Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 181644
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1144 AM CDT Tue Jul 18 2017

Updated aviation portion for 18Z TAF issuance

.UPDATE...
Issued at 630 AM CDT Tue Jul 18 2017

Backed off on pops over north central WI for the morning hours,
though convection should gradually increase there later in the
morning as the front approaches. MCV over southern MN and another
associated with the MCS in the Dakotas extrapolate into the
western part of the forecast area in the late afternoon (21z-22z),
so that is when convection should really ramp up.
&&

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 327 AM CDT Tue Jul 18 2017

The main forecast concern is severe thunderstorm and heavy
rainfall potential this afternoon and evening.

The remnants of Monday evening convection dissipated as it reach
north central WI early this morning, despite the presence of
some elevated instability (MUCAPE of 500-1000 j/kg). After a
general lull in precipitation, expect thunderstorms to redevelop
as a cold front (currently extending from the MN Arrowhead swwd
to low pressure in central SD) moves north-central WI during the
early to mid-afternoon. The highest pops will occur during the
late afternoon and evening, as a weak short-wave trof and the
RRQ of an upper jet move through the region, providing dynamic
support. Models show CAPE of 1500-2000 j/kg and deep layer shear
of 25-35 kts ahead of the cold front, which supports the potential
for severe storms. Damaging winds and large hail will be the main
threats. PWATs will approach two inches near the frontal boundary,
so torrential rain will also be possible, with local 1-2 inch
amounts this afternoon and evening. It will be warmer and much
more humid, with highs in the 80s. The shower and thunderstorm
activity will taper off from north to south overnight as the front
moves through. The arrival of drier air and partial clearing
should allow overnight lows to drop into the 50s north, but lower
to middle 60s should prevail south and east. Subsequent shifts
may need to consider adding some fog to the forecast for the
overnight period.

On Wednesday, any lingering showers will end across the south
early in the day as high pressure arrives. Abundant sunshine will
boost temperatures into the upper 70s and lower 80s at most
locations.

.LONG TERM...Wednesday Night Through Monday
Issued at 327 AM CDT Tue Jul 18 2017

A nearly zonal flow will continue to run along the northern tier
of states through the upcoming weekend, while a broad upper high
dominates most of the southern CONUS. Any cold fronts that hit
this upper high will stall and where exactly this stalling process
occurs will determine where the heavy rain would fall, along with
a severe potential. The initial cold front is progged to briefly
stall over central WI Thursday, get shoved south into northern IL
Friday and attempt to return north on Saturday. The upper high is
forecast to weaken early next week and the models show an upper
trough (of various intensity) develop over the northeast quadrant
of the CONUS. This should allow for frontal boundaries to move and
we finally get a dry period in the forecast by Monday. Warm/humid
conditions to be the rule through Sunday with several
opportunities for showers/thunderstorms.

Models continue to indicate that a MCS will develop over the
Dakotas and track southeast along an instability gradient toward
southern WI Wednesday night. Of course, exactly where this complex
develops will determine whether any part of northeast WI could get
hit by these strong to severe thunderstorms. Main concerns would
be damaging winds and heavy rainfall with 0-6km shear in the 40-60
knot range and PW values hovering around 2.0 inches over central
WI. This situation will need to be watched closely over the next
couple of model runs. Min temperatures to range from the lower 60s
north, middle to upper 60s south.

We could still be dealing with leftover showers/few storms into
Thursday morning, however the main problems for Thursday to be the
location of the quasi-stationary front and how long will it take
for the atmosphere to recover from Wednesday night`s convection.
One would have to assume the front will get pushed south toward
the IL border, but this is not guaranteed. Northern WI would have
the best chance of staying dry through the day, while central/
east-central WI are close enough to the front such that additional
showers/thunderstorms could re-fire in the afternoon. Prefer to go
this route for now, although confidence on the boundary location
is low at the moment. Warm and muggy conditions to persist
(especially south) with max temperatures in the lower 80s north-
central/near Lake MI, mainly middle 80s elsewhere.

Another thunderstorm complex is forecast by the models to develop
over eastern SD or southwest MN Thursday night and again track
southeast toward southern WI/northern IL along the stalled front.
Once again, central/east-central WI will be vulnerable if this
complex shifts north just a little. Northern WI would appear to
escape this latest round of convection with only partly cloudy
skies expected. Min temperatures to be in the upper 50s far north,
middle to upper 60s south. Models then show a mid-level shortwave
moving across the western Great Lakes region on Friday and there
are signs that the stalled boundary may lift north a bit as well.
If both of these factors do take place, this would be more than
enough to allow showers/thunderstorms to develop in this unstable
air mass. Will continue to place higher pops over central/east-
central WI closer to the frontal location. Too early yet to
determine severe potential, although instability parameters do not
look overwhelming at this time. Max temperatures for Friday to
mainly be in the upper 70s to lower 80s.

This latest bout of convection `should` again push the boundary
back south into southern WI Friday night and focus the bulk of any
further precipitation to that part of the state. As has been the
case all week, will need to carry a chance pop for central/east-
central WI, while northern WI dries out overnight. The development
of a more prominent upper trough over the Upper MS Valley on
Saturday is forecast to drive a cold front into WI and provide
another chance of showers/thunderstorms, especially during the
afternoon hours when the front arrives. Max temperatures on
Saturday will be in the middle 70s near Lake MI, upper 70s north
and around 80 degrees south.

Models disagree with the strength of this upper trough as it
swings through the Great Lakes region Saturday night/Sunday. This
would not only impact precipitation chances, but also max
temperatures on Sunday. The ECMWF is the strongest with this
trough with some agreement by the CMC (not as strong). The GFS had
been the weakest, but has now strengthened the trough (closer to
the CMC). Have therefore hedged the forecast toward a stronger
upper trough, with a higher chance of precipitation on Sunday with
cooler temperatures. Once this system clears the area Sunday
night, the mean flow to have amplified with an upper ridge over
the Rockies/Plains and the upper trough over the eastern CONUS.
The end result will be a northwest flow aloft into WI on Monday
with high pressure building across western/northern sections of
the Great Lakes. Look for humidity levels to drop and max
temperatures to be a couple of degrees below normal.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1138 AM CDT Tue Jul 18 2017

Scattered showers and storms are moving into the central and
north-central WI taf sites, ahead of a upper level disturbance. As
the airmass destabilizes this afternoon, showers and thunderstorms
are expected to become more widespread. Isolated to scattered
severe storms are possible for damaging winds and large hail. Did
speed up the timing of the storms, with the upper disturbance
moving through quicker than the trailing cold front. Storms should
be exiting eastern WI around midnight. Remained optimistic in
regards to vsbys from fog late tonight. Good flying weather
resumes for much of Wednesday.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE.........Kieckbusch
SHORT TERM.....Kieckbusch
LONG TERM......Kallas
AVIATION.......MPC



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