Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 161936

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
235 PM CDT SAT SEP 16 2017

.TONIGHT AND SUNDAY - Forecast Confidence...Medium.

A cold front roughly stretched from Duluth, MN through Des Moines,
IA at mid afternoon. It will track slowly east tonight and exit
southeast Wisconsin by early to mid morning on Sunday.
Things are fairly unstable across IA ahead of the front this
afternoon and this will help to light up a fairly solid band of
showers and thunderstorms with the front. However, as it tracks east
toward Wisconsin later this evening, we lose that instability and
this should cause the showers and storms to gradually dissipate. The
best chance for rain will be northwest of Madison after midnight,
with small chances in the southeast after about 4 am. The rain may
not even make it to the southeast given the lack of thermal support.

Once the front pushes southeast of the area Sunday morning, we will
start to get clearing from northwest to southeast with dry weather
expected everywhere in the afternoon. Highs will be more comfortable
in the 70s on Sunday.

.SUNDAY NIGHT THROUGH TUESDAY - Forecast Confidence...Medium.

By Sunday evening, weak cold front will be well east and south of
the area, however lingering southwest steering winds in the mid-
levels and weak westerlies in the low levels will ensure frontal
boundary slows up as it progresses farther south into the central MS
Valley region.  Drier air surge behind front will reduce threat for
late night fog.  In addition, may see an increase in mid-high level
clouds later in the night from developing upstream convection in IA.
Hence wl confine fog mention to susceptible WI river valley for the
late night.

Warm air advection and mid-level short wave moving slowly northeast
will trigger scattered showers and convection north of the front as
it begins shifting northward later Sunday night and Monday.  For
most of the period Sun night and Monday, the precipitation will
remain to the west and south of southern WI, but some showers and
storms may spread into the area during the aftn. There is some short
range model discrepancy, as 12Z NAM showing more short wave ridging
and stable conditions lingering into Monday evening.  The 12Z GFS
and ECMWF weaken ridging as upstream 700H short wave spreads into
southern WI or the northwest IL area by 00Z Tue.  Not surprised
about uncertainty as downstream western Atlantic tropical
disturbance typically causes issues with upstream steering winds.
Will continue chance pops over CWA Monday afternoon, especially
since all short term guidance is showing enhanced lift from right
entrance region of upper jet affecting the area.

Uncertainty continues into Monday night and Tuesday.  However model
agreement on mid-level short wave passing through southern WI in the
Monday night/Tuesday morning time frame.  Hence will have highest
chance pops during this time over the entire CWA. Column moisture
plentiful, with pwat 1-1.5 inches Monday night.


TUESDAY NIGHT THROUGH SATURDAY - Forecast Confidence...Medium.

Tropical disturbance moving northward over the western Atlantic will
continue to cause greater uncertainty with regards to strength of
weather systems affecting the Great Lakes and upper Midwest Tuesday
night into early Wednesday.  By later Wednesday however, tropical
system will have moved far enough northward to allow weakening cold
front to sweep through Wisconsin and the region Wednesday into
Wednesday night. This front will be tied to short wave trof moving
north across the northern Plains into southern Canada.  This short
wave is only the beginning of a pattern change across the western
CONUS as amplifying long-wave troffing is expected to develop
through the end of next week.  Downstream over the eastern CONUS,
high pressure ridging will expand across the area as tropical system
further weakens and moves across north Atlantic.

Medium range guidance in reasonable agreement with Wisconsin
remaining entrenched in southwest steering flow associated with the
upstream long wave trof, with stable, warm pattern locked in over
the southeast CONUS. Hence we should expect temperatures to remain
near or above seasonal normal for mid-September with several passing
weaker short waves and surface boundaries keeping chance pops going
through much of the period.


.AVIATION(21Z TAFS)...VFR conditions are generally expected through
the TAF period. A line of scattered showers and a few thunderstorms
should spread west to east across southern Wisconsin after midnight,
exiting to the southeast by or shortly after sunrise on Sunday. This
activity will be along an advancing cold front. The line of showers
will be weakening and falling apart as it approaches southeast
Wisconsin late tonight. We could see some patchy MVFR CIGS in the
wake of the front Sunday morning.


.MARINE...Conditions are just too stable over the water to support
the small craft advisory north of Milwaukee. Therefore, the small
craft advisory has been canceled. Waves will be somewhat elevated,
so very small boats should still exercise caution into the evening.
An approaching cold front will push through the nearshore waters
Sunday morning, swinging winds from southerly to northwest by
afternoon. Winds and waves are expected to remain below small craft
advisory levels through at least the middle of next week.





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