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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0904Z Aug 11, 2022)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
504 AM EDT Thu Aug 11 2022

Day 1
Valid 12Z Thu Aug 11 2022 - 12Z Fri Aug 12 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST AND EASTERN GREAT BASIN...

...Southwest & Eastern Great Basin to the Northern Rockies...

Very few changes were needed for the today into tonight period
across this region. A nearly stationary upper level pattern
featuring an upper level low centered off the coast of Washington
state and an upper level high over eastern Colorado is keeping a
steady southerly flow of monsoon moisture ongoing across this
region. Once again the greatest threat of flash flooding will be
where storms move over burn scars, slot canyons, or any more
urbanized areas. Nonetheless,  with the axis of most anomalous PWs
oriented from the Lower CO River Valley to the northern High
Plains and Rockies, any storms that form will have ample (for that
part of the country) moisture to draw on to rain out onto the
deserts and surrounding mountains. 90th percentile PW will span
across a large portion of the Intermountain West with > 3.0
standard deviation PWs values in parts of southern NV, much of
northern UT, and southeastern ID.

Actual PWs in these areas will be 1.0-1.5" with the higher end of
those values in the Lower CO River Valley. Similar to yesterday,
traversing remnant MCVs will be of interest along with any
embedded 500mb disturbances rotating around the southwest and
northwest flanks of the upper ridge. The area from the Mogollon
Rim on north up the Wasatch and into northern UT remain most
susceptible to possible flash flooding due to the favored
topography oriented quasi-parallel to the mean flow along the
highest terrain. This goes for areas as far north as Yellowstone
and the Tetons. Convection has already developed over portions of
southeastern NM, which will move and continue developing along the
Sacramento Mountains of southern NM.

The only change of significance with this forecast package was
expanding the SLGT into the Sacramento Mountains, in coordination
with the EPZ/El Paso, TX forecast office.


...Central Gulf Coast...
Persistent onshore flow converges along a weak surface trough
extending from east TX through southeast LA with PWs still ranging
between 2 to 2.25", which is ~2.0 standard deviations above
normal. This moisture and convergence along with ample instability
and very weak steering flow warrants a risk of excessive rainfall,
particularly since much of the area east of Galveston Bay to the
FL Panhandle has had above normal rainfall over the past week.
Widespread convection has developed along and just off the coast
from Vermilion Bay in southern LA eastward to off the coast of
Fort Myers, FL. While the convection along the west coast of FL
should continue inching away from the coast, that same convection
will move into the coast from Apalachee Bay south of Tallahassee
westward.  Sufficient instability and weak storm motions (as weak
as < 5 knots) are allowing torrential downpours potentially enough
residency time to cause localized flash flooding. A Marginal Risk
remains in place as confidence is on the lower side regarding
placement of the most intense thunderstorm activity. This also
goes hand-in-hand with where the heaviest rainfall both Tuesday
and Wednesday transpired, which if the heaviest rainfall for
Thursday lines up in these areas, would make those soils or
communities most susceptible to flash flooding. Once confidence in
rainfall totals increases and the heaviest concentration of storms
comes into focus, a Slight Risk upgrade could be issued with the
normally scheduled updates today.

No changes were made to the MRGL risk area.

...Southeastern MN to Eastern IA...

A new MRGL risk was issued for this area with this forecast
package. A vigorous midlevel shortwave will draw some of the
monsoonal moisture that has been over the Intermountain West and
ride the top of the ridge over to this region by tonight. A weak,
but not insignificant 35 kt southwesterly LLJ will supply
additional moisture to the atmosphere ahead of the shortwave.
LLJ's typically strengthen overnight. All of these ingredients
together will allow for showers and thunderstorms to develop in
this region, intensifying during the evening. Most of the CAMs
suggest that while any storms will move pretty quickly
southeastward with time, constant redevelopment of those storms to
the north will potentially cause some training convection,
resulting in a marginal flash flood threat. Finally, this area
recently picked up several inches of rain, which as yet to dry out
of the soils. Thus, FFG values are lower from southeastern MN from
the Twin Cities southward into eastern IA. Thus, despite some
rainfall expected over west central MN, any flash flooding should
be confined to those areas that picked up more rain in the
previous events. Since the bulk of the rainfall activity with this
feature is expected after 06Z, some flash flooding potential will
linger into D2.

Wegman


Day 2
Valid 12Z Fri Aug 12 2022 - 12Z Sat Aug 13 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, EASTERN GREAT BASIN, AND INTO WESTERN WYOMING...

...Southwest & Eastern Great Basin to Western Wyoming...

From Day 1, the only changes to the upper levels are that the
upper level low off the coast of Washington state moves inland
into southeastern British Columbia by Friday night. It will drag a
negatively tilted shortwave trough around its southeastern
periphery. This will locally reorient the upper level flow to more
of a southwesterly flow over the Pacific Northwest. Meanwhile, the
upper ridge center in response will drift southeastward to
southwestern KS, and the OK/TX Panhandles. As such, the axis of
monsoonal moisture will shift westward to southwestern New Mexico
and over essentially the entire state of Arizona. Meanwhile, the
northern end of the plume will shift eastward out of Idaho and
into Wyoming. While some rainfall is expected over Wyoming on D1,
the additional forcing from the aforementioned negatively tilted
shortwave will enhance rainfall rates over WY Friday. Thus, the
rainfall over that area today will prime the soils for increased
flash flooding risk in WY on Friday. Therefore, the SLGT was
expanded NE to encapsulate much of the western half of WY.
Meanwhile, south central NM will get a break from the rain and the
SLGT is shifted west from today. However, more moisture is likely
to impact southwestern AZ on Friday as compared with today. Thus,
the SLGT was expanded right up to the CO River. As always, the
greatest threats for flash flooding will be along the mountains
(especially the Mogollon Rim), burn scars and slot canyons.

The SLGT risk area was expanded northeastward into northeastern UT
and much of western WY in coordination with the SLC/Salt Lake
City, UT and RIW/Riverton, WY forecast offices.


...Gulf Coast to the Carolina Coast...
Continued elevated Gulf moisture persists along the Gulf coast
through the Carolina Coast (south of the Outer Banks) into the
diurnal peak with a cold front approaching the Gulf Coast late in
the day. A surface trough along the coast will help focus the
heavy rain threat to along the entirety of the northern Gulf Coast
and Southeast Coast. Given the heavy rain threats from prior days
there is likely to be a need for a Slight Risk at some point, but
the more prone areas (besides urban areas) may have to make
themselves more evident after rain today before the confidence is
great enough. A wave of moisture will increase rainfall rates into
southeastern TX, while the best remaining forcing with the front
will increase rainfall rates along the Carolina Coast as well.
Fortunately, the eastern Carolinas have been quite dry recently
with soil moisture percentiles over a large area under 10%.

The MRGL was slightly expanded both a bit south down the FL
Peninsula and nudged a little north into central SC and
southeastern NC with this forecast package.

...Southeastern Minnesota and western Wisconsin...

As mentioned in the D1 discussion, ongoing convection over eastern
Minnesota into Iowa will continue into the early morning hours on
Friday. Low FFG values from recent rains combined with the
continued potential for a bit of training convection necessitated
the issuance of this new MRGL as well. As the convection moves
east over northern WI during the day, it should diminish in
coverage and intensity. On Friday night, the convection may
reintensify over northeastern WI and the UP of MI. A new MRGL may
be required for these areas with future forecast packages. There
is considerable uncertainty still on where the convection will
develop and what form it will take. Once there is additional
certainty in these areas, there will be more confidence on
additional changes to the MRGL.

Wegman

Day 3

The Day 3 outlook will be updated by 0830Z.


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