Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 180244

Area Forecast Discussion
National Weather Service Green Bay WI
844 PM CST Sat Feb 17 2018

Updated aviation portion for 06Z TAF issuance
and new information added to update section

Issued at 840 PM CST Sat Feb 17 2018

Still plenty of details in question about the precipitation
resulting from the evolving southwest flow regime. The lead wave
heading up the front tomorrow will have limited moisture.
Significant lift will be necessary to achieve saturation, and the
precipitation will be confined to areas north of the
developing/strengthening frontal boundary. The new NAM keeps the
precipitation totally north of the forecast area. That actually
fits with the dry air (teens surface dewpoints) currently across
the Central Plains. But given the complexity of the pattern, will
forego making any changes to the precipitation forecast for Sunday
at this time, and allow the incoming shift to review the entire
00Z guidance suite before making adjustments.

Gulf moisture will eventually be drawn into the system, giving
the wave crossing the region Monday into Tuesday the potential to
be a significant precipitation producer. Precipitable water
standardized anomalies are forecast to climb to over +2 to +3
over the southeast third of Wisconsin by Tuesday morning. Although
available moisture should no longer be a concern by that point,
other critical forecast details are still highly uncertain. Those
include the location/structure of meso-scale precipitation bands,
and the thermal profile which will determine precipitation type.
The details of the precipitation type will hopefully become
clearer in the next 24 hours. But as of now the most likely
outcome seems to be for mainly SN and some FZRA/PL over the far
north, a band of mainly FZRA/FZDZ from central to northeast
Wisconsin, and mainly RA/FZRA/FZDZ over east-central Wisconsin.

Updated grids, ZFP and HWO already sent.

.SHORT TERM...Tonight and Sunday
Issued at 250 PM CST Sat Feb 17 2018

First round of light snow was moving across northeast Wisconsin as
of mid afternoon. The light snow should end around 6 pm along the
Lake Michigan shoreline. Accumulations should be around an inch or
less, Skies should clear fairly quickly this evening behind the
snow. This should allow temperatures to plummet, especially in our
typical cold spots later tonight as the winds go fairly light.
Temperatures should not do a complete tank as mid and high clouds
will be on the increase again late tonight into Sunday morning.

On Sunday, the main question is how far south snow will make into
northern Wisconsin. The NAM is farther north than the ECMWF,
GFS and Canadian model. Snow should arrive across the far north
late Sunday morning and continue through the afternoon. Currently
have an inch or two for this period. No precipitation is expected
south of a Merrill to Sturgeon Bay line. Only minor changes made
to the max temperatures on Sunday.

.LONG TERM...Sunday Night Through Saturday
Issued at 250 PM CST Sat Feb 17 2018

Snow will be winding down Sunday evening as the 850 mb low slides
over Lake Superior, dragging a cold front south in its wake. How
far south this cold front tracks through the early part of the
new work week will play a role on the precipitation type and will
present a big forecast challenge.

There is an increasing potential of mixed precipitation with ice
accumulation becoming more of a concern. Considered a watch due
to the potential for ice accumulation but precipitation may hold
off more toward Monday night. Progs this morning suggests two main
frontal waves, Monday afternoon and then again Tuesday afternoon
and evening. Even between these two primary regions of
precipitation, lighter precipitation will likely continue to be an
impact, such as continued freezing drizzle.

High pressure dropping into the mid section of the country will
bring quieter conditions to the area with the next chance for
precipitation toward the end of the week.

.AVIATION...for 06Z TAF Issuance
Issued at 840 PM CST Sat Feb 17 2018

MVFR ceilings have been a bit slower to depart the east than
earlier expected, but they seem to be on their way out now. Low-
end VFR clouds were brushing the north, but those also seemed to
be headed off to the east. That should lead to a brief period of
clear/mostly clear skies before middle clouds begin to spread
back in from the west later tonight/early Sunday. The
precipitation with an initial wave riding northeast in developing
southwest upper flow will clip the north tomorrow, leading to some
MVFR and possibly IFR conditions during the mid-day hours.
Otherwise, the main aviation weather concern will be the
development of fairly strong LLWS as winds above the surface
increase to AOA 50 kts.



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