Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
632 am CDT Thursday Sep 3 2015
Updated aviation portion for 12z taf issuance
Short term...today...tonight...and Friday
issued at 324 am CDT Thursday Sep 3 2015
Main forecast concern is precipitation chances as an area of heavy
rain tracks southward through the County Warning Area with the possibility of
additional activity out to the west this afternoon and tonight
with a lingering boundary.
A perusal of the hi-res models does not yield too much in the way
of help in the short term as the NAM seems to be the only useful
model in the initial placement of the current area of showers and
thunderstorms across northeast Wisconsin. This activity is
associated with a shortwave trough seen on water vapor imagery
over northeast Wisconsin. This shortwave is expected to track east
of the area during the morning hours...taking the showers and
thunderstorms with it. Some patchy dense fog is possible this
morning as the complex heads east...mainly across central and
north central Wisconsin where some clearing is expected before
sunrise. Will handle any fog with special weather statements.
This complex has laid out a weak surface boundary across central
Wisconsin...which will remain over the western County Warning Area during the
afternoon hours while instability builds to the west of the
boundary as MUCAPES rise to 2000 to 3000 j/kg during the afternoon
hours. Two separate shortwaves are on track to travel through the
western Great Lakes this afternoon and this evening in the
vicinity of the aforementioned surface boundary. Although the NAM
does not light this boundary up until the second shortwave tracks
through this evening...the presence of the surface boundary and
instability deserve a mention of low Chancy probability of precipitation across the
western County Warning Area this afternoon. With a very weak wind field in place
these storms are expected to be sub severe this afternoon and this
evening. Fog is again expected during the overnight hours given
the recent rains...light winds and breaks in the cloud cover.
Yet another shortwave will track through the western Great Lakes
Friday afternoon as MUCAPES rise to 1000 to 2000 j/kg. However a
layer of warm air around 800 mb will make convection less likely
than today...therefore will stay with a slight chance pop given
the cap in place.
Clouds currently in place across the western Great Lakes will
eventually erode from west to east...allowing daytime highs to get
into the lower to middle 80s during the afternoon hours. A very
warm airmass will remain over the region tonight...with lows
staying in the lower to middle 60s. Daytime highs on Friday will
again be similar to today...with highs in the lower to middle 80s
expected away from the Great Lakes.
Long term...Friday night through Wednesday
issued at 324 am CDT Thursday Sep 3 2015
500mb ridge over the central United States will flatten by early
next week...then a shift in the pattern is expected as 500mb
ridge builds across the eastern Pacific Ocean/West Coast
of North America with 500mb troughing across the central
portions of the United States and southern Canada.
Above normal temperatures should return closer to normal by
the middle of next week.
Main concern tonight is the small chances of thunderstorms Friday
night and Saturday. For Friday night...weak warm front lifts
northward with a few models depicting some light quantitative precipitation forecast amounts.
Very warm and humid conditions should prevail on Saturday.
BUFKIT soundings indicated 1500 to 2500 j/kg of cape with little
cin. Have small chances of thunderstorms on Saturday. Any storm
could produce gusty winds and locally heavy rainfall. Later shifts
may need to increase rain chances.
The best chance of thunderstorms will late Sunday afternoon into
Monday morning as cold front moves slowly through the region.
Widespread severe weather is not expected at this time. The latest
European model (ecmwf) would suggest enough lingering moisture for a small chance
of storms across east-central Wisconsin Monday afternoon. High
pressure will then dominate the weather pattern Monday night into
Tuesday morning. Some timing differences noted of the arrival of
the next system Tuesday afternoon into Tuesday night. Later shifts
can fine tune the details on the timing of this system.
Aviation...for 12z taf issuance
issued at 631 am CDT Thursday Sep 3 2015
Showers and thunderstorms have passed east of the taf sites this
morning. Some partial clearing and low stratus in the wake of this
activity will bring IFR/MVFR conditions to the taf sites this
morning until the stratus gets mixed out with VFR conditions
expected this afternoon. There is a chance for additional showers
and thunderstorms this afternoon and this evening as a shortwave
tracks through the western Great Lakes. However coverage will be
isolated enough to leave out of the current set of tafs. Partly
cloudy skies will combine with light winds and surface moisture
from recent rainfall to create dense fog at the taf sites
overnight with IFR conditions expected.