Area Forecast Discussion
Issued by NWS Green Bay, WI

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
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

FXUS63 KGRB 161722

Area Forecast Discussion
National Weather Service Green Bay WI
1122 AM CST Mon Jan 16 2017

Updated aviation portion for 18Z TAF issuance

Issued at 503 AM CST Mon Jan 16 2017

A round of wintry mixed precipitation this afternoon into Tuesday,
then temperatures warming to significantly above normal.

During the week, a strong negative upper height anomaly is
forecast to develop near the West Coast while a positive anomaly
forms over eastern Canada. Once in place, the positive anomaly
will retrograde across Canada while the negative anomaly shifts
east across the southern CONUS. The result will be split flow that
will become increasingly blocky with time.

Significant precipitation within the evolving large scale pattern
will occur if and when southern stream storm systems affect the
region. One such system will affect the area the next day or so,
with another possible late in the week or during the upcoming
weekend. As a result...precipitation amounts will probably end up
above normal for the period. The evolving split flow regime favors
much above normal temperatures during the period.

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 503 AM CST Mon Jan 16 2017

The main forecast issue was evaluating the likely precipitation
type with the incoming cyclone, and discerning the impacts that
are likely to result from it.

Model forecast soundings/thermal profiles suggest that multiple
precipitation types are likely with this system. Confidence in
the precipitation type forecast isn`t great as the models differed
some on the critical low-level thermal profile. But overall, the
best guess is that snow will be the dominant type in the far
north, though the northern extent of the precipitation is still in
question. A band from AUW/ISW/MFI/STE northeast to Wausaukee/MNM
will probably get a mix of everything. Areas from Y50/PCZ through
OSH/ATW/GRB/OCQ probably have the best chance of getting mainly
FZRA. Most of the lakeshore is likely to get FZRA then RA,
though the northern Door will get more of a mix.

As far as the warning vs advisory decision was concerned, that
was based on the expectation that air temperatures will be
marginal (or just too warm) to generate sufficient icing to cause
widespread damage to trees and power lines. Since sub-freezing
ground temperatures will result in icing on the ground where
there is little difference in impact between a few hundreths of an
inch of glaze and a quarter inch of glaze, the best course of
action seemed to be to go with a Winter Weather Advisory rather
than a Winter Storm Warning. Given the potential for mixed
precipitation to generate icy roadways, opted to extend the
advisory north into areas where snow/sleet totals are below what
would normally necessitate an advisory.

In any case, the system is still evolving. So the current
headlines should be thought of as a starting point, and may very
well need revision later today.

.LONG TERM...Tuesday Night Through Sunday
Issued at 352 AM CST Mon Jan 16 2017

Main story for this period will be the January thaw as temperatures
for most of the period will run 10 to 20 degrees above normal.
Night time low temperatures a little tricker as most of the area
will still have snow cover on the ground. The only exception may
be right near Lake Michigan. If we should get some clear and
calm nights, low temperatures will need to be lowered by at least
several degrees.

Tranquil period expected Tuesday night through Thursday night as
high pressure dominates the weather pattern. Low confidence in
the chances for rain Friday due to model differences between
the gfs/ECMWF. Higher confidence in the chances for mainly rain
Friday night into Saturday and again later in the weekend.

.AVIATION...for 18Z TAF Issuance
Issued at 1122 AM CST Mon Jan 16 2017

Flying conditions are expected to deteriorate from south to north
this afternoon as an area of low pressure approaches from the
central Plains. Surface temperatures around freezing, combined
with road/runway temperatures below freezing, will lead to mix of
precipitation types this afternoon through tonight. Icing of 1/10
to 2/10 of an inch is anticipated for all TAF sites, except RHI
where the precipitation type would be mainly snow with a light
accumulation. Cigs/vsbys are forecast to start at VFR, but quickly
drop into the IFR category by early evening and IFR conditions for
the overnight hours. As the surface low continues to track
northeast across Lower Michigan on Tuesday, the precipitation will
gradually dissipate with improving conditions later in the day.

Winter Weather Advisory from 3 PM this afternoon to noon CST
Tuesday for WIZ035>040-045-048>050.

Winter Weather Advisory from 5 PM this afternoon to noon CST
Tuesday for WIZ020-022-030-031-074.

Winter Weather Advisory from 7 PM this evening to noon CST
Tuesday for WIZ013-018-019-021-073.


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