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FOUS30 KWBC 181958
QPFERD

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

Day 1
Valid 16Z Thu Jul 18 2019 - 12Z Fri Jul 19 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN MID-ATLANTIC AND THE UPPER MISSISSIPPI VALLEY INTO
THE GREAT LAKES...

...16Z update...

Changes to the previous outlook include adding a Slight Risk from
northern MD into southern CT. Moisture tied to the remnants of
T.D. Barry was reflected in the morning precipitable water
analysis with values ranging between 2.1 and 2.4 inches. A 700 mb
trough axis was located from near ALB to IAD to just west of RNK
in 12Z RAOB data. Visible imagery showed clear to scattered cloud
cover ahead of a mesoscale circulation over northern MD at 15Z
with showers already developing near and ahead of it. Thicker
cloud cover was in place farther north. However, broad scale lift
will be in place over the northern Mid-Atlantic to far southern
New England until about 00Z which may allow for breaks in cloud
cover and increased instability to develop across northern
locations of the Slight Risk later this afternoon. Given wet
antecedent conditions skies and a signal from the 12Z hi-res
models for 1-3", locally higher, a Slight Risk was added for the
16Z update.

Changes elsewhere across the U.S. were minor, based on the 12Z
guidance.

Otto

...previous discussion follows...

...Upper Mississippi Valley into the Great Lakes...
The early Thursday morning convection across portions of the Upper
Mississippi Valley will likely undergo post 1200 utc weakening as
it pushes east across southern Wisconsin/northern Illinois.  This
will be followed by another round of potentially organized
convection beginning late Thursday afternoon and continuing into
Thursday night/early hours of Friday in the vicinity of the west
to east oriented frontal boundary expected to stretch across
northern Iowa/southern Minnesota into southern Wisconsin.
Instability will increase significantly Thursday afternoon in the
vicinity of this frontal boundary...with a large region of mu-cape
values 3000 j/kg+ as per the arw...nssl wrf and nam and li values
-8 to -12.  Moist west southwesterly low level flow...With pw
values 2 to 2.5+ standard deviations above the mean....may support
a period of training of cells in a west to east direction along
and to the north of this front.  There continues to be some
latitude differences with the axis of the heaviest precip this
period.  The Ukmet...Canadian global and regional are farther
north...while the hi res arw...nmmb and nam nest farther to the
south along with the href mean.  The 0000 UTC EC did trend farther
to the south with its heavy totals over the L.P. of MI and is
closer to the farther south hi res models and href mean.  At the
moment...the farther south axis was preferred.  Changes to the
previous outlook for this period were to orient the marginal and
slight risk areas farther to the south and west into far southern
MN into southern WI and the southern portion of the L.P. of MI.

...Mid to Upper TN Valley...
A marginal risk area was added from far northeast MS...northern AL
into central to eastern TN for locally heavy rainfall Thursday
afternoon.  Neighborhood probabilities from the arw...nmmb and nam
nest show probabilities of 10 to 40% of exceeding 1 and 3 hour ffg
values across these areas.  In addition href probabilities of 1"+
amounts in the 6 hour period ending 0000 utc Fri July 19 are
generally in the 40%+ range for 1 inch of precipitation.  Not a
lot of confidence with details of convection...but with ffg values
relatively low across this area from recent heavy
rains...additional isolated runoff issues possible.

...Northern Mid Atlantic...
Additional convection possible across portions of the Mid Atlantic
from far northern VA...across much of  MD into eastern PA and NJ.
HREF probabilities for the 6 hour period ending 0000 utc for 1"
precip are high across this area...mostly 50-90%...with a smaller
area of 2"+ amounts in the 50%+ range from north central
MD...southeast PA into central to southern NJ.  Similar
geographical distributions of neighborhood probabilities exceeding
1 and 3 hour ffg values seen in the arw...nmmb and nam nest.
Locally heavy rains possible again in the urban corridor
stretching from DC to PHL and NYC...with isolated runoff issues
possible across these regions.

Oravec




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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