Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 191008

Area Forecast Discussion
National Weather Service Green Bay WI
508 AM CDT MON SEP 19 2016

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 349 AM CDT Mon Sep 19 2016

There are two driving mechanisms for thunderstorms today, a strong
upper jet moving across northern Wisconsin and a weak cold front
moving across the entire area. The best upper support with the jet
will be north of the front, and it will lack much moisture. The
surface front will have some moisture ahead of it, and some low
level convergence, but be south of the best upper support.

So we don`t expect a lot of coverage of thunderstorms but there
should be some. There does look to be adequate instability across
east central Wisconsin for strong or severe storms if they do
materialize. Large hail and damaging winds would be the main

The front will exit the area early this evening with clearing skies
overnight. Sunny and warm weather is expected Tuesday as a weak surface
high moves across the region.

.LONG TERM...Tuesday Night Through Sunday
Issued at 349 AM CDT Mon Sep 19 2016

Models continue to show an upper ridge to build into western
Canada and eventually into the West Coast this week. This ridge
will displace an upper trough from the northwest CONUS, across the
Rockies and finally into the northern Plains next weekend. The
main problem for locations farther downstream (including northeast
WI) will be the strength of upper ridging over the east-central
CONUS and the location of a frontal boundary that will waffle over
the Great Lakes for much of the latter half of the week. Model
continuity from run-to-run has been poor, thus confidence is
rather low once we get beyond Thursday. Temperatures should be
above normal through mid-week, then slowly cool down closer to
normal thereafter.

As the Pacific Northwest upper trough continues to dig southeast,
the mean flow begins to turn southwest over the central CONUS
Tuesday night. The old cold front that had moved through northeast
WI late Monday is forecast to start lifting back north as a warm
front and is expected to approach southern WI by 12z Wednesday.
Models are now developing showers/thunderstorms along and north of
the warm front which would bring a chance of precipitation to
southern sections of the Great Lakes after midnight. At minimum,
we should see clouds increase from south to north overnight and
this will help hold min temperatures in the upper 40s to lower 50s
north, middle to upper 50s south.

The warm front is forecast to slowly lift north toward central WI
on Wednesday and be the primary focal point for shower and
thunderstorm activity. The extent of the precipitation will be
aided by a mid-level shortwave moving through the southwest mean
flow, increasing instability (steep mid-level lapse rates), right
entrance region of the upper jet and finally inflow of gulf
moisture that will raise PW values greater than 1.5 inches by 00z
Thursday. Have raised pops across the board with locally heavy
rainfall possible later in the day as deeper moisture arrives. Max
temperatures on Wednesday will be in the upper 60s to around 70
degrees north, lower to middle 70s south.

This warm front is expected to become quasi-stationary over
central WI Wednesday night and possibly get shoved a bit south by
convection on Thursday. Models suggest various surface waves will
move along this boundary, along with shortwave energy ejecting
northeast from the western CONUS upper trough. PW values to
continue in the 1.5-1.75 inch range, therefore periodic bouts of
heavy rainfall is possible. Depending on where the axis of heavy
rains line-up between Wednesday and Thursday, localized flooding
may evolve somewhere over northeast WI. Max temperatures on
Thursday will be a couple of degrees cooler than Wednesday with
mid to upper 60s north-central, lower to middle 70s over our
southern tier of counties.

Models continue to hint that the frontal boundary will get
suppressed south toward the IL border or even into northern IL
depending on which model is correct. This may allow the
precipitation to at least taper off across the north Thursday
night into Friday, however central/east-central WI will have
shower/thunderstorm chances persist through Friday. Once again,
cannot rule out additional heavy rain, especially over southern
sections Thursday night until the boundary pulls far enough away.
Temperatures will continue to slowly cool such that by Thursday,
most locations to end up in the 65-70 degree range.

As the upper trough moves through the Rockies at the start of the
weekend, the upper ridge over the east-central CONUS is progged to
strengthen a bit with an increasing southerly flow helping to once
again lift the frontal boundary back north into WI. Precipitation
chances will again increase over the forecast area for Friday
night into Saturday with another chance for heavy rain as the
atmosphere remains very moist (PW values around 1.5 inches).
Temperatures by the weekend will be closer to normal with lower to
middle 60s north, middle to upper 60s south.

There remains model disagreement with the timing of the upper
trough across the Plains and associated precipitation with an
eastward moving cold front. The GFS remains faster than the ECMWF
by about 12 hours which would mean the difference between a dry or
wet Sunday. For now, will focus higher pops across central WI on
Sunday, but this may change pending later model runs.

.AVIATION...for 12Z TAF Issuance
Issued at 507 AM CDT Mon Sep 19 2016

VFR conditions expected for most of the day today,
though scattered thunderstorms are possible. LLWS will continue
through around 15z today. Good flying weather expected tonight
through Tuesday night.

Issued at 349 AM CDT Mon Sep 19 2016

Periods of heavy rainfall is looking more likely during the
Wednesday into Thursday time frame. Thunderstorms are forecast to
develop along a nearly stalled frontal boundary and have the
potential to train over the same area. This could lead to ponding
of area roadways, minor urban flooding or even isolated flash
flooding. People living along area rivers or streams should keep
alert for rising waters as well.



LONG TERM......Kallas
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