Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 162327

Area Forecast Discussion
National Weather Service Green Bay WI
627 PM CDT Wed May 16 2018

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Thursday
Issued at 239 PM CDT Wed May 16 2018

Several minor forecast concerns to deal with ranging from small
chance of showers this evening north, to gusty winds late this
evening near the bay/lake, to low relatively humidities Thursday
afternoon across central/north-central WI.

The 19Z MSAS surface analysis showed a cold front that extended
from the Dakotas, eastward to northern sections of Upper MI, then
northeast into southeast Ontario. Earlier convection in the
vicinity of this front had diminished, however increased
instability this afternoon has allowed some cumulus to build along
the front and radar has started to pick up on new shower activity.
Across northeast WI, it was a warm afternoon with Rhinelander
approaching record max temperature territory.

The cold front is forecast to continue pushing to the south
tonight, although some acceleration is expected near the bay and
lakeshore. As winds turn to the northeast behind the front,
anticipate some gusty winds over eastern WI for a time tonight. As
for precipitation chances, models remain locked on a chance of
showers across northern WI between lingering instability/frontal
passage this evening and perhaps a stray shower overnight due to
the right exit region of the upper jet. The rest of the forecast
area should be dry under mostly clear to partly cloudy skies. Min
temperatures to range from the middle to upper 40s north, mainly
lower 50s south.

Northeast WI to reside on the southwest flank of an area of high
pressure centered over eastern Ontario/western Quebec on Thursday.
The main question is whether a return flow and increasing
instability to our west could bring a chance of showers toward
central WI during the afternoon hours. Based on the forecast
soundings, it appears the atmosphere may be too dry to allow any
precipitation from reaching any part of the forecast area. This
leads to the fire weather concern as relative humidities will only
be marginally better than today (Wednesday). However, temperatures
are expected to be cooler with readings 10 to 15 degrees lower
than today. For now, will continue to mention an elevated concern
for Thursday. Look for max temperatures to range from the lower
60s near Lake MI, to the middle 70s over parts of central WI.

.LONG TERM...Thursday Night Through Wednesday
Issued at 239 PM CDT Wed May 16 2018

Focus revolves around continued fire weather concerns and chances
for pcpn this weekend and early next week.

On Friday, northeast Wisconsin will still be influenced by the
surface high pressure across southeast Canada. Surface temps will
also rise into the mid and upper 70s with afternoon RHs ranging
from 25 to 35 percent away from Lake Michigan. Models continue to
show a fairly dry atmosphere with mixing up to about 850mb across
northern Wisconsin, which will lead to wind gusts around 20 to 25
mph. Due to these conditions, fire weather concerns will remain
elevated. See fire weather section below for more details.

Confidence has decreased on the timing of precipitation this
weekend. Latest runs of the GFS/Canadian/NAM have shifted gears
and show little if any precipitation associated with an
approaching cold front on Saturday, with a better chance of rain
on Sunday into Monday as a weak surface low and mid-level short
wave pass across the area. Whereas, the 12Z ECMWF still has a cold
front moving across the area Saturday afternoon through Sunday
followed by dry conditions early next week. Did not want to
drastically alter the forecast from the previous shift, but did
slow down timing of pcpn Saturday and lowered PoPs slightly. Pcpn
chances have been added to Monday, but confidence is low at this
time. Still agree that the best location for any thunderstorms
would be across central and east- central Wisconsin with some
instability Sunday afternoon and evening (MUCAPE up to 300 J/kg).
Greater instability and steeper lapse rates exist much further
south and do not anticipate severe weather in the Green Bay
forecast area at this time.

Long range models generally show high pressure building in
Tuesday into Wednesday which would lead to dry conditions.
Temperatures will be around normal through the period.

.AVIATION...for 00Z TAF Issuance
Issued at 619 PM CDT Wed May 16 2018

A cold front will combine with lingering daytime instability to
produce scattered showers and thunderstorms over northern WI
through mid-evening. Have added a TEMPO group for thunderstorms at
the RHI TAF site, but kept precipitation out of the remaining TAF
sites, as showers should wane with the loss of heating. The
showers will be falling from a mid-level cloud deck, so VFR
conditions should prevail. Behind the frontal boundary, winds are
forecast to shift to the northeast and become gusty across eastern
WI in proximity to the bay and Lake Michigan late this evening
and overnight. There may be a brief period of borderline LLWS at
the ATW/MTW TAF sites late tonight, as northeast winds increase to
35 knots just above the surface. Thursday should be a dry day
with SCT-BKN mid-level clouds over the region.

Issued at 239 PM CDT Wed May 16 2018

Although the duration will probably be too brief to warrant a
Small Craft Advisory, a short period of hazardous conditions is
likely on the lake and especially on the bay during the middle to
late evening as the front surges south. Northeast winds could
gust to around 25 kts for a couple hours after FROPA. If these
conditions come to fruition, a marine weather statement may become

Issued at 239 PM CDT Wed May 16 2018

Elevated fire weather conditions will continue across northern
Wisconsin. Fuel moisture values are approaching their seasonal
lows, and this coincides with a relatively dry atmosphere which
is evident on current WV imagery and time sections. The worse
conditions are expected on Friday afternoon and evening as some
locations could see wind gusts around 25 mph, low RHs, and temps
in the 70s. Fire weather conditions should improve somewhat this
weekend, although confidence of a widespread rain event has
decreased. Regardless, models still suggest more cloud cover and
overall moisture throughout the atmosphere, which should bring
some relief.



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