Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 170320
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1020 PM CDT TUE AUG 16 2016

Updated aviation portion for 06Z TAF issuance

.UPDATE...
Issued at 833 PM CDT Tue Aug 16 2016

Area radars shows clusters of showers and storms over
East Central Minnesota into West Central Wisconsin early this
evening as a short wave trough tracks through. Slow moving
convection were producing pockets of very heavy rainfall in this
region. HRRR slides this convection over the south half of the
state but still clipping areas mainly south of highway 54. Latest
radar trends suggests a more northern trend from the HRRR, but
some diurnal decline is noticed. Adjusted pops for latest
trends...increase over central WI and lower north.
&&

.SYNOPSIS...
Issued at 223 PM CDT Tue Aug 16 2016

Seasonably warm and rather humid for the rest of the work week.
Scattered showers and thunderstorms tonight and tomorrow, then
again late in the week and for at least the start of the upcoming
weekend. Turning cooler and less humid over the weekend.

Modest band of westerlies will remain across southern Canada and
the northern CONUS during the period. Amplification will begin
by late in the week as an upper trough strengthens over the
Rockies. The trough will continue to amplify as it slowly shifts
east, reaching the Ontario/Great Lakes region by early next week.

Temperatures will remain modestly above normal through Friday,
then settle back to near or a little below normal. There appear to
be sufficient opportunities for precipitation to result in near or
above normal amounts for the period. But the eastern part of the
forecast area has been dry recently, and missed out on significant
widespread rainfall despite favorable large-scale set-ups for
rain. This situation looks a little different as much stronger/
more organized synoptic scale forcing is anticipated, but still
want to be a little cautious about calling for heavy rainfall
until confidence in receiving it is very high.
&&

.SHORT TERM...Tonight and Wednesday
Issued at 223 PM CDT Tue Aug 16 2016

The latest RAP analysis and satellite/radar imagery show several
weak shortwave troughs moving across the western Great Lakes early
this afternoon.  One shortwave is moving east over Lake Superior,
while another is also heading east over southern Minnesota. Combined
with daytime heating, these shortwaves are sufficient to generate
isolated to scattered showers and storms across the region. The
latest HRRR and RAP suggest additional development over north-
central and northeast WI through the rest of the afternoon, greatest
coverage across the north. As these shortwaves continue to move
across the region, precip trends are the main forecast concern.

Tonight...Isolated or scattered showers and thunderstorms will
likely persist into the evening hours, based on recent trends over
the past few days.  Additional clusters of storms associated with
the shortwave over Minnesota will likely spread east over central
and east-central WI late in the evening and overnight.  No threat of
severe weather, due to low shear values aloft, but pwats upwards of
1.5 inches will support brief heavy downpours.  Where more
widespread rain falls, fog could develop late tonight.  Cloud cover
with the shortwave should limit how dense the fog could become
though.  Low temps mainly in the low to mid 60s.

Wednesday...The shortwave will linger over eastern WI during the
morning, which will continue the chance of showers and storms in the
forecast.  Once the shortwave pulls out by late morning, wonder if
there could be isolated showers that could pop with the heat of the
day.  Will continue to have low convective temps and ML capes
upwards of 1000 j/kg.  As a result, kept a low chance of showers and
storms through the remainder of the day.  It appears that northern
WI will have slightly higher chances than further south based on the
latest mesomodel guidance.  Little change of high temps mainly in
the 80-85 degree range.

.LONG TERM...Wednesday Night Through Tuesday
Issued at 223 PM CDT Tue Aug 16 2016

Although the bulk of the precipitation with the departing shortwave
should be over by 00z Thursday, suspect there will be at least
some lingering showers. As we`ve seen the past few days, the air
mass currently across the area is quite supportive of showers,
even at night. Since no real change in air mass is expected, see
no reason to expect anything different in the wake of this
shortwave. Adjusted forecast to linger low-end PoPs into Wednesday
night.

For the reason just mentioned, confidence in the dry Thursday we`ve
been forecasting has also waned. Will stick with the dry forecast
for now, but edge PoPs up a little to start to open the door to at
least isolated showers during the afternoon.

The main weather concern is still the precipitation/severe
weather potential late in the week into the upcoming weekend.
Amplifying upper trough will drive a seasonably strong cold front
across the region during that time. Models have been trending
toward digging an increasing amount of energy into the base of the
trough across the Rockies, which will slow the eastward advance of
the trough. That sets the stage for a frontal wave to develop and
then ride northeast along the front. Such a scenario would delay
the FROPA through the forecast area, and could result in
precipitation lingering well into the weekend. The standard
extended forecast grids have caught on to this scenario, so no
significant changes were necessary.

The severe potential is more muddled under this scenario, since
the initial convection will probably arrive while shear is still
weak. By the time stronger shear arrives, the area could be
shrouded in widespread clouds and the atmosphere may have been
overturned. SPC has handled the situation by shifting the 15
percent severe risk area to the southwest of the forecast area
Friday, and declining to depict a risk area for Saturday. Will
continue to mention the potential for strong storms late in the
week, but given the overall pattern, it really isn`t possible to
get much more specific than that this far in advance.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1020 PM CDT Tue Aug 16 2016

An upper level trough will move across the region overnight into
Wednesday morning then depart eastward. Scattered showers and
thunderstorms will be ongoing overnight especially over central
Wisconsin. Storms could redevelop late wednesday morning or early
afternoon over eastern Wisconsin as the upper level trough
departs to the east. Patchy mvfr fog possible overnight over
some locations due to rainfall and surface dewpoints in the 60s.
&&

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

$$

UPDATE.........TDH
SYNOPSIS.......Skowronski
SHORT TERM.....MPC
LONG TERM......Skowronski
AVIATION.......TDH



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