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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0845Z Aug 09, 2022)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
445 AM EDT Tue Aug 09 2022

Day 1
Valid 12Z Tue Aug 09 2022 - 12Z Wed Aug 10 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE
MID-MISSISSIPPI VALLEY TO THE CENTRAL APPALACHIANS AS WELL AS FOR
PORTIONS OF THE SOUTHWEST AND GREAT BASIN...

...Lower Mississippi Valley, Ohio Valley, and Central
Appalachians...

A front extending from the Red River of OK/TX through to New
England is acting as a focus for convection along and south of the
front this morning. It will be weak and very slow-moving today
into tonight, sagging southward as the day goes on. Nonetheless,
the front will act as a forcing mechanism for continued convection
that will develop in the moist, unstable air mass south of it.
This airmass is characterized by PWATs of 2-2.5 inches, which is 2
standard deviations above normal. Further, instability will be
quite notable, generally in the 2,000 to 3,000 J/kg range.
Finally, FFG is exceptionally low in this area, generally between
1 and 2 inches in 1 to 3 hours due to fairly persistent heavy
rainfall since July 25. Portions of eastern KY have received
10-15"+ during this time frame which is ~600% of normal...Southern
IL has fared similarly. This combination of ingredients is
forecast to have the following evolution: The periodic areas of
convection ongoing from OH through MO will maintain themselves or
very slowly weaken through the early morning. Widely scattered
slow-moving showers and thunderstorms will develop along and south
of the front in the Slight Risk area as soon as early afternoon.
The storms would develop strong updrafts/heavy rainfall rather
quickly due to the high instability. Most storms should not be
able to organize much, remaining mostly cellular, though they may
form into a line right along the front along almost the entire
length of the Ohio River by mid-to-late afternoon. Topographic
effects may also increase rainfall amounts further east into
eastern KY and WV. Enough low-level inflow is forecast that it's
possible that random pockets of loose organization/training could
occur. Slow storm movement and the possibility of random loose
organization and cell mergers could lead to isolated spots of up
to 3" an hour. Widely scattered to scattered flash flooding will
be possible this afternoon, particularly over areas with saturated
soils. 

Due to the front hanging up further north to start the day today,
the Slight Risk was expanded northward with this forecast package
to include more of southern IL, IN, OH, northern WV and the
southwest corner of PA.

...Southwest/Great Basin...

The monsoon continues in the Southwest as the combination of
southwesterly flow ahead of a strong upper level low off the
California coast and southeasterly upper level flow across NM and
AZ rounding the base of an upper level high centered over UT and
CO meet to form a corridor where heavier convection is likely to
form Tuesday afternoon. Some moisture appears to be drifting up
the Gulf of CA/Sea of Cortez around the periphery of Tropical
Storm Howard. This corridor over AZ into NV will feature much
greater instability than the surrounding area with CAPE values
between 1,000 and 2,000 J/kg. The highest values of CAPE are
expected over southern AZ. The available moisture and instability
support the potential of local amounts in the 2" range, which
could occur within an hour. The abnormally wet conditions this
region has been seeing the past few weeks means the soils in this
area are abnormally moist. The largest issues are expected in slot
canyons, dry washes, burn scars, and urban centers. Precipitable
water anomalies are as much as 4 standard deviations above normal
through northwest NV and southeast OR, making this area very
unusually prone to the potential for flash flooding.

With continued southerly flow ahead of an upper level low that
will be moving into the northern CA coast today, the Slight Risk
was expanded through northern NV into southeastern OR in
coordination with the BOI/Boise, ID forecast office.

Wegman/Roth


Day 2
Valid 12Z Wed Aug 10 2022 - 12Z Thu Aug 11 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
MID-ATLANTIC THROUGH THE MID-TENNESSEE VALLEY, PORTIONS OF THE
SOUTHWEST AND GREAT BASIN, AND THE CENTRAL GULF COAST...

...Mid-Atlantic through the Lower Mississippi Valley...

Confidence is increasing on the frontal position generally near
the OH River and interior sections of the Mid-Atlantic to southern
New England Wednesday. Continued elevated moisture (generally 2 to
2.5 sigma above normal) and ample instability (1000 to 2000 J/kg
MUCAPE), the surface front and height falls aloft as troughing
amplifies over the Northeast should allow an organized risk of
convection, particularly over the TN Valley/Cumberland Plateau
through the central Appalachians and to the Mid-Atlantic. Wherever
cell training or mergers occur, hourly rain totals to 2.5" would
be possible. The Slight Risk has been expanded eastward to the
Jersey Shore south through the Delmarva in coordination with the
LWX/Sterling, VA and PHI/Mount Holly, NJ forecast offices. FFGs
are low (generally around 1.5"/3hr) in some areas due to recent
wetness/ground saturation/urbanization. A Marginal Risk extends
from the lower MS Valley through southern New England where there
is an isolated flash flood risk.

...Southwest and Great Basin...
The monsoon plume continues to drift east over the Southwest/Great
Basin as the upper level high center drifts east over CO on
Wednesday. A corridor of continued high moisture and instability
(perhaps more unstable Wednesday than today - it depends on cloud
cover) over NW AZ, SE NV, and SW UT including portions of the slot
canyon area of southern UT continues to support a Slight Risk. The
elevated moisture plume continues north over NE NV and much of
western Idaho (1.25" PW in the Snake River Valley is 3 to 4 sigma
above normal) where there is consensus for locally heavy terrain
based activity. A low level boundary -- likely a thermal trough --
is expected to shift modestly across the region acting as the
focus for heavy rainfall. Changes with this forecast package
include expanding the Slight Risk northward into southern ID in
coordination with the BOI/Boise, ID and PIH/Pocatello, ID forecast
offices. The Slight Risk has also been expanded southwestward
along the Mogollon Rim into New Mexico as this topography
continues to be a focus for convection.

...In and near Southeast LA...
A low-level disturbance ahead of an inverted trough aloft drifts
from the eastern Gulf of Mexico into the central Gulf coast,
bringing a southerly wind surge (10-15 kts) at 850 hPa in its wake
(roughly double the mean wind due to the northerly flow aloft)
with precipitable water values of 2-2.25". As the flow is off the
warm Gulf, CAPE values of 2000+ J/kg are forecast.  This all
should contribute to rainfall efficiency with occasional loose,
short training bands and quasi-stationary/merging cells that could
bring hourly rain totals towards 3" -- well above what most
metropolitan areas can handle. Portions of Southeast LA and
southwest MS have been anomalously wet over the past week. No
changes were made to the Slight Risk for this area with this
forecast package.

Wegman/Roth

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