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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0013Z Jul 08, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
813 PM EDT Tue Jul 07 2020

Day 1
Valid 01Z Wed Jul 08 2020 - 12Z Wed Jul 08 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR NORTH
DAKOTA...

...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, thunderstorms
have evolved upscale into a forward propagating convective
line/squall line/derecho.  Mean steering flow will be rather
strong, with mean 0-6km winds 20-30 knots.  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 tonight 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 are pushing into northern IL just south of a
shortwave seen on GOES-16 water vapor imagery across the UP of MI
and northwest WI and to the northeast of another shortwave
approaching from MO.  ML CAPE of 1500-3500 J/kg to the west across
IA, effective bulk shear is a little under 25 kts, and convergent
850 hPa inflow of 15-20 kts from the west combined with
precipitable water values of 1.5-1.7" have led to thunderstorms
growing upscale into a fragmented line that has become outflow
dominant.  Considering the diffluent 1000-500 hPa thickness
pattern implying ESE motion, would expect activity to move
southeastward and possibly southward.  The 18z HREF probabilities
of 0.5"+ an hour show activity forward propagating across northern
IL with slowly decreasing hourly rain totals, fading at some point
beyond 03z.  Any activity on the western side of the broken line
needs to be watched for possible backbuilding.


...Southern Plains/Lower MS Valley...
Thunderstorms caused by daytime heating are fading across the
region.  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 and a shortwave
approaches from western TX.


...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, though activity is
currently on the wane, which led to the removal of the Slight Risk
area.  The airmass is very humid, with PW standardized anomalies
of 2.0 to 2.5 sigmas above the early July mean.  Sustained
convergent inflow around the eastern periphery of the system from
warm Atlantic waters could lead to locally intense rain rates near
the coast of the Carolinas.  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 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.


...Mid Atlantic / Ohio Valley...
A warm and unstable air mass returned northward today across the
Mid-Atlantic States in response to the approach of an upstream
trough from the Great Lakes.  Thunderstorms have started zipping
down where two outflow boundaries have converged accross northern
WV -- some showing hourly rain totals to 3" -- which could
continue to propagate southwest over the next few hours
into/within an area of 1000-1500 J/kg of ML CAPE.  Low level
inflow will again be sufficient at 15 knots from the southwest at
850mb, to sustain some thunderstorm clusters.  The event has not
been organized, given less upper support with the loss of the jet
streak. Still, isolated flash flooding is possible, including the
possibility of very heavy short term rain rates until activity
fades tonight.

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