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FOUS30 KWBC 181957
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
356 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

The Day 3 outlook will be updated by 2030Z.


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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