Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
617 am CDT Tuesday Sep 1 2015
Updated aviation portion for 12z taf issuance
Short term...today...tonight...and Wednesday
issued at 254 am CDT Tuesday Sep 1 2015
Increased surface moisture...light winds and a steep low level
inversion across the western Great Lakes has caused fog
development across the County Warning Area early this morning. This fog should
linger into the morning hours until it Burns off with daytime
heating and mixing into a dryer layer aloft. However fog should
linger over the marine areas due to cool nearshore lake
temperatures and the warm airmass overhead.
Once the fog Burns off it will be a very warm and humid day with a
combination of the aforementioned abundant low level moisture as
surface dew points stay in the 65 to 70 degree range and 850 mb
temperatures soar into the upper teens...allowing daytime highs to
climb well into the 80s...with a few 90 degree readings not out
of the question. The combination of heat and humidity will cause
heat index values to rise into the lower to middle 90s across most
areas save for north central Wisconsin during the afternoon hours.
A weak cold front approaching northwest Wisconsin this afternoon
could bring a slight chance for showers and thunderstorms across
far north central Wisconsin. Despite the weak cold front...MUCAPES
will soar to around 1500 j/kg as the right entrance region of an
upper level jet tracks through the northwestern Great Lakes. Any
storms that potentially develop should be fairly benign...as there
will be little in the way of directional and speed shear.
Tonight will pretty much be a repeat of last night as lingering
moisture and light winds cause patchy fog across the County Warning Area. A warm
airmass in place along with high dew points will generally limit
low temperatures to the middle to upper 60s across the forecast
area tonight...with some low 60s possible across portions of north
Wednesday will be similar to today with respect to temperatures as
a similar airmass will be in place across the region. However
there will be a better chance for showers and thunderstorms across
the region as a middle level shortwave tracks towards the western
Great Lakes coincident with a 850-700 mb Theta-E surge. Despite
MUCAPE values soaring to 2000 to 3000 j/kg...winds will be even
lighter with 30 knots of wind not seen until you get above 500 mb.
Therefore severe weather is not anticipated even if storms get
going on Wednesday.
Long term...Wednesday night through Monday
issued at 254 am CDT Tuesday Sep 1 2015
Models continue to show that dominant sub-tropical ridging will be
present through at least Sunday. The latest wrinkle probability of precipitation up in the
European model (ecmwf) which slows down the eastward progression of the western
trough and cold front into next Monday or Tuesday. The latest GFS
and GFS ensemble means remain close to the previous
continuity...bringing the front through Sunday night into early
Monday. Though it would not be surprising for a slower arrival of
the front given the amplitude of the pattern...will stick with the
GFS until the slower solution gets more support.
Wednesday night through Thursday night...a weak shortwave will be
moving across the western Great Lakes Wednesday night into Thursday.
This shortwave in concert with weak middle-level convergence will lead
to a small chance of showers and storms. The sref indicates that
highest probabilities will be over central and east-central WI...and
this matches where the highest precipitable waters and instability will reside.
Will modify probability of precipitation to match these thoughts. The shortwave exits on
Thursday night...which should end the chance of precipitation. No
significant changes to temperatures over the period.
Rest of the forecast...sub-tropical ridging will hang tough through
Saturday...which still looks warm...muggy...and dry. The next cold
front to move across the region Sunday night and Monday...should
provide the next widespread threat of showers and storms. Turning
cooler behind the front for early next week.
Aviation...for 12z taf issuance
issued at 617 am CDT Tuesday Sep 1 2015
IFR/MVFR ceilings/visibilities will improve rapidly this morning to VFR
conditions once southwest winds get going at the taf sites. Fog
at mtw may linger given the proximity to the Lakeshore until winds
become offshore. Fog is again expected late tonight as lingering
moisture and light winds again allow for restrictions to
ceilings/visibilities back to IFR/MVFR.