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FOUS30 KWBC 290055
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
854 PM EDT Sun Jul 28 2019

Day 1
Valid 01Z Mon Jul 29 2019 - 12Z Mon Jul 29 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 Mon Jul 29 2019 - 12Z Tue Jul 30 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN ROCKIES INTO THE SOUTHWEST...AS WELL AS THE LOWER
MISSISSIPPI VALLEY INTO THE OHIO VALLEY...

...Southern Rockies into the Southwest...
As the mid level ridge reasserts itself over the Southwest states,
the monsoonal flow becomes reestablished over the Southwest and
Southern Rockies during Day 2. While the best thrust of moisture
is expected across southern AZ (where precipitable water values
rebound to near 1.75 inches before 30/12z), the best instability
is expected across the Southern Rockies into the Sangre de Cristo
Mountains. An axis of 500/1000 J/KG of MLCAPE is expected to
develop from the Front Range of the Rockies in CO into northern
and southwest NM, peaking near 30/00z.

The developing storms will tap 1.00 inch precipitable water air on
a low level southeast flow to produce local 1.00 inch rainfall
amounts over the higher terrain, where the storms will be almost
stationary against the upslope flow. Once maximum heating occurs,
the storms will begin to move slowly west northwest with the weak
mid level flow, retaining the potential to produce hourly rainfall
rates between 0.50/1.00 inches away from the terrain. Three hour
flash flood guidance values are as low as 1.50 inches in these
areas, so a low end flash flood threat is expected. Based on the
above, a Marginal risk was placed over the higher terrain and
adjacent locations.

Further west across southeast AZ, the moisture plume from northern
Mexico possesses more moisture than
CO/NM, but instability could end up being the limiting factor for
a more organized flash flood threat. Scattered storms could
produce local 1.00 inch rainfall amounts over the higher terrain
before moving out into lower terrain, but the general lack of
forcing would suggest that the storms should be short lived. Slow
cell motions under the ridge position could allow for cell mergers
and short training. Based on the above, the Marginal Risk was
extended across southeast AZ for Day 2.


...Lower Mississippi Valley into the Ohio Valley...
Moisture and instability extending ahead of a slowing cold front
are expected to feed convection that could produce locally heavy
rainfall from the Lower MS Valley into the OH Valley during Day 2.
The best moisture feed ahead of the front is expected across MO
into AR/OK, mainly between 29/18z and 30/06z. Model soundings
showed an axis of 1500/2500 J/KG of MLCAPE in line with the
deepest moisture, and developing storms are expected to drop
southeast with the mid level flow.

Slow storm motions along the front as it becomes quasi stationary
in the 1.75/2.00 inch precipitable water air pooled near the front
could allow hourly rainfall rates to approach 2.00 inches, mainly
over AR around 30/00z. After that, there is a model signal that a
cold pool could form with storms over AR, resulting in the storms
dropping south into northern LA as they weaken.

Three hour flash flood guidance values are generally above 2.50
inches from MO into northeast OK, and above 3.50 inches over much
of the Lower MS Valley. Slow storm motions (especially early,
before any outflow driven storms form) could produce rainfall
amounts near the lower end of the spectrum, especially over
portions of AR and southern MO. Based on the above, a Marginal
Risk was placed over much of the Lower and a portion of the Mid MS
Valley for Day 2.

Further northeast along the front, the axis of moisture and
instability extends into the OH Valley, where storms along the
front across slow as the mid level flow becomes more parallel to
it. The 00z NAM CONUS Nest indicated the potential for hourly
rainfall rates to approach 1.50 inches over IN/OH and northern KY.
Three hour flash flood guidance values here are generally above
2.50 inches, but short term training in the higher moisture air
environment could allow storm totals to reach close to these
values. Based on this, the Marginal Risk was extended into the
Ohio Valley for Day 2.

Hayes

20Z Update: No noteworthy changes were necessary for the Day 2
excessive rainfall outlook, and the previous discussion remains
valid for this forecast period. /Hamrick



Day 3
Valid 12Z Tue Jul 30 2019 - 12Z Wed Jul 31 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHERN ROCKIES INTO THE SOUTHWEST...AS WELL AS A PORTION OF THE
CENTRAL PLAINS...

...Southern Rockies into the Southwest...
As the mid level ridge spreads eastward into CO/NM during Day 3,
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, peaking around 31/00z.

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 hint that weak
short wave energy (or an old MCV?) moves from northern Mexico into
southwest AZ during this time, but not all model depict this
feature. Without more in the way of forcing, the convection could
remain scattered as it expands over much of AZ and southeast CA,
peaking between 31/00z and 31/06z. There is a model signal for
local
1.00+ inch rainfall amounts, with the best consensus over south
central AZ.

Given the depth of the moisture in place, hourly rainfall rates of
an inch (or higher) are indeed possible over south central into
southwest AZ (especially where cell mergers occur in the weak
steering flow), but from this distance, it is difficult to place
where this might occur. The synoptic setup looking increasing
favorable for flash flooding, and a Slight Risk was considered for
a portion of south central and southwest AZ. However, with the
details still in doubt, and after collaborating with WFO TWC, a
Marginal Risk was placed over much of the AZ into far eastern CA
for Day 3. As details come into better focus across this area, a
Slight Risk could be needed here in later forecasts.

The flow regime is not quite 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 during Day 3. There is some indication
that weak short wave energy could top the mid level ridge in place
and provide some local 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.


...Northern and 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 3.
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, when remaining capping remains finally breaks.

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 (which is suggested by the
00z GFS simulated IR imagery) as it drops southeast from central
NE into northeast KS between 31/00z and 31/06z. There is a multi
model signal for 1.25 inches of rainfall on this axis, which is
plausible where training could occur before the system becomes
more progressive toward 31/12z.

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. Since there
is likely to be some spread concerning where the system tracks, a
Marginal Risk was placed over this area for Day 3.
It should be noted that the instability axis extends into western
SD/western ND, and there is some model support for convection
ahead of short wave energy over southern Canada. However, not all
00z model solutions include this area as a heavy rainfall/low end
flash flood threat. If the signal persists in subsequent model
runs, the Marginal Risk may need to be extended over these areas
as well.


...Northeast...
Moisture and instability streaming northeast along a slowing cold
front could feed convection producing heavy rainfall over portions
of the Northeast, mainly across western PA into western NY state
during Day 3. 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. As the storms move through a plume of 1.50 inch
precipitable water air, hourly rainfall rates could peak between
1.00/1.25 inches before they begin to weaken.

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 be close to
approaching these values here. After collaborating with WFOs
PBZ/CTP/BUF, no excessive area was assigned.

Hayes

20Z Update:  A Marginal Risk area was introduced from western PA
to western VT for the day 3 excessive rainfall outlook.  There was
a greater concentration of 1 to local 2 inch rainfall amounts
along a slow moving cold front based on the 12Z guidance,
particularly the NAM and the biased corrected QPF model, with
support from the GFS and ECMWF as well.  /Hamrick


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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