Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 140309 AAA
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
909 PM CST Fri Jan 13 2017

.UPDATE...

Area of flurries continues to shift east and out of the northern
and eastern portions of the forecast area this evening. These were
being driven by weak 500 mb differential cyclonic vorticity
advection and weak 850 mb frontogenesis response. Area forecast
soundings were showing the low levels remaining quite dry this
evening, which has limited any ground truth in those areas. Any
flurries will shift east of these areas by midnight.

Latest satellite trends suggest a fairly extensive area of clouds
remains to the west of the area. There were some breaks in the
southwestern counties, but those should fill in quickly. It
appears that these clouds will linger through the night into
Saturday morning, before the 850 mb trough moves through and
allows for some drier air to mix in Saturday afternoon. Thus,
will hand onto clouds through the night into Saturday morning.

Temperatures should remain nearly steady through the rest of the
night, given the clouds remaining over the region. May need to
adjust them upwards in the north and west to keep them steady into
Saturday morning.

&&

.AVIATION/06Z TAFS/...

Any flurries in northern and eastern portions of the area should
move to the east by midnight. Main issue will be clouds and
ceilings near Lake Michigan overnight into Saturday morning. Light
northeast winds in the low levels have allowed for MVFR category
ceilings in the 1000 to 2000 foot range to linger across the
southeastern counties this evening.

As winds veer to the southeast later tonight, these ceilings may
gradually mix out. Not confident that this will occur until winds
shift to the west northwest on Saturday morning, so may have to
hang onto these low clouds in TAFs during this period.

VFR category clouds should linger elsewhere later tonight into
Saturday morning, before gradually mixing out to scattered during
the afternoon and evening. May see clearing in the northern
counties during the evening as well. Light winds are expected
overnight, becoming west northwest on Saturday into Saturday
night.

&&

.MARINE...

Winds will be generally light overnight into Saturday morning,
becoming west northwest later in the day. Light winds are then
expected Saturday night into Sunday.

Winds should then gradually increase Sunday night into Monday
night, as low pressure moves into the region from the southwest.
Winds may approach Small Craft Advisory levels Tuesday into
Wednesday, as a tight pressure gradient develops with offshore
winds.

&&

.PREV DISCUSSION... /Issued 340 PM CST FRI JAN 13 2017/

TONIGHT AND SATURDAY - Forecast Confidence...Medium.

Flurry chances will linger into early tonight as a weak wave passes
through. There is forcing for ascent in the form of focused 700 mb
warm advection, weak differential cyclonic vorticity advection,
and some jet-level divergence from the right entrance region of
a departing speed max. Taking an isentropic perspective, we can
infer some upglide on the 295 theta temperature surface, with weak
omega co-located with condensation pressure deficits around 10 mb.
The biggest obstacle to getting much in the way of precipitation
at the surface remains a dry layer around 900 to 950 mb. So far
this layer has been slow to saturate, but we are seeing a few
flurry reports as we head into late afternoon. As for
accumulations, it`s not out of the question that some places could
see a couple tenths, but for most this system should provide a
dusting at best. Any flurries or light snow showers should exit
the area early tonight. Onshore fetch tonight should keep low-
level moisture in place, with mostly cloudy skies into Saturday
morning. With cloud cover and weak to neutral temperature
advection, temperatures should hold steady tonight. They may even
rise a degree or two in the southeast towards morning.

Expect quiet conditions for Saturday with surface high pressure in
control. Mostly cloudy skies should linger into morning, with
clearing arriving from the north during the afternoon. Temperatures
will be seasonal in the 20s to around 30.

SATURDAY NIGHT THROUGH MONDAY - Forecast Confidence...High
transitioning to Medium.

Fairly benign weather conditions to continue for the first half of
this period as a large high pressure area moves across the western
Great Lakes.  A cold night is likely Saturday night with light winds
and lack of significant cloud cover. Temps will likely be a few
degrees below seasonal normal.  Temps on Sunday helped by at least
partial sunshine should warm back up to around seasonal normal
for mid-January, into the mid 20s to lower 30s.

Short term guidance remains in good agreement /at least initially/on
closed low over Texas panhandle on Sunday taking a northeast track
through the central Plains into the western Great Lakes later Monday
into Monday night.  Some disagreement on strength of this system as
it progresses northeast due to possible phasing with upstream short
wave trof sagging into the northern Plains from Canada.  Despite
discrepancy on synoptic pattern resolution aloft, strong south
winds ahead of this short wave will transport deeper moisture and
warmer air northward into southern WI later Sunday night and
Monday. Initial dry atmosphere and maximum wet bulb aloft
remaining below zero should result in light snow developing at the
onset transitioning to a period of sleet and light freezing rain
late Sunday night and Monday morning before changing to mostly
light rain and drizzle later in the day Monday as warm air
continues to surge northward. Temperatures are likely to rise
above freezing later Monday morning and early afternoon. Pavement
temps likely to lag a few hours behind so may have a period of
light freezing rain and sleet that may impact the Monday morning
commute.

EXTENDED PERIOD...

MONDAY NIGHT THROUGH FRIDAY - Forecast Confidence...Low to Medium.

Medium range guidance diverges somewhat on amount of
reintensification and exact track of secondary low pressure area
forming along baroclinic over central Plains area Monday night. GFS
more aggressive on digging northern stream short wave into the upper
midwest Monday night and kicking first short wave rapidly northeast
thru the central GtLakes.  As a result, GFS now taking a more
easterly and quicker track of sfc low across southeast WI later
Monday night.  ECMWF well west of this track and slower, taking sfc
low across central and eastern IA.  Canadian a weaker blend between
GFS and ECMWF.  Also of note is that the ECMWF has been trending
east with track over last several runs.  WPC model blend relying
more on weaker ECMWF and ECMWF ensembles for sfc track and timing.

Canadian show more phasing of northern and southern streams
resulting in weaker low pressure tracking northeast through the area
during the period. Canadian and GFS farther east with low pressure
passage across srn WI which would bring a wintery mix transitioning
to mostly snow over western parts of CWA later Monday night into Tue
morning before precip concludes. For now, remaining with warmer
solution, but may need to introduce more of a mix changing to snow
in parts of the area Monday night into Tue. Hence considerable
uncertainty regarding track of this next system and ptype in later
periods.

No significant post-system cold push for the middle of next week as
upstream flow remains more zonal.  In fact, ECMWF and GFS show zonal
flow transitioning to stronger southwest flow ahead of pair of long
wave trofs moving across western CONUS later next week. Canadian
slower in transition to warmer southwest flow due to deeper low
pressure lingering over the Gulf Coast states.

GFS 5-day 500H anomolies show large 200m positive anomoly over
eastern Gtlakes and southeast Canada valid 00/Sat. However signs of
large positive anomoly weakening as steering flow transitions back
to more unsettled, zonal flow to start the week of fourth full week
of January.   Expect above normal temperatures for much of the
period with snow cover remaining low most areas.

AVIATION/00Z TAFS/...

Ceilings will continue to gradually lower this afternoon, but should
generally remain within the VFR to borderline MVFR flight
categories. Flurries and a few light snow showers remain possible
into early tonight. They shouldn`t be impactful, with a dusting to
perhaps a few tenths of an inch of accumulation at best. Quiet
weather is expected for Saturday with partial clearing by late
afternoon or evening.

MARINE...

High pressure will bring quiet conditions to the nearshore waters
for tonight through Sunday. Expect weak onshore flow tonight, with
offshore flow on Saturday. We may see flurries and a snow shower or
two this evening. Otherwise, the next chance for precipitation is on
Sunday night into Tuesday.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

UPDATE...Wood
TONIGHT/Saturday AND AVIATION/MARINE...SPM
Saturday NIGHT THROUGH Friday...MBK


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