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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2106Z Jul 09, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
506 PM EDT Thu Jul 09 2020

Day 1
Valid 2057Z 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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID ATLANTIC/NORTHEAST...

...Mid Atlantic/New England...
...20Z Update...
Pretty big QPF/ERO changes at this update as the newly formed
Tropical Storm Fay will track a bit closer to the coast and into
inland New England than was previous forecast. This then brings
the axis of heaviest rain farther inland; along and east of the
I-95 corridor across portions of DE, eastern PA, into NY and
points east.  As precipitable water values of over 2.25 inches
work inland aided by easterly low level flow with the track
orienting NNE over the period, expect efficient rain rates and
training (as the propagation vectors and mean wind align) not only
across much of Delaware, eastern PA (Poconos), NJ and into update
NY associated with northern quadrants but with thunderstorms
moving across Long Island and points north into CT/RI and MA. 
Areal average precipitation is around 1-3+ inches with locally
higher amounts possible.  Given much of this region has received
an abundance of rain over the past several days, especially
eastern PA/NJ and update NY, feel soils are sensitive to
additional rainfall.  Therefore, a Slight Risk was introduced
based on the latest QPF/model trends.  A Moderate Risk may be
needed, especially for the urban sector, if the QPF continues to
highlight this region and trend upward. 

...Previous Discussion...
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...
...20Z Update...
Minor changes needed to the ERO to account for organized
thunderstorm development across eastern MT and into the Northern
Plains as a potent shortwave rounds the backside of the ridge
interacting with a trough axis to the east.  Models are in fairly
good agreement on the general placement and propagation of the
activity, though more mesoscale features will be ironed out as we
approach the event.  With impressive moisture/instability ahead of
the mid-level energy, expect heavy rainfall, though the
progressive nature should limit the flash flood potential despite
the lower FFG.  So have maintained and refined the Marginal Risk
based on the latest QPF/model trends. 

...Previous Discussion...
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/Pagano


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 AS WELL AS THE MIDDLE AND LOWER MISSISSIPPI VALLEY ON
SATURDAY...

...New England...
...20Z Update...
A lot of uncertainty remains with respect to the track of TS Fay
and also the interaction with the approaching trough.  Model
spread is quite substantial with a notable westward shift among
the guidance.  While following the NHC track, also blended in some
of the other guidance to help with the expected interaction and
lingering moisture through north/central New England.   Therefore,
given the lower FFG and expected rainfall totals/rates, felt a
Marginal Risk should be expanded.

...Previous Discussion...
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 misgivings 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.

...Mid/Lower MS Valley...
Introduced a Marginal Risk to account for ongoing convection
associated with a possible MCS moving across portions of
central/southern MO and into northern AR through the morning/early
afternoon hours.  It appears this organized cluster of storms
should be fairly quick to shift southeast on the backside of the
mid-level ridge helping to progress the surface cold front east.
With precipitable water values over 1.75 inches and MUCAPE values
well over 2000 J/kg, the environment is primed for convection to
continue through much of the morning and potentially into the
early afternoon across this region as the mesoscale features would
allow.  Efficient rain rates are expected, though the quick
progression of this system and higher FFG will limit the overall
potential for flash flooding.  Training is possible across the
southern flank of the MCS, though that seems a bit uncertain at
this point. Given these factors, introduced a Marginal Risk across
the region, mainly for activity through the first half of the
period, early Saturday into the early afternoon. 


Bann/Pagano





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