Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
449 AM EDT Thu Jul 09 2020
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
...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.
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.
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
...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 atmospehere
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 approachinig 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 additonal rainfall on Day 2 could be enough
for localized excessive rainfall.
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 apparant 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
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
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