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afdgrb

Area forecast discussion
National Weather Service Green Bay WI
1147 PM CDT Fri Jul 19 2019

Updated aviation portion for 06z taf issuance

Short term...tonight and Saturday
issued at 352 PM CDT Fri Jul 19 2019

Main focus is on potential for significant severe weather this
evening.

Latest surface observations as of 20 UTC indicated surface warm
front making slow progress northward ahead of developing surface
wave currently over southwest Minnesota. Warm front stretched from
near Sheboygan to near stevene point to LaCrosse. Warm front has
been a bit slower moving northward than previously forecast.
Meanwhiile, upstream convection has rapidly developed near and
just northeast of surface low pressure system. Discrete SC
structures and small line segments already taking shape southwest
of dlh to near Alexandria Minnesota. Convection already starting
to enter far northwest Wisconsin northwest of Hayward. Appears
active convection will start to impact the far northwest portion
of the forercast area by 22-23 UTC which is a little sooner than
previous cam forecasts. Cams,particularly the hrrr, have been
very consistent in convection growing upscale to impressive qlcs
and tracking it across the northcentral portion of the forecast
area. However latest few hrrr runs have been a bit further south
with the deep convection which seems to fit the latest radar and
satellite observations. Deep layer shear over the northern half of
the forecast area is impressive especially given the degree of
instablity currently situated over the southern half of Minnesota
and over the southwest half of Wisconsin. MLCAPES are well over
4000 j/kg in this area and the instablity continues to move slowly
northward ahead of the approaching low pressure system. The
combination of 0-6 km shear greater than 40-45 kts and instablity
certainly favors initially discrete SC storms over central
Minnesota into northwest Wisconsin capable of very large hail,
significant damaging winds and perhaps a few tornadoes. There is
some uncertainty as to how quickly the convection can grow upscale
into large bowing line segment before impacting northcentral
Wisconsin. Another complicating factor is that the airmass over
far northern Wisconsin is still not as unstable as surface tds are
still only in the mid-upper 50s. Thus, the greatest severe
weather threat may end up shifting a little further south. Given
the amount of 0-3 km shear, possibity of embedded weak tornadoes
within organized qlcs is also possible especially over northcentral
part of the forecast area.

St forecast guidance has been pretty consistent on moving
convective activity east of the area around midnight.

Saturday, still somewhat uncertain for severe potential. Surface
boundary will likely extend somewhere over the central portion of
the forecast area. Models suggest another weak impulse make
interact with that boundary during the afterenoon hours which may
kick off additional convection. At this time, a slight risk for
the southern portion still seems warranted but will need to see
how overnight convection plays out as atmosphere may be pretty
worked over. Certainly will be noticably cooler for Saturday with
clouds and northerly winds especially north of i29.

Long term...Saturday night through Friday
issued at 352 PM CDT Fri Jul 19 2019

The main highlights from this forecast period are the precip
trends for Saturday night into Sunday and the upper-level pattern
change that will bring drier conditions and near seasonal
temperatures to the area.

Saturday night into Sunday...a nearly stationary frontal boundary
will propagate south over Wisconsin on Saturday. By Saturday
evening, a surface high pressure system will be moving into the
upper Mississippi Valley helping to push the slow frontal boundary
south out of the forecast area. Model guidance indicates the main
axis of showers and storms will ride along this front, with less
significant precipitation trailing behind it. The possibility for
storms over the forecast area late Saturday evening and night
will be on the decrease, as instability and lapse rates decrease
across the forecast area as the front moves south. It is possible
for some of the trailing precipitation to stick around across
central and east-central Wisconsin into Sunday morning.

Rest of the extended...the upper-level pattern will begin to
change on Sunday evening over the upper Mississippi Valley to a
northwest mean flow. At the same time, a strong ridge of high
pressure over the northern plains will continue to build. The
northwest flow aloft and a surface high pressure moving into the
northern plains from Canada will keep the forecast area dry
through the rest of the extended. There are some hints of a
shortwave impacting the area on Wednesday that could bring the
next chance for showers and storms, however, much uncertainty
remains with this feature. Will continue to monitor.
&&

Aviation...for 06z taf issuance
issued at 1147 PM CDT Fri Jul 19 2019

Shower and thunderstorm activity will continue to wind down early
this morning in the wake of an mesoscale convective system. Relatively benign conditions
are expected this morning. Showers and thunderstorms are again
possible on Saturday, mainly across central and east-central
Wisconsin during the afternoon hours. Hi-res models are in better
agreement on developing this activity during the afternoon hours,
generally in the 18-22z time frame. Therefore will place a tempo
group in the southern taf sites during this time frame as a first
guess when convection is most likely. The convection could reduce
conditions to MVFR. Although there is a marginal risk for severe
weather, the best chance looks to be to the south.

Kosh...variable winds will continue for a few more hours behind
the mesoscale convective system with lingering thunderstorms possible. Winds should then
turn southwesterly, then westerly around 11z, then northerly
around 18z. Most favorable timing for thunderstorm activity
Saturday afternoon is between 17-23z.
&&

Grb watches/warnings/advisories...
none.
&&

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