Area forecast discussion
National Weather Service Green Bay WI
234 PM CDT Wednesday Jun 19 2013
Forecast discussion for routine afternoon forecast issuance
Short term...tonight and Thursday
issued at 234 PM CDT Wednesday Jun 19 2013
Surface analysis showed an area of high pressure that continued to
shift east this afternoon. A relatively moist layer between
850-700mb combined with a weak 300mb jet diving down over Lake
Superior led to the development of fair weather cumulus in central and
north central Wisconsin. Further east...locations closer to the
center of the high pressure system remained sunny as drier air
dominated. After a cool start to the day...warm southerly flow
allowed temperatures to climb quickly and returned to near normal
levels...however dewpoints have been slow to increase. Main
forecast concern will be timing of precipitation chances on Thursday.
As mentioned before...dewpoints have been slow to increase
today...with the surge of moist holding off until Thursday
afternoon. Therefore...clipped a couple of degrees off the
overnight low in most locations. Only exception was in the
Northwoods where clouds streaming in from the next system should
keep it a bit warmer. Otherwise...not much change made to the
overnight hours from the previous forecast tonight as high
pressure will keep things dry.
A low moving through Canada will drag a cold front over the Upper
Peninsula on Thursday...and will be in the vicinity of the
WI/yupper border by Thursday evening. In addition...a weak
shortwave will slide across Wisconsin during the late afternoon.
These two features will result in scattered shower and
thunderstorm activity on Thursday. Models have been too moist
recently/overdoing the precipitation...so slowed down the progression of
chance probability of precipitation eastward from previous forecast. In fact...location
in far eastern Wisconsin may see a completely dry Thursday...with
central and north central Wisconsin sitting under mostly cloudy
skies and scattered shower activity. Temperatures tomorrow will be
warmer than today...and muggy conditions should develop by late
afternoon as dewpoints increase into the lower 60s on southwesterly
flow.
Long term...Thursday night through Wednesday
issued at 234 PM CDT Wednesday Jun 19 2013
Mean flow into early next week to consist of an upper trough over the
eastern Pacific/western Continental U.S. And a broad/somewhat flat upper ridge to extend
from the Southern Plains to the mid-Atlantic. This pattern will bring
warm...humid and unsettled weather to NE WI through much of the
extended period. Trying to find a dry period will be the biggest
forecast challenge as ripples of shortwave energy and a frontal
boundary in the vicinity act as a focus.
Prevailing west-SW flow aloft to gradually bring warmer/more moist
air mass into WI Thursday night. While relatively tame warm air advection-induced showers/
thunderstorms to be in our general vicinity Thursday evening...the potential
development of a mesoscale convective system over the upper MS valley on the nose of a 40 knots
low-level jet will need to be watched during the overnight hours. This
convective complex would be rolling toward N-cntrl/central WI prior
to sunrise with a threat of damaging winds if it were to
materialize. Have placed highest probability of precipitation generally west of an imt-sue
line with low-end chance probability of precipitation over the Lakeshore. Change in air mass
will provide an actual summerlike feel as min temperatures to only drop
into the 60-65 degree range.
Remains of the mesoscale convective system to move through the rest of NE WI Friday morning...
but should be weakening as the low-level jet weakens. The problem
then resides for Friday afternoon and whether additional convective
activity can develop. Atmosphere will be able to destabilize with
convective available potential energy surpassing 4k j/kg with little cin indicated. The key could
be if any convective boundaries are left behind by the mesoscale convective system. This
is hard to determine at this time...thus have left afternoon probability of precipitation
in the lower to middle 80s (cooler lakeside) and dew points in the middle to
upper 60s.
Persistent SW winds aloft is forecast to lift a warm front through southern WI to
central WI by 12z Sat. Gulf moisture will continue to feed into the region
with the fnt acting as a focusing mechanism. Even with the loss of
daytime heating...the atmosphere to remain unstable with middle-level
lapse rates around 7 c/km and lifted indice's around -6. Have blanketed the
forecast area with hi chance probability of precipitation for now...but expect some changes once
the location of the warm front becomes more obvious. This warm front is
then expected to either hold nearly steady over central WI or slowly
edge north toward northern WI on Sat. There could easily be a lull in
the precipitation during the morning as the atmosphere reloads from earlier
convection...however as the atmosphere destabilizes once again
during the afternoon (capes around 2k j/gk/Li's around -5)...a new
round of showers/thunderstorms should develop especially near the warm front. 800 mb
temperatures in the +15 to +20c range should yield maximum temperatures to reach
into the Lower-Middle 80s inland/75-80 near Lake Michigan on Sat.
Subtle model differences occur Sat night as the GFS sharpens the upper
ridge over the Great Lakes and forces the next shortwave well to our
northwest. Meanwhile...the European model (ecmwf) keeps the upper ridge somewhat flat and
allows this shortwave to enter the western Great Lakes. Hard to
determine which solution is correct...but with the warm front still
lurking over northern WI...will place highest probability of precipitation over the north with
slightly less probability of precipitation farther south. Much like Sat...any overnight
convection would tend to weaken in the morning on Sunday...only to
refire in the afternoon as instability soars. Precipitation for Sunday
would be scattered at best due to the warm front to have lifted north
and no prominent shortwave advertised by the models. If we could
stay pcpn-free...a few locations could reach the 90 degree mark as
the heat and humidity continue to overspread the area.
A new Twist occurs Sunday night as a cold front attempts to push toward
WI. This fnt will lose the battle against the entrenched upper ridge and
eventually stall (or even washout) somewhere over northern or central WI
by Monday. This fnt could still bring another round of showers/thunderstorms to
parts of the forecast area Sunday night and depending on where the fnt
hangs up on Monday...bring additional precipitation chances. Too much uncertainty
here to try and pinpoint higher probability of precipitation...so essentially followed a
consensus solution which brings chance probability of precipitation to all of NE WI. While
the extended models agree that the upper ridge will eventually rebuild
over The Rockies by the end of the period...how we get there is a
matter of conjecture. The European model (ecmwf) subtlely flattens the mean flow
with the GFS showing a substantial shortwave trough to move through WI
around Tuesday night. NE WI to remain in the warm sector through Tuesday...so
cannot rule out at least a chance of shwrs/tstms. By Wednesday...the cold front
pushes through the region with a continued chance for more precipitation.
&&
Aviation...for 18z taf issuance
issued at 1140 am CDT Wednesday Jun 19 2013
High pressure over the western lakes will continue to shift east
allowing southerly winds to bring in warmer and more humid air
into the region. VFR conditions will continue for today and
tonight. A disturbance will approach the area out of the
northern plains on Thursday....bringing with it increasing VFR
clouds through the day from northwest to southeast. Scattered
showers possible in the northern taf sites late in the morning.
&&
Marine...
issued at 234 PM CDT Wednesday Jun 19 2013
A warm and humid air mass will overspread the cooler waters of
the Bay of Green Bay and Lake Michigan Thursday night or
Friday...leading to the formation of locally dense marine fog.
This locally dense marine fog will continue at times into early
next week as the warm and humid air mass remains firmly entrenched
across the region. Several complexes of thunderstorms are expected
from time to time Friday into early next week. These storms will
pose a significant risk for mariners with strong winds...locally
higher waves...large hail and torrential rains which will reduce
visibilities below one mile.
&&
Hydrology...
issued at 234 PM CDT Wednesday Jun 19 2013
Several complexes of thunderstorms are expected from Friday into
early next week. A very warm and humid air mass will become
entrenched across the region late this week and persist into early
next week. Precipitable water values should run 150 to 200
percent of normal...leading the likelihood on torrential rains and
potential for urban flooding due to the intense rainfall rates.
Any training or slow moving thunderstorms will increase the risk
of flooding.
&&
Grb watches/warnings/advisories...
none.
&&
$$
Short term.....Wolf
long term......kallas
aviation.......Wolf
marine.........Kallas
hydrology......kallas
