800
FXUS63 KGRB 162324
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
624 PM CDT Wed Aug 16 2017

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Thursday
Issued at 254 PM CDT Wed Aug 16 2017

Primary issue during this period is the passage of a deep upper
trough and frontal system, and associated precipitation trends
and heavy rainfall potential.

The forecast area remained under the influence of high pressure
over the northern Great Lakes and southeast Ontario, and upper
level ridging. This was keeping the region dry this afternoon.
However, a band of showers and thunderstorms was organizing over
IA and northern IL, in response to the approach of an 850 mb warm
front and a mid-level short-wave trof. These showers should reach
the southwest part of the forecast area by early evening.

The H8 warm front and short-wave trof will lift through the
region tonight, with a 30-40 kt LLJ resulting in strong moisture
convergence during the late evening and overnight hours. PWATs
are expected to increase to around 2 inches, with H8 dew points
climbing to around +15 C, so the air mass will be quite juicy.
Will go with categorical pops with the main band of showers and
storms overnight, and also increase QPF amounts. Not too
concerned with a threat of severe weather tonight, despite some
decent shear and marginal instability. The timing and elevated
nature of the storms will mitigate the threat of damaging winds,
and high wet bulb zero heights of 12K+ feet will be unfavorable
for large hail. Heavy rainfall will be an obvious threat, but
since the rain band is expected to shift north with the H8
boundary, most places should only receive an inch or less of
rain.

On Thursday, the upper level trof will move into the region,
and the surface cold/occluded front will track through during
the day. CAPE will increase to 500-1500 j/kg ahead of the
front, so a few strong storms could develop in the late morning
or afternoon, but the overall severe threat should remain low.
Most of the region should see a quarter to a half inch of rain,
with the most significant amounts over northern WI, closest
to the track of the surface and upper lows. Widespread cloud
cover and showers will hold temperatures back a bit, so have
lowered them a few degrees.

.LONG TERM...Thursday Night Through Wednesday
Issued at 254 PM CDT Wed Aug 16 2017

500mb trough will exit the region Thursday night with a second
trough expected to swing across the area Friday night into
Saturday. Zonal flow pattern expected over the weekend with
a 500mb ridge buildings over the plains by the middle of next
week.

First system will move away from the region Thursday night.
Gusty west to northwest winds are expected behind the system along
with some lingering showers. The highest chances will be across
the north. A chance of showers lingers into Friday morning as high
pressure brings drier conditions to the region Friday afternoon.
The next weather maker is expected to now pass south of the area,
thus have gone with a dry forecast. A little leary about pulling
the rain chances completely as the 500mb trough passes over the
western Great Lakes region. The next system is expected to
approach the area late Sunday afternoon. Have a small chance of
showers and storms across the far north.

The weather becomes more unsettled Sunday night through Tuesday as
a slow moving cold front slowly sags south across the area. There
should be at least two rounds of showers and storms as the front
sags slowly southward. High pressure builds into the region
Tuesday afternoon into Wednesday. Made minor adjustments to
account for latest temperature trends and to account for our
warmer locations during the day and our typical colder spots
at night.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 624 PM CDT Wed Aug 16 2017

A warm front will produce MVFR/IFR conditions and
showers and thunderstorms as it moves north across the region
tonight. Showers and a few thunderstorms will continue Thursday as
low pressure moves slowly across the area, along with IFR ceilings
in most places.
&&

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

$$

SHORT TERM.....Kieckbusch
LONG TERM......Eckberg
AVIATION.......RDM