Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Thu Jul 09 2020

Day 1
Valid 12Z Thu Jul 09 2020 - 12Z Fri Jul 10 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL AND SOUTHERN PLAINS AS WELL PORTIONS OF THE UPPER
MIDWEST...

...Portions of the Upper Midwest...
The main change to the previously issued Excessive Rainfall
Outlook was to expand the Slight Risk across southern and eastern
WI, northern-central IL, and into far northwest IN. MCS-aided
shortwave energy tracking from the lower Missouri Valley to the
Upper Great Lakes will interact with deep moisture and instability
ahead of a frontal boundary and lead to organized/widespread
convection during peak heating hours this afternoon and evening.
Lingering MCS debris will likely maintain some CIN early, however
by afternoon mixed-layer CAPEs should average between 2500-3500+
j/kg per the models, just as the deep-layer forcing increases
ahead of the approaching shortwave. Deepening moisture
(pre-convective environment PWATs around 1.75") coupled with the
anticipated instability will likely lead to hourly rainfall rates
of 1.5 to 2.0+ inches underneath the heaviest cells -- especially
given the enhanced potential for training given the alignment of
low-mid level flow and weak Corfidi vectors (under 10 kts). Most
of the CAMs have localized maximum totals >3" within the Slight
Risk area, which would be especially troublesome across portions
of northern IL, far northwest IN, and southern-eastern WI where
over much of this area hourly FFG values are between 1-1.5".

...Eastern Kansas-Oklahoma southeast through portions of the Lower
Mississippi Valley and the Deep South...
Accelerating and weakening MCS early this morning may lead to some
localized runoff issues after 12Z this morning across eastern KS
into the MOKSAROK area, however the wetter soils would prime this
region for an enhanced potential for flash flooding from
additional organized convection ahead of the approaching front
(pre-frontal trough) this evening and overnight ahead of the next
MCV. High-res CAMs show a narrow swath of 3-5 inches of 12-18hr
QPF within the Slight Risk area.

...Mid Atlantic...
There continues to be a trend with the model guidance for the
plume of deeper moisture associated with the compact coastal low
to be a bit further east, with the highest QPF confined over the
offshore waters.  However, the coastal areas of northeast North
Carolina and along the Chesapeake Bay in far eastern VA should
have enough instability and forcing for scattered storms that may
be heavy rainfall producers, and locally produce 1 to 3 inches of
rain. Based from the growing model consensus, the Marginal Risk
was pushed farther east of the Washington-Baltimore urban
corridor, while also south of Philadelphia. There is still a fair
amount of spread with the guidance concerning the northward and
westward extent of impactful rainfall today and tonight, with most
of the high-res CAMs a bit farther west and north with the heavier
rainfall while the 00Z ECMWF is the driest solution. Given the
trend to keep the highest totals offshore, there is no need to
introduce a Slight Risk area at this time.

Hurley


Day 2

The Day 2 outlook will be updated by 0830Z.


Day 3
Valid 12Z Fri Jul 10 2020 - 12Z Sat Jul 11 2020

...A MARGINAL RISK OF EXCESSIVE RAINFALL LINGERS OVER PORTIONS OF
THE SOUTHEAST U.S. AND THE MID-ATLANTIC REGION...

The front which helps focus conveciton on Day 2 will become
quasi-stationary and provide the focus for additional convection
from Friday into early Saturday morning.  Deep moisture, as
characterized by precipitable water values 1.8 to 2.2 inches, will
be pooled along and east of the boundary early Friday.  However,
the front should begin to shift eastward.  This results in the
focus for heavier amounts to be along and east of Interstate 95
and with a gradual shift southward with time.  Still appears that
some brief intense rainfall rates and spotty rainfall totals in
excess of 2 inches during the 24 hour period.  As a result, the
Marginal Risk area is largely an extension of the area from Day 2.

Bann



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