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FOUS30 KWBC 190055
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
854 PM EDT Thu Jul 18 2019

Day 1
Valid 01Z Fri Jul 19 2019 - 12Z Fri Jul 19 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MIDWEST INTO THE GREAT LAKES AND PARTS OF THE CENTRAL
AND SOUTHERN MID ATLANTIC REGION

...Upper Mississippi Valley into the Great Lakes...
Expanded the Slight Risk area over parts of southern Minnesota
where convection was building in a region of deep moisture and
strong instability.  The area was north of a warm front draped
along the Minnesota/Iowa border...and it separated an airmass with
dewpoints in the mid 70s to lower 80s over parts of northern Iowa
from an airmass with dewpoints in the lower 70s over the southern
part of Minnesota.  Instability and deep moisture (as shown by PW
forecasts in excess of 2 inches immediately along and north of the
front) will persist overnight and is expected to spread into
southern Wisconsin and may even reach western lower Michigan by
early morning.

...Mid-Atlantic Region...
Adjusted to the Marginal Risk area along the Mid-Atlantic region
where convection was waning...and moved the focus into the
southeast part of Virginia and northern North Carolina where 00Z
soundings still showed CAPE values of 2500 to 4000 J/kg and
Precipitable Water values were at or above 2.1 inches across the
region.  Am expecting the convection to weaken an dissipate by
late this evening.  Even so, brief period of intense rainfall may
still lead to localized problems from flooding or ponding of water
in areas of poor drainage.

Bann





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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EAST CENTRAL
MINNESOTA INTO WEST CENTRAL WISCONSIN...

...Northern Plains into the Upper Midwest...
The main change for the Day 2 Excessive Rainfall Outlook was the
addition of a Slight Risk area over portions of east central MN
into west central WI. The focus for the change is the increasing
model signal for an MCS to develop on the nose of a strengthening
low level jet along a frontal boundary extending from the Northern
Plains into the Upper Great Lakes.

There is an increasing signal for an MCS to form on the nose of a
30/40 knot low level jet along and just ahead of weak short wave
energy in the fast mid level flow across the Northern Plains. The
system is expected to develop on the northern edge of a 3500/4500
J/KG MUCAPE axis, just north of where model soundings showed the
presence of a fairly strong mid level cap. The MCS is expected to
follow the instability axis as it contracts during the evening and
early overnight, eventually becoming more outflow driven before
exiting through southern WI by 20/12z.

The mid level flow becomes almost parallel to the mid level flow,
allowing the propagation vectors to become better along with it.
This should foster an environment where training occurs,
especially on the southern edge of the MCS. Given the depth of the
moisture in place (with 1.75 inch precipitable water air, which is
between two and three standard deviations above the mean), hourly
rainfall rates of 1.50+ inches are possible (supported by the 12z
NAM CONUS Nest and the 12z WRF ARW) over far east central MN into
far west central WI before 20/06z. Three hour flash flood guidance
are as low as 1.50 near KMSP, and after collaborating with WFOS
MPX/ARX, a Slight Risk was placed over this area to cover the
threat.

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 anticyclonically 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/Hayes

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...
20z update...Not much change was to the current Slight Risk area.
The changes that were made were based primarily on guidance
trends, which suggested that the best threat for flash flooding
will occur ahead of the frontal boundary extending from NE across
IA into southern WI. Based on this, the Slight Risk area was
expanded to include more of this area, and account for the
continued model spread concerning the placement of the highest
rainfall amounts.

Previous discussion...
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...
20z update...Not much change made to the Marginal Risk in place,
as the upslope flow in the wake of the frontal boundary feeds 1.00
inch precipitable water air across the High Plains into the front
Range of CO/WY. The deepening moisture, combined with MLCAPE
values between 1500/2500 J/KG, will support convection capable of
at least isolated flash flooding.

As it typical, the 12z GFS had much higher rainfall amounts in the
upslope areas of NM/CO/WY that the remainder of the 12z guidance.
It appears too aggressive with the upslope flow, especially over
portions of northeast CO. Seven day rainfall totals here have been
300+ percent of normal, resulting in 3 hour flash flood guidance
values as low as 1.50 inches. If there is a better upslope
signature from other models in later forecasts, particularly
across northeast CO, a Slight Risk could be needed here.

Previous discussion...
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/Hayes


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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