Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 171730

Area Forecast Discussion
National Weather Service Green Bay WI
1230 PM CDT Thu May 17 2018

Updated aviation portion for 18Z TAF issuance

Issued at 420 AM CDT Thu May 17 2018

Cooler today, and breezy near the lake and bay. Rain chances for
the weekend are now looking less robust than they have been.

The main belt of westerlies will remain across Canada and the far
northern U.S. during the forecast period. A somewhat weaker branch
of the flow will exist across the the rest of the CONUS. That
branch will be better defined across the West where a substantial
trough will exist, and more diffuse in the East.

A cooler Canadian air mass will settle across the area today, with
cooling augmented by a trajectory across the Great Lakes.
Temperatures are likely to remain near or below normal through the
weekend and into the start of next week, then warm to above normal
by the middle of next week. Rain chances for the weekend continue
to be tied to the arrival of a frontal system from the west. But
it now appears additional support from energy ejecting out of the
trough over the southwestern CONUS will be delayed and weaker
than previously expected. So the likelihood in receiving a
widespread significant rainfall during the weekend has lessened
considerably. It`s back to looking like amounts for the period
will end up below normal.

.SHORT TERM...Today...Tonight...and Friday
Issued at 420 AM CDT Thu May 17 2018

The showers that developed within the baroclinic zone sagging
south across the area dissipated late yesterday evening, but have
now reappeared. There`s even been a lightning strike within one
county of the forecast area during the past hour. Added a mention
of the showers back into the forecast until mid-morning. The
original plan was to keep the afternoon dry. But after watching
what`s happened during the past couple hours and seeing some
patchy light precip across the N/NW on a few of the latest model
runs, have been having second thoughts about that. One key
difference by this afternoon is likely to be elevated instability.
The NAM indicated several hundred J/kg MUCAPEs across the north
right now, but showed those decreasing to zero by afternoon. If
that`s right, precipitation this afternoon would likely need to be
driven by larger scale forcing, mainly isentropic lift back over
the frontal zone. That seems more likely to stay focused to our
west. So stuck with a dry afternoon, though will continue to
monitor and pass concerns about that aspect of the forecast on to
the next shift.

Otherwise, cooler temperatures are expected today. Leaned heavily
toward a blend of the guidance products that performed best for
Tuesday`s highs, when a similar NE flow was present.

Temperatures should rebound some on Friday. Dry air aloft
circulating back around a departing cyclone will swing through the
area during the afternoon. That is likely to once again drop
relative humidities during the afternoon--especially across the
north which is still in the process of greening up. But east flow
will limit the depth of the the mixing, limiting the potential for
strong wind gusts.

.LONG TERM...Friday Night Through Wednesday
Issued at 420 AM CDT Thu May 17 2018

Nearly zonal pattern at 500mb at the start of the period will
gradually become northwesterly over the western Great Lakes
region late next week as a 500mb ridge builds across western
Canada. If we do not get any rain across the north this weekend
into Monday, the next chance of rain will not be until late next
week as the Hudson Bay high will dominate the weather pattern.

The weekend system has become muddled compared to the runs
yesterday. It appeared there would be a rainfall event across the
north to help alleviate the abnormally dry conditions in this
region over the last month. For Friday night, models have come
into better agreement with system passing to the south of the area
which could trigger a few showers across the Fox Valley and
lakeshore region. Per coordination with NWS Milwaukee and Chicago,
have increased rain chances late Friday night into Saturday
morning to account for this feature.

Now for Saturday afternoon into Sunday morning, the models have
differing solutions on the precipitation trends. Current ideal is
that showers and storms will develop west of the area Saturday
afternoon and then weaken and dissipate as it moves into the area
Saturday evening. Some of the models now suggest it could remain
dry through Sunday afternoon across the north. Since a few of the
models continued to indicated some light precipitation, left small
chances of rain. The uncertainty in the forecast lingers into
Sunday morning as how far north and east any rain makes it into
the area. Will have a chance of showers during the morning, but
the dry air will end any rain chances across much of the area
Sunday afternoon. With a little more sunshine and less rain, have
raised high temperatures by a few degrees on Sunday.

Models show another system moving across the area Sunday night
into Monday, however the ECMWF differs by leaving much of the
area dry as the system passes to the south of the area. Only
minor changes made to the remainder of the forecast as the Hudson
Bay high is expected to dominate the weather pattern. In this
pattern, very low relative humidity readings can be expected
during the afternoon and early evening hours each day.

.AVIATION...for 18Z TAF Issuance
Issued at 1230 PM CDT Thu May 17 2018

VFR conditions are expected over the next 24 hours as northeast WI
remains on the southwest flank of an area of high pressure
situated over eastern Canada. Mid deck of clouds should gradually
dissipate this evening, while high clouds to persist due to a
system located well to our south. Gusty northeast winds should
also diminish this evening, however LLWS was added to all TAF
sites for portions of tonight as winds aloft remain strong.

Issued at 420 AM CDT Thu May 17 2018

Elevated fire weather conditions will continue across northern
Wisconsin today and Friday. Pine needle moisture is approaching
its seasonal lows, and this coincides with a relatively dry
atmosphere. The worst conditions are expected on Friday afternoon
into early Friday evening as some locations could see wind gusts
to around 20 mph, low relative humidity values and temperatures
in the 70s. A few days ago, the models were suggesting a
widespread rainfall event over the weekend across northern
Wisconsin. This appears unlikely now and it is possible that
portions of northern Wisconsin will not see any rain this weekend.
There is a chance of rain Sunday night and Monday, but there is a
great deal of uncertainty if rain would make it into northern
Wisconsin. Beyond Monday, Hudson Bay high is expected to dominate
the weather pattern through the end of the next week, bringing dry
conditions and low relative humidity readings.



SHORT TERM.....Skowronski
LONG TERM......Eckberg
FIRE WEATHER...Eckberg/Skowronski is the U.S. government's official web portal to all federal, state and local government web resources and services.