Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
807 PM EDT Fri Jul 26 2024
Day 1
Valid 01Z Sat Jul 27 2024 - 12Z Sat Jul 27 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MIDDLE/UPPER TEXAS COAST, THE COASTAL CAROLINAS, AS WELL AS COASTAL
SOUTHEAST LOUISIANA AND MISSISSIPPI...
...01z Update...
Diurnal convection is waning across much of the country, but a
Slight Risk has been maintained where convection remains active
across coastal portions of LA/MS where convection (see MPD #740).
While this activity is poised to weaken and diminish with the loss
of daytime heating, there should be reinitiation of convection
late tonight over portion of the Middle and Upper TX Coast (where a
Slight Risk has been maintained and expanded). The 18z HREF
indicates 40-50% odds for localized 3" exceedance (per a 40-km
neighborhood method), mostly after 06z. Broader Marginal Risk areas
were maintained across the Southeast, Mid Atlantic, Southwest, and
Intermountain West (where localized flash flooding will continue
for at least a few more hours).
Churchill
...16Z Update...
Several changes to note with today's midday update:
...Southeast Texas & The Mississippi Delta...
The Slight across southeast Texas has been expanded to include
much of Louisiana and the Mississippi Gulf Coast. Ongoing heavy
rainfall has resulted in numerous Flash Flood Warnings across east
Texas and northwestern Louisiana. A plume of deep tropical moisture
is surging northward across the Slight Risk area. CAMs guidance
shows with peak heating this afternoon, numerous showers and
thunderstorms will form over the Gulf and move inland into southern
Louisiana and the Mississippi Gulf Coast. With unidirectional flow,
it's probable there will be training storms embedded within the
broader field of storms. Meanwhile, the plume of moisture over
northern Louisiana now should decrease in intensity as peak heating
results in universal increases in instability, resulting in
dissipating storms reliant on a coherent plume of moisture.
PWATs are near their climatological maximum across Louisiana as
they approach 2.25 inches this afternoon. This will allow the
expected afternoon convection plenty of moisture to translate into
efficient rainfall rates. Urban areas including New Orleans,
Gulfport, and Baton Rouge are at a higher threat for flash
flooding.
...Carolinas...
In coordination with RAH/Raleigh, NC; CAE/Columbia, SC; and
ILM/Wilmington, NC forecast offices, the Slight for the coast was
expanded westward to cover the ongoing convection along the NC/SC
border and aligned with MPD 734. While the convection has been
shallow, very efficient warm rain processes and training has
resulted in multiple inches of rain in the impacted areas. The
focus going into this afternoon will be much closer to the coast,
but any disorganized afternoon convection that impacts this same
area...with sufficient clearing of the cloud cover between now and
then, could result in widely scattered instances of flash flooding.
...New Mexico...
In coordination with ABQ/Albuquerque, NM forecast office, a Slight
Risk upgrade was introduced with this update for the afternoon
convection in the Sacramento Mountains. Increasing instability this
morning and continued above normal atmospheric moisture should
result in a renewed round of afternoon storms. The Sacramento
Mountains have been both hard hit in recent weeks with heavy rain,
and have very sensitive and vulnerable burn scars around Ruidoso
that would enhance the impacts of any potential flash flooding in
that area.
Wegman
...Previous Discussion...
...Portions of Southeast Texas...
Radar was showing increasing coverage of showers in a region of
confluent flown off the Gulf of Mexico in the early morning
hours...and the expectation is that convection will be persisting
beyond 12Z. Given some overlap with a region of lower Flash Flood
Guidance values and potential for 1 to 2 inch per hour rates
redeveloping later in the day as shown by the HREF 40 km
neighborhood probability guidance, maintained a Slight Risk from
coastal Texas northeastward along the axis of highest precipitable
water/instability. There is some upper support in the form of a
trof axis between 200 mb and 300 mb with a 50 kt speed max rotating
around the east side of the trof axis that looks help draw the
moisture towards the Southern Mississippi valley late in the
period,
...Portions of the Southeast US Coast...
Surface low pressure will linger along the Carolina coastline for
much of the period with an associated frontal boundary helping
provide some focus for convection as mid-level height falls
approach from the north and west. Much of the operational guidance
showed the heaviest rainfall along or immediately off the Carolina
coastline...although the NAM maintained some threat of 2 to 5 inch
amounts falling inland. Between that and the fact that the 26/00Z
HREF showed 40 to 60 pct neighborhood probabilities of 1- and
3-hour QPF exceeding flash flood guidance along the immediate
coastline at time of maximum daytime heating...maintained the
Slight risk but continued to shrink the area somewhat from the placement
of the inherited Slight Risk area.
Surrounding the two Slight Risk areas was an expansive Marginals
Risk area. Models depict abundant moisture and instability in this
area but weak forcing mechanism and meager steering flow. Thus
cells that form will be slow moving and efficient rainfall
producers.
...Southwest...
Enough moisture and instability will still be in place over
portions of the Southwest and the Intermountain basin to support
the development of convection in the afternoon and which persists
into the evening...although the best focus will have shifted
eastward since Thursday. There is still concern that any storm
which forms will have the potential for 1+ inch per hour rates and
isolated storm total amounts in the 1 to 2 inch range...enough to
result in flooding and run off problems with the greatest risk
being over recent burn scars and in normally dry washes. Some minor
adjustments were made to the previous MRGL risk area but overall
there was a fair degree of continuity.
Bann
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sat Jul 27 2024
Day 1
Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TEXAS GULF COAST|...
...Texas Coast to Southeast US...
Maintained the Slight Risk area with onshore southerly flow of
deep tropical moisture off the Gulf set to bring another day of
scattered convection capable of producing heavy rain over an area
where flash flood guidance has been lowered by several days of
moderate to heavy rainfall...with the expectation that the
convective organization should be less today compared with previous
days.
Elsewhere, only modest adjustments were made to the expansive
Marginal Risks surrounding the Slight Risk area and eastward to the
mid-Mississippi Valley given persistent south to southeasterly
flow on the east side of an upper level trough/closed low. The
trough/upper low could lead to enough upper support to focus and
sustain storms that produce localized downpours...but even
convective initiation along local outflow boundaries will be
forming in an environment supportive of downpours from the
Mississippi Valley into parts of the Southeast U.S...so isolated
instances of multi- inch rainfall totals are quite possible.
...Southwest United States...
Signals from the 27/00Z suite of numerical guidance continues to
show decreasing threat of excessive rainfall as mid level flow
increases across the Intermountain region. The exception remains
over parts of Arizona and New Mexico where enough
moisture/instability linger in a region with meager southwesterly
steering flow...so will be maintain at Marginal risk area.
...Upper Midwest...
Maintained the Marginal Risk area mainly over northern Minnesota as
given the presence of increasing southwesterly low level flow
feeding an airmass with precipitable water values locally in
excess of 1.75 inches ahead of a cold front making its way across
the area. Given spread in forward speed of the front...expanded the
Marginal Risk area but largely maintained continuity in terms of
the area covered.
Bann
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sat Jul 27 2024
Day 2
Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST...
...Northern Minnesota...
Rain associated with a cold front moving across the northern tier
of the country should be ongoing across northern Minnesota. While
there is likely to diminish in intensity for a period early in the
period...there should be a renewed risk of excessive rainfall later
in the day and evening. With precipitable water values increasing
to 1.75 inches or greater continuing to be fed into the region by
south to southwesterly 850 mb flow with divergence aloft being
provided by the right entrance region of an upper level
jet...the 27/00Z HREF neighborhood probability values of 10 to 15
percent for 2+ inch amounts in an hour seem pretty reasonable.
Assuming that the GFS was too fast with the forward speed of the
front during the period...he risk of training cells and repeat
convection are additional concerns for excessive rainfall in
addition to the intense rainfall rates.
...Tennessee Valley and Surrounding Areas...
A stationary front turned warm front will track northeastward up
the Tennessee Valley through late Sunday night/early Sunday morning.
Increasing atmospheric moisture associated with the advance of an
airmass originating off the Gulf will allow for potential training
and backbuilding of strong storms capable of increasingly heavy
rainfall. The surface warm front will likely slow as it approaches
the southern Appalachians...supporting slower moving storms that
with interactions with the terrain will be capable of flash
flooding. With very efficient warm rain processes at
work...multiple inch per hour rainfall rates . Urban areas are at
a higher risk for localized flash flooding with any repeating
storms.
...Southwest US...
The risk of excessive rainfall continues to get less conducive with
time and gets shunted eastward as mid-level westerly flow
strengthens over much of the West outside of Arizona and New
Mexico. Maintained a Marginal Risk here given the lingering
moisture.
Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sat Jul 27 2024
Day 3
Valid 12Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY FOCUSED OVER
THE TENNESSEE VALLEY...
...Southeast US...
Focus for excessive rainfall shifts eastward and is mainly focused
in the central and eastern portion of the Tennessee Valley as mid
level height falls approach from the west and precipitable water
values at or above 1.75 inches will already be in place. While the
height falls are associated with a trough that was beginning to
fill...the diffluent flow located above an instability axis should
be enough to support downpours from slow moving storms in the
afternoon and evening. Elsewhere in the Southeast US...the forcing
remains weak and nearly impossible to time with any accuracy at
this range but localized downpours from any convection that can
develop and was covered by an expansive Marginal Risk area.
...Upper Midwest to far Western Great Lakes...
The same cold front helping to focus heavy to potentially excessive
rainfall on Sunday/Sunday night will continue to make its way
eastward on Monday. The pattern becomes somewhat less favorable for
heavy to excessive rainfall but the potential for some isolated
enhanced amounts linger...especially from far northern Minnesota
into the western Great Lakes where a Marginal Risk was placed.
...Southwest US...
Steering flow in the mid and upper levels begin to weaken as
heights begin to rise while precipitable water values linger at or
above an inch (even 1.25 inches in places over the desert)...meaning
a slight westward expansion in the Marginal risk area over
portions of Arizona and New Mexico compared with the previous
couple of days.
Bann
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 AM EDT Sat Jul 27 2024
The Days 4-5 Excessive Rainfall Outlooks covering Tuesday-
Wednesday night show fairly broad marginal risks across parts of
the Midwest to the Northeast where above-average precipitable water
values will be in place along weak frontal boundaries moving
through the flow. There was enough agreement in the guidance for
small slight risks across portions of the central Appalachians on
each day where flash flood guidance is typically lower and
antecedent conditions are wet. Much of the Eastern third of the
country should remain generally unsettled the rest of the week as a
upper trough is slow to budge out of the region while it generally
weakens. Elsewhere, monsoonal convection over the Southwest should
stay confined to southern areas through midweek, though it may
eventually expand a little northward depending on the shape of the
southern Rockies/Plains ridge and upper level energy rotating
around its western periphery.
The forecast pattern evolution will support an expanding area of
warmer than average temperatures across the Lower 48, possibly into
the Ohio Valley/Mid-Atlantic by next Thursday. The most persistent
and extreme high temperature anomalies should be over the Central
Plains early on where some locations should see multiple days with
highs 10-15F or so above normal and high temperatures broach 105F.
An incoming front will cause some easing of the heat late next
week. The experimental HeatRisk likewise reflects an expanding area
of Moderate to Major risks of heat- related impacts through next
Wednesday, and even some pockets in the Extreme category over the
central U.S.. Much of the West will likely see near to slightly
below normal highs through the first half of next week before the
strengthening ridge pushes temperatures above normal by next
Thursday, and more significantly so next Friday into Saturday with
lower elevations seeing 100F+ high temperatures. The Northeast
should also trend warmer with time next week.
Santorelli
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 AM EDT Sat Jul 27 2024
The Days 4-5 Excessive Rainfall Outlooks covering Tuesday-
Wednesday night show fairly broad marginal risks across parts of
the Midwest to the Northeast where above-average precipitable water
values will be in place along weak frontal boundaries moving
through the flow. There was enough agreement in the guidance for
small slight risks across portions of the central Appalachians on
each day where flash flood guidance is typically lower and
antecedent conditions are wet. Much of the Eastern third of the
country should remain generally unsettled the rest of the week as a
upper trough is slow to budge out of the region while it generally
weakens. Elsewhere, monsoonal convection over the Southwest should
stay confined to southern areas through midweek, though it may
eventually expand a little northward depending on the shape of the
southern Rockies/Plains ridge and upper level energy rotating
around its western periphery.
The forecast pattern evolution will support an expanding area of
warmer than average temperatures across the Lower 48, possibly into
the Ohio Valley/Mid-Atlantic by next Thursday. The most persistent
and extreme high temperature anomalies should be over the Central
Plains early on where some locations should see multiple days with
highs 10-15F or so above normal and high temperatures broach 105F.
An incoming front will cause some easing of the heat late next
week. The experimental HeatRisk likewise reflects an expanding area
of Moderate to Major risks of heat- related impacts through next
Wednesday, and even some pockets in the Extreme category over the
central U.S.. Much of the West will likely see near to slightly
below normal highs through the first half of next week before the
strengthening ridge pushes temperatures above normal by next
Thursday, and more significantly so next Friday into Saturday with
lower elevations seeing 100F+ high temperatures. The Northeast
should also trend warmer with time next week.
Santorelli