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FOUS30 KWBC 200818
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sat Jul 20 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 Sun Jul 21 2019 - 12Z Mon Jul 22 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
PLAINS EAST INTO THE OHIO VALLEY...

...Central Plains, Mid-MS Valley, OH Valley...
A front will continue to slowly move into the Central Plains,
Mid-MS Valley into the OH Valley Sunday into early Monday. Models
are coming into slightly better agreement on the orientation and
timing of the front, especially across the OH Valley.  The GFS and
now CMC are showing signs, however, of more organized convection
as mid-level energy moves atop the aforementioned front.  The rest
of the model guidance remains weaker with the impulses and the
position of the jet axis a bit farther removed to the north and
east.  Thus the GFS and CMC have higher QPF amounts across eastern
KS into northern MO as an organized complex moves from SE NE into
MO Sunday evening/night. Given the GFS has shown run to run
continuity, the CMC is now showing similar features and GEFS
probabilities support the higher QPF amounts, felt it should be
incorporated into the model blend.

With respect to the ingredients in place, moisture and instability
will continues to pool right along and south of the boundary, with
precipitable water around 2 inches and MUCAPE values over 3500
J/kg. As previously mentioned, there continues to be uncertainty
with respect to the interaction between the mid-level energy and
the surface features, thus resulting in questionable organized
convection in the central CONUS.  At this time, forecasting weak
to modest mid-level impulse atop this surface boundary which
should promote convection along the front from the Central
Plains/Mid-MS Valley across the OH/TN Valley.

Areal average QPF generally ranges between 1-2+ inches with
locally higher amounts associated with convection. The two main
areas of concern are across east KS/northern MO with the potential
for organized convection and portions of the OH Valley where FFG
are lower. Rain rates could climb over 1.5 inches/hour. The
propagation of the convection along the front should limit the
flash flood potential as it sinks south. However, if a complex
forms as depicted by the GFS, some training could occur. There is
enough signal to support isolated to potentially more widespread
flash flooding concerns, therefore a Slight Risk was introduced to
encompass these regions.

...Southern Rockies...
As a front continues to dive south through the Northern Plains,
settling into the Central Plains, Mid-MS Valley and OH 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 flow during
the afternoon. Moisture that pooled south of the aforementioned
front will bank up 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 2+ inches when all is said and done.

Models have trended down with QPF for the 00Z forecast suite and
thus have trimmed amounts slightly.  Though ensemble
probabilities, albeit somewhat low, still highlight this region as
exceeding 2 inches.  Models tend to do poorly within this flow
regime/pattern and thus will maintain the Marginal Risk across
this area with some refinements made based on WPC QPF.

Pagano

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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