Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 210601 AAA

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1201 AM CST Sun Jan 21 2018

Starting to see some fog develop in areas that have cleared out
here and in Iowa. A lot of uncertainity about how much and whether
it will be dense. Freezing fog is a concern in areas that are
below freezing.


A lot of uncertanity about how much fog tonight and Sunday morning and
whether it will be dense. Freezing fog is possible with
temperatures right around freezing. For now going with a
conservative 2SM in TAFS because the stratus approaching from the
south may mitigate some of the fog. 1500ft ceilings continue to
push in from the south. But, could see visibility get below 1 or 2
SM for a time early this morning.

Drizzle with IFR ceilings and visibility are likely later
Sunday. Have better confidence in this because it is already
happening upstream.


.PREV DISCUSSION... (Issued 943 PM CST Sat Jan 20 2018)

High clouds are moving east allowing for some clearing and maybe
an additional lowering of temperatures a degree or two. But
stratus deck is moving into southern WI which should mitigate
temperature drop. Fog and freezing fog in the areas that drop
below freezing is still a possibility for later tonight into
Sunday morning. With the stratus deck overhead there is some
question as how to much fog we get. Not see much development yet

A small craft advisory may be needed for Monday and Tuesday. Brisk
easterly winds on Monday will turn southerly with building waves.
The passage of low pressure and a cold front will then bring
northwesterly winds Monday night into Tuesday. Waves will build
once again with the northerly fetch across Lake Michigan.

PREV DISCUSSION... (Issued 558 PM CST Sat Jan 20 2018)

Not much change in the thinking from the previous discussion. The
main concern is the potential for dense fog, which may be freezing
fog in some areas. The freezing line will likely set up somewhere
across southern Wisconsin with temperatures above freezing across
far southern Wisconsin. Cross over temperatures were upper 20s to
just above freezing in a north-south gradient, and we will have
weak warm air advection overnight so lows could be a tad higher
than those cross over temps. There is a very small chance for
drizzle/freezing drizzle but a better chance for rain showers
Sunday afternoon when there are better upper level dynamics.

See discussion above about low temperatures and potential for
freezing fog. Main concern for freezing fog will be north of TAF
sites but will have to watch how pavement temperatures respond to
frozen subsurface temperatures.

There is a wall of stratus approaching from the south. Ceilings
and visibilities are expected to drop rapidly as it moves across
southern WI overnight. IFR/LIFR conditions are likely after
midnight and through Sunday morning. Conditions may improve
somewhat for a time Sunday afternoon.

PREV DISCUSSION... (Issued 312 PM CST Sat Jan 20 2018)


Low pressure currently developing over the southern plains will
gradually push northeast, resulting in weak WAA across southern
WI through tonight and Sunday. The gentle southerly flow will
help advect the stratus deck currently over southern IA and
central IL into the area, reaching southern WI sometime around
midnight. The WAA, coupled with overcast skies for a portion of
the night, will help buoy temperatures tonight, especially in the
south where clouds will arrive first. I don`t this our southern
counties will fall below freezing, but the freezing line will
likely set up somewhere across the CWA. With this backdrop, there
are 2 primary concerns with this forecast. i

The first concern is for the potential for fog or freezing fog.
Increasing low level moisture and very weak surface winds will
likely result in some fog across the area. Questions remain with
regards to how bad the fog will get, but may meso models are
showing overnight visibilities less than 1SM across much of the
area, and the SREF is showing probabilities in excess of 80% for
visibilites to fall below 1SM. Therefore, at least patchy fog
seems likely, and the potential for more widespread or dense fog
is indeed on the table. Obviously, if fog develops in an area with
temps below freezing, freezing fog is possible.

The second concern is for the potential for some freezing
rain/drizzle. Eventually, the low level WAA and moisture will
become sufficient for precipitation, with most guidance pushing
the precip into far southern WI sometime around sunrise. As the
precip arrives in the morning, there may be a narrow window in
which temperatures will be cool enough to allow precipitation to
freeze on some surfaces. However, uncertainty remains as to when
exactly the precip will arrive. Moreover, if precip arrives early
enough to support freezing rain/drizzle, it will be very light and
only last for an hour or two at a given location.

Precip will gradually push north and increase in coverage through
the day Sunday. Temperatures will struggle to get out of the upper
30s, resulting in a dreary January day.

Sunday night through Tuesday...Forecast confidence medium.

The areas of fog and drizzle on Sun will continue into Sun eve.
More organized pcpn will then arrive late Sun nt as the deepening
low pressure area moves into srn IA/nrn MO, and the diffluent and
divergent upper low begins to affect srn WI. Low to mid level
warm advection and frontogenesis will also increase during this
time. Rain is mostly expected but sfc temps and road temps will
likely be near freezing toward central WI thus some light ice
accum could occur.

A mid and upper level dry slot will then move into the area for a
period of time in the afternoon with the occluded low to then
track across se WI from late afternoon into the evening. A low
level TROWAL and sfc trough will hang back from the center of the
low bringing more pcpn with rain changing to snow. 1 to 3 inches
of snow accum is forecast Mon nt. Nnwly winds and cold advection
will increase late Mon nt and continue into Tue.

LONG TERM...Wednesday through Saturday...Forecast confidence

A progressive, yet meridional pattern will continue for this
period. A couple shortwave troughs within nw flow aloft will
reinforce polar air for Tue nt-Wed. Upper ridging will then
develop for Thu-Fri as a longwave trough moves from the wrn USA
into the Great Plains. The longwave trough and a cold front will
then bring chances of rain/snow for Sat. Above normal temps will
return with the approach of the longwave trough for Thu-Sat.


Confidence remains high that a low stratus deck currently over
southern IA and central IL will push into WI overnight. We will
likely see a dramatic drop from VFR and IFR/LIFR within a few
hours, but given some uncertainty on the exact timing, I didn`t
feel comfortable putting that tight of a drop in the TAFs quite
yet. Most high res guidance also showing visibilities tanking as
this stratus deck moves in.

Beyond the very low cigs and vsbys, some concern remains for very
light freezing drizzle or freezing fog Sunday morning. However,
temps will be very close to freezing in southern Wisconsin, and
may not actually make it to the freezing mark at the TAF sites.
Given just how close I expect temps to be to freezing, and the
fact that the best precip chances don`t move into the area until
the afternoon, so I`ve got pretty low confidence that fog will
freeze or that freezing precip will materialize at the TAF sites.

MARINE...A Small Craft Advisory may be needed for Mon-Tue. Brisk
ely winds on Mon will turn sly with building waves. The passage of
low pressure and a cold front will then turn the winds to brisk
nnwly Mon nt into Tue. Waves will build once again with the nly
fetch across Lake MI.




Tonight/Sunday and Aviation/Marine...BSH
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