Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
652 am CDT Wednesday may 6 2015
Updated aviation portion for 12z taf issuance
Short term...today...tonight...and Thursday
issued at 405 am CDT Wednesday may 6 2015
Precipitation trends...temperatures and fog/stratus potential are the main
A 800 mb warm front was lifting north across the region early this
morning...and generating scattered/numerous showers over the forecast area.
Have not seen any lightning anywhere in the state...though a few
strikes were noted in Iowa. High pressure was shifting east
through the Great Lakes...with strong return flow bringing
moisture into the upper Mississippi Valley. Dew points in the
50s and lower 60s were reported in in southern Minnesota...Iowa and
Illinois...along with widespread low stratus and fog.
The elevated warm front will shift north of grb County Warning Area later this
morning...so showers should gradually lift north and weaken.
Models continue to show instability increasing over our SW
counties this afternoon...which may lead to scattered thunderstorm development
later in the day. This will be dependent on whether or not there
are enough breaks in the stratus clouds to allow sufficient
heating. Used a blend of the top performing guidance sets for maximum
temperatures...but lowered readings a few degrees in the south due to
Any lingering convection in our SW counties should tend to weaken
after sunset...but an increasing low-level jet and a weak short wave trough
will keep a small chance of rain showers/thunderstorms and rain going across our western
counties overnight. Low temperatures will be mainly in the
50s...with middle to upper 40s in far NE WI and along the lkshr.
A 35-45 knots low-level jet will reside over the region on Thursday...
with daytime mixing leading to windy conditions by late morning.
Moisture and instability will increase across our northwest counties
during the afternoon...as a cold front approaches from Minnesota. Have
trended probability of precipitation from likely northwest to slight chance or dry in far eastern
WI. The strong south winds will pull warmer air into the region...
allowing temperatures to rise into the upper 70s and lower 80s before the
Have added some fog to Lake Michigan and some of the shoreline
areas later today through Thursday...as low-level moisture
advects over the cold waters. Confidence is not high enough to
issue marine fog advisories yet. Have held off on mentioning
fog for the rest of the inland areas...as MOS guidance shows
substantial temperature/dew point spreads continuing overnight...and
boundary layer winds increasing to 25 to 30 kts. This should
support continued stratus development instead of widespread fog.
Long term...Thursday night through Tuesday
issued at 405 am CDT Wednesday may 6 2015
Focus was given in the long term forecast to the Thursday night
and Friday period to try and resolve precipitation chances with a frontal
passage. After Friday...the long term becomes muddled and could
not add much detail with certainty.
A surface cold front will be passing through the Great Lakes region
Thursday night and Friday. Models continue to have issues with precipitation
ahead of the front Thursday night. Without any significant forcing
and a broad warm air advection regime in place over the state...cannot provide
much detail from previous forecast. The better chances for showers
and thunderstorms looks to be in north central WI before midnight
as a weaken shortwave passes through and washes out.
Models have at least some agreement on the timing of a cold front
on Friday...with the latest gfs40/gefs being a good compromise.
The 00z European model (ecmwf) is still a bit fast...and the nam80 is too strong
based on the lack of upper level support. Best chances for precipitation
Friday morning through early afternoon with the actual frontal
passage. Dry air near the surface should result in just light precipitation.
BUFKIT soundings through Thursday night and Friday morning do show
a moderate midlevel instability...though the amount of instability
and the strength of a capping inversion varies from model to model.
Some of the storms Thursday night and Friday morning could be
strong...with higher wind gusts possible...but without any strong
forcing mechanism the severe threat remains minimal.
After the front sweeps through the state...weakening as it moves
east...it should then stall out and settle somewhere in northern
Illinois through lower Michigan. The rest of the forecast will depend on
the placement of this weak boundary and waves of low pressure
riding northeast along it out of the Southern Plains. Expect
active weather with occasional precipitation starting this
weekend...however it doesnt looks like a wash out through the
beginning of next week thought not enough confidence in when this
would occur to highlight a drier periods. The upper levels also
become more unsettled and would support more widespread and
heavier rains with any rounds of precipitation that do move through.
Given the placement.
After one last day with well above normal conditions
Friday...temperatures should fall back to near normal after the cold
front sweeps through Friday night and will stay near normal into
Aviation...for 12z taf issuance
issued at 639 am CDT Wednesday may 6 2015
A band of showers will continue to shift north this morning...and
should lift into the Upper Peninsula of Michigan by midday. Additional
showers and isolated thunderstorms are possible in central and parts of north
central WI later this afternoon and tonight...but precipitation chances are
small. Increasing low-level moisture and overnight showers have
caused ceilings to lower to MVFR across central and parts of north
central and east central WI. The low ceilings should persist for
much of the morning before daytime heating causes them to lift to
Expect widespread MVFR/IFR stratus to develop again tonight as
moisture continues to increase. Some fog is also possible...but
strong boundary layer winds may prevent visibilities from dropping too
much at inland locations. Marine fog will likely impact the Lake
Michigan shoreline locations...including the mtw taf site...
later today into tonight...and conditions could drop to IFR/LIFR
Have added some low level wind shear to the rhi/auw/cwa taf sites late tonight...
as south-southwest winds increase to 35 kts just off the surface.