Area Forecast Discussion
Issued by NWS Green Bay, WI

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268
FXUS63 KGRB 220359
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1059 PM CDT Thu Sep 21 2017

Updated aviation portion for 06Z TAF issuance

.UPDATE...
Issued at 932 PM CDT Thu Sep 21 2017

Updated forecast grids to include widespread dense fog over the
east, lake and bay. This matches the dense fog/marine dense fog
advisory areas. Also downplayed the PoPs for at least the next
several hours.
&&

.SHORT TERM...Tonight and Friday
Issued at 258 PM CDT Thu Sep 21 2017

A warm front was lifting into southern MN and southwest WI
this afternoon. A band of low clouds was persisting north
of this boundary, and moving northward through GRB CWA.
Temperatures were in the low to mid 70s within the cloud band, but
in the upper 70s to lower 80s where there was sunshine. Despite
SBCAPE of 1000-1500 j/kg across the region, CINH of 75-100 j/kg
was capping off any convection.

The warm front is expected to lift north across the region tonight
and Friday morning. The front, combined with a developing 30-35 kt
low-level jet, should trigger isolated elevated thunderstorms
north of the boundary, as H8 LI`s of -3 to -5 C advect into the
region. Storms may be a bit more numerous over north central WI,
as some weak short-wave energy and the RRQ of 50-60 kt jet brush
past later tonight. SPC still has a small part of north central WI
in the Marginal risk for severe storms tonight, and this seems
reasonable, as deep-layer shear is strongest there (around 30
kts). Have added patchy fog across most of the region overnight,
with areas of fog in far northeast WI and on Lake Michigan, where
marine fog is already occurring.

The warm front will lift into the Upper Peninsula of Michigan
on Friday, ending the threat of thunderstorms. The resulting
clearing and gusty southwest winds will lead to strong heating,
with mixing through 900-875 mb supporting high temperatures
in the middle 80s to lower 90s, much above normal.

.LONG TERM...Friday Night Through Thursday
Issued at 258 PM CDT Thu Sep 21 2017

An amplified pattern will be in place for the conus until early next
week, thanks to a deep trough over the western U.S., and a ridge
over the east. The pattern does translate east, which will signal a
pattern change by the middle of next week across the region.
Although there are some timing differences in regards to when this
pattern change occurs, models are generally in good agreement this
far out, so will continue to use a multi model blend for this
forecast.

Friday night through Sunday...With a cold front nearly stationary
over Minnesota, and a warm front extending east over Lake Superior,
the entire region is expected to be located in the warm sector
during this period.  With mostly clear conditions and a breezy south
wind, temperatures will likely range from 15 to 20 degrees above
normal. Saturday should be warmer than Sunday, when a few 90s are
possible in the warm spots.  Heat indices in the low to mid 90s on
Saturday will fall back to the upper 80s on Sunday.

Rest of the forecast...Sunday night continues to look quiet and
warm. Then precip chances will gradually increase from west to east
on Monday and Tuesday when a cold front drifts closer to the region.
Storm chances look the highest on Tuesday along this front, but its
too early to assess a severe threat.  More seasonable temps return
behind the front for midweek.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1059 PM CDT Thu Sep 21 2017

Plenty of issues just in the short term make this a low confidence
forecast. Rain that most of the synoptic scale models had reaching
the area by 06Z at the latest, has not yet developed. HRRR had a
few specs in central/north central Wisconsin on the simulated
reflectivity product, but just for a couple of hours and then
nothing moves in until around 14Z when showers and storms reach
Vilas County. In the mean time mostly clear skies prevailed, but
dew points were well into the 60s and it will not take much for
temperatures to drop enough for fog development. IFR/LIFR
conditions in dense fog prevailed close to Lake Michigan and the
fog could advect farther inland overnight.

Will go with mostly MVFR and/or IFR conditions in fog at most
locations overnight. Some low ceilings may also develop. Plan to
bring showers and storms in during the morning and continue them
east into the afternoon.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 10 AM CDT Friday for WIZ022-040-050.
&&

$$

UPDATE.........MG
SHORT TERM.....Kieckbusch
LONG TERM......MPC
AVIATION.......MG



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