Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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387
FXUS63 KMKX 222118
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
318 PM CST Thu Feb 22 2018

.SHORT TERM...

Tonight and Friday...Forecast Confidence is High...

As has been typical of late, a rather complex wintry precipitation
forecast. At this hour, mid level moisture continues to stream
over the area. The most we could muster so far is sporadic reports
of a few flakes/rain drops as snow aloft evaporates in the dry
lower levels. In spite of abundant cloud coverage and easterly
flow, temperatures have steadily warmed today, though dewpoints
are lagging a bit in this dry low level flow which will have an
impact on our forecast as we head into this evening.

Low level moisture will steadily increase over the next few hours
and lift will continue to increase in a warm advection regime.
Persistent isentropic upglide is expected from late this afternoon
and evening which should eventually produce light precipitation.
Initial precip type will likely be snow/sleet given sufficient
moisture in the ice growth region aloft and low level wet bulbing
effects. With time, though, the presence of cloud ice will become
questionable if not downright variable. By this evening, there is
enough variability in the presence of cloud ice that a light mix
of snow/sleet/rain is expected. Initial light precipitation will
primarily be along and west of a line from Monroe to Madison to
Fond Du Lac. Precipitation amounts this evening do not look
robust, but any areas of temperatures at/below 32 will pose a risk
for glazing and slick spots on roads.

Overnight, cloud ice aloft remains variable (at best) and a low
level warm nose will increase as surface low pressure passes just
to our northwest. As a result snow will become less likely and
rain/some pockets of sleet will become dominate. The best risk of
at/below freezing temperatures will be in the current advisory
area where a glazing to up to one tenth of an inch of ice
accumulation is forecast. Locations along and southeast of a line
from Monroe to Janesville to Waukesha to Port Washington look to
remain at or above freezing by the time precipitation moves in, so
at this point just rain is expected overnight (perhaps a few
sleet pellets/snow flakes at the onset as we moisten up).

The good news is that the freezing line will be on the move
northward overnight. So the most dicey period for glazing/light
mix accumulation will be this evening through about 3 AM, with the
freezing line nearly north of the area by daybreak. Given
nighttime timing and low level temperature variability, there is
no need to alter the going advisory. The main message will be
travel impacts that will be variable - some locations wet, others
slick - and prepare for a slower morning commute.

Lingering light rain exits to the east during the daylight hours
of Friday morning with at least partial clearing expected in the
afternoon. Highs will be above average in the 30s to around 40
degrees.

Friday night through Sunday...Forecast confidence medium.

Polar high pressure will shift from the nrn Great Plains to the
nrn Great Lakes for Fri nt-Sat AM. Afterward, cyclogenesis will
begin from the central and southern Great Plains to nrn MO by 00Z
Sun as a sharp upper trough ejects newd from the 4 corners region.
The low will then track newd and rapidly deepen to 982 mb near
the WI and Upper MI border by 12z Sun. Low to mid level warm,
moist advection and the approaching sharp upper trough will begin
the rain by late Sat afternoon or early evening. An occluded
front will then pass through the evening with at least slight
chances for tstorms. The fairly fast movement of this system only
results in forecasted rainfall amounts around 0.25 inches at this
time. The occluded low may then deepen to 979 mb as it moves from
Lake Superior into Ontario, Canada. The strong low and steepening
lapse rates via cold advection will allow wswly wind gusts to
approach Wind Advisory levels for Sun AM.


.LONG TERM...Monday through Thursday...Forecast confidence medium.

A broad but low amplitude ridge will prevail over the ern USA for
early in the week. Persistent swly flow will occur for Mon-Tue.
High temps for those days will range from the middle 40s to the
lower 50s. Afterward, confidence lessens due to the uncertain
interaction of upper waves in the nrn and srn streams of a split
flow jet stream. Weak or fairly strong cyclogenesis may occur and
bring mixed pcpn to srn WI for Tue nt-Thu. Polar high pressure
will then return for Fri.



&&

.AVIATION(21Z TAFS)...
The going forecast is largely on track. The biggest change with
this update is to shrink the duration of light freezing rain and
sleet at MSN to just this evening. Slow warming is expected
during the overnight hours. Precipitation type at UES, MKE and ENW
still looks to be of the liquid variety with air temperatures
above freezing.

Current VFR conditions will gradually deteriorate to MVFR this
evening and then IFR/LIFR overnight. Visibility restrictions will
be most common with precipitation and generally in the MVFR range.
Winds will be easterly, shifting to southerly overnight and
westerly Friday morning as low pressure exits to the east.

Conditions will improve during the daylight hours of Friday
morning with partial clearing during the afternoon.


&&

.MARINE...
East-northeasterly winds will shift to southeasterly this evening
and continue to increase with time. 3 to 5 ft waves build to 4 to
7 feet tonight. A Small Craft Advisory remains in effect.

Winds will become westerly Friday as low pressure exits, but
become east-southeast again later Saturday and Sunday in advance
of our next low pressure passage. Still looking at the potential
for strong low pressure to move through the Great Lakes region
Sunday, with increasing winds and waves in response.



&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Friday for WIZ046-047-056-057-062-067.

     Winter Weather Advisory from 9 PM this evening to 6 AM CST
     Friday for WIZ051-052-058-059-063-064.

LM...Small Craft Advisory until 8 AM CST Friday for LMZ645-646.

     Small Craft Advisory from 6 PM this evening to 8 AM CST Friday
     for LMZ643-644.

&&

$$

Tonight/Friday and Aviation/Marine...Gagan
Friday Night through Thursday...Gehring



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