000
FXUS63 KGRB 220235
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
935 PM CDT Wed Jun 21 2017

Updated aviation portion for 06Z TAF issuance
and new information added to update section

.UPDATE...
Issued at 921 PM CDT Wed Jun 21 2017

Convection has been slow to develop across the area this evening,
likely due to stable air lingering across the area. Local meso
plots indicate destabilization has occurred over about the
southwest quarter of the state. That will expand northeast during
the night, possibly quite rapidly as the LLJ increases. Storms
over northern Minnesota have weakened as they shifted east of the
main axis of instability. Increasing isentropic lift near the nose
of the LLJ still looks favorable for thunderstorms across the
area, but have delayed their onset and the highest PoPs until
later in the night. The severe risk is also somewhat uncertain.
Storms to the west have shown no sign of upscale growth. Right now
the most likely path to getting severe weather tonight would be
for a few individual storms to get strong enough to generate
severe hail after midnight.

Grids/ZFP with updated PoPs/Wx were just sent, will tweak/update
the wording of the HWO and have that out shortly.
&&

.SHORT TERM...Tonight and Thursday
Issued at 239 PM CDT Wed Jun 21 2017

The main concern for tonight and Thursday is timing of
thunderstorm activity along with the potential for severe
weather. 19z surface analysis indicated a surface warm front
draped from western Minnesota to south-central Iowa. Temperatures
on the north side of the front were in the 70s across much of our
area along with dewpoints in the middle 40s to middle 50s. To the
south of the warm front, temperatures were in the middle 80s to
lower 90s with dewpoints in the 60s.

For tonight, models depict low level jet increasing to 50 to
55 knots as the warm front lifts northward. Elevated convection
is expected to develop across central and east-central Wisconsin
during the mid to late evening hours. Some differences by a few
hours noted between the HRRR and the NAM Nest Models. The latest
RAP 13 splits the difference in timing. Models also indicated
strong 0-6 km shear of 50 to 60 knots, mid level lapse rates
around 6 c/km to support rapid development of storms. Current
thinking there would be some large hail at the onset, however
wetbulb zero heights on the bufkit soundings were near twelve
thousand feet that may lower the number of severe hail reports
over an inch. Once the storms form, damaging wind gusts are
possible as the storms organize. Most of the activity should be
out of the forecast area shortly after sunrise. Low clouds will
prevail tomorrow morning which should limit any convective
activity. Torrential rainfall can be expected with some of the
stronger storms which could lead to urban street flooding and
ponding of water on area roadways.

Did keep a small chance in the morning then increase chances
again Thursday afternoon. Some of the meso models depicts a
northern band of showers and storms heading for northern
Wisconsin while another band of showers and storms develop across
our southern forecast area and then merge late Thursday
afternoon. Some of the storms could become severe with damaging
winds and large hail, especially south of a Marshfield to Green
Bay to Algoma line. High temperatures tricky depending on how
much sunshine we get in the late morning and afternoon.

.LONG TERM...Thursday Night Through Wednesday
Issued at 239 PM CDT Wed Jun 21 2017

Focus in the long term will be ongoing convection Thursday night
and on and off chances for more showers and thunderstorms through
the forecast period.

Thunderstorms (some potentially severe) are likely to be ongoing
at 00Z Friday, especially south of a line from Marshfield to
Green Bay closer to the location of the surface cold front.
Thunderstorm chances will end from NW to SE overnight, with models
showing the front and instability moving south of the forecast
area by 12Z Friday.

The rest of Friday looks mainly quiet. Models still show a weak
shortwave to potentially produce a few showers in northern
Wisconsin during the afternoon, with perhaps isolated thunder at
best with MLCAPE and MUCAPE not more than 200 J/kg.

GFS, ECMWF, and Canadian all depict an upper level trough over
Ontario and the Great Lakes dominating the pattern across
Wisconsin from Saturday through early next week. This flow will
continue to bring mid- level shortwaves across the area and result
in scattered shower and thunderstorm chances. Models are in less
agreement on the exact timing and location of these shortwaves.
The best chances seem to be in the Saturday afternoon/Saturday
night and Monday afternoon/Monday night time periods. Stuck with a
blended model solution which features chance and slight chance
PoPs that coincide well with when the best energy moves though.

On Tuesday the upper level trough moves east as an upper level
ridge builds over the Upper Midwest. This is accompanied by high
pressure at the surface and the best-looking day to see dry
weather at the moment. Models continue to diverge on timing and
track of a surface frontal system to approach the area around
midweek. Continued with the blended model solution for now which
brings in higher PoPs Wednesday.

Temperatures will be running below seasonal normals through most
of the long term period, especially across northern Wisconsin
where highs will be about 10 degrees below normal Saturday and
Sunday. Temps moderate some for the start of next week.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 921 PM CDT Wed Jun 21 2017

The main aviation forecast concern continues to be the
thunderstorm potential overnight and Thursday. It still looks like
2 rounds of storms are likely, but the onset of the first round
has been delayed a bit by lingering stable air across the area.
Will continue to try and indicate the most likely time for storms
in the TAFs, though it certainly is possible those may need to be
adjusted as the storm development unfolds.
&&

.HYDROLOGY...
Issued at 239 PM CDT Wed Jun 21 2017

Another round of heavy rain expected tonight into Thursday night.
Average rainfall totals of 1 to 2 inches are possible south of a
Merrill to Sturgeon Bay line. Locally higher amounts are possible
if thunderstorms train over the same area.
&&

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

$$

UPDATE.........Skowronski
SHORT TERM.....Eckberg
LONG TERM......KLB
AVIATION.......Skowronski
HYDROLOGY......Eckberg