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Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI
929 PM CST sun Dec 21 2014

Update...

00z NAM model soundings show the low level saturation to around 4
thousand feet with dry layer above until 10 thousand feet. Then soundings
saturate again. This is in the dendridic growth zone...however too
much separation for any seeder feeder mechanism.

00z NAM tracks the initial surface low across southern Wisconsin
just to the west and north of Madison. Despite the 18z GFS moving
the secondary low back westward...the 00z NAM continues the east
track of the secondary low Wednesday night and Thursday morning.

&&

Aviation/06z tafs/...

00z NAM model soundings show the low level saturation to around 4
thousand feet with dry layer above until 10 thousand feet. Then soundings
saturate again. IFR ceilings will spread slowly northeast across the
forecast area tonight and into Monday. Overall models keep most of
the measurable precipitation across western and northern WI with less in the
southeast. With low pressure and a more organized shortwave approaching
Monday afternoon/evening expect more widespread rainfall moving
into southern WI. The area of above zero celsius temperatures increases
towards Monday night.

&&

Previous discussion... /issued 303 PM CST sun Dec 21 2014/

Rest of this afternoon and tonight - confidence...medium
lead wave combined with warm air advection generating light precipitation returns across
western County Warning Area. However 88-d trends show coverage decreasing rapidly with this.
Spotty light rain or light snow with this band. Moist flow continues
ahead of advancing low pressure trough in the plains. Surface temperatures
not expected to drop much due to southerly winds and cloud cover. Middle
level shortwave expected to amplify as it draws closer. Weaker shortwave
to brush southern WI after 6z. Will keep probability of precipitation confined to northwest County Warning Area closer to
850 low level jet axis and area of model quantitative precipitation forecast. With middle levels showing a dry look
on BUFKIT soundings and anticipated temperatures remaining above zero not
expecting any freezing rain/drizzle.

Monday - confidence...medium
nearly stacked low movers into western Iowa. Shortwave energy wrapping
around this upper low shifts into southern WI and coincides with increasing
850 millibar wind maximum during the afternoon hours. Mild airmass with highs
in the u30s/l40s expected. NAM sounding in northwest County Warning Area look a smidge cold
in the low levels. Prefer the better handling of low level temperature profile
on the GFS sounding. But precipitation type still questionable into the afternoon
across the northwest County Warning Area. Area soundings showing snow potential with saturation in the
dendridic growth zone. However low levels will determine eventual precipitation
type. So will still keep mixed precipitation into the afternoon there with very light
accums. Much of County Warning Area expected to be rain in the afternoon though the northwest is still
vulnerable with lowest 850 temperatures and coldest 1000-500 millibar thicknesses.

Monday night and Tuesday...forecast confidence is high.

Fairly strong dynamics arrive Monday night as the main closed middle
level low spins over Iowa into southern Minnesota...ejecting decent short
wave energy northeast into Wisconsin. The associated surface low
will track from northwest Iowa to north central Wisconsin between
6 PM Monday through middle-day on Tuesday. Decent warm air advection
ahead of the low...resulting in strong q-vector convergence when
coupled with the middle level short wave...supports very high probability of precipitation
into the evening. Temperature profiles support all rain with the
possibility of a rain/snow mix northwest of Madison.

We should get dry slotted later Monday night through Tuesday with
low probability of precipitation moving in. Likely just another cloudy day with a small
chance of rain or some drizzle as this first low moves off to the
northeast. Temperatures will be mild as we remain on the warm side of
the low/trof.

Tuesday night through Wednesday...forecast confidence
increasing.

If you/ve been following the discussions...you know there has been
an incredible amount of model variability in the handling of the
secondary low pressure development coming out of the lower
Mississippi Valley for this period. The new 12z runs are much
tighter and the lower spreads in possible solutions provides
higher confidence. The consensus low track is from the Ohio Valley
near Louisville to southeast lower Michigan...then northeast from
there. The southeast Wisconsin forecast area...including the
Milwaukee metropolitan area could get into the far western precipitation shield
and generate some light snow. But temperatures would probably still be
warm enough to melt most of it.

The GFS/ECMWF/NAM/Gem keep the highest impact weather well east
of US. Of minor concern is a 12z sun parallel high resolution run
of the GFS that shows a major low moving from St. Louis to
Milwaukee during this time. It has been assessed as a significant
outlier...not close to any other ensemble runs...and is being
discarded. I mention it as this solution may make it/S way out
into the social media world and raise questions. And the
caveat...we can/T let our guard down. The tracks of these lows can
change...so remain up to date on the latest forecast through
Christmas evening.

Christmas day...forecast confidence is high.

For all those planning to travel on the Holiday...it looks like a
decent day with dry weather as high pressure briefly builds in.
Temperatures will be in the lower to middle 30s.

Friday...forecast confidence is medium.

Late Thursday night and Friday...a fast moving short wave arrives
in the zonal flow and could bring around an inch of snow. The GFS
is much farther north with the surface low track compared to the
European model (ecmwf) and this has been the case for the last few runs. Wpc is
favoring the European model (ecmwf) which will support a colder solution...with
that inch of snow possible.

Saturday and Sunday...forecast confidence is medium.

Looks like high pressure returns for next weekend. It looks dry
with temperatures running below normal...generally highs in the lower to
middle 20s.

Aviation/00z tafs/...combo of shortwave trough moving east from western
Minnesota and western Iowa along with sustained warm air advection...resulting in most
widespread precipitation across ec Minnesota into northwest WI. Band of more scattered
precipitation extends southward through SC WI. This will continue to
translate eastward this afternoon/Erly evening moving through the rest of southern WI.
Surface temperatures above freezing. Rap BUFKIT soundings show the low level
saturation with dry layer above and hints of middle level saturation
within dendridic growth zone...so cannot rule out some snow...though
seeder feeder dry depth is fairly large. IFR ceilings and visibilities will
prevail with the NAM MOS and consshort having the best handle on
ceiling/visibility trends. Overall models keep most of precipitation this period
across western and northern WI with less in the southeast. With low pressure and
more organized shortwave approaching Monday afternoon/evening expect
more widespread rainfall moving into southern WI. Any mixy potential
looks to be north or northwest of taf sites.

Marine...a Small Craft Advisory is in effect from Port Washington
and northward and starts at 10 PM south of Port Washington. As low
pressure approaches from the northern plains...breezy south to
southeast winds will continue to ramp up into Monday night. The
southerly fetch will generate high waves during this period. The
winds and waves will subside on Tuesday as the weakening low
pressure area moves across the region. The Small Craft Advisory will
expire early Tuesday am.

&&

Mkx watches/warnings/advisories...
WI...none.
Lm...Small Craft Advisory until 3 am CST Tuesday for lmz644>646.

Small Craft Advisory until 6 am CST Tuesday for lmz643.

&&

$$

Update...hentz
tonight/Monday and aviation/marine...collar
Monday night through Sunday...Davis

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