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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0012Z Jul 09, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
812 PM EDT Wed Jul 08 2020

Day 1
Valid 01Z Thu Jul 09 2020 - 12Z Thu Jul 09 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF
NEBRASKA AND KANSAS...UPPER MS VALLEY TO THE UPPER LAKES...AND
ALONG THE OUTER BANKS OF NORTH CAROLINA...

...Upper MS Valley to Upper Great Lakes...
Convection is expected along and near the warm front in northeast
MN to northern WI and the the western UP of Michigan.
A well defined corridor of enhanced southwesterly low level inflow
will come into the warm front, generating additional
showers/storms this evening.
Cell mergers and upscale growth could lead to runoff problems and
isolated flash flooding. Strong CAPE along with precipitable water
anomalies to +2.5 (actual PW values above 1.50 inches in the
pre-convective environment) will yield intense short term rain
rates. Expect hourly rates at least 1.5 to 2.0 inches, while
mergers and brief bouts of backbuilding/training could lead to
accumulations over 2.50 inches in an hour. The hi-res models are
unanimous in forecasting spotty 3-inch event totals. 

...Central Plains...

Along roughly the edge of the +12C temperatures the models are all
consistent in developing thunderstorms over central Nebraska late
today. These may then show a tendency to propagate both
east-northeastward along the trailing portion of the northern
Plains front, but also southeastward into the low level inflow. In
Nebraska and Kansas this has the look of an MCS riding along the
edge of the cap, fed by inflow from the capped region, with
redevelopment favored on the south to southwest flank of the cold
pool. Flash flooding could occur early as cell mergers lead to the
MCS formation, and also as training and redevelopment occur along
the edge of the cap. We maintained a Slight Risk area, maintaining
lower Marginal Risk probabilities in the Nebraska Sand Hills where
some of the early development is likely to take place. 

The NMMB and ARW are showing the heaviest QPF riding along the
capping inversion a bit farther east as compared to the other
guidance.  Though, based on alignment of the low level jet and the
boundary layer convergence, kept the heaviest QPF mainly across
central NE into eastern KS. There is uncertainty with respect to
the propagation and how long the MCS progresses through the early
morning hours.  

...Coastal North Carolina...

Low pressure will gradually develop near the Carolina coast. The
airmass here is rich with tropical moisture, many RAOBS sampling
2.15 inch PW or greater.
The convection developing int he Gulf Stream is expected over the
coastal waters with a few cells making it onshore. Overnight
rainfall, however, does not look especially concentrated. The
overall coverage and intensity will be limited by low level inflow
and weak instability, despite higher precipitable water values.
The Slight Risk over OBX appears warranted as the strongest
moisture convergence should continue to feed moderate/heavy rain
into the region, especially between Cape Lookout and Cape Hatteras
overnight. Most areas are expected to received 1 to 3 inches of
rain, with isolated totals to 5 inches possible. Most models keep
the heavier rain over the coastal waters.

...Pennsylvania/adjacent western MD/northern WV...

Activity has produced multiple bands and clusters of
showers/storms over PA  this afternoon and evening in the vicinity
of the decaying frontal boundary.
These cells have produced multiple outflow boundaries.  Boundary
collisions may produce additional cells that result in isolated
heavy rainfall.  Isolated flooding may occur of these mergers
occur where earlier showers/storms produced rainfall.   Once these
mergers occur, we will run of real estate where instability has
not been tapped, with the expected overall trend towards less
activity overnight.  We maintained Marginal Risk probabilities due
to limited areas of instability that remain.

Burke/Pagano/Petersen


Day 2
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 AS THE MID AND UPPER
MISSISSIPPI VALLEY...

...Southern/Central Plains into the Upper Great Lakes...
20z update...
The main change to the previously issued Excessive Rainfall
Outlook was to introduce a Slight Risk across the Mid and Upper MS
Valley. Short wave energy tracking from the Central Plains to the
Upper Great Lakes interacts with deep moisture and instability
along a frontal boundary to support convection capable of
producing excessive rainfall extending across IA into southern WI
and far northwest IL.

Ahead of the short wave, model soundings showed strong instability
along the boundary, which should be sufficient to support storms
developing on it after 09/16z. As the short wave approaches,
synoptic scale ascent should allow storms to become better
organized. Deepening moisture on a low level 20/25 knot inflow)
supplies the storms that produce hourly rainfall rates near 1.50
inches (based on several 12z high resolution model solutions) as
they move east. There is a model signal that clusters of storms
cross much of IA/southern WI and far northern IL between 09/18z
and 10/02z, before they start to weaken. The 12z NAM CONUS Nest
and FV3CAM suggest local 3.00 inch rainfall amounts are possible,
especially where short term training occurs.

Three hour flash flood guidance values are as low as 1.50/2.00
inches over southern WI, which are certainly attainable in the
deep moisture plume. Based on the high resolution signal, and
after collaborating with WFOs DVN/ARX, A Slight Risk was
introduced for portion of IA/southern WI/northern IL for Day 2.

Previous discussion...
There is decent overall model signal in the 00z regional/global
guidance concerning the track of two MCSs that are likely in mid
level northwest flow ahead of a cold front from southeast Nebraska
to northwest Arkansas, with the 00z NAM among the most robust
solutions with areal QPF on the order of 2 to 3 inches.  The
second MCS during the overnight period Thursday night will likely
have the greatest QPF, and after coordinating with the local
forecast offices, a Slight Risk area was introduced from eastern
Kansas to extreme northwest Arkansas, where the best prospects for
potential cell training will exist. 

Farther to the northeast across the Upper Midwest and western
Great Lakes, scattered to numerous showers and storms are likely
as the cold front approaches with ample instability of 2500 to
3500 J/KG of MUCAPE ahead of it.  This should be sufficient to
support multi-cell convection with support from mid level short
waves and remnant MCVs.  A 20 mph low level inflow transports PW
values approaching two inches into the developing storms, which
could support hourly rainfall rates on the order of 1.5 inches or
greater with the strongest convection.  However, this activity
will likely not be as organized as the MCS activity farther to the
southwest, thus keeping this a Marginal Risk level for now.


...Mid Atlantic...
20z update...
Model continuity concerning the surface just low off the Mid
Atlantic coast on Day 2 has not improved, which results in a below
average forecast for the region. Early in the period, there is
better agreement taking the surface low and its deep moisture
(with precipitable water values over 2.00 inches) along the the
NC/VA coasts, especially after 10/06z. Bands of convection with
hourly rainfall rates of 2.00 inches are possible across the
southern DelMarVA Peninsula between 10/06z and 10/12z. Though
instability could remain below 1000 J/KG of MUCAPE, the deep warm
cloud layer could result in lower topped storms that are highly
efficient rainfall producers.

Collaboration was conducted with WFO AKQ, as the banded rainfall
could affect the eastern portions of their CWA before 10/12z.It
was agreed to hold onto the Marginal Risk, rather than upgrade at
this time. There are still enough of a model spread in timing to
hold off on the upgrade. However, should there be better timing in
later model runs, a Slight Risk could be needed for portions of
the DelMarVa Peninsula in subsequent forecasts.

Previous discussion...
There has been a general trend in the model guidance over the past
24 hours for the coastal surface low, and the plume of deeper
moisture associated with it, to be a bit further east with the
highest QPF likely to occur over the offshore waters.  However,
the coastal areas of eastern North Carolina and locations east of
Interstate 95 in Virginia should have enough instability and
forcing for scattered storms that may be heavy rainfall producers,
and locally produce 1 to 3 inches of rain. The Marginal Risk area
also encompasses the greater Washington, DC metro area and
extending northward to Philadelphia, and points eastward.  There
is still a fair amount of spread concerning the northward extent
of impactful rainfall on Day 2, but given some of the impressive
rainfall totals over the past 48 hours, flash flood guidance is
reduced and it will not take as much rain to result in flooding
problems.  The UKMET is farther southeast with the low and has
little in the way of significant QPF inland, whereas the NAM and
GFS have the core of the heavier rainfall closer to the Delmarva
Coast.  Given the trend to keep the highest totals offshore, there
is no need to introduce a Slight Risk area at this time.

Hamrick/Hayes

Day 3
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...
20z update...
Not much change to the previously issued Excessive Rainfall
Outlook, as there is still quite a bit of model spread concerning
the track and strength of the surface low starting Day 3 along the
Mid Atlantic coast. The 12z NAM/GFS are still taking the surface
low and turning northwest it northwest ahead of the broad trough
to the west. The 12z NAM is particular seems to be too strong with
the surface low, as it wraps mid level energy around the surface
low, allowing convection to track northwest across portions MD
into PA. At this point, these solutions seem to lift the surface
out too quickly inland, and were not much of a factor in the
forecast decision.

The WPC QPF and Excessive Rainfall Outlook were based more closely
on a 12z ECMWF/UKMET solution, though the 12z ECMWF QPF
distribution appears to take into account too much moisture and
lift in a trowal like feature that sets up over NJ during that
time (despite the surface low moving northeast like the remainder
of the 12z non-NCEP guidance). Because of this, the 12z ECMWF was
used sparingly across the Mid Atlantic states.

In any event, low topped convection rotating around the surface
low and mid level center becoming involved in mid level
frontogenetic forcing on the western side of the circulation.
Where this sets up and pivots will be the key to the rainfall
forecast (and flash flood threat). The 12z ECMWF QPF amounts
aside, the best rainfall amounts are expected from southeast NY
into southern and central New England, as the surface low tracks
across eastern New England. Even without strong instability, the
deep moisture plume should be able to support low tops storms that
are efficient rainfall producers.

Because of the continued model spread, no upgrades were deemed
viable with this forecast. Should there be better model agreement
in later runs, a Slight Risk could be needed anywhere from NJ into
portions of southern and central New England in subsequent
forecasts.

Previous discussion...
The coastal low will continue making slow progress up the coast
Friday night and into early Saturday.  A veering low level jet
from southeasterly to southerly on the order of 30-40 mph at 850
mb will advect an anomalously moist airmass quickly northward,
with PWs progged by the ECMWF to potentially reach into the 2.1 to
2.4 inch range in the warm sector of this low across southern New
England.  These values approach daily PW records for early July,
and thus deep warm cloud layers and high precipitation efficiency.
 A corridor of enhanced 300 mb divergence will aid in deep layer
ascent, along with low level frontogenesis across much of this
region.  The NAM and CMC are most aggressive in bringing excessive
rainfall amounts inland from New Jersey to Rhode Island, where the
ensemble means suggest that the core of the heaviest rainfall
should continue to remain offshore. 


...Northern and Central Plains...
20z update...
Not much change to the previously issued Excessive Rainfall
Outlook, other than to extend it further into the Central Plains,
as seen in much of the 12z model guidance. Short wave energy tops
the building mid level ridge across the Southwest and Southern and
Central Rockies during Day 3, riding in the northwest flow across
a frontal boundary extending from the Dakotas into eastern NE.

Ahead of the short wave energy, a low level southerly flow
transports 1.50/1.75 inch precipitable water from eastern NE/KS
into SD. The combination of instability and moisture to support
convection that tracks southeast along the instability axis. Just
how developed the convection becomes could be dictated by how much
the short waves can erode the mid level capping associated with
the building ridge. There has been an overall increase in QPF
across this area, suggesting regional/global models are breaking
the cap more readily with time. Right now, there is not enough
support for an upgrade anywhere along the instability axis.

Previous discussion...
A cold front currently approaching the Pacific Northwest is
expected to reach the northern Plains by Friday and become an
occluded surface low by Friday evening, with the warm sector
extending from central North Dakota to northern Nebraska.  A
secondary low will likely develop along the cold front across
western Nebraska as the boundary tracks eastward.  Warm air
advection should begin in earnest by Friday night as the low level
jet intensifies and advects a corridor of 1.5 to 1.8 PWs northward
over eastern South Dakota and much of central and eastern
Nebraska.  Mixed layer CAPE is also expected to rise to at least
2000 J/kg, and combined with increasing moisture will provide fuel
for a pair of MCSs that will likely develop late Friday afternoon
and track southeastward during the overnight hours.  Some training
of convection is possible with these complexes and raises the
potential for flash flooding.  However, there is still enough
spatial variability in the models to support just a Marginal Risk
area at this time.

Hamrick/Hayes


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