Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
337 am CST Monday Dec 29 2014
Today and tonight... forecast confidence high.
Radar returns along the WI/Illinois border back to the west over NE Iowa
from light snow aloft...forced by weak warm air advection/isentropic
lift and middle-level moisture ahead of weak short wave. Forcing
weakens as wave shears out later this morning as it rides 120 knot
jet streak into confluent zone over the eastern Great Lakes.
Surface observation not reporting any snow under returns...with lowest cloud
heights around 8k-9k feet. This supports dry layer below 10k feet on
forecast soundings that is evaporating/sublimating any snow falling
Cold air makes it/S entrance into the region during this period as
slow moving cold front moves through southern WI today as polar high
builds down into the plains...with 925 mb temperatures falling about 7c-8c
to-15c to -17c by 12z Tuesday. Current cloud cover has limited temperature
fall overnight so highs should be able to reach low to middle 20s with
slow cold advection process. Lows tonight will tumble into the
single digits...with low teens along the lake and near the Illinois border
in southeast WI.
There is a very low chance of some lake effect flurries over far
southeast WI with some weak 1000=900 mb convergence that is mainly
focused over the far southern end of Lake Michigan...affecting NE Illinois/northwest in
for a brief time this morning. While the lake water temperature/850
mb temperature differential is somewhat favorable the shallow cloud
depth will limit potential. Will not mention at this time.
Tuesday - confidence...medium
steady cold advection continues. Upper flow is strong though
shortwave dropping southeast will shift into northwest WI...keeping any
forcing well north of southern WI. 925 temperatures slide into the -15c to
-18c range. This yields highs only in the teens. Low level relative humidity
forecasts keep the better moisture across northern WI. So expect a good deal
of sunshine but a steady northwest wind will add extra bite.
Tuesday night and Wednesday - confidence...medium
a very cold period persists with pressure gradient tightening up
between high pressure passing to our south in the plains and low
pressure moving through Canada. Low level thermal trough will be
in place with 925 temperatures averaging -18c with 925 winds expected at or above
20 knots. Expect some mixed conditions with the evolving tighter
gradient. This should result in some wind chills dropping into the
teens below zero for a time late Tuesday night into Wednesday
morning. Cips analogs showing a 50-60 percent shot at dropping
below -20. Surface temperatures will be modified by lack of
snowcover...so leaned a bit towards warmer guidance options. 925
winds really ramp up as the day GOES on. 925 temperatures modify to -11
to -13c...suggesting temperatures only reaching a bit deeper into the
teens and with gusty west-southwest winds...wind chills likely to show a slow
Thursday - confidence...medium
rather strong northwest upper flow. Any vorticity within this flow looks
largely sheared. Weak front is proggd to sag into the area with
the GFS now showing light quantitative precipitation forecast potential and the European model (ecmwf) has trended
dry. Soundings look overly moist on the GFS and prefer the drier
look of the NAM soundings. Will elect to keep the dry forecast
going. Expecting highs to moderate back into the 20s as low level
westerly flow continues to modify the 925/850 cold pool.
Friday - confidence...medium
upper flow remains strong across the upper Midwest with southern
stream energy in the SW US. Parallel GFS/Gem and European model (ecmwf) all show a
surface low of varying strengths developing across the upper lakes
and shifting eastward. These forecasts suggest some light quantitative precipitation forecast across
northern/central WI so having some probability of precipitation in our northern County Warning Area seems reasonable
Saturday and Sunday - confidence...low
the GFS is hanging onto the scenario of bringing the energy from
the southern stream northeastward towards the Midwest bringing precipitation. In
fact this outlier solution shows some pretty warm 850 temperatures and
1000-500 millibar thicknesses Sat and Sat night. However other
models including parallel GFS suggest this southern stream low
will stay well south and east with northern stream jet stream
dominating. Superblend probability of precipitation retained which keeps the chance probability of precipitation
going for the weekend.
Aviation/12z tafs/...MVFR cloud deck along leading edge of cold front
has dissipated...though a brief period of broken 1800-2k feet is possible
in the far southeast with western edge of expected lake-effect cloud band/flurries
brushing Lake Shore locations this morning. Will go with VFR ceilings/visibilities
through the forecast with polar high pressure building into the plains.
Middle-high clouds with weak short wave today will scatter out tonight.
Marine...north winds will stay below small craft levels...with waves
generally under 4 feet behind a cold front slowly dropping through
the region today. Winds will backing to the northwest Tuesday...then
west Tuesday night with winds and waves expected to remain below
Small Craft Advisory levels.
Winds increase Wednesday/Wednesday night as western Great Lakes will
be in the tightening pressure gradient between the broad high
centered over the Southern Plains and low pressure crossing Canada.
Gusts could approach gale force during this period.
Today/tonight and aviation/marine...rem
Tuesday through Sunday...collar