Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 170250
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
850 PM CST Fri Feb 16 2018

.UPDATE...

Have raised PoPs a bit Saturday. HRRR and new NAM has increased
QPF a few hundreths. Still less than an inch of snow expected.

&&

.MARINE...

A small craft advisory is in effect Saturday due to increasing
south winds. Winds will diminish Saturday night, but then
increase again from mid-day on Sunday through late Sunday night.
Another small craft advisory may be needed for that period.

&&

.PREV DISCUSSION... (Issued 456 PM CST Fri Feb 16 2018)

UPDATE...

Increasing cirrus later tonight.

AVIATION(00Z TAFS)...

VFR conditions are expected tonight and through much of the
morning on Saturday. A rather narrow band of light snow is
expected to push through the area from mid/late morning into the
mid/late afternoon on Saturday. Temperatures will be around
freezing, so accumulations on paved surfaces may be tough. But,
any accumulations will stay below 1/2 inch. We could see a brief
period of MVFR CIGS with the snow, then VFR conditions prevail
through Saturday night. Look for light winds tonight to back to
the southwest to south for Saturday.

PREV DISCUSSION... (Issued 245 PM CST Fri Feb 16 2018)

DISCUSSION...

Tonight...Forecast confidence is high.

High pressure slides east of the area overnight, exiting to the
east by sunrise. Just some high clouds spilling in late tonight.
Temps will be a bit tricky with light winds and radiational
cooling. Favored low spots could bottom out lower than forecast.

Saturday...Forecast confidence is high.

A weak mid level wave and frontogenesis will bring a rather
narrow band of snow showers across southern Wisconsin from late
morning through about mid afternoon on Saturday. Temps near the
freezing mark will make any accumulations on hard surfaces
difficult, but we could see 1/2 inch or less in some spots.

Saturday night and Sunday...Forecast confidence is high.

High pressure will again roll through the area. Low pressure will
already be nosing into the area by Sunday afternoon with
southerly winds increasing quickly. Those southerly winds should
push temps to around 40, possibly higher, depending on how that
low level thermal profile responds. This period will be dry with
increasing clouds on Sunday.

Monday through Wednesday...Forecast confidence is medium.

This is an incredibly active and potentially impactful period of
weather.

An elongated surface trough and cold front will stretch from
Oklahoma to Wisconsin by Monday morning, with a surface low than
tracking northeast along that axis into Monday night/Tuesday. The
southwesterly upper level flow oriented along that axis will help
ensure any east to southeast progress of this system will be slow
at best. This sets up the potential for some episodic rainfall
with high totals by Tuesday. Model guidance is anywhere from 1 to
2 inches across parts of the far southeast, primarily Monday and
Monday night. This rain, coupled with a lingering high moisture
content snowpack on mostly frozen ground could result in some
localized flooding.

The other major concern is the cold thermal profile at the start
of this system and on it`s way out. We could see some freezing
rain north of Milwaukee and Madison later Sunday night into early
Monday morning where temps should be below freezing. Those temps
will quickly warm on Monday. Southern areas should stay above
freezing at this point. The freezing rain threat then returns
later Monday night into Tuesday.

We warm up quite a bit on Monday with highs ranging from the
upper 30s north to near 50 in the far southeast. This is a really
tight thermal gradient and minor shifts north or south will have a
big impact on local temps. Since all areas will be above freezing
Monday, expect just rain. There is some elevated instability
Monday afternoon/evening across the far southeast, therefore
tossed in an isolated thunderstorm mention.

Temps begin to collapse from northwest to southeast Monday night
and this is where we have subtle, but important model
discrepancies. We could see a period of freezing rain Monday night
into Tuesday morning, but much depends on how quickly the cold
air will infiltrate the area. Again, minor temp adjustments will
make for big shifts in any freezing rain threat. The GFS continues
to be quicker than the ECMWF on this cooling trend. Right now, it
looks like ice accums would be at 1/10 inch or less, but enough
to cause slippery conditions.

Given the model differences, chances of snow continue into
Wednesday, but the trend is beginning to look dry. We`ll work this
out with subsequent forecasts, but all the precip may exit by
sunrise on Wednesday.

Thursday and Friday...Forecast confidence is medium.

Thursday looks dry under high pressure. A system passing by mostly
to the south on Friday could bring a chance of snow, or rain/snow
to the area.

AVIATION(21Z TAFS)...

VFR conditions are expected tonight and through much of the
morning on Saturday. A rather narrow band of light snow is
expected to push through the area from mid/late morning into the
mid/late afternoon on Saturday. Temperatures will be around
freezing, so accumulations on paved surfaces may be tough. But,
any accumulations will stay below 1/2 inch. We could see a brief
period of MVFR CIGS with the snow, then VFR conditions prevail
through Saturday night. Look for light winds tonight to back to
the southwest to south for Saturday.

MARINE...

A small craft advisory is in effect on Saturday due to increasing
southerly winds. Winds will diminish Saturday night, but then
increase again from mid-day on Sunday through late Sunday night.
Another small craft advisory may be needed for that period.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory from 6 AM to 6 PM CST Saturday for
     LMZ643>646.

&&

$$

Update...Hentz
Tonight/Saturday and Aviation/Marine...Hentz
Saturday Night through Friday...Davis


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