Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
328 PM CDT Wednesday Oct 1 2014
Forecast discussion for routine afternoon forecast issuance
Short term...tonight and Thursday
issued at 328 PM CDT Wednesday Oct 1 2014
Precipitation timing is the main concern during this part of the
Radar time lapse indicated showers moving into central Wisconsin
this afternoon ahead of a middle level short wave. These showers were
decreasing in coverage and intensity as they continued into north
central Wisconsin. Hrrr model appeared to do a better job of
depicting this than the NAM/GFS/ec/sref. Still...have left showers
with a slight chance of thunder due to the approaching cold front
and some elevated instability that is forecast for tonight.
Surface flow becomes quite light toward daybreak...due to a weak
surface pressure gradient. Have areas of fog around daybreak due
to the combination of moisture...light or calm winds...and 12z MOS
guidance showing low visibilities in fog.
The better chance for showers and thunderstorms on Thursday should
be in the afternoon as a low pressure system moves along the front
toward Wisconsin. Have rain and thunder chances increasing during
the afternoon as the low approaches.
The wet conditions should keep low temperatures above normal...and
warm advection ahead of the low pressure system will lead to warmer
than normal conditions on Thursday.
Long term...Thursday night through Wednesday
issued at 328 PM CDT Wednesday Oct 1 2014
Precipitation trends and temperatures will be the main forecast concerns.
Models have come into good agreement on a surface low lifting north-northeast along
a strong cold front Thursday night...and spreading widespread showers
across the forecast area. Will carry categorical probability of precipitation for Thursday
evening. A few thunderstorms should be embedded within the shower activity
east of the frontal boundary...over roughly the southeast half of the forecast
area. A decrease in the shower activity is expected over central and
north central WI overnight...and the rest of the area on Friday
morning...but scattered showers will continue into Friday afternoon as
a deep upper trough/upper low approaches. In the wake of the cold
front...friday's high temperatures should be 10-15 degrees cooler
than on Thursday.
The upper trough will shift east across the region on Friday night...
so scattered showers will continue. Showers will likely become lake-
enhanced over NC WI...where cold northwest flow will move across Lake
Superior. Will keep likely probability of precipitation there. Will continue the mention
of mixed rain/snow showers over far NC WI late...as 800 mb temperatures drop
to -2 to -4 c. A light mix of rain/snow showers should continue into
Saturday morning over northern WI...then taper off during the
afternoon as wind trajectories become west-northwest.
Saturday night through Monday night...the weather pattern will be
dominated by an upper low over Ontario..with several short wave troughs
rotating through WI. Moisture will be limited and low level winds will
be too westerly for any significant lake-effect. Will have low
probability of precipitation (15-30 percent) over most of the region during this period...
along with chilly temperatures. If precipitation occurs over northern WI...
a light mix of rain/snow will be possible during the late
Expect drier conditions to prevail into midweek...with a bit of
a warmup possible. This is a low confidence forecast though...
as models show a wide range of 800 mb temperatures over the wstrn Great
Lakes. Have discounted the 800 mb temperatures of +16 to +20 off the
00z European model (ecmwf)...as they seem anomalous compared to the ensemble
means...and unlikely with a fairly zonal flow pattern.
Aviation...for 18z taf issuance
issued at 115 PM CDT Wednesday Oct 1 2014
Ceilings were mainly MVFR across the area with the
exception of some clearing between low clouds over the eastern
part of the state and the clouds approaching from the west. Ceilings
may improve to VFR at times this afternoon...but prevailing should
be mostly MVFR. Showers that moved into western Wisconsin earlier
in the day were making slow eastward progress so have kept central
and north central Wisconsin tafs dry until late afternoon and
left the east dry until this evening. Hrrr suggests a relatively
short period of rain while the NAM/GFS/sref/ec all hang on to
measurable precipitation for quite a bit longer. Right now the
hrrr is looking better but have decided to err on the side of
caution and keep the chances for showers around for longer. 12z
MOS guidance from the NAM and GFS showed low visibilities in fog around
12z Thursday so have tempo group with 1/2sm fog at the taf sites.
Based on what I saw from this mornings observations there did not
appear to be widespread dense fog...so did not use it as the
prevailing condition. Have left showers without thunder tonight
but went with a prob30 for thunder on Thursday as the atmosphere
becomes more unstable.