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FOUS30 KWBC 181600
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM 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 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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