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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1946Z Aug 08, 2022)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 PM EDT Mon Aug 08 2022

Day 1
Valid 16Z Mon Aug 08 2022 - 12Z Tue Aug 09 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN ROCKIES...MID-MISSISSIPPI VALLEY...AND NEW ENGLAND...

...Desert Southwest through the West...
An anomalous plume of tropical Pacific moisture rides north from
the Desert SW, through the western Great Basin and through central
Oregon today/tonight as the upper high center settles over the
Four Corners and an upper low off northern CA drifts east.  Recent
thunderstorm development in the Lower CO Valley appears to be
instigated by an MCV moving by the the north.  Skies across
central and southeast AZ topography is partly cloudy -- debris
clouds don't appear to be a hindrance to the usual convective
cycle.  Diurnal scattered convection should break out this
afternoon along the Mogollon Rim, SE AZ, and southern NV terrain
and over the peninsular ranges of southern CA and the Sierra
Nevada, with convection in the terrain of AZ moving into deserts
as low-level inflow is too weak to contain/pin them in the
mountains.  Ongoing thunderstorms in the deserts could migrate
over to the CA Peninsular Ranges with time, moving along with the
MCV.  ML CAPEs are already 500-1000 J/kg across southwest AZ and
southeast CA -- they should peak at over above 2000 J/kg this
afternoon.  Along with the increase in PW standardized anomalies
(+ 2 to +2.5 per the 00Z GEFS), the environment supports peak
sub-hourly rainfall rates of 0.50"+ and hourly totals up to 2"
underneath any slow moving and merging cells.  The monsoonal
moisture plume arrives into OR late this afternoon with potential
for early evening activity similar to areas farther south along
the OR Cascades and western High Desert.


...Southern Rockies...
Enough confidence remains in 00Z CAMs to warrant a low end Slight
Risk over portions of northern NM into far southern CO. Initially
terrain-based diurnal activity propagates slowly under a very
light steering flow on the near-east side of the upper high
settling near the Four Corners this afternoon. 00Z HREF probs are
still likely for 2" -- which could fall as quickly as in an hour
-- and since much of this area has 3hr FFG around 1.5", there is a
risk for scattered instances of flash flooding this afternoon into
this evening.


...Mid-South to Midwest...
The 12z mesoscale guidance has also honed in on the southern
extent of a cold front, near a current cloud edge on satellite
imagery, moving across the mid-MS Valley where there is risk for a
narrow swath of repeating heavy rain to set up this evening. 
Upper level divergence couldn't be much better as the Westerlies
to the north and an upper low slowly retreated over northern AR
provide nearly pure divergence aloft.  PWs of 2 to 2.25" ahead of
the slowing front along with CAPE values of 3000+ J/kg should help
with the maintenance and organization of convection and pose a
scattered flash flood risk.  The St Louis metro area through
southern IL - areas that have received excess rainfall over the
past week or two, remain in the Slight Risk area.  Hourly rain
totals to 3" are possible wherever cells move slowly, merge, or
train.  There has been uncertainty with the position of the cold
front in peak heating, so further adjustments are possible. 
Generally speaking across the Mid-South through the Midwest the
frontal activity will be heavy with fairly light steering flow
becoming more oriented with the front raising a widespread heavy
rain threat.  Looped the Marginal Risk area south to near the
southwest MS/LA border where it's been quite wet this past week.


...Northeast/Northern New England...
Northern portions of the Northeast will greater forcing than the
Midwest, with a closer proximity to the mid-level wave and upper
jet. PW values of 2 to 2.25" ahead of the front are generally 2.5
to 3 sigma above normal and will be near-record for portions of
the Northeast tonight. In addition to ample Gulf-moisture, the
source of this plume can be traced back to the Monsoon surge that
has been over the Southwest for several days being pulled up and
over the ridge and then eastward across the northern tier of the
country.
The system is fairly progressive, but a corridor of enhanced 850mb
moisture transport will be strung out enough to allow for an
elongated corridor of increased convergence. This could allow for
some west to east training of cells as the system as a whole
progresses eastward. A longer duration rainfall with 2-3" totals
is expected across portions of northern Maine down to far northern
VT/NH. There is limited instability, so rates should generally not
exceed 0.5"/hr, but the duration still warrants a Slight Risk
which has been honed a bit based on good agreement among the 00Z
CAM and global consensus.


...Mid-Atlantic to southern Appalachians...
Abundant moisture in the airmass ahead of the northern-tier cold
front along with light steering flow and moderate instability
warrants an isolated flash flood risk from the Atlanta Metro, up
the Appalachian chain.  While terrain effects look to be a better
factor for development than bay/river breeze boundaries,
precipitable water values of 1.75"+ being concentrated into the
convergence within the lee trough where a cumulus field is already
developing.  The forecast 850 hPa inflow of 20-25 kts is about
double the mean 850-400 hPa wind, which would lead to rainfall
efficiency for thunderstorms that form.  The 700 hPa temperatures
at or below 9C imply minimal capping aloft, and SPC mesoanalyses
show that overnight CIN has eroded.  Went ahead and shifted the
eastern edge of the Marginal Risk area to the I-95 corridor to
cover any isolated flash flood potential in the lee trough.

Roth/Jackson


Day 2
Valid 12Z Tue Aug 09 2022 - 12Z Wed Aug 10 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR
KENTUCKY ALONG WITH PORTIONS OF THE SOUTHWEST & GREAT BASIN...

...In and near KY...
A front draped across the eastern half of the country from New
England southwestward to the mid-Mississippi Valley will act as a
focus for convection along and south of the front. The front will
be weak and very slow-moving by Tuesday and Tuesday night, sagging
southward as the day goes on. Nonetheless, it will act as a
forcing mechanism for the convection that develops in the moist,
unstable air mass south of the front. 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 outcome: 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.  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 Tuesday afternoon over
areas with saturated soils. 


...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.  Coordination with the LKN/Elko NV,
REV/Reno NV, and VEF/Las Vegas NV forecast offices led to a
northwest extension to the previous Slight Risk area across NV. 
The largest issues are expected in slot canyons, dry washes, burn
scars, and urban centers.

Roth/Wegman


Day 3
Valid 12Z Wed Aug 10 2022 - 12Z Thu Aug 11 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, CENTRAL GULF COAST, & SOUTHERN
APPALACHIANS/CUMBERLAND PLATEAU...

...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 Tuesday - it depends on
cloud cover) over NW AZ, SE NV, and SW UT including portions of
the slot canyon area of southern UT warrants a Slight Risk, which
was extended farther into NV on this issuance per coordination
with LKN/the Elko NV forecast office.  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.
 A broader Marginal Risk continues to surrounding the growing
Slight Risk, but given these PW values are over the max moving
average for places like KBOI, a Slight Risk may be considered.


...Lower Mississippi Valley through southern New England...
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 1500 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 remains relatively
unchanged.  FFGs are low (generally around 1.5"/3hr) in some areas
due to recent wetness/ground saturation.  A Marginal Risk extends
from the lower MS Valley through southern New England where there
is an isolated flash flood risk with guidance currently
highlighting the Northeast/I-95 urban corridor for enhanced rain
which may warrant a Slight Risk should this focus persist.


...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. 
Coordination with LIX/the Slidell LA forecast office led to the
introduction of a Slight Risk area for their county warning area.

Roth/Jackson


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