Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
943 PM CDT Fri Aug 23 2019
Update...clear skies now over all of southern WI. Decoupling
winds have allowed temperatures to fall rapidly this evening.
Marine...warmer lake surface with temperatures in the 60s to
around 70 should continue to allow gustier winds to reach lake
surface through the night.
Previous discussion... (issued 630 PM CDT Fri Aug 23 2019)
Update...added fog to WI River Valley for after midnight.
Otherwise, sct-bkn cumulus thinning nicely, and should have mostly
clear conditions for much of the night. Boundary layer mixing just
off the surface should prevent much fog elsewhere late tonight.
Never the less, with 925h temps around 9-12 degrees, lowered
temps in susceptible eastern areas overnight, with a few spots
possibly getting into the 45 to 49 range.
Aviation(00z tafs)...VFR conditions to continue. May see some
late night shallow ground fog in low areas, and especially the WI
River Valley. More sct to occasional bkn cu expected to develop
Previous discussion... (issued 315 PM CDT Fri Aug 23 2019)
today through Sunday night...forecast confidence is high
Big bubble, no trouble. Broad ridge of high pressure slowly moving
across the Great Lakes will lead to dry and pleasant conditions
through the weekend. Main forecast challenge is with cloud cover
as as the 23.18z rap soundings have been fairly dry, but lapse
rates right under the inversion are just steep enough to produce
a weakly unstable profile from about 925-800mb which in turn has
produced a decent cu field. There actually appears to be some Lake
Michigan influence with some slightly higher moisture content air
to produce some better clouds there. Should see something similar
through the weekend as a similar setup is in place through the
rest of the Great Lakes, but shouldn't see deep enough of lift to
produce any rain showers.
Monday through Thursday night...forecast confidence is medium
Main shot at getting some rain over the next week comes on Monday
as a northern and southern stream system come together to create a
deep/closed off trough in central Canada by mid-week. Southerly
low level flow will help to push a more moist/humid air mass into
southern Wisconsin starting on Monday. This system comes in two
waves, the first being a southern stream system that pushes
northward across Illinois on into the central Great Lakes region.
This could clip southeast Wisconsin Monday morning, but the
23.12z European model (ecmwf) has no precipitation in southern WI while the 23.18z
NAM/GFS start streaming some light precipitation in across the
state. Thermal profiles are not overly impressive with warm
temperatures aloft and a skinny cape profile. The second wave
comes with the main mid level/northern stream trough which pulls a
cold front through during the evening hours. Instability still
isn't looking very strong with this period but forcing could end
up being pretty decent due to the strengthening trough. Wind shear
looks good with this feature, but the instability may again be
the limiting factor.
After this cold front passes early on Tuesday, another round of
cooler air gets advected into the region on the southern periphery
of the deepening trough. 850mb temperatures will drop back down
into the 5-7c range through much of the rest of the week. The
23.12z European model (ecmwf) pulls a cold front through Thursday evening, so added
in some pops for that period as well.
VFR conditions are in place across southern Wisconsin and are
expected to remain that way through Saturday. Some few/sct cumulus
are possible late Saturday morning into the afternoon, but should
stay at 4kft or higher. Winds will be gradually shifting from the
east-northeast to the east-southeast into tomorrow as a ridge of
high pressure drifts across the Great Lakes.
north-northeast winds will gradually become more east to
southeasterly through the weekend as a ridge of high pressure
moves through the region. Some stronger south-southeasterly winds
are expected going into Monday as the next low pressure system
tracks through the Dakotas. This low pressure system will then get
stuck in central Canada and deepen going into mid-week. This
should lead to some stronger westerly winds across the Great
Lakes, particularly the northern portion of Lake Michigan going
into Wednesday. Can't rule out some gales for this period with
some colder air getting pulled in along with it.