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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Jul 08, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Mon Jul 8 2024

Day 1
Valid 16Z Mon Jul 08 2024 - 12Z Tue Jul 09 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR MOST OF
EASTERN TEXAS INTO SOUTHWESTERN ARKANSAS...

...16Z Update...

No huge changes, but several noteworthy ones to highlight for this
midday update:

First the good news. The risk areas are being trimmed on the
southwest side of Beryl's circulation along the Upper Texas Coast.
The northerly flow of drier air will continue as a northwesterly
offshore wind along the coast through the afternoon. This should
notably tamp down the potential for convection in the area, with
even lesser chances that the convection that does form can produce
flash flooding given the low expected rainfall rates. Thus, despite
obviously rock bottom FFGs from Beryl, the lack of additional
rainfall lends confidence to full downgrades to zero behind Beryl's
circulation. With the heaviest rainfall now clear of Houston
proper, the ERO was downgraded accordingly. Any additional rainfall
in Houston will be over the next hour or two and will be much
lighter in comparison to this morning's rain due to the bands
southwest of center are weakening due to dry air entrainment.

Now the not so good news. Starting in eastern Oklahoma...increasing
moisture convergence from the southeasterly flow ahead of Beryl's
center will clash with the predominant northerly flow over much of
the state. PWATs across eastern Oklahoma now well ahead of Beryl
are generally above 1.5 inches. Thus, with ample moisture in place
now and more to come, a front is expected to intensify across
eastern Oklahoma that with peak diurnal heating will lead to a line
of strong thunderstorms capable of heavy rainfall. There is
considerable uncertainty as to how quickly this line of storms will
move...but the clashing air masses tends to keep it slow.
Thus...expect a narrow area of heavier rainfall totals in eastern
Oklahoma. For this reason, the Slight with Beryl was expanded
northwest for this potential.

Further along to the north and east, the inherited Slight risk area
was expanded well to the north and east to include the southern
third of Missouri with this update. An expanding jet streak over
the northern part of the state will put the southern part of the
state in the favorable right entrance region. This will result in a
rapidly expanding precipitation shield with Beryl as it approaches
tonight. With rain, heavy at times, moving in earlier than
previously forecast, the Slight was expanded accordingly.

No big changes were made on the eastern side of the risk areas as
southerly flow now and through tonight as the storm's center lifts
northward will mean favorable southerly flow off the Gulf continues
behind the circulation. This will keep moisture and instability
advecting across Louisiana, so the threat remains largely the same.

The daily convection across much of the Southeast will also
continue today, including across the Carolinas. The Marginal was
expanded, primarily in deference to the potential for another round
of daily afternoon isolated cells forming, especially at the head
of Pamlico and Albemarle Sounds in North Carolina...though recent
rainfall supports an isolated flash flooding risk once again across
the Southeast, even if the coverage today will be notably lower
than previous days.

A Marginal Risk area was added for portions of west Texas with the
ongoing storms near Midland and expected renewed daily convection
in flood sensitive areas of eastern New Mexico.

Wegman

...Previous Discussion...

The National Hurricane Center anticipates Hurricane Beryl to make
landfall near Matagorda Beach early this morning and continue to
track northward. It is expected to be a fairly compact storm as it
tracks through eastern Texas so the QPF gradient will be somewhat
tight with its 3 to 8 inches swath slightly east of the track with
local maximums possibly approaching 15 inches. The shift in Beryl's
track resulted in an east and northward adjustment to all of the
inherited Excessive Rainfall Outlook areas. The Moderate Risk
spans from Brazosport Area to south of Lake Calcasieu and northward
into southwest Arkansas. The Slight Risk spans from Matagorda Bay
to west of Vermilion Bay and northward into central Arkansas. The
Marginal Risk spans from San Antonio Bay to Atchafalaya Bay and
from the Texas Panhandle to Southern Missouri. Tropical moisture
from Beryl into a cold frontal boundary draped over the Southern
Plains should focus scattered heavy rainfall over much of Oklahoma
into the Texas Panhandle.

...Southeast...

Convection in proximity to the lingering frontal boundary will
persist across the Southeast. PW values on the order of +2 to +3
standard deviations will be pooled over the region and will
bolster rainfall efficiency. Recent rains have increased soil
saturation levels and with additional showers and thunderstorms
expected the threat for isolated flooding concerns will remain
elevated for this part of the country. The latest guidance trended
higher with the thunderstorm potential and QPF amounts across
portions of northeast/central Florida. The Marginal Risk area was
expanded south to account for this trend and new WPC forecast.

Campbell


Day 2
Valid 12Z Tue Jul 09 2024 - 12Z Wed Jul 10 2024

...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL ACROSS THE MIDDLE
MISSISSIPPI VALLEY AND PORTIONS OF THE LOUISIANA COAST...


...Mississippi Valley and Midwest...

Beryl is still expected to quickly phase with an upper trough and
surface front to the north while in transition to being extra
tropical. This should lead to the potential for heavy rain to
blanket part of the central U.S. causing flash flooding concerns
from Arkansas to Indiana during this period. Some of the highest
QPF amounts of 3+ inches are likely to focus over the Ozarks
region and points northeast to the Illinois/Indiana border. Areal
averages are expected to be around 2 to 4 inches with local
maximums upwards of 8 inches. A Slight Risk extends from central
Arkansas to western Indiana. A Marginal Risk stretches from the
central Gulf Coast northward to southern Michigan. Again, an
future adjustments will likely be dependent on the track for
Beryl.

In addition, some of the CAMs hint that some type of southwest to
northeast band could set up along or just inland of south-central
Louisiana coast to produce a few inches of rain over some areas
that could be a little more sensitive to heavy rain impacts. A
Slight Risk area was raised for this stretch of the coastline north
to about I-10.

...Southwest...

Monsoonal moisture feeds over eastern Arizona through the southern
Rockies warranting maintenance of the Marginal Risk for most of
New Mexico, southeast Arizona into southern Colorado with
particularly considerations for burn scars and areas that have
seen ample rainfall recently.

...Southeast...

Convection will persist during this period thanks to the abundant
moisture hanging around and diurnal forcing. Some of the guidance
continues to suggest areas of locally heavy rainfall especially
along the eastern coastline of Florida. General consensus suggests
1 to 2 inches although local maximums of 3 to 4+ inches will
certainly possible. A Marginal Risk remains in effect for portions
of central/northern Florida and southeast Georgia.

...Northeast...

Interaction of moisture ahead of Beryl and a front settling over
the Mid-Atlantic/southern New England may favor heavy rainfall
potential over this region; although guidance is rather light with
activity mainly in New England. Some focus of the convective rain
ahead of the front(s) and sufficient instability for heavy rain
rates in the northern Mid-Atlantic to Northeast favor of a Marginal
Risk.

Campbell


Day 3
Valid 12Z Wed Jul 10 2024 - 12Z Thu Jul 11 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT LAKES REGION AND THE NORTHEAST...

...Midwest to Northeast...

There is still a fair amount of spread in the guidance with respect
to the evolution/track for post-tropical Beryl and exactly how
this upper level energy ejects into the overall Great
Lakes/Midwest mean trough. The flow of tropical moisture should
bring the highest precipitable water anomalies into the Lower
Great Lakes and Northeast during this period. Some PWAT anomalies
within this region will likely exceed the 90th percentile and west-
east oriented fronts north/east of post- tropical Beryl may promote
training. The exact location of the heaviest rainfall will be
dependent on the location of the frontal boundary but there is a
growing signal for a broad area to be impacted by the heavy rains.
A Slight Risk is in effect from northern Indiana to New Hampshire
while a Marginal Risk area spans from Illinois to Maine. As details
resolve and get closer in time, there may be the need to upgrade
the risk level, but at this time there is too much uncertainty.

...Central Rockies and portions of the Southwest...

Continuation of the monsoonal moisture and diurnal heating will
keep convection possible across portions of the Central and
Southern Rockies and southwest New Mexico during this period. The
inherited Marginal Risk was maintained with westward modification for
New Mexico. Rain in proximity to burn scars and other sensitive areas
that have seen ample rainfall recently will have an elevated risk
for flash flooding.

Campbell

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt