Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 212347
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
547 PM CST Thu Feb 21 2019

.UPDATE...
High clouds will persist through the night with some low to mid
level moisture possibly building in overnight. As of right now
most of the moisture looks to remain to the north and west,
however if some of that moisture does push in we could see some
overnight development of low clouds. Otherwise conditions are
expected to remain quiet through tonight with low temperatures
falling to the low to mid teens.

&&

.AVIATION(00Z TAFS)...
VFR conditions are expected through the TAF period, however there
are some concerns for some low CIGS overnight. Low to mid level
moisture primarily to the north and west could push into the
Madison area and bring CIGS to 1-3 kft. At this point it is
unlikely given that moisture should remain to the north and west
of the region. Winds from the west will become calm tonight and
remain calm to light throughout the TAF period with light winds
from the ESE by Friday afternoon.

&&

.PREV DISCUSSION... (Issued 328 PM CST Thu Feb 21 2019)

SHORT TERM...

Tonight and Friday...Forecast Confidence is High.

Surface high pressure will move over region during the next 24
hours. Mainly clear skies will persist into much of tonight, with
a gradual increase in mid/high level cloud coverage late tonight
into tomorrow. Given clear skies, light winds and mainly clear
skies, temperatures will be tricky tonight. Have gone with lows
around 10 (give or take) but valleys and sheltered areas can
easily drop well into the single digits at any given point.

Little change to the forecast for Friday with a mix of mid/high
clouds and sun. Temperatures should warm into the low/mid 30s in
most locations.

LONG TERM...

Main forecast challenge is the strong storm system for this
weekend.

Saturday and Sunday...Forecast Confidence is Medium.
Robust upper level wave over the Four Corners region will quickly
move northeast and become negatively tilted as it moves out of
the southern Plains. Guidance is in general agreement that the
850mb and surface low will move over southern Wisconsin. Precise
track will be a defining feature of this event. The 12z suite of
model output features only small deviations in track, but even a
small deviation will have substantial consequences on
precipitation type. Bottom line, we will have to watch the low
track very carefully with this system.

Have sided with the ECMWF/GFS for the passage of this system,
which moves the low from TOP to SBM in about a 12 hour period from
18z Saturday to 06z Sunday. Initial wave of moisture late Friday
night into the daylight hours of Saturday morning will encounter a
rather potent dry layer. Precip onset has been delayed a few hours
and the influence of this dry layer in delaying onset even further
needs to be monitored.

Temperature profiles Saturday morning support initial freezing
rain (maybe a little sleet), though QPF amounts are generally
light with pockets of higher amounts. The GFS QPF has been
discounted as an outlier for 12-18 UTC Saturday. Light QPF
continues through Saturday afternoon as temperatures warm to at or
above freezing. Current freezing rain estimates are between 0.05"
and 0.15" for Saturday, and will likely need to consider a winter
weather advisory for slick spots on area roads. Given the
temperatures going into the event, the higher sun angle of late
February and mainly light rates, the accumulation may be most
noticeable on elevated objects.

The main push of moisture arrives Saturday evening. Temperature
profiles look to be at or above freezing for nearly the entire
area. One area of uncertainty and concern is from (roughly) Lone
Rock through the Dells. If they are unable to warm above freezing,
there is a risk of additional ice accumulation. There is ample
moisture availability, so there will be a period of moderate to
heavy rainfall rates. The good news is this system is progressive.
As it moves through overnight Saturday night, the dry slot
overtakes a good part of the area and precipitation rates will
lessen considerably. A brief period of light snow is expected as
the upper wave moves through. A little too early to get into
amounts, but it appears any accumulation should be on the light
side (an inch or two from Lone Rock to Fond du Lac and points
northwest).

Sunday will be raw. Light snow will be winding down in the
morning, but winds will be on the increase. The surface low will
be deepening as it moves through and strong/gusty northwest winds
are expected. Expected sustained NW winds of around 30mph with
gusts to around 45mph. Wind advisory may be needed for a good
portion of the area.

Bottom line with this system...initial freezing rain Saturday
morning changes to rain Saturday afternoon and evening. Ice
accumulation on the order of 0.10" (give or take). Rain changes
to snow from west to east overnight Saturday night, ending Sunday
morning with only minor accumulation expected. Heaviest rainfall
Saturday evening. Runoff will lead to flooding in low spots and a
within bank rise on most area rivers. Highest area of uncertainty
is LNR to DLL where temperatures may struggle to warm above
freezing (depends on surface low track).

Monday onward...Forecast Confidence is Medium to Low.
A pattern shift to a more zonal/northwesterly flow is expected.
This will result in a decidedly colder pattern. Another upper wave
passage is expected in the Tue/Wed time frame, but it will have
far less moisture to work with. Bottom line, well below average
temperatures.

AVIATION(21Z TAFS)...
VFR conditions are expected for the next 24 or so hours. High
pressure moving over the region will result in light and variable
winds tonight through much of Friday. Mainly clear skies are
expected today and this evening, with the occasional patch of
cirrus here and there. Mid and high level moisture begins to
increase overnight tonight and especially Friday. Looking beyond
the TAF cycle, the weekend weather conditions look active as a
strong storm system moves overhead.

MARINE...

Lake Ice...
A clear satellite view of Lake Michigan showed a narrow and rather
scattered axis of lake ice pushing off the western shore of the
lake, out into the open waters. This should mostly dissipate as
winds increase this weekend.

Expect light winds across Lake Michigan through Friday as high
pressure drifts across the Great Lakes. A major storm system will
then begin to organize and head toward the Great Lakes this
weekend. East to southeast winds will begin to increase Friday
night, remaining elevated through Saturday. A very intense low
pressure system will track from northern Missouri Saturday evening
to northern Lake Michigan by sunrise Sunday morning.

Those easterly winds will ramp up very quickly late Saturday
afternoon into Saturday night...gradually veering to the south
then southwest into early Sunday morning. Gale force southerly
winds are expected by early Sunday morning. Gales will continue
through Sunday night as winds continue to veer to the west on
Sunday and northwest Sunday night. Mariners should be aware that
winds will approach storm force levels at times on Sunday, before
gradually diminishing Sunday night.

Winds should begin to settle down by Monday with no major wind or
wave concerns into the middle of next week.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Gale Watch from late Saturday night through late Sunday night
     for LMZ080-261-362-364-366-563-565-567-643>646-669-671-673-
     675-777-779-868-870-872-874-876-878.

&&

$$

Update...ARK
Tonight/Friday and Aviation/Marine...ARK
Friday Night through Thursday...99


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