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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1535Z Jul 09, 2020)
 
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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
1135 AM EDT Thu Jul 09 2020

Day 1
Valid 16Z 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...
There is a Slight Risk of excessive rainfall across southern and
eastern WI, northern-central IL, and into far northwest IN. 
MCS/MCV-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 2500-4000 j/kg per deterministic and conceptual
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 2-2.5"
underneath the heaviest rainfall -- 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/Southeast...
Accelerating and weakening MCS early this morning may lead to some
localized runoff issues late this morning across OK and AR,
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.  While the Slight Risk area
has shrunk due to radar trends, have left the Marginal Risk intact
across KS for the possibility over overnight convection.  The
possibility of training showers and thunderstorms from MS and the
southern Appalachians through the Southeast has led to some
expansion of the Marginal Risk area per signals seen in the 12z
HREF guidance.

...Mid Atlantic...
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.  Spread
remains within 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 appears to be no
need for a Slight Risk area at this time.

Roth/Hurley


Day 2
Valid 12Z Fri Jul 10 2020 - 12Z Sat Jul 11 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID ATLANTIC/NORTHEAST...AS WELL AS THE NORTHERN AND CENTRAL
PLAINS...

...Mid Atlantic/New England...
The range of options offered by the 00Z suite of numerical
guidance remained large as low pressure hugs the Eastern Seaboard
on its way north from the Mid-Atlantic region towards New England.
The NAM continued to appear to be too strong with the system and
the associated QPF because it wraps mid-level energy around the
surface low...which allows convection to rotate northwest across
portions of MD into PA.  Given the amount of mid- and upper-level
ridging north of the low, think these models tend to be too fast
with their forward speed of the system. 

The WPC QPF and Excessive Rainfall Outlook were able to maintain a
high degree of continuity with the previous forecast and remained
more closely aligned with the 09/00Z  ECMWF/UKMET
solution..although the actual QPF amounts from the ECMWF still
looked to be too high and were used sparingly.

Because of the continued model spread, no upgrades were deemed
viable with this forecast. At least conceptually, the atmosphere
should be capable of supporting storms which produce local
downpours...but agree with the previous shift to hold off issuing
a Slight Risk at this time until the signal becomes stronger. 

...Northern and Central Plains...


Not much change to yesterday's Day 3 Excessive Rainfall Outlook as
it becomes today's Day 2 Excessive Rainfall Outlook.  Did expand
the eastern side of the area a bit given a signal from the
operational GFS, several GEFS members as well as SREF runs with
the NMM core which favored far southeast South Dakota into Iowa.

Ahead of short wave energy approaching from the west, a low level
southerly flow transports 1.50/1.75 inch precipitable water from
eastern NE/KS into the region.  The combination of instability and
moisture should support convection that tracks southeast along the
instability axis. Even though the deterministic QPF is not
terribly great, parts of the outlook area...especially over
eastern Nebraska...did get some decent rainfall tonight and the
cumulative effect of additional rainfall on Day 2 could be enough
for localized excessive rainfall.

Bann


Day 3
Valid 12Z Sat Jul 11 2020 - 12Z Sun Jul 12 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
NEW ENGLAND ON SATURDAY...

...Eastern Portion of New England...
Issued a Marginal Risk area downstream of the Marginal Risk area
from Day 2 in response to an area of low pressure moving northward
along the coast. The forecast period begins with some large model
differences apparent in the placement of the surface low moving
northward...the NCEP guidance favoring a solution that takes the
surface low well inland across New York while the ECMWF leads a
pack of models keeping the area of low pressure hugging the coast
all the way into the Gulf of Maine by 00Z Sunday. 

The NCEP-led models results in more QPF across New England due to
persistent on-shore fetch of Atlantic moisture while the ECMWF-led
guidance does develop some moderate QPF in a deformation zone area
across New England that would result in a lesser risk of excessive
rainfall. Despite some mis-givings we have about the deterministic
ECMWF QPF along the coast and how much QPF it wants to generate in
the deformation zone, the ECMWF track was favored over the NCEP
guidance. 

To cover the possibility that the more-inland solutions are
correct and parts of eastern New England get 1 to 3 inches of
rainfall across inland areas with terrain, a Marginal Risk area
was issued.  Subsequent outlooks will be able to upgrade the risk
area, or remove the area all together, as the track becomes more
clearly defined.

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