Area Forecast Discussion
Issued by NWS Green Bay, WI

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KGRB 281153

Area Forecast Discussion
National Weather Service Green Bay WI
553 AM CST Tue Feb 28 2017

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 354 AM CST Tue Feb 28 2017

Precipitation trends, type and snowfall accumulations are the main
forecast concerns.

A surface warm front was situated near the WI/IL border early this
morning, with temperatures in the upper 40s in the warm sector.
An 850 mb warm front was shifting through southeast WI, and
generating a band of rain showers over parts of central and east
central WI. Fortunately, road temperatures were above freezing
where the rain was falling, and latest radar trends suggest that
pcpn will not fall on the colder road surfaces of north central
WI early this morning, so the threat of freezing rain is low.

After the initial band of showers with the H8 front shifts
northeast this morning, there may be a little break before pcpn
increases again. The increase should occur this afternoon as
a cold front sags southeast across the area and a coupled
upper jet develops. The most significant pcpn should occur
over east central WI, where a 30-40 kt LLJ will take aim in
the afternoon. Elevated instability (H8 LI`s od 0 to -2) will
alsO push into Fox Valley and Lkshr areas, bringing a chance of
thunderstorms. Outside of our far northwest counties, suspect that
pcpn will be all rain. As higher dew points arrive, patchy/areas
of fog should develop, especially across the southeast half of the
forecast area. This should linger at least into the evening hours
before drier air arrives and results in some improvement.

Colder will push southeast in the wake of the cold front tonight,
but most of the pcpn will continue in our southeast counties,
where pcpn type will remain as rain until late. Things get more
interesting early Wednesday, as forcing increases over the Fox
Valley and lkshr areas as pcpn finally changes to all snow.
A couple short-wave trofs and the LFQ of an upper jet will
provide the lift. Have increased snowfall amounts substantially
over our southeast counties, enough so that we may need to
consider a winter weather advisory for that region with
subsequent forecasts.

Winds will pick up tonight and remain gusty into Wednesday evening.
Looks like a good bet for a Small Craft Advisory on Lake Michigan,
but will hold off on the headlines for now.

.LONG TERM...Wednesday Night Through Monday
Issued at 354 AM CST Tue Feb 28 2017

Mean flow initially to be dictated by an easterly Pacific upper
ridge and an eastward moving mid-level trough across the CONUS.
This will bring a northwest flow to northeast WI for the end of
the work week, along with a clipper system and below normal
temperatures. A stronger mid-level trough is forecast to drop
south toward the Pacific Northwest and turn the mean flow to a
more zonal direction across the CONUS, thus sending temperatures
above normal this weekend. As a piece of this trough shifts east
early next week, this will spin up a system that could bring
mainly rain (maybe storms) to the Great Lakes.

A brief break in the precipitation is expected for most of
northeast WI Wednesday night as weak high pressure passes by to
our north. The exception would be north-central WI where
northwest winds will combine with 8H temperatures around -15C over
western Lake Superior, to produce at least a chance of lake effect
snow showers. The one big negative for accumulation is the low
inversion heights, thus do not anticipate much snow to fall from
this event. The rest of the forecast area should be mostly cloudy
with min temperatures mainly from 10 to 15 above north, 15 to 20
degrees south.

A cool northwest flow aloft will be over the Great Lakes on
Thursday with the American models (NAM and GFS) sending a clipper
system that passes to our south, yet brings a chance of light snow
to most of the area. The Canadian (CMC) model and ECMWF send this
clipper so far south that we would not see any snow at all. For
now, will focus higher chance pops over central/east-central WI,
although confidence is rather low. Another weather feature to
watch on Thursday would be a better chance for lake effect snow
showers over north-central WI as colder air gets pulled into the
region (8H temperatures over western Lake Superior drop to around
-20C) which allows inversion heights to rise. May need to raise
pops for the Vilas County snowbelt for later Thursday as the CAA
takes over. Max temperatures on Thursday to range from 25 to 30
north, mainly lower 30s south.

Lake effect snow threat will continue into Thursday evening, but
will weaken later Thursday night as winds begin to back ahead of a
ridge of high pressure approaching from the west. Any additional
accumulation over Vilas County appears to be light at this point.
The rest of northeast WI should see skies become partly cloudy as
the surface ridge approaches and with relatively light winds in
place, min temperatures could easily fall into the single digits
above zero north, 10 to 15 degrees south. This would likely make
Thursday night the coldest night of the extended forecast. This
surface ridge axis slides across WI on Friday and eventually
allows winds to back southwest and start WAA by late in the day.
After a mostly sunny start to the day, expect to see mid/high
clouds start to overspread the area Friday afternoon. Look for max
temperatures to reach the upper 20s far north, to the middle 30s
over parts of central WI.

The surge of WAA continues over WI Friday night and if the
atmosphere can saturate sufficiently, we could see a chance of
light snow lift across the region. Models are not all in agreement
with the saturation as the GFS and CMC are dry, but the ECMWF does
bring light snow to northeast WI. Prefer to keep the small chance
pop wording in the forecast for now. A prevailing south-southwest
wind will help usher much warmer air into WI on Saturday. Exactly
how warm we can get will depend on cloud cover as a warm front
approaches from the south. The optimistic approach would take max
temperatures into the lower to middle 40s north, middle 40s to
around 50 degrees south. More clouds would easily chop 3 to 5
degrees from these numbers. Either way, we would be well-above
normal for temperatures on Saturday.

Forecast becomes more uncertain for the end of the weekend as the
models are struggling with the locations of the warm front and the
timing of a weak cold front. The CMC is much more aggressive by
bringing a swath of rain into northeast WI Saturday night/Sunday
morning, while the GFS and ECMWF are essentially dry. If
precipitation were to develop, this would lead to precipitation
type issues as temperatures fall a few degrees either side of the
freezing mark. Due to the uncertainty, have kept pops in the
slight or low chance categories for now and hope for better model
continuity in later model runs. Northeast WI appears to clear
these frontal boundaries by Sunday allowing for at least partly
cloudy skies. Max temperatures to be in the middle to upper 40s
north/lakeshore, around 50 degrees south.

The next system of interest exits the Rockies Sunday evening, but
the models differ greatly from each other, not to mention from
their own respective run-to-run as to the eventual track of the
surface low. The CMC is south and fast with little precipitation
for the forecast area. The GFS is north and would keep most of the
precipitation as rain. The ECMWF has shifted north and now is
following the lead of the GFS. Such a variety leads to little
confidence in any solution at this time, thus have followed the
consensus solution which brings mixed precipitation to much of the
area during the late Sunday night/Monday time frame.

.AVIATION...for 12Z TAF Issuance
Issued at 538 AM CST Tue Feb 28 2017

Moisture will surge north into the region ahead of a cold front
this morning, leading to a significant lowering of the ceilings,
and eventually some fog. Expect conditions to deteriorate to at
least IFR, with LIFR possible in fog this afternoon and evening,
especially in the southeast part of the forecast area. Conditions
may improve to MVFR across the northwest half of the region
overnight as increasing north winds bring colder and drier air
to the region.

Precipitation trends are going to be harder to pinpoint, as
several weak upper disturbances move through the region during the
TAF period. One area of pcpn will lift out of NE WI this morning,
but additional shower activity is already forming over Iowa.
The most significant rain should occur in the Fox Valley and
Lakeshore areas this afternoon, as a low-level jet brings moisture
and instability into the region. Have added a PROB30 group for
thunderstorms at MTW late in the afternoon, and we may need to add
thunder at ATW/GRB too.

Precipitation will change over to snow from northwest to
southeast tonight as the colder air arrives. The change to
snow should occur early to mid evening at RHI, toward midnight
at AUW/CWA and late tonight at GRB/ATW/MTW. Accumulating snow
is expected to develop across much of central and east central
WI by early Wednesday.




SHORT TERM.....Kieckbusch
LONG TERM......Kallas
AVIATION.......Kieckbusch is the U.S. government's official web portal to all federal, state and local government web resources and services.