Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 192337
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
537 PM CST Mon Feb 19 2018

...Evening and Aviation Update...

.UPDATE...

No major changes to forecast expectations at this point, with a
large area of rain and occasional thunderstorms pushing across the
region at this hour. North of the surface front, northerly to
northeastern flow is ushering in slightly cooler and drier air,
and we`re very closely watching temperature trends across the
north--from roughly Baraboo to Fond du Lac to Sheboygan.
Temperatures across this area will likely hover right around
freezing tonight. As precipitation moves overhead, freezing rain
and/or freezing drizzle is expected. Some light accumulations are
possible this evening, but the better potential will be overnight
into the early morning hours on Tuesday. There have been no
changes at this point to the ice totals, with 0.1 to 0.2 inches
expected. While this likely won`t be enough to cause any sort of
widespread power infrastructure impacts, travel will likely become
extremely hazardous once the freezing rain and drizzle develops.
And, given the variability to the temperatures, drivers should be
prepared for rapid and unexpected changes to road conditions.

For the southern half or so of the forecast
area, hydrologic concerns will remain high, with persistent
moderate rainfall over frozen ground and snowpack.

&&

.AVIATION(00Z TAFS)...

IFR to LIFR flight conditions are expected across the area
tonight and for at least the first half of the day on Tuesday.
Freezing rain and/or freezing drizzle will be possible at times
for areas north of roughly a Baraboo to FDL to Sheboygan line,
but for the four TAF terminals, precipitation should remain all
rain.

For the TAF sites, expect low cigs and fog to persist through the
night. Generally brought vis down to 1/2SM overnight, though some
variability between 1SM and M1/4SM is certainly possible as
precipitation moves in and out of the area. Winds will be somewhat
variable, with a surface frontal boundary overhead. They should
generally remain under 10kt, however. Finally, LLWS will be a
concern at times tonight and early Tuesday morning, with a strong
low level jet overhead.

On Tuesday, fog and low ceilings will persist, before conditions
begin to improve mid day into the afternoon.

&&

.PREV DISCUSSION... (Issued 331 PM CST Mon Feb 19 2018)

SHORT TERM...

Tonight And Tuesday...Forecast Confidence Is High.

The details and reasoning of the forecast continue to tighten up.

The biggest challenge has been locking in the expected surface
temps in the region hovering near or just below freezing northwest
of Milwaukee and Madison. Then the total precipitation amounts
across the area.

Fortunately, those precip amounts have dropped considerably
across Columbia, Sauk, Marquette, Green Lake and Fond du Lac
counties. That allowed us to downgrade the Ice Storm Warning to a
Winter Weather Advisory. Colder temperatures are pushing out of
the northeast and into Sheboygan and Fond du Lac counties as the
northeast gradient flow increases. Sheboygan county needs to be
watched closely, as temps could start to fall quicker than
expected there. Roads should still become quite hazardous
overnight into Tuesday morning, but the potential damage to power
lines, etc has diminished considerably.

We`ll see rain move through this afternoon and early evening,
with a diminishing trend for a while, then another surge of rain
pushes in later tonight into Tuesday morning. This is a result of
the fluctuating low level jet.

The focus for the heaviest rainfall remains across the far
southeast, south into Illinois. We`re still looking at totals in
the 1 1/2 to 2 inch range over southern Milwaukee, Racine,
Kenosha, Walworth and Rock counties.

Additionally, a few thunderstorms may occur tonight into early
Tuesday and this focused heavy rainfall could result in some
flooding.

Dense fog will be a problem for far southern WI overnight due to
light winds, very moist conditions and an existing snowpack. It
may lift from time to time as rain moves through. It will be more
of a problem in rural areas vs urban.

We finally see a diminishing trend from west to east on Tuesday
with most areas drying out during the evening on Tuesday.

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

Any lingering precipitation is expected to wind down Tuesday
evening from northwest to southeast across the area. There is some
potential for light freezing rain or drizzle to occur during this
time, as drier air mixes out ice crystals. However, upward
vertical motion will be weakening, so not sure if this will even
materialize.

Strong cold air advection continues Tuesday night into Wednesday,
as high pressure moves eastward toward Wisconsin. The high will
then shift east across the region Wednesday night and Thursday.
Quiet weather is expected Wednesday into Thursday morning, with
temperatures a little below seasonal normals.

The dry easterly flow in the low levels Thursday afternoon will
probably hold off the next system until Thursday night. The
current low PoPs in areas west of the Dells to Janesville may be
removed in later forecasts. Temperatures should climb back to near
or a little above normal on Thursday.

&&

LONG TERM...

Thursday Night Through Monday...Forecast Confidence Is Medium.

Models have the next system moving east through the region
Thursday night into Friday. There is modest upward vertical motion
with this system, with modest warm air advection. The low level
jet nose is pointing into central and northern Wisconsin, though
there is a decent feed of moisture in the low to middle levels.
GFS forecast soundings are showing potential for some freezing
rain to occur, with a lack of ice crystals in the air column. For
now, will just mention snow Thursday night with a change over to
rain Friday. This may need to be watched for impacts for the
Friday morning commute.

Models then have low pressure moving northeast from the southern
Plains to the eastern Great Lakes region Saturday into Sunday.
There is some variation with the track, but each does give the
area some precipitation mainly Saturday night into Sunday morning.
Kept blended PoPs going for now. This could bring some snow to the
area Saturday night. Temperatures look mild later in the week and
over the weekend. How these temperatures impact precipitation
types will be interesting to see.

&&

MARINE...

Small Craft Advisory remains in effect from 6 PM CST this evening
until 6 AM CST Tuesday morning for areas north of Port Washington.
Northeast winds of 10 to 20 knots are expected during this time,
before weakening Tuesday morning. This will bring building waves
of 3 to 5 feet this evening, lingering overnight before subsiding
by 6 AM CST Tuesday morning.

Dense Fog Advisory continues south of there into tonight, with
light winds and saturated low levels. This may linger into
Tuesday morning, before improving during the afternoon.

&&

HYDROLOGY...

The heavy rainfall expected in the southeast parts of the area
tonight into Tuesday may lead to excessive runoff and localized
flooding, as well as some rivers rising above flood stage. Will
continue to monitor area river gauges, as well as low lying areas
for flooding potential.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Flood Watch through Tuesday afternoon for WIZ064>066-068>072.

     Dense Fog Advisory until 6 AM CST Tuesday for WIZ062>072.

     Winter Weather Advisory until 3 PM CST Tuesday for WIZ046-047-
     051-052-056>058.

LM...Dense Fog Advisory until 6 AM CST Tuesday for LMZ644>646.

     Small Craft Advisory until 6 AM CST Tuesday for LMZ643.

&&

$$

Update...Boxell
Tonight/Tuesday and Aviation/Marine...Boxell
Tuesday Night through Monday...99


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