Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 290216 AAA

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
916 PM CDT SUN AUG 28 2016


Uncertainty remains about widespread dense fog tonight. Still
expect dew point depressions to lessen with nocturnal cooling, with
the best chance at saturation and dense fog over South Central
WI, which agrees with the higher SREF probabilities for
visibilities falling under one mile.

However cloud debris from convection from the south and southwest
is overspreading the area, limiting radiational cooling. There is
also a bit more surface pressure gradient than last night that is
keeping surface winds in the 5 to 8 knot range in the west, though
they should lower. RAP forecast soundings also show 10 to 12 knot
winds at the top of the shallow low-level inversion between 300
and 500 ft through at least 08Z providing some stirring that
would prevent dense fog formation.

Will keep the current widespread fog with patchy dense fog going
for now and monitor trends.



With a bit more surface pressure gradient than last night that is
keeping surface winds in the 5 to 8 knot range in the west, though
they should lower, and RAP forecast soundings also showing 10 to
12 knot winds at the top of the shallow low-level inversion
between 300 and 500 ft through at least 08Z providing some
stirring, widespread dense fog formation is still uncertain.
Some patchy fog is beginning to develop in the east with lesser
surface winds with 6sm vsby reported at KSBM. Should see IFR
conditions with 1 mile fog and low cigs around or after 08Z until
14-15Z. VFR conditions expected after the fog lifts through the
end of the forecast. Low precipitation chances between 06Z Tuesday
and 12Z Tuesday stay over the western CWA.



Light easterly winds and a moist boundary layer brings prospects
for fog back to the nearshore waters after midnight and into
early Monday morning. Web cams are not showing any fog at this
time with dew point depressions still in the 2 to 4F range at
near shoreline ob sites. Will keep mention of widespread fog but
no marine fog advisory at this time. Winds veer to the
southeast... Monday...then south and southwest Monday night with
the approach of a surface trough. Wind speeds and wave heights
will remain below Small Craft Advisory criteria through Monday


.PREV DISCUSSION... /Issued 335 PM CDT SUN AUG 28 2016/

TONIGHT AND MONDAY - Forecast Confidence...Medium.

Mainly scattered diurnal cumulus should linger until sunset, with
light easterly winds. Any convection to the south and southwest of
the forecast area will remain in those areas into this evening.

High pressure will slide slowly to the east tonight into Monday.
Decoupling winds and dew points remaining in the 60s should lead to
another good chance for fog later tonight into Monday morning, with
dense fog possible. Went with areas of fog, with patchy dense fog,
in the forecast for later tonight into early Monday morning.

More widespread dense fog is possible during this period, with not a
whole lot changing from last night. May see some low stratus
development with the fog as well. A headline may be needed by later
shifts for dense fog later tonight into early Monday morning.

It may take awhile during the morning hours for the fog and any low
stratus to dissipate across portions of the area. Things should
become partly sunny by the afternoon, with some diurnal cumulus
development in the warm and humid airmass. Went with highs in the
mid 80s inland, with mid 70s to around 80 lakeside.

Any convection Monday afternoon should remain south and southwest of
the area, as there is not much in the way of forcing for upward
vertical motion over the forecast area. Some uncertainty here, but
left the forecast dry for now.

MONDAY NIGHT THROUGH WEDNESDAY - Forecast Confidence...Medium.

Focus of this period on small chance for convection developing ahead
and along cold front that will be moving through southern WI Tue
aftn/eve.  Column moisture will remain in place until the front
moves through, with precipitable water values remaining 1.25 to 1.5
inches.  Synoptic scale forcing remains weak Monday night through
Tuesday night.  Thinking best chance for convection will be Tuesday
into early Tue evening when some increase in upper level divergence
affects southern WI due to strengthening right entrance region of
upper jet.  0-6km bulk shear to remain weak, below 20 kts, however
Cape likely to increase to over 1500 j/kg.

Some concern over weakly channeled vorticity spreading into southern
WI later Monday night at the same time as some weak low level
moisture flux convergence in the area.  Added schc pops west of
Madison for late in the night.

Drier and less humid air will settle in late Tue night and Wed
behind departing cold front.  Cooler temperatures are expected
beginning Wednesday.  High temps likely to remain in the 70s on


WEDENSDAY NIGHT THROUGH SUNDAY - Forecast Confidence...Medium
to High.

Good consistency and agreement in Medium Range guidance showing more
amplified weather pattern developing mid-week and persisting into at
least the start of the holiday weekend.  Amplifying longwave
troffing over eastern Pacific and western CONUS will move slowly
eastward later in the week.  Downstream blocking ridge will expand
into the Upper Midwest and western Great Lakes which will result in
dry and comfortable conditions for much of this extended period.  A
reinforcing push of cooler air in strengthening northwest flow will
drop 925h temps into the 12 to 16C range on Thu and Fri. Pending
sunshine, daytime temps may only peak in the low to mid 70s both
days, with warmer temps returning over the weekend. A few low spots
may fall below 50 Thursday night.

Tropical system still expected to affect southeast CONUS slowing
down steering level pattern change across Upper Midwest over the
weekend.  Hence upstream ejection of mid-level short wave from long
wave trof delayed in helping to flatten ridge and may not bring a
chance for precipition to southern WI until Sat night or Sunday.
However, noticed 12z ECMWF not nearly as amplified with this feature
crossing FL late in the week. Will watch other medium range guidance
to see if this trend continues. Enjoy the great weather this
upcoming week!


Scattered diurnal cumulus should dissipate by early evening across
TAF sites. VFR conditions should prevail for a time this evening,
with light winds.

Another round of fog, with dense fog possible, should occur by later
this evening into Monday morning. High pressure moving slowly east
of the area will bring light/calm winds once again, with a moist
airmass lingering. Visibilities below alternate minimums, down to
1/2 to 1 mile, seem reasonable between 06Z and 14Z Monday. It may
begin earlier than that.

1/4 mile or less visibilities are possible with dense fog, but may
leave out of 00Z TAFs for now, until better handle on timing and
areal extent is attained. May also see low stratus develop with the
fog, with very low ceilings near airport minimums. Some uncertainty
here as well, so will keep any low clouds scattered for now.

Once the fog and low stratus mix out by late morning on Monday, the
rest of the day should be VFR category, with scattered to broken
diurnal cumulus cloud development. Light southeast to south winds
are expected.


Nearshore waters will remain in a light wind regime through Monday.
Moist air over the cool nearshore waters will result in periods of
fog, possibly into Monday night. Best shot will be later tonight
into Monday morning, and again Monday night.

MODIS satellite imagery of sea surface temperatures still show mid
to upper 50s lake temperatures in the nearshore waters of Lake
Michigan along the Sheboygan and Ozaukee County shoreline, with
lower to mid 60s south of there. Given the mid 60s dew points
expected later tonight into Monday morning, there should be fog,
with patchy dense fog. More widespread dense fog may develop, and a
headline may be needed in later forecasts.





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