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FOUS30 KWBC 290813
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Mon Jul 29 2019

Day 1
Valid 12Z Mon Jul 29 2019 - 12Z Tue Jul 30 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN
PORTIONS OF THE NORTHERN PLAINS TO THE UPPER MIDWEST...

...Upper Mississippi River Valley/Upper Midwest to Western Great
Lakes...
Realigned the Slight Risk area a bit considering short term radar
trends during the evening.  Removed the Slight Risk from areas
close to the Great Lakes where rainfall intensities have been held
in check by modest values of CAPE.  Realigned the Slight Risk
areas farther to the south along and ahead of an approaching cold
front moving across Nebraska and Iowa where CAPE values were in
excess of 2000 J/kg and where the Precipitable Water values were
in the range of 1.4 to 1.8 inches.  That combination should be
enough to support rainfall rates approaching an inch per hour.
Thinking is that the storms should be progressive enough to
preclude too many problems with flooding, although there is the
potential for a short period of cell training.  However, the 18Z
run of the HREF showed over 30 percent neighborhood probabilities
of 1 hour QPF exceeding an inch per hour through at least 07Z
making their way eastward across Iowa.  As a result of those
probabilities and the CAPE forecast by multiple models, opted not
to remove the Slight Risk. Maintained the Marginal Risk area over
portions of South Dakota ahead of a separate shortwave trough
embedded within northwest flow behind the main cold front.  The
SPC Mesoanalysis showed that these storms were approaching a
region where CAPE values were approaching 3000 J/kg.  This could
sustain brief intense rainfall rates for a couple hours beyond
01Z...but the HREF showed little in the way of probabilities of 1
inch per hour rates persisting beyond 03Z.  That was consistent
with the idea that the RAP and the HRRR were showing a sustained
decrease in CAPE during the late evening hours.

Bann



Day 2
Valid 12Z Tue Jul 30 2019 - 12Z Wed Jul 31 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER THE
SOUTHERN ROCKIES INTO THE SOUTHWEST...A SMALL PORTION OF THE
CENTRAL PLAINS...AS WELL AS PARTS OF THE NORTHEAST...

...Southwest and Southern Rockies...
As the mid level ridge spreads eastward into CO/NM during Day 2,
the return southeast flow focuses the deeper monsoonal moisture on
portions of southern AZ into far southeast CA. Precipitable water
values of 1.75/2.00 inches (which is between two and three
standard deviations above the mean) are transported into these
areas, with the best thrust of moisture expected after 31/00z.
Model soundings from the 00z GFS showed an axis of 1000 J/KG of
MUCAPE extending across much of southwest AZ into far southeast
CA.

Convection is expected to develop across the terrain far southwest
NM and northern Mexico between 30/18z and 31/00z, tracking
northwest in the mid level flow. There is some model support for
weak short wave energy (or an old MCV?) to move from northern
Mexico into southwest AZ during this time, but not all models
depict it. Without more in the way of forcing, the convection
could remain unfocused as it expands over much of AZ and southeast
CA, peaking between 31/00z and 31/06z. There is a model signal for
hourly rainfall rates near an inch, with the best consensus over
south central AZ, especially where short term training occurs, as
the mid level flow becomes better aligned with the propagation
vectors.

The synoptic setup looks increasing favorable for flash flooding,
and a Slight Risk was considered for a portion of south central
and southwest AZ. However, with some details still to be worked
out, and after collaborating with WFOs TWC/PSR, a Marginal Risk
was placed over much of the AZ into far eastern CA for Day 2. As
details come into better focus across this area, a Slight Risk
could be needed here in later forecasts.

The flow regime is not as favorable for portions of southwest into
northern NM and southwest CO, where the upslope flow carries
0.75/1.25 inch precipitable water air (highest over the plains)
across the area. There is some model support for weak short wave
energy topping the mid level ridge to provide some lift for
convective clusters. The best model signal is over the San Juan
Mountains in southwest CO and the Sangre de Cristo Mountains in
NM.

However, instability could end up being the limiting factor for a
more organized flash flood threat, suggesting that storms could
remain too scattered for more than a localized flash flooding.
Some of these areas have received 300+ percent of normal rainfall
over the past 7/14 days. The wet antecedent conditions leave
portions of eastern CO and eastern NM more susceptible to even
scattered storms with heavy rainfall, so these areas were included
in the Marginal Risk was well.


...Central Plains...
Moisture and instability on the nose of an increasing low level
flow could feed a developing cluster of storms (or an MCS) over
portions of NE into northeast KS during the second half of Day 2.
Deepening moisture and instability is drawn northward across the
Central and Northern Plains on the eastern edge of the mid level
ridge cresting over the southern Rockies after 31/00z. There is an
increasing model signal for convection to develop on an
instability axis north of a frontal boundary extending from
western ND/western SD across central NE into northeast KS just
before 31/00z.

A 20/30 knot low level south southeast flow transports 1.75 inch
precipitable water air (which is approaching two standard
deviations above the mean) along the instability axis. The
deepening moisture  feeds the developing convective clusters,
which could grow upscale into the MCS. However, there seems to be
less model support for this scenario than this time yesterday.
Instability could end up being the limiting factor for a greater
threat, both spatially and temporally across NE/KS.

Three hour flash flood guidance values here are generally above
2.50 inches, though a portion of southeast NE has seen 300+
percent of normal rainfall over the past seven days. Based on the
above, a Marginal Risk was placed here for Day 2, but if model
trends continue, this Marginal Risk could be eliminated in later
forecasts.


...Northeast...
Moisture and instability streaming northeast along a slowing cold
front could feed convection producing heavy to locally excessive
rainfall over portions of the Northeast, mainly across western PA
into western NY state during Day 2. An axis of 1000/2000 J/KG of
MLCAPE could support scattered to broken areas of convection
extending from WV across western PA into western NY state during
the 30/18z to 31/00z time frame. Propagation vectors drop below 10
knots across western PA into western NY state ahead of the front,
which could promote short term training or cell mergers in the
afternoon hours. As the storms move through a plume of 1.50 inch
precipitable water air, hourly rainfall rates could peak between
1.00/1.50 inches, especially where training occurs.

The lowest flash flood guidance values were scattered over
portions of northern WV into southwest PA (generally between
1.50/2.00 inches), and slow moving storms could produce rainfall
amounts that could approach these values here. Since here has been
an uptick in model QPF amounts with the storms across much of
western and northern NY with the convection, a Marginal Risk has
been extended from eastern
OH/northern WV across western PA into much of western and northern
NY state for Day 2.


...Lower Mississippi Valley...
Convective clusters (or a small MCS, per 00z NAM simulated IR
imagery) are expected to drop south across far southern AR into
northern LA during the first part of Day 2. The storms are
expected to remain south of the slow moving (or quasi stationary)
front here, as low level feeds the weakening convection in 2.00
inch precipitable water air. There is a multi model high
resolution model signal for the storms to drop slowly south as
they weaken, and the 00z NAM CONUS Nest showed the potential for
hourly rainfall rates near 2.00 inches across northern LA before
this occurs.

The 00z NAM CONUS Nest also showed that storms or small MCS
becoming outflow dominated in the 30/12z to 30/18z time frame,
with the storms fanning out into eastern TX and far west central
MS. Outside of cell mergers or short term training occurring along
the outflow, this does not appear to be an organized flash flood
threat, especially since three hour flash flood guidance values
are as high as 3.50 inches. For now, no excessive area was
assigned, as the threat appears nebulous. If there is better
agreement on the evolution of the storms, a Marginal Risk could be
needed here in later forecasts.

Hayes

Day 3

The Day 3 outlook will be updated by 0830Z.


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


$$




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