Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 170232

Area Forecast Discussion
National Weather Service Green Bay WI
932 PM CDT SUN OCT 16 2016

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

Issued at 912 PM CDT Sun Oct 16 2016

Local meso plots indicate that the air across the forecast area is
still quite stable. But unstable air with MUCAPEs around 1500
j/kg was advancing toward the area from the southwest, and should
overspread the area after midnight as the LLJ shifts into the area.
Shear will be considerable, so the strongest storms will likely
produce hail. But at this point the risk of getting severe (1
inch) hail looks much lower. The marginal risk on the SPC Day 1
Convective Outlook seems right on target. Given current radar
trends, will need to adjust the forecast to delay the arrival of
the precipitation a little. It still looks like the greatest
coverage will be across the north.

Fog could also become an issue overnight. Dense fog was being
reported over southern Lake Michigan and at some sites in southern
Wisconsin. That may tend to advance north as the warm front lifts
into the area overnight. But on the other hand, strengthening
pressure gradient and mixing with the arrival of precipitation
will work against dense fog. Lakeshore areas are probably at the
greatest risk of having dense fog due to the arrival of air with
higher dewpoints from the southeast, and that area probably having
fewer showers and storms overnight (compared to areas to the north
and west).

Convective initiation tomorrow afternoon is in question, mainly
due to the expected widespread cloud cover lingering from tonights
storms. If substantial breaks develop in the clouds and the
atmosphere can destabilize, shear will be favorable for severe
storms, especially near the warm front over northern Wisconsin.
But at this point, it seems much more likely that widespread
clouds will limit the convective potential.

Updated product suite will be out ASAP.

.SHORT TERM...Tonight and Monday
Issued at 327 PM CDT Sun Oct 16 2016

Precipitation trends and potential for severe storms are the main
concerns during this part of the forecast.

A warm front is forecast to move into the area from the south, strong
warm advection will develop as 850mb winds increase into the 35
to 40 knot range, and a mid level short wave will move into the
region. These factors will combine to bring a chance for showers
and thunderstorms to the forecast area starting late this evening.
12Z NAM and GFS showed MU CAPE values in excess of 1000 j/kg
across at least the southwest half of the forecast area with wet
bulb zero heights near or below 10 kft. SPC day 1 convective
outlook had the forecast area in a marginal risk through 12Z
Monday, mainly focusing on hail.

A surface low moving along the frontal boundary will combine with
an approaching mid level short to keep the chance for
thunderstorms going through Monday but temperatures should still
reach highs in the mid 60s to mid 70s.

.LONG TERM...Monday Night Through Sunday
Issued at 327 PM CDT Sun Oct 16 2016

Impressive dynamics for this time of the year which continue into
Monday night will be the primary issue with this forecast period.

A marginal risk of severe storms over the northwest half of
Wisconsin Monday into Monday night or day 2 is well deserved. For
starters, 130 knot upper jet is progged to be positioned over
northern Wisconsin in addition to a frontal boundary. The
unseasonably warm moist and unstable air mass for this time of the
year will be working with a deep sheared environment. Mu capes
1000 to 1500 J/kg at the onset Monday evening before subsiding
from north to south overnight. SB capes slightly lower but severe
potential likely to be depend on the mid level cap eroding monday
afternoon. Still even the elevated convection could be capable of
producing large hail. A short wave trough with 850 low will track
over northern Wisconsin Monday night for a focus of heavy rain
across the north in addition to the severe weather potential.

Most of the precipitation will depart to the northeast with the
short wave Tuesday morning while the surface and 850 mb cold
front slides south over the area through Tuesday. This frontal
passage will lead to the start of a cooling trend.

For the rest of the week, an overall drier period with a
continued cooling trend as a series of short wave troughs with
reinforcing cold fronts will track over the areas. Precipitation
chances will be very spotty in the drier airmass, but the -4 C
850 MB air mass by late in the week may produce isolated light
diurnal pcpn.  Obviously timing differences can be expected with
these short wave troughs in the fairly fast northwest to west
flow. Compared to 24 hours ago, do note progs have backed off on
the deep upper trough just east of the area by the end of the

.AVIATION...for 06Z TAF Issuance
Issued at 912 PM CDT Sun Oct 16 2016

A band of SHRA/TSRA now developing over MN will move across the
area after midnight. Will attempt to highlight the most likely
time for TSRA in the 06Z TAFs. Then there will be a break in the
precipitation before additional storms are possible tomorrow
afternoon, especially across the north. LLWS will also a factor
overnight as the LLJ develops eastward across the area.



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