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Area forecast discussion 
National Weather Service Green Bay WI
1056 PM CDT Wednesday Aug 20 2014

Updated aviation portion for 06z taf issuance

Short term...tonight and Thursday
issued at 337 PM CDT Wednesday Aug 20 2014

Precipitation timing is the primary concern during this part of
the forecast as a surface low and upper cyclone continue to slowly
move away from the area. Most of the clouds across the southwest
1/2 to 2/3 of Wisconsin should dissipate with loss of daytime
heating...and the clouds across much of the forecast area will
move away with the departing systems. Dew points were mainly in
the upper 50s and lower 60s early this afternoon so there was
plenty of low-level moisture. The moist conditions...departing
clouds and weak surface flow should result in at least patchy fog

12z versions of the NAM/GFS/ECMWF all brought quantitative precipitation forecast into the
forecast area between 06z and 12z ahead of an approaching warm
front. There is also some short wave energy propagating through a
weak upper level ridge that should reach the state on have gradually spread and increased probability of precipitation from
southwest to northeast overnight through Thursday. Could be some
heavy rainfall with the storms considering the fact that precipitable water from
the 12z grb sounding was 1.41 inches...which is in the 75th
percentile climatologically. There continues to be some uncertainty
regarding severe potential with Storm Prediction Center showing a 5 percent chance for
severe across much of the forecast area.

Long term...Thursday night through Wednesday
issued at 337 PM CDT Wednesday Aug 20 2014

The 500 mb level forecasts build a ridge late this week and into the
weekend. Warmer temperatures are expected under the building ridge
along with several chances of rain. The most significant rain will
likely be near or along a frontal boundary but location of this
boundary is still in question but latest model runs are hinting
the surface boundary will sag a bit further south in northern
Illinois late this week and into the weekend. As the ridge breaks
down next week...the pattern looks to be setting up with a trough
settling into the northern plains and the southwest flow aloft and
likely short wave troughs will bring several chances of rain...however
timing these precipitation chances becomes challenging.

For Thursday night...there is some potential for thunderstorms to
at least produce heavy rainfall as well as strong to severe
storms. The surface warm front lifts into southern Wisconsin while
the h850 front lifts into the southwest Wisconsin along with a
modest 30 knot low level jet which eventually veers to the west. An upper
jet lifting northward as the upper builds may provide a few jet Couplets
to enhance the convection...and the latest European model (ecmwf) suggest a rrq
region lingers into Friday over southeast Wisconsin including
portions of east central Wisconsin. Precipitable waters climb to a healthy 1.60
inches which may produce rounds of heavy rain...however since the
frontal boundary and higher precipitable waters of 2.00 inches are trending
southward and the likely location of sig rains...will not issue an
esf for now and highlight in the severe weather potential statement.

The focus of any convection Friday into Saturday night will likely be
any northern plains short wave and convection working over the
ridge...and especially in the region of upper jet dynamics over
the northern Great Lakes region. But in general will go dry or
low probability of precipitation Friday into Saturday.

The warm front is then prognosticated to move northward Saturday into
Saturday night followed with a cold front Sunday night into
Monday...however model differences remain with the timing of these
features due to timing of the breakdown of the upper ridge.

Temperatures potentially be rather warm along with increased
humidity...especially if convection becomes sparse and clouds

Aviation...for 06z taf issuance
issued at 1051 PM CDT Wednesday Aug 20 2014

Stratocumulus with MVFR ceilings persisted over far northern WI late
this evening...but breaks in the clouds were allowing patchy fog
to develop elsewhere across the region. The fog is expected to
become more widespread overnight...with visibilities/ceilings lowering to
IFR/LIFR at many places. The thicker fog should dissipate early
Thursday...but light fog or haze may linger through the day due to
a moist air mass.

Clouds will increase again from the south late tonight as a warm
front creeps northward toward far SW WI. This front may bring scattered
rain showers/isolated thunderstorms and rain to parts of c WI and the southern Fox Valley late
tonight into Thursday morning...and a better chance of rain showers/thunderstorms and rain
over the entire region as an upper level disturbance passes through
Thursday afternoon and evening. Have added a tempo group for
thunderstorms in the tafs during the middle afternoon to early evg period...
but the timing may need to be adjusted with subsequent forecasts.

issued at 337 PM CDT Wednesday Aug 20 2014

Rivers rose a little a few days ago with the spotty
heavy rainfall...but overall was not impacted by the heavy rain
and can handle more rain. Potential of more heavy rainfall next
several days. Trends of the models suggest the deep moisture with
precipitable waters of 2.00 inches have trended south of the area while the
primary front with the deep moisture prognosticated to drop into Illinois
on Friday. But precipitable waters of 1.60 inches can still produce heavy rain
and needs to be watched...especially when the upper jet lifts
north and winds become lighter in the ridge...and potentially
slower movement of the convection.

Grb watches/warnings/advisories...


Short term.....Mg
long term......tdh

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