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Area forecast discussion 
National Weather Service Green Bay WI
404 am CDT Thursday Aug 21 2014

Forecast discussion for routine morning forecast issuance

Synopsis...
issued at 353 am CDT Thursday Aug 21 2014

Periods of thunderstorms today and tonight. Heavy rainfall
possible...mainly across central and east-central Wisconsin.

Upper pattern across noam undergoing amplification...with trough over
the intermountain west and a ridge from the lower Mississippi Valley
north-northeastward toward James Bay. Amplitude should peak early in the
weekend...then energy ejecting out of the western trough will flatten the
northern portion of the ridge/shove it eastward. Though the exact details
are uncertain...upper pattern will likely evolve toward a lower
amplitude regime with fairly broad band of westerlies across southern
Canada and the northern Continental U.S. By the middle to latter part of next
week.

Ample moisture will be across the area and available to be acted upon
by shortwaves riding northeastward out of the western trough...so considerable precipitation
is likely during the period. The main precipitation events will be
tda/tngt...and late in the weekend/early next week. The pattern
favors above normal and possibly even excessive rainfall...especially across
the southern/southwestern part of the forecast area. The air mass across the area will
be warm and humid...but daily temperatures will depend heavily on the amount
of cloud cover and precipitation. It looks likle the warmest temperatures...
possibly at or above 90f in some places...will occur late in the weekend and
early next weak...with readings returning to normal by the end of
the period.
&&

Short term...today...tonight...and Friday
issued at 353 am CDT Thursday Aug 21 2014

Ingredients of excessive rainfall event coming into place across
the region...though the focus appears to be just S/SW of the forecast
area. Baroclinic zone stretched out from the northern plains...esewd...to
the lower Great Lakes region. 850 mb jet is directed into the
baroclinic zone across Minnesota at this time...where an mesoscale convective system is in the process of
organizing. A narrow zone of convection also stretches from the Minnesota
convection all the way to northern in. Ongoing weather not handled
especially well by the models... which complicates the rest of the
forecast and makes adding temporal resolution to the probability of precipitation a real
challenge.

Expect Minnesota convection to continue to congeal...heading eastward and then
southeastward following corfidi vectors. It is almost certain to affect
the SW part of the forecast area...though the northeastward extent is in
doubt. Structured probability of precipitation with a sharp decr to the NE middle-day
today...though that may need adjustment. The complex will probably
arrive before the atmosphere has much chance to destabilize...and the more
stratiform NE quadarant of the complex is what will affect the forecast
area...so severe weather seems unlikley this morning.

Back out to the west...shortwave ejecting out of the western trough should
reach western WI around 00z. Assuming some air mass recovery occurs in
the wake of the lead complex...additional storms and probably
eventually another mesoscale convective system will likely form. Deep layer shear will be
increasing by then as shortwave drives through the crest of the upper
ridge...increasing westerly flow at mid-levels. Think the greatest severe
threat will be with these storms...late this afternoon into this evening.
Primary risk will be damaging winds...though hail will also be
possible. Low-level directional shear will be considerable...but its
doubtful individual cells will remain discrete long enough to have
enhanced tornado potential.

Forcing then wanes later tonight as the shortwave shifts east and 850 mb
flow veers westerly. Upper heights across the area build Friday and the upper
flow becomes increasingly anticyclonic. Hard to absolutely rule
out precipitation given the available moisture and possible boundaries across
the region...though large scale forcing is definitely unfavorable. Will
cover with slight chance probability of precipitation.

Temperatures tricky due to amount of clouds and precipitation. Trimmed maxes some
today...but stayed closer to the warmer guidance products Friday as not
much convection is expected then.

Long term...Friday night through Wednesday
issued at 353 am CDT Thursday Aug 21 2014

500mb ridge expected to dominate across the eastern United States
this weekend. Main question if quick ridge will flatten next week
as the Canadian/GFS/ECMWF has various solutions on how quick ridge
will flatten by the middle of next week.

For Friday night...it should be fairly quiet although kept a
small pop due to lingering precipitation from Friday afternoon or
shortwave energy that could kick off an isolated storm. Warm front
should lift northward on Saturday and Saturday night...bringing
warm and humid conditions and the chances for thunderstorms. Some
concern on northward push of warm front as low will moving across
the northern plains/northern Minnesota. Depending on where front
sets up...will determine how warm we get Sunday afternoon. Beyond
Sunday... a lot of uncertainty in frontal positions due to the
differences in breaking down the 500mb ridge...and where outflow
boundaries may enhance the southward push of the front.

Did raise maximum temperatures several degrees Sunday and a few
degrees on Monday. Did add the local influences to maximum/min
temperature grids during the period...adding a few degrees to the
typical warm spots for highs each day and nudged down lows a
degree or two in the typical colder spots.
&&

Aviation...for 12z taf issuance
issued at 353 am CDT Thursday Aug 21 2014

SC deck wtih mvf ceilings has proven very tough to dislodge from northestern
WI. Where it has exited some IFR St/fog have formed in its wake.
Expect to trend toward mainly VFR ceilings during the day...but with
much poorer conditions in vicinity of shra/tsra. Anticipate MVFR or maybe
even IFR conditions tonight as low-levels will be even more moist than
they are now.
&&

Hydrology...
issued at 353 am CDT Thursday Aug 21 2014

Luckily...we are now a couple days removed from the last heavy rain
event. That combined with the focus of heaviest rains mostly
likely being S/SW of the area keep the threat from rising to the
level of requiring a Flash Flood Watch. Will handle by issuing an
esf and mentioning heavy rain/flooding potential in the severe weather potential statement.
&&

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

$$

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

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