Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
1141 am CDT Friday Sep 19 2014
Updated aviation portion for 18z taf issuance
Short term...today...tonight...and Saturday
issued at 402 am CDT Friday Sep 19 2014
Low level moisture trapped under an inversion is
once again producing low clouds across much of the area early
today. It will probably mix out by midday in most places. Strong
warm advection at 850mb today but there is little moisture below
500mb so we do not expect any precipitation except maybe in the
far north during the afternoon. Highs today will depend on how
quickly the clouds depart. Temperatures will be cooler along the
There is a better chance of showers and thunderstorms tonight as a
weak coupled jet moves across the region while strong low level
warm advection continues. This should produce a broken area of
precipitation that moves slowly south across the area. Temperatures
will not fall much due to cloud cover and a steady breeze.
A cold front will move across the area Saturday. There should be
abundant low level moisture available ahead of the front but the
question is whether there will be enough instability to eliminate
an inversion and get thunderstorms before the front GOES by.
Forecast soundings suggest that there may be 1000 j/kg of cape by
afternoon with middle level winds of 40 to 50 knots. So an isolated
severe thunderstorm with strong winds is not out of the question.
Long term...Saturday night through Thursday
issued at 402 am CDT Friday Sep 19 2014
Middle/upper trough will slides over the area on Saturday night and
Sunday...with a northwest flow dominating the western Great Lakes.
This will be the best period for rain in the extended. The
amplified patterns returns by early next week...with strong
ridging over western Canada and a trough over eastern Canada and
the eastern Great Lakes. The ridge will slide east early next
week...and bring a fairly quiet upper level pattern to the
eastern 2/3 of Canada and U.S. However...models now show a weak
disturbance in the middle-levels crossing into the plains by middle-
week. This could throw some issues into the dry forecast as we
head toward late Wednesday and Thursday. At the surface...a
sprawling high pressure will slowly build into the western Great
Lakes early next week...with return flow setting up as the high
spreads toward the eastern Seaboard.
Precipitation chances will be the main forecast challenge to start off
the extended forecast...as upper trough slides across the region.
Ingredients seem to be coming together for more of a widespread
precipitation event...especially over norther Wisconsin...on Saturday
night as synoptic lift will be enhanced with the area in the left
front quadrant of the upper jet and two potent shortwaves will be
crossing the region. The European model (ecmwf) and Gem are on board with this
thinking...but the GFS and NAM are a little less bullish. Moisture
seems more than sufficient under the cyclonic flow. Will keep the
probability of precipitation mainly in the high chance category...but can see them needing
to be bumped up...especially over the north.
The best forcing and moisture exit the early Sunday morning...as
drier air and ridging begin to work into the western Great Lakes.
Cyclonic flow...lingering moisture...and cool air aloft will keep
at least a chance of showers through the day. Dry weather is then
expected Sunday night into early next week...as upper and surface
ridging dominate the region. Some questions enter the forecast
late Wednesday and Thursday...as models now show a weak middle-level
shortwave trough and increasing moisture pushing from the northern
plains into the western Great Lakes. Will hold on to the dry
forecast...however if models continue to hang on to this
trend...probability of precipitation will be needed by Thursday.
Temperatures look to start off a little below normal...with some chilly
nights expected...then slowly moderating temperatures are expected early
to middle next week.
Aviation...for 18z taf issuance
issued at 1140 am CDT Friday Sep 19 2014
Gusty southwest winds up to 25 or 30 kts will continue
until early this evening across the region but with VFR conditions. Then
showers and thunderstorms are expected to develop along a cold front
over Minnesota and Iowa late this afternoon or this evening. Some
of these storms will make a run towards the central Wisconsin taf
sites and possibly north-central Wisconsin as well. Gusty winds and
small hail are possible within any storms. These storms are expect
to weaken late tonight as they approach the Fox Valley and Lake
Shore taf sites. Behind the storms...cigs/vsbys will be dropping
quickly to IFR and MVFR levels. Improvement to the ceilings and visibilities
will take place by middle to late morning on Saturday.