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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1429Z Jul 07, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1029 AM EDT Tue Jul 07 2020

Day 1
Valid 1426Z Tue Jul 07 2020 - 12Z Wed Jul 08 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE NORTHERN
PLAINS AND NORTHEAST TEXAS...

...Montana / The Dakotas / Western Minnesota...
A relentlessly active pattern continues across the northern tier
of states. The latest medium wavelength shortwave to eject from
the mean longwave position over the Northwest U.S. will carry
substantial height falls into Montana today. It is fairly clear
cut that clusters of strong thunderstorms will develop, quickly
evolving upscale into forward propagating convective lines and
clusters. Mean steering flow will be rather strong, with mean
0-6km winds 20-30 knots. Supercells could move slower, especially
if they are able to form in the upslope regime over southeast
Montana. Still, event rainfall totals will be held in check by
forward progression. The main concern will be high short term
rates owing to strong instability. And much of eastern Montana has
seen 1.50 to near 4.00 inches of rain in the past week, thus
lowering flash flood guidance to as low as 1.00 inch in 3 hours,
or less than an inch needed in one hour to cause bankful
conditions.

The event, although progressive, will become more worrisome as it
moves into North Dakota where 1-week precipitation anomalies are
even greater, sitting at 4 to 6 times normal for the time of year.
The hi-res guidance is in relatively good agreement as to a swath
of organized moderate rainfall with embedded heavy rain emanating
from southeast Montana toward central North Dakota. We chose to
upgrade to Slight Risk here, essentially as an extension of Slight
Risk out of the area of greatest concern - which is eastern North
Dakota. The WRF-ARW2 and NAM CONUS Nest have generally performed
the best in this pattern over the northern Plains, and they both
signal an intense round of rainfall Tuesday night at the nose of
the developing 40-50 knot low level jet, as the wave ejects from
the Rockies. There are some questions about latitude, however, as
much of the global guidance and a few hi-res models place this
rainfall farther north. Looking this morning at how far south the
recent convective outflows have pushed the effective frontal zone,
it would be a feat for the capped airmass and effective 850-700 mb
front to migrate all the way up to northeast ND by Tuesday night.
Judging by GFS mass fields, the WRF-ARW2 and NAM Nest solutions
seem to make sense. In coordination with WFO Grand Forks we leaned
toward the somewhat more northerly 00Z NAM Nest solution. It is
worth noting that the 06Z NAM Nest again supported the 00Z
solution. This region, eastern ND from near Fargo southward, and
perhaps including adjacent South Dakota or Minnesota, will have to
be watched closely. This is where cells initiating along the nose
of the low level jet will have potential to merge and mill about
until the upstream forcing comes along to budge the MCS out of its
place. While the HREF probability matched mean predicts local
maxima near three inches, an isolated higher-end rainfall event is
not out of the question.

...Southern Plains/Lower MS Valley...
...14Z Update...
Extended the Marginal Risk south and east to account for expected
convection along a stalled boundary draped from northern LA into
southern MS/southeast LA.  With this region observing above normal
precipitation over the past couple of days, any additional
rainfall of slow moving convection and training may result in
isolated flash flooding, especially over any urban areas. 

Pagano

...Previous Discussion...
North Texas sits at the tail end of a ribbon of mid level
vorticity getting pinched off the back of a broad southeast U.S.
trough. Inflow atop the nocturnal boundary layer looks sufficient
this morning to lead to an expansion of slow moving thunderstorm
activity, some of which was already ongoing in northeast Texas.
The trough extends back toward southwest Texas, and will migrate
slowly south / east today, with the possibility of intense local
downpours all along it, from southwest Texas to southern Arkansas
and northern Louisiana. Some of the hi-res QPF guidance is
particularly heavy, with solutions indicating 7-plus inches of
rain near an embedded vort max just east and south of Dallas.
These solutions include the NAM Nest and multiple runs of the
HRRR. Consistency of their QPF placement, along with radar trends
at 08Z, prompted us to upgrade a portion of Texas to Slight Risk,
with the risk perhaps peaking during the morning hours, before the
boundary layer process disrupts things and the trough begins to
slide east.

...Southeast / Including the Tennessee Valley...
...14Z Update...
Introduced a Slight Risk across portions of eastern GA into
west-central SC as the surface low continues to pivot moderate to
heavy rain over this region through the next several hours. 
Multiple runs of the HRRRv3 and HRRRv4 illustrating higher 6 hour
QPF totals with probabilities suggesting the 3 hourly rates could
result in more scattered flash flooding across this region.  The
24-hour MRMS storm total QPE also denotes that this region has
received 2-5+ inches making soils sensitive to additional
rainfall.  Rain rates could approach 2 inches/hour, though this
will likely accompany any convection that is able to form along
differential heating boundaries. The area of concern is mainly
with respect to the pivot point near the GA/SC border as rain will
be focused just north/northeast of the surface low with better
moisture flux convergence and slow surface low movement.  
Therefore, a Slight Risk was introduced. This corresponds well
with the Flash Flood Watch that was also recently introduced by
the local offices. 

Pagano

...Previous Discussion...
Broad troughing that was nearly cut off from the westerlies had
gradually developed greater vorticity over Georgia. The system,
although of baroclinic origin, may behave a bit like a hybrid or
warm core system as it evolves. It appeared somewhat warm core in
700mb plan view data, but not at 500mb. The cyclone phase spaces
diagrams initiate the developing coastal surface low as warm core
before then transitioning back, as it moves up the coast.
Regardless, there will be spiraling areas of convergence and local
thunderstorm activity out at a pretty good radius around this
system today. The airmass is very humid, with PW anomalies of 2.0
to 2.5, which is impressive for mid summer. Instability is more
limited, but not entirely, and any pockets of heating, along with
sustained convergent inflow around the eastern periphery of the
system - should lead to locally intense rain rates. The main thing
keeping this in the Marginal Category right now is that a bulk of
the heaviest model QPF is just offshore of the Carolinas, with 2
to 3 inch areal averages bleeding onshore. It is not a great setup
for training, although a few areas could get multiple
thunderstorms in the 24 hour period. The Carolinas had also been
dry recently, and it seems the main threat would be to urban areas
that are less capable of handling 2-inch per hour rain rates,
especially from Charleston to near Wilmington. While the emphasis
for synoptic lift is along the coast, certainly any of the
thunderstorm clusters back inland toward the Blue Ridge could
produce isolated flash flooding.

Meanwhile, we also added Marginal Risk for parts of TN / KY. That
area has been active the past few afternoons, but with generally
short-lived and isolated flash flood threats. Today looks to have
a bit greater potential, given the breadth of a developing
2000-plus J/kg MUCAPE region and further weakening of deep layer
steering flow on the back side of the southeastern U.S. low. The
NAM Nest, in particular, shows strong clustering of storms, likely
to lead to cell mergers that could boost local 2 or 3 hour
rainfall totals toward multiple inches.

...Mid Atlantic / Ohio Valley...
A jet streak diving off the southern New England coast, along with
thunderstorm outflows, had pushed a well of higher moisture
content and instability down toward central and northern Virginia.
This air mass will return northward today in response to the
approach of an upstream trough from the Great Lakes. By midday
expect that thunderstorms could become most numerous in the warm
advection / upslope regime over southern PA, and perhaps along a
remnant outflow boundary in the DMV. As occurred on Monday, low
level inflow will again be sufficient at 15 knots from the
southwest at 850mb, to sustain some thunderstorm clusters. The
event should not be quite so organized, given less upper support
with the loss of the jet streak, and some of the air mass having
been overturned on Monday. Still, isolated flash flooding is
possible, including the possibility of very heavy short term rain
rates.

Burke


Day 2
Valid 12Z Wed Jul 08 2020 - 12Z Thu Jul 09 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN THE UPPER
MIDWEST AND ACROSS PARTS OF THE CAROLINAS...

...Minnesota to eastern South Dakota, eastern Nebraska, northwest
Iowa, and adjacent northeast Kansas...
Showers and thunderstorms are expected to increase in coverage
Wednesday afternoon and evening from Minnesota into eastern
Nebraska and extending south across central Kansas in a
moisture-rich environment, ahead of a cold front moving across the
region.  By the time the models initiate convection, precipitable
water values should be on the order of 1.75 inches in a corridor
from eastern Nebraska into Minnesota, and south to southwest flow
at 850 mb will continue to advect moisture north ahead of the
front.  Showers and storms may back build into the low level flow
across northeast Kansas.  The model agreement is pretty good in
developing convection ahead of the front, and some of the typical
model spread begins to appear with the GFS, NAM, ECMWF all
differing some where the heavier showers/storms develop.  However,
there was more of an enhanced signal in the guidance for an MCS to
develop across southeast Nebraska to northeast Kansas to merit the
introduction of a Slight Risk area.

...Carolinas...
The surface low and its accompanying shortwave is forecast to
continue drifting northeast across the southeast states, with the
surface low expected to move just off the coast of the Carolinas. 
Heavier showers are expected along the track of the 850 mb low in
northeast South Carolina and especially across eastern North
Carolina.  Along and east of the low track, the 12z GFS and NAM
show high precipitable water values on the order of 2-2.25 inches.
 Due to the low level jet forecast by the models, 850-700 mb
convergence is modest for now.  Model QPF has trended higher
across eastern North Carolina, with aerial amounts potentially in
the 1.5 to 3 inch range for the Day 2 period.  A Slight Risk area
was added to account for the increased model agreement and high
rainfall efficiency given the tropical airmass expected.

Hamrick

Day 3
Valid 12Z Thu Jul 09 2020 - 12Z Fri Jul 10 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS AND MIDWEST AND ACROSS PARTS OF THE
MID-ATLANTIC...

...Northern Oklahoma to Wisconsin and the U.P. of Michigan...
There will likely be two separate areas of active weather, with
one MCS associated with a developing surface low across the
central Plains, and a second corridor of enhanced rainfall ahead
of a low moving front that is progged to be crossing the Upper
Midwest and western Great Lakes.  Models are indicating a surge of
PWs on the order of 1.75 to 2 inches ahead of this boundary, and
the presence of a mid-level vort max crossing Iowa and Illinois
will help to organize convection.  Much of this region has
received above normal rainfall in recent weeks, and thus flash
flood guidance values are reduced in many areas.  Aerial QPF is
expected to range from half an inch to an inch, but there are some
indications in the guidance for localized 2+ inch rainfall
amounts, and this could lead to some flooding problems.  However,
there is enough spread in the models to support a broad Marginal
Risk area for now.

...Eastern North Carolina to New Jersey...
Showers and thunderstorms are expected to continue across the
coastal plain and the piedmont of the Mid-Atlantic as the surface
low slowly moves up the coast.  The majority of the heaviest
rainfall should remain east of the low center, however there will
likely be bands of enhanced rainfall feeding into the low, mainly
along and east of the Interstate 95 corridor.  There is some
indication in the guidance for rainfall to reach into the 2 to 4
inch near the immediate coast of North Carolina, and if these
trends continue, an eventual upgrade to a Slight Risk may be
warranted in future outlooks. 

Hamrick



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