of an advancing frontal boundary both within a convergent
axis from an old polar boundary stretching from the Greater
Antilles into the central Gulf coast -- which shifts east with
time -- and along the front and any prefrontal outflow boundaries.
A complicating factor will be an initial convective line that
moves across LA and MS in the morning which drops an outflow
boundary near or past the Gulf Coast. Since Gulf waters are warm
now, the instability gradient should re-establish itself at the
coast and shift north ahead of the cold front, leading to a second
wave of heavy rains over southern LA. With MUCAPE values
approaching 3000 J/kg and 25-40 kts of 850 mb inflow oriented
nearly parallel to storm motions, convection with the potential of
backbuilding, training, and embedded mesocyclones increases the
possibility for higher totals both on an hour and multi-hour
basis. Hourly rain totals up to 2.5" are expected with local
24-hour amounts of 3-7" likely. Some of this rainfall could fall
on areas that have been a bit wetter recently,
especially near and southwest of Baton Rouge and -- over the past
six hours -- around Pascagoula and Mobile. This led to the
introduction of new Moderate Risk areas across south-central LA
and southernmost MS and AL, which was coordinated with the
LCH/Lake Charles LA, LIX/Slidell LA, and MOB/Mobile AL forecast
offices. Due to recent heavy rains, a Moderate Risk area remains
across western TN per coordination with the MEG/Memphis TN
forecast office as a course of least regret -- some guidance still
brings 3"+ amounts to that area.
Wisconsin/Michigan...
The surface low track through the Upper Midwest during the period
will bring periods of showers and thunderstorms, some of which
will interact with building instability and increasing moisture
characterized by MUCAPE rising to 500-1000 J/kg as far north as
southern WI/MI and PWs of 1.5" or so. The storm motions are
expected to be fairly progressive which should limit hourly rain
totals 1-2" but some isolates instances of flash flooding will be
possible due to heavy rains over the past week. The Marginal Risk
area has wavered somewhat eastward across MI and MN to better fit
the recent guidance.
Roth
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Wed May 25 2022
Day 2
Valid 12Z Thu May 26 2022 - 12Z Fri May 27 2022
...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE CENTRAL GULF COAST...
...Ohio Valley and Southern Appalachians...
Closed mid/upper level low moves slowly eastward through the
period, reaching the lower Ohio Valley Friday morning. Pockets of
heavy rainfall will be possible underneath its increasingly moist
center where the latest guidance suggest 1-2" (isolated 3") totals
will be possible over the 24-hour period. HREF probabilities
indicate hourly totals likely to remain 1" or less but there is
some signal for higher rates across eastern KY into WV and eastern
OH. A Marginal Risk was maintained for that area.
Further south, moist southerly flow upsloping across the southern
Appalachians is likely to lead to higher rainfall amounts over the
course of a few rounds of thunderstorms. Some potential for hourly
totals to approach 2" and this could lead to isolated/scattered
instances of flash flooding given the anomalously high PWs and
sufficient instability in place. 24-hour totals may approach 3-5"+
in the favored upslope areas. Antecedent conditions are very wet
given the recent heavy rainfall in the past week. The latest
percent of normal is 300-500 percent and relative soil moisture is
70-80 percent in the top 40 cm layer. As a result, the additional
rainfall could lead to isolated/scattered instances of flash
flooding and the Slight Risk ERO was maintained.
...Gulf Coast...
Slow moving cold front is likely to be across southern LA at the
start of the period with ongoing convection along/ahead of it.
Warm, moist air continues to surge onshore with favorable
ingredients in place for heavy rainfall including higher than
normal PWs and more than sufficient instability. The organized
convection should track eastward through the morning and early/mid
afternoon hours and likely result in hourly totals up to 2"
(isolated signal for 3" hourly totals along immediate coast). This
will fall over areas that are increasingly saturated due to recent
heavy rainfall and the latest HREF probabilities show a moderate
signal for exceeding 5" totals over the course of the day,
particularly across western FL Panhandle. As a result, the
Moderate Risk was expanded a bit to the east while the Slight Risk
was maintained from far southeastern LA to the Apalachicola, FL.
Roth/Taylor
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
356 PM EDT Wed May 25 2022
Day 3
Valid 12Z Fri May 27 2022 - 12Z Sat May 28 2022
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE EASTERN
UNITED STATES...
Ahead of the slow moving closed mid/upper low and a surface
boundary advancing eastward during the day, warm/moist air will
quickly surge northward along the East Coast bringing precipitable
water values 1.5-1.75", which is near 2 standard deviations above
normal for portions of the Mid-Atlantic. Peak heating will allow
for modest destabilization and MLCAPEs of 1500-2000 J/kg.
Soundings show unidirectional flow and surging low level flow at
850 mb. There should be widespread/numerous showers/thunderstorms
with pockets of stronger clusters. Potential exists for hourly
totals 1-2" at times during the afternoon hours and these storms
are likely to fall along the urban corridor from central NC to
eastern PA/western NJ. Much of the region has seen much above
normal rainfall over the last week with 300-600 percent of normal
departures and the latest relative soil moisture in the top 40 cm
layer is 70-80 percent from NASA SPoRT. A Slight Risk was
introduced to account for the threat of isolated to scattered
instances of flash flooding Friday afternoon/evening.
Taylor
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
