Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
641 PM CDT Friday Aug 28 2015
Updated aviation portion for 00z taf issuance
Short term...tonight and Saturday
issued at 219 PM CDT Friday Aug 28 2015
Main forecast issue continues to focus on the extent of the precipitation shield
across NE WI tonight/Sat as two separate systems converge on the
Great Lakes. The only thing of certainty is that the heavier quantitative precipitation forecast
will be focused on the southern half of WI tonight before the precipitation gradually
pulls away on Sat.
The 19z msas surface analysis showed a weak area of low pressure on the
Nebraska/Iowa border with a weak warm front stretched eastward across central
sections of Iowa. Radar mosaic indicated a large area of light to
moderate rain across SW WI...southeast Minnesota...NE Iowa and northwest Illinois. This precipitation
shield was having a hard time moving northward...away from the better
The weak surface low over the Midwest is forecast to weaken further tonight
and drift southeast toward the Iowa/MO/IL border. Despite the weak surface low
and lack of a significant low-level jet...plenty of moisture to be
available with dew points in the 50s to around 60 degrees and precipitable water
values around 1.25". Farther aloft...models continue to show a
shortwave trough (with embedded shrtwvs) pushing across WI tonight...as
well as a favorable right entrance region of the upper jet adding
lift especially over the southern half of the state. There is
sufficient low-level moisture convergence and middle-level forcing to
bring/maintain this large band of showers across roughly the southern 3/4
of the state late this afternoon through most of tonight. Likely or
categorical probability of precipitation placed generally along and south of a line from
Merrill to Wausaukee with scattered probability of precipitation farther north. Instability
remains weak...thus nothing more than isolated thunderstorms expected
tonight. Have also added patchy fog to the forecast as abundant moisture
to be present with light winds at the surface. Min temperatures to range from
the lower to middle 50s north-central WI...around 60 degrees east-ctnrl WI.
Shower activity should still be impacting eastern sections of WI Sat
morning as the shortwave trough slowly pushes to the east. Upper heights
begin to rise over the area by Sat afternoon as the central Continental U.S.
Upper ridge tilts northeastward toward the western Great Lakes. Most if not all
precipitation should be over with by early Sat afternoon with gradual
clearing skies from northwest to southeast. Any clearing in eastern WI may not occur
until late in the day. Maximum temperatures on Sat are forecast to be close to
seasonal normals with most locations in the lower to middle 70s.
Long term...Saturday night through Friday
issued at 219 PM CDT Friday Aug 28 2015
A return to summertime warmth is expected during the period.
The upper trough and associated rainfall is expected to be east of
the forecast area by Saturday evening...so have continued the
previous dry forecast. With clearing skies and light winds anticipated
overnight...combined with moisture from earlier rain...fog looks
like a good bet. Have increased fog coverage from patchy to
areas...and suspect that at least locally dense fog will be a
concern. Dry conditions are anticipated for Sunday and Monday as
a strong upper ridge becomes established over the region.
Developing south winds will mark the beginning of a warming
trend...with highs warming into the lower to middle 80s by Monday.
Small precipitation chances return to the forecast for the rest of the work...
due to a weakening frontal boundary that arrives midweek...
diurnal instability...and another cold frontal boundary next
Friday. Overall...this still appears to be a relatively dry period
for most of the area...as the presence of the upper ridge should
limit the coverage of any convection that does develop. 800 mb temperatures
stay fairly steady in the +18 to +21 c range...so maximum temperatures should
be mainly in the 80s...with perhaps a 90 degree reading or two at
the typical warm spots in the Sandy soil areas in central
Aviation...for 00z taf issuance
issued at 641 PM CDT Friday Aug 28 2015
Light to moderate rainfall will overspread the southern 3/4 of WI
tonight. Ceilings will be lowering into the MVFR range this evening
and then either high end IFR or low end MVFR conditions late
tonight into Sat morning. Visibilities will lower to MVFR levels due to
a combination of rain and fog. Conditions are not expected to
climb back to VFR levels until late Saturday afternoon.