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Area forecast discussion 
National Weather Service Green Bay WI
546 PM CST sun Dec 21 2014

Updated aviation portion for 00z taf issuance

Short term...tonight and Monday
issued at 313 PM CST sun Dec 21 2014

First round of precipitation currently moving across WI...mainly in the
form of light snow. Main forecast issue for tonight is how long to hang
on to the snow before tapering down to freezing drizzle or
flurries. Main forecast issue for Monday will be precipitation type in the afternoon
as the next surge in moisture overspreads the region.

The 19z msas surface analysis showed hi pressure just east of the Great
Lakes with low pressure over the central hi plains. A broad S-SW wind
was pumping warm/moist air all the way to the Canadian border with
dew points in the lower 30s and temperatures above normal. Radar mosaic
indicated light snow over the northwest half of WI associated primarily
with warm air advection.

This initial band of mainly light snow (mixed with a little rain
perhaps over central/E-cntrl wi) will continue through the evening hours with
the greatest impact north of a line from isw-mnm. Potential for
around an inch or two of additional accumulation in these
locations with little if any accumulation for east-central WI. Forecast
soundings indicate a loss of saturation in the middle-levels of the
atmosphere later tonight...thus the snow should taper off to patchy
freezing drizzle or flurries. Due to the coninuation of warm air advection through
the not expect temperatures to fall much with upper 20s to lower 30s

NE WI to reside in between areas of lift Monday morning as the
initial warm air advection weakens a bit and the main/forcing associated with a
longwave trough still to our south and west. Thus...have toned down
morning probability of precipitation especially across eastern WI. By Monday afternoon...a jet
streak rounds the base of this upper trough and helps to lift an 800 mb
warm front northward to the WI/Illinois border by 00z Tuesday. Increasing lift/forcing
will bring precipitation from south to north into NE WI with the highest probability of precipitation
over central/E-cntrl WI for the afternoon. The main forecast issue to be
precipitation type as the 800 mb 0c isotherm approaches central WI and surface temperatures
rise into the lower to middle 30s north...middle to upper 30s south. By the
time the precipitation arrives in earnest...cntrl/E-cntrl WI would see a
mix or even change to all rain. Northern WI would be mostly snow through
the day...but with temperatures at or above freezing...snow accumulations
would be limited to under an inch.

Long term...Monday night through Sunday
issued at 313 PM CST sun Dec 21 2014

Forecasts last few days were hinting of deepening storm system by middle
week with high impacts for the Great Lakes. Morning runs using a
better sampling over western states has backed off on impacts for
the area this week.

From Monday night into Tuesday...brunt of the warm air advection surge of precipitation
from around 0.20 to 0.45 inches water equivalent during a 24 hour
period spreads northward over the area. The primary issue with
this system is precipitation type as forecast soundings suggests mixed
precipitation. Parts of NC and central Wisconsin could turn to more snow as
upper heights fall and temperatures cool aloft and could reach the better
snow growth region aloft. Otherwise snow ratios are expected to be
low so a compact snow may be the rule. The snow mix may be enough
for advisory of some type for the forecast area during this
period. Meanwhile over eastern Wisconsin...precipitation appears
to be mostly rain.

The system is prognosticated to evolve east of the area Wednesday into
Thursday. Colder air wrapping around the system will turn the mix
back over to snow over at least the northwest half of the state.
Much of east central Wisconsin remains in the dry slot. Southern
stream energy with a surge of precipitation which eventually phases into
the northern low...occurs a bit further east of the area for a
diminished threat of accumulating snow over the area. Even winds
expected to be less but still blustery over eastern Wisconsin
through Thursday...with the trend to less snow to drift around.

Forecasts have been going back and forth with a weakening system passing
over over the region Thursday night into Friday that could
produce an advisory type of snow over the southeast two thirds of
the state.

Aviation...for 00z taf issuance
issued at 542 PM CST sun Dec 21 2014

Poor flight conditions expected to persist for the next 24 hours and
beyond. Low clouds with generally IFR/LIFR ceilings will remain widespread
across the area. Precipitation will be more widespread across the north...but
possible across the area. Hard to see a sig long-lived improvement
in conditions until the main upper trough axis shifts east of the area

issued at 313 PM CST sun Dec 21 2014

Relatively warm slowly winter storm system will track over the
Great Lakes region from tonight into Wednesday. While snow or a
snow mixture is expected over north central...central
Wisconsin...mainly rain is expected over eastern Wisconsin. Latest
model data suggests up to a half inch of rain is expected from
Monday evening into Tuesday morning with temperatures possible
rising to near 40 on Tuesday. While the rain will produce some
runoff...primary issue is the potential affects on ice conditions
on area rivers and lakes.

Grb watches/warnings/advisories...


Short term.....AK
long term......tdh

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