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FOUS30 KWBC 300106
QPFERD

Excessive Rainfall Discussion...Corrected
NWS Weather Prediction Center College Park MD
904 PM EDT Fri Jun 29 2018

Corrected for geographic reference in the second line of the
second paragraph

Day 1
Valid 01Z Sat Jun 30 2018 - 12Z Sat Jun 30 2018

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PARTS OF THE
SOUTHWEST, PLAINS/MIDWEST, & GREAT LAKES...


...Central High Plains/Upper Midwest/Great Lakes...
As upper troughing continues to slide eastward out of the Rockies
during this period, clusters of thunderstorms formed in WY and CO
which have moved into portions of SD, NE, and KS. Deep layer wind
profiles will provide a fair amount of forward motion, with mean
0-6 km winds at least 15 to 20 knots. The region has been wet
early this summer, however, and FFG is on the low side, such that
any of the typical merging cells or brief training could result in
isolated flash flooding.

Across MN and MI, there is more uncertainty. Either additional
clusters of thunderstorms or a portion of the ongoing convection
in SD is expected to develop and move across Minnesota and the
Upper Peninsula of Michigan/Lake Superior. The most likely region
for organized redevelopment will be from central to northeast
Minnesota, near the edge of the Great Plains capping inversion
based on the position of the forecasted 12C isotherm at 700 hPa.
A shortwave seen in water vapor satellite imagery across the
Dakotas is expected to be the instigator for new convection in MN
which has brought sufficient 850 hPa inflow into the region. The
effective bulk shear of 25-50 kts combined with MU CAPE currently
4000-5000 J/kg would result in some fairly organized convection
and locally heavy rainfall. The combination of bulk shear and MU
CAPE could lead or further support forward propagating
lines/QLCS/LEWP modes which could limit the overall rainfall
threat. The hi-res model signal is weak/skimpy based on the low
18z HREF probabilities of 1"+ in an hour, with an unclear signal
whether or not convection with heavy rainfall reaches portions of
the Upper Peninsula of Michigan which recently saw heavy rainfall.
After coordination with Marquette MI/MQT, felt it was best to
include more of the Upper Peninsula within the marginal risk area
as a precaution.


...Southwest NM/Southeast AZ...
Ahead of the upper level trough, thunderstorms have been
percolating -- and at times backbuilding -- across Southeast AZ
and Southwest NM. MU CAPE values in the region are around 1000
J/kg, which considering cell coverage, could allow for another
four hours of issues. Close to 25 kts of effective bulk shear
appears to be leading to some level of organization. Precipitable
water values are ~1", which considering the terrain in the area is
sufficient for heavy rain concerns. Prudently added this area as
a precaution based on radar trends.

Roth/Burke


Day 2
Valid 12Z Sat Jun 30 2018 - 12Z Sun Jul 01 2018

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
IOWA....

...Central Plains into the Upper Great Lakes...
Ahead of a weakening long wave trough moving across the Rockies,
mid level lift aids in spinning up a series of surface waves on a
front extending from the Upper Great Lakes into the Central
Plains. In advance of the surface waves, a 30 to 35 knot low
inflow transports 2.00 inch precipitable water air (which is
between two and three standard deviations above the mean) along
the front over northeast KS/eastern NE into much of
IA/southeast MN and the western half of WI, peaking between 01/00z
and 01/06z.

During that time, model soundings along and east of the front
showed MUCAPE values between 2000-2500 J/KG, with the best
instability on the nose of the low level jet. Short wave energy
ahead of the mid level system could provide sufficient synoptic
scale lift for clusters of storms to form near the surface waves.
As a surface wave approaches eastern NE/western IA (generally
during the 01/00z to 01/06z time frame), propagation vectors
briefly oppose the 850-300 mb mean wind, suggesting the
possibility of backbuilding over portions of IA, before the
surface wave tracks along the front into western WI.

Model soundings also showed high wet bulb zero values (as well as
a moistening column), suggesting that warm rain processes will
dominate the convection, resulting in efficient rainfall
producers. There was some spread in the guidance regarding the
placement of the highest rainfall amounts, and the 12z WRF ARW was
used to place the highest amounts over western and central IA.
This area has seen much above normal rainfall the past seven days,
and after collaborating with WFO DMX, a Moderate Risk was placed
over that area. The combination of heavy rainfall over saturated
ground should produce an enhanced flash flood threat. The Slight
Risk was stretched into western and northern WI, where training of
efficient rainfall makers could result in hourly rainfall rates
near 2.00 inches.

Hayes

Day 3
Valid 12Z Sun Jul 01 2018 - 12Z Mon Jul 02 2018

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER THE UPPER
GREAT LAKES AND CENTRAL GULF COAST...

...Upper Great Lakes...
Convection is expected to be ongoing across WI/the UP of MI during
the first part of the period, as the last in the series of surface
waves tracks along the slow moving front in the region. Ahead of
the front (and the weakening mid level trough crossing the Upper
Great Lakes), a 30 to 40 knot low level southwest flow continues
to pump 1.75/2.00 inch precipitable water air across WI into much
of MI. Much of the instability ahead of the front should have been
consumed by earlier convection, but model soundings indicated
500-1000 J/KG of MUCAPE in place, which should be sufficient to
support convection ahead of the front, mainly before 02/00z.

There is a small window of opportunity for training across WI as
the 850-300 mb mean wind becomes better aligned with the
propagation vectors early, during which time efficient rainfall
makers could support local hourly rainfall rates near 1.50 inches.
After that, the mid level flow becomes increasingly fast with
time, which should help the storms move, reducing the flash flood
threat. Given the flash flood threat early, the Marginal Risk was
retained for much of northern and central WI into the western UP
of MI.

...Gulf Coast...
There is an increasing model signal for a closed mid level low
over the central Gulf Coast states, accompanied by a surface wave
(which is not present in all of the 12z guidance). The presence of
the mid level and surface systems will aid in focusing tropical
moisture (with precipitable water values near 2.25 inches, which
is between two and three standard deviations above the mean) near
the coast. At this point, much of the guidance suggests that the
heaviest rainfall associated with the surface and mid level
systems remains near or just off the coast.

However, any banded convection that rotates onshore over far
southeast LA/southern MS/southern AL/the western Fl Panhandle
brings the threat of hourly rainfall rates of 2.00 inches or
greater, especially where cell mergers or short term training
occurs. Since the highest rainfall amounts are expected to remain
offshore, a Marginal Risk was maintained for the region. If better
model agreement brings higher rainfall amounts onshore, this area
could see an upgrade to a Slight Risk in subsequent forecasts.

Hayes


Day 1 threat area: http://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: http://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: http://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


$$




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