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FOUS30 KWBC 180754
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Thu Jul 18 2019

Day 1
Valid 01Z Thu Jul 18 2019 - 12Z Thu Jul 18 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY, THE NORTHERN MID ATLANTIC INTO
SOUTHERN NEW ENGLAND, AND THE MID SOUTH...

...Upper Ohio Valley into the northern Mid Atlantic and southern
New England...
No significant changes were made to the previous ERO across this
region; the primary change was to start trimming the northern
extent of the Slight Risk area given the decrease in areal
coverage and rainfall amounts shown in the hi-res models for the
overnight hours.  The southern extent of the Slight Risk area
still looks to be in shape as storms in the area have abundant
moisture to work with in an unstable environment.  00Z soundings
showed Precipitable Water values were in excess of 2 inches from
Pennsylvania into southern New England...which was on par with the
numerical guidance.  Additional pockets of heavy rainfall will be
possible further south and southwest, from Ohio and West Virginia
into Pennsylvania and New Jersey will still be within a moisture
plume overnight. Hourly rain rates may exceed 2 inches in the
strongest convective rain bands and thunderstorms.

...Mid South to Lower Ohio Valley...
A Slight Risk that was initially focused northern Alabama earlier
in the day has been expanded once again to cover convection
dropping southeast over Illinois and eastern Missouri.  On the
current trajectory, the will bring that convection into the parts
of western Tennessee and northern Alabama/Mississippi overnight.
Any convection would be likely to be slow-moving with relatively
weak mid-upper level winds and an upper-level anticyclone centered
near MO. The slow movement of thunderstorms embedded within an arc
of deeper moisture (PW around 2 inches) could lead to localized
flash flooding particularly given recent rainfall, greater soil
moisture across the region and rainfall rates at or above 1.5
inches per hour.

...Upper Mississippi Valley...
Opted to remove the Slight Risk area from Iowa now that the
complex which covered the state has shifted south and east.
However, with the hi-res guidance still suggesting that there
could be additional rainfall overnight, was reluctant to remove
the Marginal Risk area from Iowa northward across Minnesota.

...Georgia to Florida...
Locally heavy rainfall will linger this evening into the early
overnight hours across parts of the Southeast U.S. in a convergent
area along a shear axis. Precipitable Water values in the area
ranged from 2.0 to 2.2 inches according the 00Z soundings.  Given
very light winds, cells that produce very heavy rainfall may be
moving so slowly that isolated flash flooding and localized
problems due to ponding of water.


...Southeast AZ...
No changes across southeast Arizona.  The threat appears pretty
localized, but should have at least scattered convective coverage
this afternoon, with instability probably getting just high enough
to support rainfall rates strong enough to produce a localized
flash flood risk across any more sensitive areas.

Bann



Day 2
Valid 12Z Fri Jul 19 2019 - 12Z Sat Jul 20 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
NORTHERN PLAINS INTO THE CENTRAL GREAT LAKES...


...Northern Plains into the Upper Midwest...
Exiting shortwave will be well east into south-central Canada at
the start of the forecast period with an unseasonably strong 250mb
130kt jet to the northwest of the frontal zone allowing for deep
unidirectional flow in the vicinity of the boundary.  Low level
moisture flux throughout the day will support steady increase of
total PWats along the boundary, but given the strong ridge and
deep EML with 30C 85H temps there providing strong capping.
Strong insolation and lapse rates with building low level moisture
will support extreme instability values over 5000 J/kg, but will
need significant upper-level support or deep moisture convergence
to break the cap.  Toward over night, LLJ is expected to increase
to 30-35kts and with weak late evening cooling should spark an
overnight MCS that will progress across the frontal zone into MN/N
WI.  Still, the later start, weaker angle of convergence from the
LLJ may limit overall coverage with time.  Additionally, given
proximity to the core of the mid-level speed max, convection is
likely to be fast moving limiting overall duration of heavy
rainfall cores in one given location.  Still, given the strength
of updrafts and depth of available moisture over 1.75" TPWs,
increasing through the late period, the sheer rain-rates may be
impressive enough to yield short-duration exceedance of FFG
values.  Additionally, given orientation of LLJ, additional
upstream redevelopment that has the chance for repeating suggests
isolated areas that may even exceed 3hr FFG values that may have
been reduced from Day 1 period.

While model guidance has seen a better overall agreement in the
placement of the frontal boundary, and therefore corridor of heavy
rainfall, areal averages of 1-2" and isolated 2-3" remain spotted
across a corridor from central SD into N WI/UP of MI through the
remainder of the overnight period into day 3, and the WPC QPF
forecast is generally a blend of the global guidance supporting a
broad Marginal Risk of Excessive Rainfall across the stationary
frontal zone.


...Central Great Lakes
Hi-Res CAMs driven by the ARW and NAM-NEST support lingering
ongoing convection at the end of day 1 to support outflow as the
some shortwave energy anticylconically rounds the NE side of the
ridge to drop south across the LP of MI into NW OH/NE PA/W NY.
Quick rebuild of instability with early morning insolation will
support enhanced redevelopment by midday that will progress
southeast with sufficient rainfall rates capable of pressing 1
hourly  FFG values (1-1.25"...as such have expanded the Marginal
Risk to incorporate the 1-3" totals and isolated flash flooding
possible in this area.

Gallina


Day 3
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...

...Midwest into Michigan...
At the start of the forecast period (20.12z), ongoing isentropic
ascent across the stationary boundary will be growing upscale to a
solid MCS with and MCV crossing the North Central Great Lakes.
Unseasonably strong 250mb jet will shifting northeast centered
into Ontario, this will provide solid left entrance region
dynamics across the Upper Midwest and support height-falls and
tail end of shortwave energy to amplify and press eastward across
the Dakotas by 21.00z.  This will enhance strong post-frontal
northerly flow across the Northern Plains and Upper Mississippi
River Valley, sharpening the frontal zone both north-south but
also vertically.  Deep moisture will remain pooled along the
lagging frontal zone across SE SD, but with pressing height-falls
will quickly back the LLJ more southerly, weakly at 10-20kts near
the inflection surface to 850mb trof extending out of the Colorado
High Plains.  Moisture/instability will build at this
intersection/SE into warm sector reaching 3000-3500 J/kg by 21z.
By mid-day winds will increase and given nearly 180 degrees of
convergence across a steepened boundary, should spark strong/deep
convection.  Given exiting shortwave/jet energy, the resulting
convective complex will be slower moving eastward along the
boundary before the frontal zone as a whole drives south.  Very
strong updrafts, deep moisture should result in strong rain rates
and given slightly veered 850mb flow, supporting some weak
backbuilding upstream, cross-track/short-term training should
exist across SE NEB into S IA and result in areal averages of 2-4"
with hourly rates of 2-2.5"/hr yielding a solid potential for
scattered flash flooding to result.  This is, generally, in line
with the 00z GFS/NAM and to account for some variation in timing,
a slightly northern ECMWF blend was implemented for WPC QPF.  All
in all, a broad Slight Risk of Excessive Rainfall was placed from
the surface to 850mb inflection point eastward across IA and
downstream into S WI and NW IL.

A Marginal Risk extends into Central MI, where sandy soil
conditions and slightly reduced instability/moisture flux
intersecting with the frontal boundary should limit overall
coverage for flash flooding, but still remains a non-zero threat
given the environment.


...Front Range of Colorado/High Plains of NW Kansas...
Global guidance, in particular the GFS continues to suggest as the
front drops south across the High Plains, while reduced to further
east, remaining moisture banked up along the Range, day-time
heating supporting upslope flow has the potential for scattered
heavy rainfall producing thunderstorms, with some suggestion of
initially slow development moving off terrain with chaotic cell
motions and intersections of outflow boundaries that may pose
short-term stalled downdrafts capable of isolated rainfall totals
of 2-3".   The 00z ECWMF trended this direction as well but with a
more consolidated complex (likely due to slower timing and
allowance for stronger/broader overall development than the
earlier developing GFS/NAM).  While confidence is not particularly
high in one given location, the environment is supportive of this
flooding threat and so a Marginal Risk was extended west to
incorporate this threat, particularly along the Front Range of CO
and SE WY.

Gallina












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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