Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 201712

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1112 AM CST Fri Jan 20 2017

.UPDATE...Based on webcams have decided to cancel Dense Fog
Advisory early. Rain area diminishing and shifting north but still
seeing a few shra keeping developing. Will keep cloud cover
intact and cannot rule out a bit of drizzle but not much in
upstream obs. Will have to monitor trends for more dense fog
development as day and night wear on. Another headline cannot be
ruled out.



.AVIATION/18Z TAFS/...Stratus and fog will be widespread this
period. While many vsbys have improved across srn WI expecting the
potential for more dense fog to take shape later tonight into
Saturday morning. Cigs to remain in the LIFR category and SREF
cig/vsby prog showing high confidence in dense fog...90 pct
plus...later tonight into Saturday morning.



.MARINE...A relatively mild and moist flow over the cold waters of
Lake MI will continue fog at times through Sun. Dense fog with
visibilities under 1 mile or much less will also occur at times.



.PREV DISCUSSION... /Issued 332 AM CST Fri Jan 20 2017/

TODAY AND TONIGHT...Forecast Confidence Is Medium.

Area of rain continues to shift north northwest across the eastern
half of the forecast area. This was being driven by differential
cyclonic vorticity advection with advancing negatively tilted 500 mb
shortwave trough. This rain should continue to gradually shift north
northwest and out of the area by later this morning.

Rain has helped temporarily improve visibilities with fog across the
eastern portions of the area. However, as the rain gradually shifts
out of the area, light winds and a very moist boundary layer should
allow for dense fog to return this morning. Will keep the Dense Fog
Advisory going for the eastern portions of the area. Dense fog may
develop further to the west as well, with light winds and the very
moist low levels. For now, will leave these areas out of an advisory
and continue to monitor. Fog is expected to linger today and tonight
across the area.

Second 500 mb vorticity maximum shifts north through the area this
afternoon. However, ice crystals are lost as the air column dries out
from top down. Area forecast soundings have a drizzle look to them,
with saturated low levels and dry sounding above that. Thus, went
with drizzle this afternoon into tonight.

Main 500 mb shortwave trough slides northeast into Iowa and far
southern Minnesota later tonight. Some differential cyclonic
vorticity advection may clip the western counties, so kept PoPs
going for that period. Mild temperatures are expected to continue
during this time.

SATURDAY AND SUNDAY...Forecast confidence is medium.

A deep low pressure system lifts north through Iowa and southern
Minnesota on saturday. The low becomes largely removed from the mean
flow by Sunday, while pinwheeling near the Minnesota Arrowhead. An
associated warm front pushes north through our area Saturday
morning. Weak thermodynamic support accompanies the front as
evidenced by low to mid-level frontogenesis response. Isentropic
omega peaks during the morning to afternoon, with rain chances
during this time. Continued weak lift warrants keeping slight
chances into the evening. Any rain that does fall should be very
light, with only trace amounts.

Given wet ground, a very moist boundary layer, and light winds,
areas of fog are expected from Saturday into Sunday morning. Dense
fog is possible given small dewpoint depressions and high SREF
probabilities of visibilities under 1 mile.

925 mb temperatures remain quite warm for Saturday, in the
neighborhood of 5 to 8 degrees Celsius by evening. The ECMWF is even
warmer, cracking 10 degrees in the far southeast. Even taking a
conservative approach to surface temperatures given moist low
levels, would expect highs to climb into the 40s north to the
lower 50s in the southeast. Cooler air arrives on Sunday as the
low pulls away, but still expecting highs well into the 40s.

SUNDAY NIGHT THROUGH TUESDAY...Forecast confidence is medium.

A vigorous low pressure system will advance through the Tennessee
valley on Sunday, moving into the Carolinas on Monday, before
lifting into the Mid Atlantic States on Tuesday. We could see a bit
of light rain or drizzle in our east Sunday night or Monday morning
before shortwave ridging works in for Monday into Tuesday morning.
Mid level heights will begin falling Tuesday afternoon ahead of
the next low pressure system approaching from the central Great
Plains. Above normal temperatures will continue, with mostly
cloudy skies.

TUESDAY NIGHT THROUGH THURSDAY...Forecast confidence is medium.

There is general model agreement in lifting a strong low pressure
system out of the lower Missouri valley Tuesday evening and bringing
it into the western Great Lakes on Tuesday evening into Wednesday.
Models differ on the exact track of the deepening surface low and
the resulting thermal profile over southern Wisconsin.
Precipitation may start as a rain/snow mix Tuesday evening and
perhaps transition to all snow on Wednesday. The details
surrounding this system should come into better focus over the
next few days, so stay tuned. Temperatures will cool a bit from
readings over the weekend, but should remain above seasonal


Rain is expected to gradually push north northwest of the area this
morning, with a passing trough of low pressure. Dense fog and some
drizzle should develop once again after the rain ends in eastern
portions of the area. Ceilings below alternate minimums and
approaching airport minimums are expected to continue today and
tonight, especially in the eastern portions of the area.

Areas to the west of there may or may not drop to 1/4 mile into this
morning. Confidence is low here with dense fog potential further to
the west this morning. Light easterly winds are expected today into
tonight as well.


Marine Dense Fog Advisory will continue across the nearshore waters
into this morning. Relatively light winds and ample low level
moisture helping with the maintenance of fog over the waters. The
passing rain may be improving visibilities temporarily, but should
drop once again after the rain ends. May need to extend the advisory
into the afternoon and tonight. Otherwise, no marine hazards are
anticipated at this time into the upcoming weekend.


LM...DENSE FOG ADVISORY until noon CST today for LMZ643>646.


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