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Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI
935 PM CST Sat Dec 20 2014

Update...

700 mb relative humidity remains rather dry tonight. 850 mb layers are saturated
but dry from the south Sunday afternoon as the warm air advection
increases. 925 mb relative humidity increases east late and stays high trough the
day. Therefore expect stratus to lower. Main concern is
drizzle/freezing drizzle. Very weak 700 mb upward motion until
after sunrise. Therefore main concern of freezing drizzle/drizzle
will be after sunrise...but temperatures will rise. In addition
surface dewpoints do not saturate so do not expect any headlines
at this time.

&&

Aviation/06z tafs/...

700 mb relative humidity remains rather dry tonight. 850 mb layers are saturated
but dry from the south Sunday afternoon as the warm air advection
increases. 925 mb relative humidity increases east late and stays high trough the
day. Therefore expect stratus to lower east with MVFR ceilings
becoming IFR across southern Wisconsin...with some LIFR Sunday
morning. Main concern is drizzle/freezing drizzle. Very weak 700
mb upward motion until after sunrise. Therefore main concern of
freezing drizzle/drizzle will be after sunrise...but temperatures
will rise. In addition surface dewpoints do not saturate so do not
expect any headlines at this time.



&&

Previous discussion... /issued 255 PM CST Sat Dec 20 2014/

Rest of this afternoon through Sunday - confidence...medium
shortwave continues to pull east with widespread very light...low
dbz returns. Precipitation expected to gradually diminish into the evening
hours. Middle levels then become rather broadly anticyclonic. Key for
any additional forcing will be largely in the low layers. This will
be caused by tightening baroclinic zone with increasing cross-
isothermal flow...isentropic lift. Moisture depth is limited and so
the layers that are moist may squeeze out some fzdz or sleet later
in the night into early. Question is there as to whether the
soundings are just too moist resulting in the trace/light quantitative precipitation forecast
generation but there is the decent isentropic lift signal so will
keep the low MOS probability of precipitation going. BUFKIT soundings show the dry air aloft
which keeps the dendridic growth zone free of saturation which
limits the introduction of ice into the column. Model quantitative precipitation forecast output
Keys on southern and southwest County Warning Area after 6z...and then the western
1/2 to 2/3rds for 12z-18z Sunday. Sustained southerly airflow will
allow for continued warming of the airmass through Sunday. However
subzero 850 temperatures will linger especially across the northwest County Warning Area so will
keep potential of a light rain/light snow mix going there.

Sunday night through Tuesday...forecast confidence medium.

Strong short wave energy will push out of the Pacific
northwest into the northern and Central Plains by Sunday night through
Monday. Surface low pressure will subsequently form from the
Dakotas...stretching south through Oklahoma during this same time.
Through Sunday night...any precipitation chances will be small and
limited to areas mostly west of Madison...and could be a rain/snow
mix. By Monday...increasing southerly flow coupled with the
arrival of the first middle level short wave trough ahead of the main
low should spread a band of rain across the forecast area during
the day. Temperatures will be warm enough to keep it all rain...except
for a potential rain/snow mix north of Madison early.

There should be a diminishing trend in precipitation Monday night and
Tuesday as the surface low splits...with one low center moving off
into northern Wisconsin while secondary development occurs over
the southern Mississippi River valley. Temperatures will still be warm
enough for all rain up to this point. In fact temperatures should climb
into the 40s on Tuesday.

Wednesday and Thursday...forecast confidence is medium.

A strong low pressure system is forecast to move from the Ohio
Valley into the Great Lakes region during this time. The models
have yet to converge on a similar track...very typical at this
stage. The GFS operational run...which is an outlier to the GFS
ensemble solutions...produces plenty of snow across the central
forecast area Wednesday into Wednesday night. This would have big
impact. However...as mentioned...this is a big outlier to the
ensembles which suggest a solution closer to the European model (ecmwf) is a higher
probability. The European model (ecmwf) takes the low from the Ohio/ind border to
Northern Lake Huron vicinity. This would keep most of the higher precipitation
much farther east...giving our area about an inch of snow. The
Canadian Gem is...again...in between these two. At this point the
consensus approach is best and it gives about 3-4 inches of snow
across southern Wisconsin Christmas evening...drying out on Christmas.
So...a white Christmas is still possible. The biggest travel
concerns will be Christmas evening...especially if you/re traveling
east and north of the area.

Friday and Saturday...forecast confidence is medium.

Another weak low...associated with another Pacific northwest
short wave...will arrive on Friday and could bring US another
round of snow. This is a fast mover...so accums should stay fairly
low. Dry weather is expected on Saturday as high pressure finally
moves in. We might see some sunshine...but temperatures will be much
cooler.

Aviation/00z tafs/...water vapor loop shows shortwave moving east
into lower Michigan. Widespread very light snow/flurries expected to persist
into the early evening hours in the eastern County Warning Area. Little if any
accumulation. Remaining VFR ceilings will likely transition to MVFR as
the afternoon wears on. Sref ceiling prbs indicate increase potential
for IFR ceilings later tonight in SC WI and Sun morning with MVFR ceilings hanging
on in the eastern areas. With isentropic ascent increasing expect
some zl to develop after 06z and linger into early Sun morning before temperatures
modify above freezing. Enough middle level saturation in the dendridic growth
zone to suggest some snow mixing in as well on Sunday...but precipitation
potential becomes confined to northwest County Warning Area...north and northwest of taf
sites....esp after 18z Sunday.

Marine...hoisted Small Craft Advisory for Sunday night through
Monday night due to strong south-southwest winds ahead of approaching low
pressure. Highest waves will be in the northern nearshore waters but
looks like all of the nearshore area will get in on some of the
gusty winds.

&&

Mkx watches/warnings/advisories...
WI...none.
Lm...Small Craft Advisory from 10 PM Sunday to 3 am CST Tuesday for
lmz643>646.

&&

$$

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

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