Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Green Bay WI
1257 PM CDT Wednesday Aug 20 2014

Updated aviation portion for 18z taf issuance

Synopsis...
issued at 425 am CDT Wednesday Aug 20 2014

Relatively quiet today...then the chance of thunderstorms returns
for tonight and Thursday. Remaining rather humid the next fseveral
days.

The upper pattern across noam will amplify the next several days as a
trough deepens over the intermountain west and a downstream ridge
builds from the lower Mississippi Valley...nnewd...acrs the upper
Great Lakes region. This will leave the forecast area in the ring of
fire region for convection firing around the perhphery of the upper
ridge. Though differing on the exact timing...medium range models
agree that energy shearing out of the western trough will first flatten
the eastern ridge...followed by the main trough itself eventually
shiftg into the plains.

This looks like a wet pattern...as ample moisture is acted upon by
shortwaves riding northeastward out of the western trough and around the eastern ridge.
Though timing of individual convective events will be difficult...
the pattern favors above normal and possibly even excessive
rainfall. The air mass across the area will be warm and humid...so
temperatures should be above normal for the most part. Of course temperatures
could be held back on any days with widespread thunderstorms.
&&

Short term...today...tonight...and Thursday
issued at 425 am CDT Wednesday Aug 20 2014

Fairly quiet weather expected today. The north/NE will have to contend with
lingering precipitation wrapping around the western flank of the upper low that will
be slowly edging eastward out of the region. Precipitation character may be that of
a fine light rain or drizzle...but coverage will be limited...so
opted to handle in the forecast as isolated light showers. St advancing
across the region this morning will probably take a little time to
break up...so adjusted skycon grids accordingly.

Upper low will continue to edge away tonight. Meanwhile...convection
driven by isent lift in the frontal zone liftg northeastward toward the
region could affect the area...especially after midnight. First thought
was to slow the arrival of the precipitation a little as departing upper low
may hold things up a bit. But precipitation really isn/T that far away
already...so stuck close to previous forecast. New European model (ecmwf) also fit well
with the going forecast.

Frontal wave supported by shortwave ejecting out of the western trough
should reach the northern plains Thursday afternoon. Combined with backing upper
flow...that should allow warm front to lift into WI by afternoon.
Stuck with the likely probability of precipitation from the previous forecast...though timing of
best precipitation chances is somewhat uncertain. It could be during the
morning as the NE portion of nocturnal mesoscale convective system from the plains mvs
through the area...or it could be during the afternoon when the front
nears the area. Shear will be increasing as westerly flow aloft
strengthens. There will also be ample moisture...with precipitable waters
approaching 2 inches and surface dewpoints approaching 70f. Main unknown
for severe will be the amount of destabilization that can occur. Storm Prediction Center went
with 5 percent chance for severe in the day 2 convective outlook. Thats good
for now and will begin mentioning in the severe weather potential statement. Risk will likely need to
be be upgraded if confidence increases that there will be enough
sunshine to allow the atmosphere to significantly destalilize. Precipitable waters around
2 inches support heavy rain/flooding potential...especially since many
areas received sig rains the past few days.

Long term...Thursday night through Tuesday
issued at 425 am CDT Wednesday Aug 20 2014

An unusual 500mb pattern for this Summer is setting up across the
United States late in the work week into the upcoming weekend.
500mb ridge expected to build across the south central United
States while 500mb trough enters the West Coast by Friday. 500mb
ridge will be firmly entrenched across the Missouri Valley this
weekend. This pattern should bring above normal temperatures to
the area. The ridge will not last long across the western Great
Lakes as ridge will break down by the middle of next week.

For Thursday night...there is some potential for strong or severe
thunderstorms Thursday evening as warm front moves across the
region. 850mb jet modest around 30 knots which should veer to the
west overnight. Think most of the convection would be during the
evening with only small chances after midnight (highest across
northeast wisconsin). Friday could end up mainly dry as some drier
behind departing system arrives. Also...modified soundings
indicated convective temperatures in the lower to middle
90s...which we would not reach. Have lowered chances of storms
substantially for Friday and Friday night as some of the models
push boundary far enough south to keep the region dry. Later
shifts may be able to pull the slight chances for rain if this
scenario holds true. New European model (ecmwf) now slower with the return of
precipitation. 12z run indicated Saturday while 00z run tonight
now taking aim for Saturday night. Will continue with Middle Range
chances for rain. A lot of uncertainty in how quick the 500mb
ridge will break down early next week. Latest GFS would suggest a
ton of rain...but think outflow boundaries will eventually push
southward at some point late this weekend into early next week.
Did not stray far from previous forecast as due to the uncertainty
in the timing of showers and storms and will boundary will set up.
Could be a very active pattern with Summer time heat and humidity
at times.
&&

Aviation...for 18z taf issuance
issued at 1257 PM CDT Wednesday Aug 20 2014

Mostly MVFR ceilings across the area should gradually improve to VFR
from south to north during the day before clouds start to clear.
Surface high pressure...light winds and low level moisture will
allow fog to develop overnight with widespread MVFR and IFR visibilities.
A warm front will push northeast into Wisconsin on Thursday as low
pressure over the plains approaches the state. Weak short wave
energy...increasing instability and the surface boundary will lead
to the development of showers and storms...beginning to impact
central Wisconsin toward morning and continuing northeast
throughout the day. Prevailing ceilings should be mostly VFR but
showers and storms will bring MVFR...and some IFR...ceilings and
visibilities.
&&

Grb watches/warnings/advisories...
none.
&&

$$

Synopsis.......skowronski
short term.....Skowronski
long term......eckberg
aviation.......mg

National Weather Service Glossary of Abbreviations