Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
333 am CDT Sat Jul 4 2015
Today and tonight...forecast confidence...high
An upper level trough will exit the upper Great Lakes today with a
ridge building across Wisconsin and the upper Mississippi Valley
with weak 250 mb flow. Weak upper divergence today and tonight.
Very weak 700 mb cooling this morning...then weak warming by
tonight. 700 mb drying by afternoon. Fairly neutral 700 mb upward
motion. 850 mb dew points are around 11 celsius. Forecast soundings
have a fairly moist layer around 800 mb. GFS forecast sounding have
strong low level lapse rates with surface to 3 km values of 8
celsius/km and around 9.7 from the surface to 850 mb. However the
middle levels are fairly stable with 3-6km lapse rates of 5.9
celsius/km...with a rather high middle level cap developing late this
afternoon from 700 to 600 mb.
Little in the way of a pressure gradient with a large high across
the middle Mississippi Valley and upper Ohio Valley. A weak trough axis
/cold front extends across northeast Wisconsin but that weakens with
mainly a weak west flow inland areas...with a lake breeze pushing
inland during the day east.
Surface dew points rise slowly today. With the steep low level
lapse rates and low level moisture...zero to 1 km cape rises to
around 800 joules/kg. The mesoscale models are generating isolated
showers this afternoon...with the hrrr and GFS keying more on the
lake breeze front. Will keep this possibility...especially across
Expect dry weather Sunday under high pressure centered over the
eastern Great Lakes. Southerly low level flow will continue to
bring milder air into the area...with highs likely a couple to few
degrees warmer than normal.
Monday through Friday...forecast confidence...medium
Aside from the quicker GFS...models have slowed a bit with the timing
of a low pressure system and associated cold front on Monday.
Should be a good increase in moisture ahead of the front...with
the GFS increasing precipitable water values to around 2-2.25 inches.
Cape values...0-6 km bulk shear...and middle level lapse rates not
particularly impressive...but some stronger storms possible given
the front timing. Storm Prediction Center has the forecast area under a marginal risk
for severe storms. Either way...looks like a decent chance for a
round of afternoon and evening Summer storms though...so kept
higher probability of precipitation going
Slower frontal timing should allow for more heating. Model 925 mb
temperatures generally in the 22-24c range...so went with middle to even
upper 80s in a few spots. Looks breezy for much of the day as well
ahead of the approaching front.
Any lingering precipitation should wind down by early Tuesday
morning...with dry weather then expected though Wednesday. Models
diverge beyond that...with the GFS and Canadian bringing a return
of moisture and shower/storm chances at times Wednesday night
through Friday...while the European model (ecmwf) remains dry. Have some low probability of precipitation
going at times to account for the GFS and Canadian solutions.
Somewhat of a spread in model temperatures in the extended...with the
European model (ecmwf) maintaining cooler temperatures aloft behind the departing Monday
system...while the GFS has less of cool down and recovers temperatures
faster. Stuck near consensus of model temperatures due to the
uncertainty...resulting in forecast temperatures within a couple degrees
of normal values.
Patchy IFR fog mainly in low areas ending by 13z. Otherwise VFR.
Light winds will continue overnight as a ridge of high pressure sits
over the region. A weak cold front will stall out across central
Wisconsin and weaken...so potential isolated showers or stray
thunderstorm should stay mainly to the north of the local taf sites.
Expect elevated smoke layer to continue from the Canadian/alaskan
Expect a lake breeze with light east winds by afternoon. Dew points
rise slowly so fog potential increases...but do not expect any
widespread dense fog.
Today/tonight and aviation/marine...hentz
Sunday through Friday...ddv