Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
926 PM CDT Monday Aug 31 2015
Update...temperatures dropped rapidly early this evening so lowered
minimum several degrees most areas. Areas of fog likely to
develop...but boundary layer mixing should keep widespread dense
fog at Bay. Likely to be some patchy dense fog however...especially
in the east closer to Ridgeline and moist marine layer.
Aviation/06z tafs/...fog likely to affect taf sites overnight.
Widespread fog over the nearshore waters will likely affect
Lakeshore for a time...however Lakeshore winds expected to turn to
the south to southwest...keeping most of the lake fog at/off
shore. Dewpoints about 3 degrees warmer than yesterday and high
afternoon cross over temperatures and clear skies likely to allow fog to
drop visibilities to MVFR. Will continue IFR visibilities at kenw due to high
cross over temperature and closer proximity to Ridgeline. Better low
level mixing likely to keep dense fog at Bay elsewhere. Recent
ascent kmke tamdar sounding shows strong mixing about 1k feet off
Marine...latest 11-3.9 micron imagery showing dense fog rapidly
expanded and now covers most of the nearshore waters...which is
confirmed by Lakeshore webcams. Modis imagery estimated nearshore
lake surface temperatures in the low 50s early today...but then warmed
into the middle to upper 50s during the afternoon. Recent upwelling
incident has caused the cool water to surface as off Shore Lake
surface temperatures mostly in the 60s.
With the warm...moist air sticking around for much of the coming
week...bouts of dense fog are likely to continue until the surface
water temperature warms at least 5 to 10 degrees.
Previous discussion... /issued 252 PM CDT Monday Aug 31 2015/
Tonight - confidence...medium
stratus deck morphed into cumulus field. Expect this to diurnally dissipate.
Fog potential still there with overall light wind regime and high dew
points. However there is a bit more stirring off the deck so not
expecting widespread dense fog to be a concern. MOS temperatures appear
Tuesday - confidence...high
after any morning fog erodes 925/850 thermal ridge leans into the County Warning Area
with 925 temperatures reaching the middle 20s celsius. So expect highs to reach
well into the 80s with a few spots reaching 90. Evolving onshore
component will drop temperatures back a bit Lakeside as the afternoon wears on.
Tuesday night through Thursday nt..
A large scale upper trough will prevail over western Canada and the
Pacific northwest with ridging over much of the central and eastern USA
during this period. However...weak shortwave troughs will exist
within the ridging area. One such shortwave trough is expected to
track across central or southern WI for late Wednesday into Thursday. MLCAPES are
expected to climb to 1500-2000 j/kg for Wednesday after with little
capping...but also little to no surface convergence. Some models do
have quantitative precipitation forecast at times although model and MOS pop guidance is very low
during this period. Thus went with slight chance probability of precipitation for the
afternoon over south central WI and over the entire County Warning Area for Wednesday nt
High temperatures the last couple days have been cooler than
forecast due to stratus and fog and cannot completely rule this
out for this period...although still believe temperatures will be able to
warm into the middle-upper 80s. Heat index values will reach the
Long term...Friday through Monday...forecast confidence medium.
A very warm...humid...and unstable airmass will continue for the
weekend. Thus small chances of thunderstorms are forecast at times but
will increase on Labor Day as a cold front approaches. The models
differ on the strength of the cold front and associated precipitation at
Aviation/00z tafs/...MVFR cumulus field expected to dissipate with loss
of diurnal heating. Airmass remains moist and guidance still suggesting
fog develops as the night wears on. Leaned a little closer to the
metmos on visibility trends. 850/925 thermal ridge starts to lean into the
area Tuesday with increasing SW flow. NAM soundings not showing much
Marine...weak flow continues tonight into Tuesday with high dew
point airmass over cool waters of the lake. Some uptick in south
then southeast winds on Tuesday but traj favors keeping fog...some
dense...in place through at least Tuesday morning. With potential
lingering longer in northern zones into Tuesday night. With gradient
looking overall weak for the balance of the weak may be seeing more
of the dense fog advisory headlines as the week moves along.
Lm...dense fog advisory until 10 am CDT Tuesday for lmz644>646.
Dense fog advisory until 7 am CDT Wednesday for lmz643.