Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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142
FXUS63 KMKX 172018
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
218 PM CST Sat Feb 17 2018

.DISCUSSION...

Rest of this afternoon - Confidence...Medium
The snow area will clear the far east/southeast late this
afternoon or very early this evening. Some pockets of higher
DBZ`s/lower vsbys, though moving through at a pretty good clip.
The meso models and extrap of radar show that the heavier 88-d
returns should stay just south of the WI/IL border though some of
this sneaking into the or barely sneak into the far south. Nudged
amounts up but just a couple tenths or so.

Tonight and Sunday - Confidence...Medium to High
After this snow associated with the mid level wave and surface
trough moves through, there is clearing skies upstream which
should move in this evening. Weak ridging settles in. The high/ridge
axis shifts east for Sunday. A gusty southerly wind will set up.
Temps should respond and rise to near 40. Precipitation associated
with surface/850 low moving across nrn MN should stay to our
north. All progs are in good agreement on this.

Sunday night...Forecast confidence is high.

We transition into a very strong and increasingly moist
southwesterly flow aloft this period. The evening should remain
dry, but the surge of warmer air and moisture should begin to
translate into small precip chances by morning. The low level
temperature profile would support some freezing rain northwest of
Madison.

Monday through Tuesday...Forecast confidence is high.

This is a really active period with impacts that could be quite
large. There are two main threats. First, we have the potential
for some major ice accumulation (approaching 0.40" of ice) north
of a Port Washington to Spring Green line primarily Monday night
into Tuesday. Second, there is the potential for prolonged
moderate to heavy rain across the southeast with many areas
getting between 1 1/2 to 2 inches of rain. This will fall on an
existing icy snowpack over mostly frozen ground. Runoff should be
efficient and quick, potentially leading to some flooding of
smaller rivers and streams. If the rain becomes focused under
thunderstorms, flash flooding is possible.

The deep vertical motion setting up between a powerful low level
jet pointed into the region and persistent through this period,
coupled with the right rear quad of the upper jet is impressive.
The low level jet across a very tight thermal boundary over the
region will produce very focused frontogenesis. The precip being
kicked out by many of the models highlights high QPF over far
south central and southeast Wisconsin. Given the sensitive focus
of this band, it has the potential to shift north and south
resulting in locally big changes in amounts. Also, there is some
elevated instability Monday afternoon and Monday evening that
could allow some thunder activity across the southeast.
Thunderstorms with heavy rain would be a big flooding threat.

The ice set up still looks good as the low level cold temps will
infiltrate the northern areas mainly on Monday night and Tuesday.
We could see some ice initially late Sunday night and Monday
morning, but temps will warm enough to push it north by Monday
afternoon. Those cold temps then push back in Monday night and it
is at this point the ice will start to accumulate. The amounts
could be in the ice storm levels. It`s too early for any watches
at this point and it will be good to get another run or two of
the model before committing to amounts/location.

The offending low pressure and frontal boundary will finally push
east Tuesday night. At this point, some freezing rain could drift
into southeast Wisconsin before it all exits by sunrise on
Wednesday.

Wednesday and Thursday...Forecast confidence is high.

High pressure returns, bringing a period of quiet weather with
seasonable temperatures.

Friday and Saturday...Forecast confidence is low.

The GFS and the ECMWF are in disagreement with the timing of a
late week system. The GFS is more south and brings it in on
Friday, while the EC is north and slower, pushing it through on
Saturday. This would likely have a mix of rain and snow at times
given the marginal temp profile. Stay tuned.

&&

.AVIATION(18Z TAFS)...
Snow moving through with some vsbys aob
1sm. Still looking at accums of around 0.5 inch with a smidge
higher closer to the Illinois border. Quick exit early this
evening with upstream clearing. A gusty southerly wind will set up
tomorrow with VFR conditions. Any precipitation will stay up
across northern WI.

&&

.MARINE...The small craft advisory is pretty borderline at best.
Will let it expire naturally at 6 PM. If anything gradient is
tending to loosen as trough axis actually gets closer.

Another Small Craft Advisory will likely be needed Sunday
afternoon and night for breezy southerly winds and high waves. A
few gale force wind gusts could occur north of Port Washington.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory until 6 PM CST this evening for LMZ643>646.

&&

$$

Tonight/Sunday and Aviation/Marine...Collar
Sunday Night through Saturday...Davis



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