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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0813Z Jul 08, 2020)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Wed Jul 08 2020

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


...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.


Day 2
Valid 12Z Thu Jul 09 2020 - 12Z Fri Jul 10 2020


...Mid Atlantic...
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.

...Southern/Central Plains into the Upper Great Lakes...
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 aerial 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.


Day 3
Valid 12Z Fri Jul 10 2020 - 12Z Sat Jul 11 2020


...Delmarva to southeast New England...
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. 

...Central Dakotas and northern Nebraska...
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.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: