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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0837Z Aug 07, 2022)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
437 AM EDT Sun Aug 07 2022

Day 1
Valid 12Z Sun Aug 07 2022 - 12Z Mon Aug 08 2022

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHEAST IOWA, SOUTHERN WISCONSIN AND NORTHWEST ILLINOIS...

...Great Lakes...
An ongoing flash flood risk exists to start the new day 1 period
across portions of eastern IA, southwest WI and northwest IL.
Northwest to southeast Training convection will likely continue
into the morning hours, although the organization and intensity of
activity should generally be on a downward trend after ~15z or so.
Leftover outflows and/or MCV will track downstream across the
Great Lakes through the day and should trigger some additional
convective development across portions of MI/IN and OH this
afternoon and evening. This convection should generally be
progressive, but quick bursts of heavy rainfall and some brief
training could still result in isolated flash flood concerns.

Then attention turns to what happens tonight over portions of
IA/MN/WI and northern IL. Much stronger synoptic forcing pushes
across the area, with a well defined mid level shortwave and upper
jet favorably positioned to the north. Low level moisture
transport ramps up as well, increasing lower level convergence and
helping lift the stalled out boundary and instability pool back
north. The system is progressive as a whole, which will put a
limit on rainfall duration. However thickness diffluence and weak
Corfidi Vectors support some backbuilding/training into the
increasing low level jet...so some increased rainfall duration
appears probable. Meanwhile PWs should remain over 2" and near
record values, with a continued connection in the mid/upper levels
to the previous Monsoon moisture that made its way around the
mid/upper ridge. Thus the ingredients seem to be there for another
round of potentially significant rainfall tonight.

There remains above average uncertainty with the details of this
overnight convection. The better forcing should end up over
eastern MN into central and northern WI, but instability recovery
here is questionable. So while we will see locally heavy
convection here, the lower instability and progressive cell
motions could limit event total magnitudes. If there is some
better west to east training it will probably be on the southern
edge of the forcing, along the instability and low level moisture
gradient. Convection this morning may play a role on where this
ends up and exactly how strong of a gradient we end up with.
Possible that the convection this morning limits recovery to some
degree, keeping the upstream instability pool a bit muted and
reducing the significant training threat tonight. However
certainly a chance we are able to regenerate significant
instability and a stronger convergence/instability axis results.
In this case a more pronounced training scenario could evolve
somewhere across northeast IA, southern MN and central/southern
WI. Given antecedent rainfall, this scenario would result in the
likelihood of more widespread hydrologic concerns.

We opted to expand the MDT risk with this update into portions of
northeast IA, southwest WI and far northwest IL. This is based on
the rain that has already fallen, the additional rain that is
expected through mid morning, and then the additional convection
forecast tonight. With hydrologic conditions becoming more
sensitive with time today into tonight, at least scattered
instances of flash flooding appear likely. The threat of a higher
end event is probably dependent on what happens tonight, and as
mentioned above that remains uncertain. Either way locally
significant flash flooding could still evolve.

...West...
An isolated to scattered flash flood risk exists from portions of
AZ/NM into CO. One Slight risk this period will be focused over
portions of northeast NM into eastern & central CO.  This is
likely being driven by an approaching front enhancing low level
convergence and some easterly upslope flow in the wake of the
front within an exceptionally moist airmass for the area of
1.25-1.75". Combine that with upper level divergence in the right
entrance region of the upper jet and the synoptic setup looks
pretty good for heavy rainfall. Sufficient instability and PWs
increasing over the 90th percentile should support heavy rainfall,
which combined with the expected convective coverage, continues to
support the Slight risk over this area. HREF 3" neighborhood
probabilities are over 40% across this area, indicating the
likelihood of scattered 3"+ rainfall totals with this event. FFG
exceedance probabilities from the HREF are also over 40%, and
portions of this area received heavy rainfall yesterday. Thus
seems likely that scattered flash flooding will occur today over
this region, with a minor eastward adjustment to the Slight risk
to account for HREF trends.

A Slight risk remains over portions of central and southern AZ. 
PWs are forecast to increase quite a bit over southern AZ compared
to Saturday, with ~1000-2000 J/KG of CAPE forecast as well. The
overall synoptic pattern will remain favorable for terrain based
convection, potentially aided by any shortwaves rounding the base
of the upper ridge, with some eventual propagation into the valley
locations given mean easterly flow. The overall signal today
appears better than Saturday, so the Slight risk seems warranted.
The risk was expanded a bit based on trends seen in the 00z HREF.

...East...
A Marginal risk was maintained across portions of the OH and TN
valley into the central and southern Appalachians. Continued
isolated to scattered convection is expected today, and while the
coverage and magnitude of rainfall will probably be less than
Saturday, indications are that localized 1-2" rainfall will again
be possible (locally 3" in the southern/central
Appalachians)...which may approach or exceed some of the
compromised FFG values over sensitive hydrologic areas.

Chenard


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: 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