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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2115Z Jul 07, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
515 PM EDT Tue Jul 07 2020

Day 1
Valid 2045Z Tue Jul 07 2020 - 12Z Wed Jul 08 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
NORTHERN PLAINS AND THE SAVANNAH RIVER VALLEY...

...Montana / The Dakotas / Western Minnesota...
The latest shortwave to eject from the mean longwave position over
the Northwest U.S. is moving across MT.  So far, clusters of
strong thunderstorms have developed, which should quickly evolve
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 of average. 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 guidance signals 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.  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.


...Upper Midwest...
Thunderstorms have formed across southern WI this afternoon just
south of a shortwave seen on GOES-16 water vapor imagery across
the western UP of MI and northwest WI.  ML CAPE of 1500-2500 J/kg,
effective bulk shear around 25 kts, and convergent 850 hPa inflow
up to 20 kts combined with precipitable water values of 1.5-1.7"
have led to occasional cell mergers which have caused hourly rain
totals up to 2.5" per radar estimates.  With the latest radar
imagery showing an outflow boundary pushing southward, and a
diffluent 1000-500 hPa thickness pattern implying ESE motion,
would expect activity to cluster somewhat further before moving
ultimately southeastward.  The 18z HREF probabilities of 0.5"+ an
hour show activity forward propagating into northern IL by 03z
with slowly decreasing hourly rain totals beyond 00z.  A new
Marginal Risk area was added here per radar and satellite trends.


...Southern Plains/Lower MS Valley...
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.  Activity was more impressive earlier, and outflows are
pushing a slowly declining coverage of thunderstorm activity
southward.  Lowered the Slight Risk as a bulk of the issues have
passed, lowered the risk level and shifted the southern edge
southward to account for radar trends.  Overnight thunderstorm
activity from the ArkLaTex to the ArkLaMiss ahead of shortwaves
moving eastward across OK and MO could renew flash flood concerns
from northeast TX eastward into western and northern MS by
Wednesday morning as ample moisture and instability linger to the
south overnight.


...Southeast / Including the Tennessee Valley...
Broad troughing aloft nearly cut off from the westerlies had
gradually developed greater vorticity over Georgia, which has
acted to "hybridize" what was once warm core system that developed
across the north-central Gulf of Mexico which has since linked up
with a section of the polar front.  There will be spiraling areas
of convergence and local thunderstorm activity around this system
this evening, tonight, and Wednesday morning. The airmass is very
humid, with PW standardized anomalies of 2.0 to 2.5 sigmas above
the early July mean.  Any pockets of heating, along with sustained
convergent inflow around the eastern periphery of the system from
warm Atlantic waters - should lead to locally intense rain rates. 
The Slight Risk was maintained but shifted southward to account
for radar trends.  A bulk of the heaviest model QPF tonight into
Wednesday morning 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.  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.


...Mid Atlantic / Ohio Valley...
A jet streak diving off the southern New England coast, along with
thunderstorm outflows, pushed a well of higher moisture content
and instability down toward central and northern Virginia this
morning. This air mass returned northward today in response to the
approach of an upstream trough from the Great Lakes. 
Thunderstorms are becoming numerous across southern OH, northern
WV, and soon across southwest PA.  Thunderstorm coverage has been
slow to expand near the Nation's Capital due to morning stratus
that was stubborn to burn off.  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.

Roth/Burke/Pagano


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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NEBRASKA AND KANSAS...NORTHERN MINNESOTA...AND  EASTERN NORTH
CAROLINA...

...Central Plains into the Upper Mississippi Valley...
20z update...
The main change here was to introduce a Slight Risk across
portions of northern MN. A weakening MCS is expected to cross that
area through 08/15z or so, before it outruns its instability
source. However, there should be sufficient moisture to support
hourly rainfall rates between 1.00/1.50 inches, which is shown in
much of the high resolution guidance across northern MN. After the
the first round exits, the airmass will require time to recover
from the morning overturning. Much of the high resolution guidance
shows instability recovering quickly enough during the afternoon
hours to support another round of storms, mainly after 08/21z.

The second round of storms could produce storms with hourly
rainfall rates of 1.50 inches, with precipitable water values
between two and three standard deviations above the mean. Between
the two rounds of convection, much of the high resolution guidance
indicated local 2.00/4.00 inches of QPF. Much of northern MN has
been fairly dry the past 7/14 days, but after collaborating with
WFO DLH, it was determined a small Slight Risk would be needed for
northern MN for Day 2.

Previous discussion...
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.


...Lower Mississippi Valley...
A west to east oriented band of storms should be ongoing across
portions of southern AR into west central MS, following the
northern edge of the 1000/2000 J/KG MUCAPE gradient across the
region. Instability and moisture (with precipitable water values
near 2.00 inches) appear to be focused on a low level boundary
across this area, and the storms are expected to follow the
frontal boundary until they outrun their instability, probably
after 08/15z.

Before that time, the moisture in the column (as well as a deep
warm cloud layer) could support hourly rainfall rates near 1.50
inches (which is supported by much of the high resolution
guidance), as training could be an issue. Where training occurs,
local 2.50 inch rainfall amounts are possible. Three hour flash
flood guidance values here are generally above these values, so
for now an enhanced flash flood threat is not expected. After
collaborating with WFO JAN, a Marginal Risk was placed here to
cover the threat.


...Carolinas...
20z update...
Not much change was made to the previously issued Excessive
Rainfall Outlook here. Much of the guidance has been easing just
to the east with the surface reflection associated with the broad
mid level trough attempting to close off over the Mid Atlantic
states. Bands of convection in the deep tropical moisture (with
precipitable water values between two and three standard
deviations above the mean) moving west northwestward could allow
training across eastern NC. Much of the high resolution guidance
showed the potential for hourly rainfall rats near 2.00 inches
where the bands train, which is certainly plausible given the
depth of the moisture in the column.

Previous discussion...
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.


...Northeast...
Short wave energy tracking from upstate NY across central and
northern New England pushes a frontal boundary across the
Northeast during Day 2. Ahead of the front, a low level southwest
flow transports 1.75 inch precipitable water air (which is between
two and three standard deviations above the mean) from central PA
into northern New England, mainly before 09/00z. Model soundings
ahead of the front showed 1000/2000 J/KG of SBCAPE in place, and
the combination of moisture and instability should be sufficient
to support a scattered to broken line of storms extending from ME
into central PA, peaking before 09/00z.

While the storms should be generally progressive (especially over
northern New England), as the short wave shears out to the
northeast, storm motions could begin to slow from MA southwest
into PA. Hourly rainfall rates of 1.50 inches are possible where
the storms slow, and short term training and cell mergers are
possible from southwest New England into central PA, with the last
of the storms weakening toward 19/04z. Local 2.50 inch rainfall
amounts are possible where short term training occurs, and this is
indicated by high resolution guidance across portions of western
MA into Southeast NY and possible northwest.

Three hour flash flood guidance values are generally above 2.50
inches (excluding urban areas), and training would be required to
initiate flash flooding in most places. Since these values are
attainable locally, and after collaborating with WFOs BOX/GYX, a
Marginal Risk was extended from ME to portions of PA on Day 2 to
cover the flash flood threat.

Hamrick/Hayes

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...

...Mid Atlantic...
20z update...
Not too much change to the previously issued Excessive Rainfall
Outlook was made with this update. There has been a general trend
to take the surface low and its deeper moisture a bit further east
with time. Should this occur, it is possible that the deepest
moisture remains just off the NC coast. Heavy rainfall is expected
here on Day 2, which could prime the soils for flash flooding
during Day 3.

However, the trend has been to inch the highest QPF amounts just
offshore of the Mid Atlantic coast during Day 3. This would
mitigate the threat to eastern NC to some degree, and that is the
primary reason this area was not upgraded to a Slight Risk. There
is still a fair amount of spread concerning northward and westward
extent of the rainfall, with the 12z GFS probably too fast moving
the heavy rainfall into southern New England, while the 12z UKMET
may be too suppressed, keeping almost all of the heavy rainfall
just offshore.

Previous discussion...
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. 


...Southern/Central Plains into the Upper Great Lakes.....
20z update...
No significant changes made to the previously issued Excessive
Rainfall Outlook, with changes limited to trends in the model QPF.

There is a fair amount of spread in the 12z regional/global models
concerning the track of MCSs in the mid level west northwest flow
along a surface boundary extending from the Upper Great Lakes into
western KS. Along and ahead of the front, model soundings showed
2500/3500 J/KG of MUCAPE, which should be sufficient to support
convection forming clusters with support from mid level short
waves and/or MCVs. A 20 knot low level inflow transports 1.75/2.00
inch precipitable water air into the developing storms, which
could support hourly rainfall rates near 1.50 inches as the
convection peaks.

The best track for MCSs could be from NE across eastern KS into
eastern OK, based on 12z model solutions. The Marginal Risk was
left here to cover this threat. Should any of these locations have
MCSs track across them, a Slight Risk could be needed in the
abovementioned locations during Day 3.

Previous discussion...
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.  Areal 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.

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