Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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701
FXUS63 KMKX 150615
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1215 AM CST Thu Feb 15 2018

.UPDATE...

Visibility has improved over the past few hours, so cancelled the
Dense Fog Advisory. Some of the guidance does suggest visibility
could lower again late tonight along the Illinois border, but not
enough confidence to keep the Advisory going for this area. We`ll
continue to monitor for any changes.

&&

.AVIATION(06Z TAFS)...

Some erosion of the low stratus is occurring across our western
areas this eyeing, but clouds will likely hang tough in our
eastern areas overnight. Areas of fog will also persist, with
another period of lower visibility possible along the Illinois
border late tonight.

&&

.PREV DISCUSSION... (Issued 919 PM CST Wed Feb 14 2018)

UPDATE...

So far mixing has been just a tad too strong for widespread dense
fog. The exception is in Green, Lafayette, and Iowa Counties
where visibilities have consistently been around 1/4 mile or less.
Still think there is potential for visibility to lower later this
evening as the low-level speed max promoting our deeper mixing
pushes off to the east. Given this, no changes planned to the
Dense Fog Advisory area at this time.

Areas of drizzle have also developed this evening, and added this
to the forecast grids from Waukesha eastward to the lake.

MARINE...

A Dense Fog Advisory is in effect for the nearshore waters through
tomorrow morning. Expect periods of poor visibility along with
occasional southwesterly wind gusts to around 20 knots this
evening. Wind gusts on Thursday night may approach Small Craft
Advisory criteria.

PREV DISCUSSION... (Issued 616 PM CST Wed Feb 14 2018)

UPDATE...

With visibility tanking along the state line, went ahead with a
Dense Fog Advisory for much of the area tonight. As previously
noted, there is some variability with visibility over northern
Illinois right now, but feel confident that we`ll at least see
areas of dense fog given upstream trends. Low confidence in the
end time for the advisory (9 AM tomorrow), but we can adjust
tonight and/or cancel early if trends support it.

AVIATION(00Z TAFS)...

Low stratus and fog are pushing north into the area this evening,
with ceilings quickly dropping into the IFR to LIFR range south
of Milwaukee and Madison. The ceilings will drop shortly along and
north of these areas. Visibilities are bouncing around quite a
bit this evening, but plan for them to drop to 1 SM or less later
this evening, with periods at or below 1/4 SM possible.

There remains uncertainty about when the low stratus and fog will
clear out, with some models keeping it in place into late
Thursday morning. Slowed the ceiling and visibility improvements a
bit in the TAFs, but further adjustments in timing will be
necessary tonight as things become more clear.

Given the westerly 50 knot low-level jet, low-level wind shear is
also possible for tonight.

PREV DISCUSSION... (Issued 351 PM CST Wed Feb 14 2018)

SHORT TERM...

Tonight and Thursday...Forecast confidence is low.

Fog and low stratus is continuing to make its way north and should
reach southern Wisconsin within the next couple of hours. The
intensity of the fog in northern IL varies quite a bit with obs
ranging from a quarter mile to 4 miles. Much of the high res
guidance is showing an abrupt drop to less than a mile, but that
doesn`t match the obs, so my confidence in these models is not
particularly high.

Another source of uncertainty in this forecast is how long the
fog and low stratus will hang on for. Much of the guidance shows
the fog/stratus creeping into southern WI though midnight before
being gradually swept away west to east, clearing all but the far
south by 12z. Given the very thin layer of moisture and gusty
westerly winds just off the deck, this seems like a reasonable
solution.

Finally, there is some question as to just how long fog and low
stratus will linger into tomorrow in the far south. Guidance
appears to be split, with some members keeping our south socked
in, while others push everything out by mid morning. The synoptic
models keep the low levels pretty dry beginning tomorrow morning,
so I`m leaning toward a more optimistic forecast.

On top of all this, we do have a few periods with a slight chance
of precipitation. Tomorrow morning, a band of mid level
frontogenesis will become established across southern Wisconsin,
and could result in some light precip, though the previously
mentioned dry low levels could prohibit that. If it does manage
to precipitate, temperatures will be right around freezing, so a
mix of rain and snow is possible. Then, a cold front will swing
through tomorrow evening, bringing a second chance of
precipitation. The front will drop temps below freezing, but it
looks like we may struggle with moisture in the DGZ, which could
result in freezing rain mixed with snow.

Friday...Confidence is high.

Friday will begin with brisk northwest post-frontal winds, keeping
temperatures quite cool. As the flow aloft transitions from NW to
more zonal, high pressure will pass through the region. This
should reduce winds by Friday afternoon, gradually swinging them
to the SW by Friday evening. Despite these more southerly light
winds, temperatures Friday night and Saturday morning will be
quite cold thanks to the newly arrived arctic air mass and clear
skies.

LONG TERM...

Saturday...Forecast confidence is medium.

Low pressure will pass through southern Canada while high pressure
is shunted to the southeast. This will set up a decent pressure
gradient and result in breezy southwesterly winds through the day
before the cold front extending from the low sweeps through the
region. Fropa should hold off until later Saturday afternoon,
allowing temps to rebound to near freezing. Some light QPF is
also expected with the fropa, and some of the guidance is
suggesting that we may have a lack of moisture in the DGZ. Held
off on the mention of freezing rain for now, but it is worth
monitoring going forward.

Sunday through Tuesday...Forecast confidence is medium.

High pressure will zip through the region on Sunday morning as low
pressure develops over the Southern Plains. A significant
baroclinic zone will become established over the Northern Plains
and Upper Great Lakes, resulting in several hundreths of QPF.
Right now, it looks like southern Wisconsin will be on the
southern fringes of this warm frontal precipitation Sunday
afternoon through Monday morning Temps will be right around
freezing on Sunday afternoon, so precip type will depend on what
time of day we see it.

Precip becomes more likely on Monday afternoon as the parent low
moves through southern Wisconsin. Models in surprisingly good
consensus with the timing and placement of the low, swinging it
right through our CWA around 00z on Tuesday. That said, storm
total QPF amounts vary quite a bit at this point making it tough
to get very specific. None the less, it does look like this will
be a significant event somewhere in the Upper Midwest.

AVIATION(18Z TAFS)...

An area of low stratus and fog continues to march north across
central IL, and it still appears likely that it will impact much
of southeastern Wisconsin this evening into tomorrow morning.
Guidance is in good agreement bringing it in sometime around 00z,
and pretty much everyone keeps it in place through midnight. Cigs
are pretty consistently below 500 ft within the low stratus
upstream, but visbys are bouncing quite a bit this afternoon.
However, given the snow melt occuring across southern Wisconsin
this afternoon, the low levels should be primed to drop visbys to
1SM or less this evening.

Unfortunately, there is not really a consensus on when the low
stratus and fog will clear out, with some models pushing much of
the low level moisture out in the early morning and others
keeping it in place through noon Thursday.

With gusty winds just off the deck and only a thin layer of
moisture responsible for the low stratus and fog, I`m inclined to
lean toward the more optimistic solution, but I do not have high
confidence in this forecast.

MARINE...

Fog and low stratus continues to push north through northern IL
and souther Lake Michigan. It should reach our nearshore waters
within the next few hours and may drop visibilites to below 1SM.
Fog will linger though at least midnight, though there is some
question as to when exactly it will clear out.

Otherwise, winds will be generally offshore for the next several
days and will remain below SCA criteria.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Dense Fog Advisory until 9 AM CST this morning for LMZ643>646.

&&

$$

Update...SPM
Tonight/Friday and Aviation/Marine...BSH
Friday Night through Thursday...BSH



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