Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 170427
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1127 PM CDT Thu Mar 16 2017

.UPDATE...First swath of wintry mix spreading east across western
CWA attm, as a result of initial push of 700H warm air advection.
Light precip should begin to reach surface in Madison shortly.
Mostly light rain being reported but increasing intensity in spots
results in cooling and rain changing to mix. Several mPing reports
from south side of Madison reporting light rain and sleet.
Temperatures remain below freezing farther east and will be
expanding Winter Weather Advisory to include several more counties
farther east and south as light rain spreads east into these
areas. Called Sauk county as temperatures have fallen to around
33-34 degrees but no reports of any issues there.

&&

.AVIATION(06Z TAFS)...First swath of wintry mix spreading east
across southern WI attm. Mix of light rain, snow and light sleet
should be pushing into Madison shortly. More concern farther east
as surface temps remain below freezing. May be a period of light
freezing rain in parts of southeast and eastern central WI.
Otherwise, expect cigs to lower through the morning with areas of
fog lingering for a time after precipitation diminishes.

&&

.PREV DISCUSSION... (Issued 954 PM CDT Thu Mar 16 2017)

UPDATE...Temperatures have fallen below freezing across eastern
CWA where cloud cover has been thin through the evening.
Temperatures remain above freezing in the west, however dewpoint
depressions remain 5 to 10 degrees. Initial push of 700H warm air
advection triggering patchy light rain and snow over west central
Wi, soon to be reaching the surface in southwest CWA. Secondary
reinforcing push of 850-700H WAA will cause more expansive area of
light precipitation to develop over south central WI through 08Z
and then spread into southeast and east central WI later in the
night. Initial push of snow and sleet may mix with a period of
light freezing rain over northern and eastern areas. After
collaboration with GRB, decided to pull trigger on Winter Weather
Advisory for portions of the north. Held off on including areas
farther east and southeast, but may need to expand advisory as we
get better handle on ptype and impact through 08z. Also introduced
some fog to the forecast as sfc winds decrease Friday morning as
sfc front/occlusion moves through the ara.

MARINE...Tightening pressure gradient ahead of approaching warm
front and low pressure will cause increasing onshore winds through
the night. Gusts to 25 knots are expected late tonight into Friday
morning. These onshore winds will cause waves to build as well.
The Small Craft Advisory has been expanded to include all
nearshore areas, from Sheboygan to Winthrop Harbor.

PREV DISCUSSION... (Issued 625 PM CDT Thu Mar 16 2017)

AVIATION(00Z TAFS)...Lowering cigs expected tonight as period of
synoptic scale forcing sweeps across southern WI. Expect surface
wet bulb temperatures to fall to near or slightly below freezing
when precipitation commences overnight. Precip likely to begin as
-sn in the east and a mix of -ra and -sn in the west. Pavement and
surface temperatures will likely be near or below zero in some
central and eastern areas for a time. Hence timing is everything
and some areas will likely experience a wintery mix including
sleet or light freezing rain. Wintery mix to transition to rain
and end as occluded front moves through on Friday. Added fog
mention due to lighter surface winds as front moves through and
warm dewpoints moving over fresh, widespread snow field.

PREV DISCUSSION... (Issued 334 PM CDT Thu Mar 16 2017)

SHORT TERM...

An initial band of precipitation due to warm air advection and
frontogenesis aloft will reach Madison around 10 or 11 pm and
Milwaukee just after midnight tonight. This initial round should be
in the form of snow or perhaps sleet, but the question is whether or
not it will reach the ground, especially west of Madison. There is a
lot of dry air in the lower levels to overcome. This band will move
through quickly, so less than an inch accumulation is expected.

Then the main precip associated with the lower level warm air
advection and upper trough will arrive in southern WI shortly after
that initial band. Top-down methodology would describe the precip
type to be rain due to the increasing warm layer, but the surface
temperature and boundary layer temperature would suggest a chance
for freezing rain or maybe sleet.

There is still uncertainty about temperatures for tonight. Clear
skies should allow for cooling temperatures in southeast WI this
evening, then a slight warming as the clouds and precip move in with
the warm front. We will be right in that 31 to 34 degree range, so
thus not sure about the impacts from the brief period of mixed
precip.

No winter weather advisory planned at this time. We will have to see
how temperatures go this evening. Precip will be light, but it does
not take much to make untreated surfaces slippery.

LONG TERM...

Friday night through Sunday...Forecast Confidence is High:

A little rain or snow is possible in the northeast Friday night as
low pressure exits the area.

High pressure will then bring quiet weather for the weekend.
Temperatures will be around normal for Saturday, warming to a few
degrees above normal on Sunday.

Sunday night and Monday...Forecast Confidence is Medium:

Low pressure and an associated cold front is expected to bring
rain to southern Wisconsin Sunday night and Monday. Rain does
seem likely for this event, but there are some model timing
differences that are leading to a little uncertainty with when
the highest precip chances will be. Also seeing a range in model
highs due to the different front timing. Went with a blend of
model temps for now.

Tuesday through Thursday...Forecast Confidence is Medium to High:

High pressure will return for Tue/Wed, bringing mostly sunny
skies. Temps will be back around normal behind the departed cold
front.

Models are in pretty good agreement showing strong low pressure
heading for the area late in the week. This will likely bring
precip chances back into the area by Thursday. Looks like mainly
rain for this system, though could see a brief period of snow if
the precip arrives later Wed night or early Thu morning.

AVIATION(18Z TAFS)...

Initial band of precipitation will be in the form of snow or
sleet. It will take a little while to saturate from the top down,
so the best chance for precip reaching the ground with the initial
returns on radar will be from Madison and east from around 11 pm
and later.

The rest of the precip will arrive shortly after, and this will
mainly be rain. There could be some freezing rain mainly east of
Madison overnight, but this will depend heavily on surface
temperatures. Right now, there is some uncertainty about
temperatures.

Ceilings are expected to be low-end MVFR during rain early Fri
morning, but there is a chance for IFR.

MARINE...

Southerly winds due to low pressure tracking into Ontario Canada
will become gusty late tonight. This will build high waves north
of Port Washington. A small craft advisory is in effect for late
tonight through midday Friday.

Then westerly winds will likely reach Small Craft Advisory
conditions for all NSH areas of southeast WI Friday night through
Saturday, on the back side of the low.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until 9 AM CDT Friday for WIZ046-047-051-
     057-058-059-064-065.

LM...Small Craft Advisory from 4 AM to 1 PM CDT Friday for LMZ643>646.

&&

$$

Update...MBK
Tonight/Friday and Aviation/Marine...Cronce
Friday Night through Thursday...DDV



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