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New Day 1 Outlook >
WPC Day 1 Excessive Rainfall Outlook Valid Through 12Z Today Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 0824 UTC Wed Sep 27, 2023
Valid: 12 UTC Sep 27, 2023 - 12 UTC Sep 28, 2023
Forecast Discussion
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
830 PM EDT Tue Sep 26 2023
Day 1
Valid 01Z Wed Sep 27 2023 - 12Z Wed Sep 27 2023
...A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS
OF ILLINOIS, WISCONSIN, AND NORTHWEST INDIANA...
...01z update...
Trends in RADAR/Satellite and recent Hi-Res CAMs continue to trend
southward across IND with convective coverage and have adjusted in
turn. The largest change was an expansion of the Marginal Risk of
Excessive Rainfall upstream through the Lower Missouri River
Valley and into SW & S IL.
The trailing tail-end of an internal shortwave trof aloft in
combination with consolidating outflow boundaries with return flow
across the Ozark Plateau has supported a moderately defined
boundary across the Missouri River Valley. The LLJ is likely to
respond to the slowly wobbling upper-low to the north with
increasing southerly flow that will initially be oriented
orthogonal to the sharpening boundary. Initially winds will be
light, so thunderstorm activity may be initially scattered in
nature but have ample elevated CAPE to around 1750-2000 J/kg and
modest moisture up to 1.25" through depth. As the LLJ strengthens
toward 25-30kts by 06z, it is expected to veer with fairly broad
confluence in the 850-700mb depth, likely to increase convective
coverage. While moisture is generally lacking for very intense
rainfall rates of 1-1.5"/hr (with HREF probs reaching 30-50% of
1"/hr between 06-12z), there is favorable steering/orientation for
upstream redevelopment to potentially repeat/train, particularly
across east-central MO into SW IL. Isolated spots of 1-3" may be
possible through 12z, so a spot or two of low-end flash flooding
may be possible overnight, with best potential for increased
runoff if crossing urban areas, including metro St. Louis.
Gallina
...Midwest...
Upper low over the Midwest will continue to pivot southeast with
convective development underneath the closed circulation and
within the difluent area downstream of the main low. PWATs are
elevated with 12z sounding out of KDVN indicating moisture
extending pretty solidly through the boundary layer, up towards
the tropopause. Strong ascent under the ULL will generate plenty
of lift within a zone of modest instability to create a scattered
heavy rainfall risk within the QPF footprint. Hi-res deterministic
was consistent in the axis of where the heaviest rain would fall,
even extending a little more into northwest IN and southwestern
corner of MI. Considering the urbanized area surrounding Lake
Michigan, this is the primary area of flash flooding potential,
but thunderstorms across northern IL over into northern IN will
create isolated flash flood risks as well thanks to hard soils
from very low soil moisture (0-4%) as depicted by the latest NASA
SPoRT soil moisture analysis. 12z HREF neighborhood probability of
2"/3 hrs indicates areas of 15-20% with a max of 30-40% near the
Quad Cities later this afternoon.
In coordination with the Chicago and Milwaukee WFO's, have
introduced a SLGT risk for the urban/suburban areas of northeast
IL and southeast WI as well as northwest IN. Heavy rainfall this
morning across portions of the above area has created a higher
risk for flash flooding concerns within the urbanized areas of
Chicago and Milwaukee. MRMS Multi-Sensor readings have pockets of
1-2" within the past few hours and considering the complex
environment around our ULL to the west, there's concern for
locally heavy rainfall training over the same areas this afternoon
and evening over places impacted this morning. Local WFO's are in
agreement for the targeted area of interest, leading to an upgrade
beginning at the top of the hour.
...Florida...
12z sounding from KJAX showed a tall, skinny CAPE signature within
a zone of very high PWATs running between 2.1-2.2" which is within
the 1.5 deviations above normal climatologically. The primary
target will be within the urban corridor stretching from Savannah
down through Jacksonville into northern Daytona Beach. Weak
ridging to the north will shift winds near the coast out of the
northeast with sea breeze progression inland after 18z. Surface
trough to the east, in tandem with a weak shortwave traversing
overhead has created a conglomeration of heavy rainfall within the
Panhandle thanks to a solid convergence zone from Tallahassee and
points southwest. Expecting the convective energy remnants to
shift eastward and interact with the sea breeze located over
northeast FL which will generate a second area of very heavy
rainfall within the aforementioned corridor. 12z HREF neighborhood
probabilities for both 2" and 3"/hr are incredibly high within the
northeast FL Panhandle up into southern GA with 2"/hr signals
exceeding 60-70% for a large chunk of the area and 3"/hr potential
between 40-50% across Jacksonville and surrounding suburbs. Local
FFG's are still high considering FL standards, but even so the
risk will be higher within that corridor today given the expected
interaction of convergence within a zone of deep tropical moisture
and modest instability. Urban flooding will the primary focus due
to the runoff potential, but even some areas within the coastal
plain will have an opportunity for flash flooding concerns if
training storms becomes an issue. Only adjustment made for FL was
to bring in the western fringes of the MRGL to match current radar
trends and 12z HREF blended mean of anything >1.5".
Kleebauer
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
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