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FOUS30 KWBC 200056
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Fri Jul 19 2019

Day 1
Valid 01Z Sat Jul 20 2019 - 12Z Sat Jul 20 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE UPPER
GREAT LAKES...PORTIONS OF NEW YORK AND PENNSYLVANIA...AS WELL AS
SOUTH DAKOTA...

...Upper Great Lakes...
The main focus for flash flooding is tied to the MCS growing
CROSSING  WI this evening. Ahead of the MCS, the low level flow is
transporting 2.00+ inch precipitable water from southern MN (which
is between two and three standard deviations above the mean) into
southern and central WI. The deep moisture is supporting hourly
rainfall rates in excess of 2.00 inches with the activity, though
dry air in the mid levels (per the 00z GRB sounding) suggest that
hail could be contaminating these values.

The MCS maintain itself for at least the next 2-4 hours in the
presence of the very strong instability (with MLCAPE values in
excess of 5000 J/KG just upstream over southern WI (where surface
dew points are in the upper 70s to lower 80s). While hail and wind
will continue to be threats with the growing system, there is a
window for a flash flood threat (generally before about 20/03z) on
the southern edge of the MCS, where a training west to east band
of convection could allow for short term training and/or
backbuilding.

This scenario is supported by the most recent HRRR run, which
showed the potential hourly rainfall rates in excess of 2.00
inches over portions of central WI. These amounts appear plausible
where training occurs, as the deep warm cloud layer could support
efficient rainfall makers before 20/03z. Three hour flash flood
guidance is as low as 1.50 inches over central WI, and these
amounts could be exceeded, especially where training occurs.

Once the MCS matures, it is expected to to become more outflow
dominated, and the flash flood threat will diminish once this
occurs. Before that time, flash flooding is considered possible.


...Lower Great Lakes...
Convective clusters across southeast MI and northwest PA are
following axes instability (where MLCAPE values are still between
2000/3000 J/KG) early this evening, spreading out along outflow
boundaries. While mid level forcing is not strong in the flat
ridge position, the instability is strong enough to support
following the outflow boundaries into the late evening hours. The
storms are also embedded in an axis of 1.75/2.00 inch precipitable
water air, which could support hourly rainfall rates near 2.00
inches.

Three hour flash flood guidance values are as low as 1.50/2.00
inches in these areas, and given the deep moisture and place, and
the potential for training on outflow boundaries, the threat of
flash flooding is expected to continue through through about
20/03z. After that time, the instability is expected to be
exhausted, or become more elevated, probably resulting in the
activity becoming less organized. This would effectively end the
flash flood threat.


...Northern Plains...
Convection developing north of a frontal boundary becomes focused
on a axis of instability and increasing moisture oriented west to
east across portions of SD into far southwest MN, mainly between
20/06z. A subtle short wave over west central WY is expected to
cross SD after 20/03z. strengthening the low level flow ahead of
it. The low level flow is expected to transports 1.25/1.50 inch
precipitable water air in a ribbon of of 2500/3500 J/KG of MUCAPE
tracking north over the warm front.

After convective initiation, the activity is expected to spread
out in a west to east long extending across north central SD into
far southwest MN. As the 850-300 mb mean wind becomes better
aligned with the propagation vectors, training becomes more
likely, especially after 20/06z across western into central SD.
The deepening moisture plume (resulting in rising warm cloud
heights) could support hourly rainfall rates near 1.50 inches over
central SD.
Through 20/12z, a few pieces of high resolution guidance (most
notably the most recent HRRR) indicated the potential for local
3.00+ inch rainfall amounts, which could occur in areas of
training.

Three hour flash flood guidance values are generally between
1.00/1.50 inches across much of SD, due to heavy rainfall in the
past seven days (where 7 day totals have been as much as 400+
percent above normal). While there is some spread in the high
resolution guidance in the placement of the highest rainfall axis
(north versus south, due primarily to how they resolve the mid
level capping), the ingredients for flash flooding, superimposed
over wet soils, necessitated an upgrade to a Slight Risk over much
of central SD. The upgraded was collaborated with WFOs
UNR/ABR/FSD.

Hayes




Day 2
Valid 12Z Sat Jul 20 2019 - 12Z Sun Jul 21 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST CENTERED ON IOWA AND SOUTHERN WISCONSIN...

...Midwest into Michigan...
2000 UTC Update -- Based on the model trends, including the
high-res CAM windows which now extend through the entire day 2 ERO
period (12Z Sat-12Z Sun), have adjusted the Slight and Marginal
Risk areas a bit farther south compared to the previous outlook.
This aligns well with the favorable pre-frontal thermodynamic
profile per the growing model consensus (robust deep-layer CAPE
along an axis of PWs aoa 2.00". Strong deep-layer shear should
limit the potential for cell training; however, the strength of
the thermodynamic profile will likely lead to robust short term
rainfall rates -- likely aoa 2.0"/hr underneath the strongest
cells.

Previous discussion...

Early Saturday morning,  ongoing isentropic ascent across the
stationary boundary will be growing upscale to a solid MCS with
and MCV crossing the North Central Great Lakes with ample
mid/upper level support.  Unseasonably strong 250mb jet will shift
northeast centered over Ontario which will provide solid right
entrance region dynamics across the Upper Midwest and support
height-falls and tail end of shortwave energy to amplify and press
eastward across the Upper Midwest. This will help the front to
advance south through the day.  The challenge will be the
convective initiation during the afternoon which will be largely
focused along the boundary, but also any residual outflow
boundaries left during the aforementioned convective activity.
There is no shortage of moisture and instability pooling along and
just south of the front.  Precipitable water values surge as high
as 2+ inches in the early morning associated with the MCS into the
Great Lakes region.  During the afternoon/evening with strong
moisture transport out of the Gulf and strengthening low level jet
there is strong evidence of 2.25" precipitable water centered in
Iowa into Wisconsin which is 2 standard deviations above the mean.
 Instability is also a factor as MUCAPE values climb to over 4500
J/kg across this region.

Areal average QPF based on a blend of the GFS, ECMWF and our
in-house bias corrected model results in values of 1-1.5 inches
with locally higher amounts expected.  While training is possible
as determined by the mean wind aligning with the corfidi vectors
for a time, cold pool/outflow dominated convection in the late
evening into the overnight hours may limit this potential. Also,
mean wind between 850mb-300mb is generally 20-25 knots which is
another limiting factor, though it is hard to discern the
propagation pattern of convection this far out.  Regardless, with
expected convection and hourly rain rates at times well above 1.5
inches/hour, anticipate localized flash flooding across portions
of Iowa into Wisconsin during the late afternoon through the
overnight hours. This is further evident by flash flood guidance
as low as 1 inch/hour.  Therefore, kept the Slight but modified
for per the latest QPF and model trends.  If models come into
better agreement in terms of placement of the heaviest rain and
the overall rain amounts, especially if they go up, a Moderate
Risk may need to be considered.


...Front Range of Colorado/High Plains of NW Kansas...
As the front drops south across the High Plains, remaining
moisture banked up along the Range will be available for diurnal
convection.  Therefore any thunderstorms that do form assisted by
upslope flow has the potential for producing heavy rainfall.
Initially slow development moving off terrain with chaotic cell
motions and intersections of outflow boundaries may pose
short-term stalled downdrafts capable of isolated rainfall totals
of 2"+. While confidence is not particularly high in one given
location, the environment is supportive of this flooding threat
and so a Marginal Risk was maintained to incorporate this threat,
particularly along the Front Range of CO and SE WY.

...Portions of the Lower Tennessee Valley to the Gulf Coast...
Included a Marginal Risk across eastern MS, much of AL, and far
western portions of GA and the western FL Panhandle. A weak
mid-level shear axis/vort lobe is indicated on the global runs,
and is reflected in the sensible data with the uptick in QPF. The
activity will be heavily weighted toward the peak heating hours
later Sat morning through the afternoon given the weak lapse rates
aloft (<6.0 C/KM between 700-500 mb). Moreover, the 0-6km bulk
shear values aob 20 kts would suggest more pulse-type activity --
which would limit the duration of any intense rainfall rates.

Hurley/Pagano

Day 3
Valid 12Z Sun Jul 21 2019 - 12Z Mon Jul 22 2019

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
CENTRAL AND SOUTHERN ROCKIES AS WELL AS THE CENTRAL PLAINS INTO
THE OHIO AND TENNESSEE VALLEYS...

...Southern Rockies...
As a front continues to dive south through the Central Plains and
Mid-MS Valley, a trailing boundary along the Front Range will act
as a catalyst for convection during the afternoon into the
overnight hours Sunday into Monday.  Mid-level high pressure will
build overhead with impulses rounding this feature which too will
promote vertical motion. At the surface, as the Canadian high
builds south into the Northern Plains, easterly flow will enhance
upslope during the afternoon. Moisture that pooled south of the
aforementioned front will bank against the high terrain with
instability of over 2000 J/kg available during the
afternoon/evening hours.  These factors should promote convective
initiation along the windward slopes of the Rockies in CO and NM.
Given fairly weak flow, convective cells should have fairly slow
storm motion along the slopes with more erratic propagation as
they grow upscale.  Regardless, expect heavy rainfall is given
decent moisture transport across this region.  Hourly rain rates
may approach 1 inch/hour which is just shy of the one hour FFG
(1.5 inches) across this region.  Again, given the uncertainty
with respect to the evolution and storm motion, feel there could
be some locations that see as much as 2-3+ inches when all is said
and done.  Thus a Marginal Risk was introduced across this region.
 If QPF amounts come up, a Slight Risk may be justified.

...Central Plains, OH/TN Valley...
2000 UTC Update -- Based on the latest model trends, removed the
Marginal Risk area north of ~42N, as the expectation is that the
surface front and best pre-frontal instability and moisture
transport will have pushed to the south.

Hurley

Previous Discussion...

A front will continue to slowly move south and east Sunday into
early Monday. While all model guidance illustrates this feature,
the overall orientation and timing of the front varies.  The GFS
and the ECMWF phase the mid-level energy round the closed low in
Ontario differently which plays a large role in the surface front
and expected organized convection.  Regardless, the focus for
shower/thunderstorm activity will be along this feature.  Moisture
and instability will pool right along and south of the boundary,
with precipitable water as high as 2 inches and MUCAPE values over
3500 J/kg. Since there is bit of uncertainty with respect to the
mid/upper level pattern, it is hard to discern if organized
convection will actually occur.  The 00Z GFS has a stronger signal
for mid-level vorticity and upper level divergence across the
Central/Southern Plains into the Mid-MS Valley as compared to 00Z
ECMWF or even the 00Z NAM which is reflected in the QPF output.
The GFS is by far higher in its precipitation amounts given its
overall synoptic forcing resultant MCS.  While there is little
model consensus for this and thus no strong signal to suggest such
organized convection along the frontal boundary, it is still taken
into consideration given many of the ingredients are available.
We will have to monitor the evolution of the upper trough axis and
mid-level energy which would most certainly help to initiate
organized convection.  At this time, forecasting weak mid-level
impulse atop a surface boundary which should promote convection
along the front from the Central Plains across the OH/TN Valley
and into the northern tier of New England.

Areal average QPF generally ranges between 0.5-1.5 inches with
locally higher amounts associated with convection.  The most
sensitivity areas will likely be across portions of the OH/TN
Valley into the central Appalachians where FFG values are as low
as 1 inch in 3 hours. A Slight Risk was considered across this
region, though confidence in amounts and coverage are too low at
this point.  Also, with respect to the potential MCS across the
Central/Southern Plains, the soils are not as sensitive per FFG
and the NWM Soil Moisture. Therefore, felt a Marginal Risk would
suffice for this forecast package with the understanding that a
Slight Risk will likely be hoisted if models come into better
agreement about organized convection and thus higher QPF amounts.

Pagano



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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