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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0827Z Aug 08, 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
427 AM EDT Mon Aug 08 2022

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


...Desert Southwest through the Western Great Basin...Sierra
Nevada...and Central Oregon...
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. Typical
diurnal isolated to scattered convection breaks out this afternoon
along the Mogollon Rim, SE AZ, and southern NV terrain as well as
over the peninsular ranges of southern CA and the Sierra Nevada.
MUCAPEs peaking between 500-1000 J/kg on average along with the
increase in PW standardized anomalies (+ 2 to +2.5 per the 00Z
GEFS) will support peak sub-hourly rainfall rates of 0.50+ inch
underneath the strongest cores. The monsoonal moisture plume
arrives into central 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" 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...
00Z CAMs have honed in on the southern extent of a cold front
moving across the mid-MS Valley where there is risk for a narrow
swath of repeating heavy rain to set up this evening. PWs of 2 to
2.25" ahead of the slowing front under diffluent upper level flow
which should help with the maintenance and organization of
convection and pose a scattered flash flood risk. As of now, the
St Louis metro area through southern IL - areas that have received
excess rainfall over the past week (and on the heels of the
historic flooding in StL two weeks ago). In coordination with WFOs
LSX, ILX and PAH, a narrow Slight Risk has been raised. 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. Recent radar trends suggest the Chicago/Milwaukee
area will still have a risk around 12Z, so the Marginal was
expanded back northwest over those metro areas and retained
farther east across the rest of the Midwest and OH Valley.

...Northeast/Northern New England...
Northern portions of the Northeast will see even 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. As of now terrain effects look to be a
better factor for development than bay/river breeze boundaries
down in the I-95 corridor, so a Marginal Risk was held off for the
Mid-Atlantic coast.


Day 2

The Day 2 outlook will be updated by 0830Z.

Day 3

The Day 3 outlook will be updated by 0830Z.

Day 1 threat area:
Day 2 threat area:
Day 3 threat area: