Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 170457
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1057 PM CST Sat Dec 16 2017

.UPDATE...

Forecast is on track. Still looking for some fog development later
on.

&&

.AVIATION(06Z TAFS)...

IFR CIGS are becoming dominant across the area late this evening.
The low level moisture will remain high through the TAF period and
therefore don`t see a lot of reason for improvement to better
flight categories. Will keep the TAFs generally IFR through the
TAF period. Some light fog can be expected across the area as
well, mainly from about 10-17z Sunday.

&&

.PREV DISCUSSION... (Issued 920 PM CST Sat Dec 16 2017)

UPDATE...

A low level cloud deck has moved over nearly all of the CWA and
will continue to gradually fill in the spots not yet socked in.
Given these clouds, and considering the fact that much of the
short term guidance has lows in the upper 20s and low 30s, I`ve
brought overnight temperatures up a few degrees across the CWA.
Otherwise, the previous forecast is on track.

AVIATION(03Z TAFS)...

The stratus deck completely stalled as it got to the southern
tier of counties and briefly retreated northward. The clouds seem
to be headed back south now, so nearly all of the CWA will be
socked in within the next few hours. However, the fun doesn`t stop
with forecasting the spatial extent of the clouds. Cigs have been
gradually lifting from high-end IFR to low-end MVFR across the
area. At the moment, I have a hard time seeing why the gradual
lifting trend would stop, so I brought all the TAF sites to MVFR
for the next few hours. However, forecast models remain more
pessimistic, keeping everyone in, at best, IFR. Considering the
disagreement between current trends and model guidance, and given
the fact that we`re right on that IFR/MVFR threshold, forecast
confidence regarding IFR versus MVFR cigs is pretty low.

MARINE...

Conditions on Lake Michigan will remain somewhat tame for this
time of year. The next period of interest will be on Monday night/
Tuesday morning when offshore winds may start gusting to just
above SCA criteria. Beyond that, little to no significant waves
or winds are expected on the lake until late week when a strong
low pressure system will move through the midwest.

PREV DISCUSSION... (Issued 536 PM CST Sat Dec 16 2017)

UPDATE...

Previous forecast is on track. Many of the meso models are showing
some fog in our SW and the SREF is highlighting that same area
with at lest a 50% chance of visibilities below 3SM. Therefore
left the mention of freezing fog in the forecast and will keep a
close eye on that going forward.

AVIATION(00Z TAFS)...

Much of the CWA is socked in beneath IFR cigs with the exception
of the far south. That cloud deck is gradually pushing south so
the first question is when will it reach ENW. The meso models
have it staying north of ENW until nearly 06z, but that seems
unreasonable given the current position of the cloud deck (near
RAC) and its slow southward progression. That said, the southward
progression does appear to be stalling a bit, so the arrival of
the IFR cigs at ENW may need to be adjusted.

Otherwise, there is a pretty strong signal in the meso models for
LIFR cigs for a period late tonight and into the morning away
from Lake Michigan. Additionally, much of the guidance is showing
fog in the same area. Temperatures across the area are already
right around freezing and will drop a bit as we move through the
night. Therefore, there is a potential for freezing fog away from
Lake Michigan.

PREV DISCUSSION... (Issued 343 PM CST Sat Dec 16 2017)

SHORT TERM...

Tonight And Sunday...Forecast Confidence Is Medium.

Patchy freezing fog still looks possible across areas away from
Lake Michigan later tonight into the middle morning hours Sunday.
Low stratus clouds under inversion should help trap moisture
during this time, with rather light easterly winds inland. Closer
to the lake, the winds will be a little stronger, and should
limit any freezing fog potential there.

The freezing fog may cause slippery spots on roads later tonight
into Sunday morning, especially in low lying areas. Not confident
enough for a Freezing Fog Advisory at this time, but will need to
be watched by later shifts.

There may also be some flurries or light freezing drizzle in
northeast parts of the area later tonight into Sunday morning.
This would be from some frictional convergence with the easterly
flow coming off of Lake Michigan.

Forecast soundings are indicating a lack of ice crystals into the
air column, so light freezing drizzle may result. Not confident
enough to mention in the forecast at this time, but will also need
watching by later shifts.

Low stratus deck should gradually slide southward this evening,
and linger over the area Sunday, as current west-to-east-oriented
quasi-stationary surface boundary sags to the south. A 500 mb
shortwave trough and associated weak surface low slide northeast
through central and northeast Illinois Sunday. These features do
clip southern portions of the area. However, not seeing enough
upward vertical motion fields and deep moisture to mention PoPs
at this time.

Lows tonight will be tricky, with the low stratus perhaps keeping
temperatures a few degrees warmer than currently forecast. Highs
Sunday should reach the middle 30s in most areas.

Sunday Night Through Tuesday...Forecast Confidence Is Medium.

Low stratus should linger into Sunday night and Monday morning at
least, with low level moisture trapped under inversion. May see
another round of patchy freezing fog during this time, with light
winds and moist low levels under inversion. Again, this may cause
some slippery spots on roads.

Models then show warm air advection developing for Monday into
Monday evening, before a cold front slides southeast through the
area later Monday night. The front looks to be a dry passage, with
a lack of moisture on GFS soundings. The NAM has more moisture,
but remains dry with QPF. Left this period dry. Temperatures
should be above seasonal normals through this period.

LONG TERM...

Tuesday Night Through Friday...Forecast Confidence Is Medium.

Main issue this period continues to be the chances for several
inches of accumulating snow for Thursday into Friday morning. The
GFS/Canadian/ECMWF are trying to come into a consensus with the
track of the surface low. They are taking it northeast from the
southern Plains Thursday into either far northern Illinois by 12Z
Friday per ECMWF/Canadian, or further east into northeast
Indiana/northwest Ohio per GFS.

The GFS track would bring the best mid level frontogenesis band
and snow probabilities over the area Thursday into Thursday night,
with the ECMWF/Canadian shifting this band a little to the
northwest. It still would bring decent snow amounts to northern
and western portions of the area.

Still used the consensus blended model PoPs for this period,
which are high. This system will certainly need watching and
messaging about it may have to ramp up soon, if the models
continue to move toward a consensus on the low track. This would
impact travel as early as Wednesday night and perhaps into Friday
morning, so stay tuned for later updates.

AVIATION(21Z TAFS)...

Main concern is potential for light freezing fog on runways later
tonight into the middle morning hours Sunday away from the lake.
Patchy freezing fog is anticipated, which may cause slick spots on
runways. Visibility values should generally be in the 3 to 6 mile
range in this area.

Area of IFR stratus clouds should gradually shift southward into
this evening. These clouds should then hang around later tonight
into Sunday, remaining in the 500 to 900 foot range.

Easterly winds are expected tonight into Sunday morning, becoming
light Sunday afternoon as low pressure moves northeast into
portions of the area. Another round of light freezing fog may
occur Sunday night into Monday morning across the area.

MARINE...

Easterly winds of 10 to 20 knots are expected tonight and Monday.
Thus, the waves will continue to increase into the 3 to 4 foot
range in this area. No Small Craft Advisory is anticipated at
this time, as most gusts should remain around 20 knots, with
waves at or below 4 feet.

Gusty southwest winds are possible later Monday into Monday
evening, becoming west to northwest into Tuesday, as a tight
pressure gradient develops over the region. High waves should
remain over the open waters of Lake Michigan. A Small Craft
Advisory may be needed for this period.

Winds and waves may reach Small Craft Advisory levels again
Thursday night into Friday.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

Update...Davis
Tonight/Sunday and Aviation/Marine...Wood
Sunday Night through Saturday...Wood



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