Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Mon Aug 08 2022
Valid 12Z 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 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.
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
Valid 12Z Tue Aug 09 2022 - 12Z Wed Aug 10 2022
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN
KENTUCKY AND PORTIONS OF THE SOUTHWEST U.S...
...Eastern Kentucky and Nearby Surrounding Areas...
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. 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 will be locally strong due to the high instability. The
storms will not be able to organize much, remaining mostly
cellular. However their slow movement, intensity, and abundant
moisture to feed on will mean any of the more intense storms will
be able to produce a quick inch or two of heavy rain over the
local area, meeting the FFG. HREF guidance strongly supports this
forecast through 00Z Wednesday. Thus, widely scattered flash
flooding will be possible Tuesday afternoon. Unfortunately, any
flash flooding on Tuesday will prime many of the same areas for
potentially worse and more widespread flash flooding on Wednesday.
Few changes were needed to this Slight Risk area for this forecast
package. Some uncertainty remains especially with how far north
flooding concerns extend into NV and OR. Topographically forced
convection is highly likely, but how much rainfall this convection
can produce remains uncertain. Sparse population in that area will
also limit impacts any flash flooding would cause.
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. This corridor over AZ into southern 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 broad southerly flow
will also advect significant monsoonal moisture north up the Gulf
of California into AZ and NV. The moisture may be further enhanced
by Tropical Storm Howard moving off the coast of Baja California.
Finally, the abnormally wet conditions this region has been seeing
the past few weeks means the soils in this area can't take much
rainfall before runoff causes flash flooding, especially in the
slot canyons and other typically susceptible areas.
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