Scientific Forecaster Discussion
fxus63 kmkx 072350
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
550 PM CST Sat Dec 7 2019
Update...widespread cirrus over southern Wisconsin early this
evening as pressure gradient tightens ahead of low pressure riding
along the U.S./Canada border. Upstream upper level warm air
advection should continue to bring sct-bkn cirrus to the area much
of the night. The clouds along with gusty south winds will keep
temperatures holding mostly in the upper 20s to low 30s
overnight. Will need to lower a few spots in the east into the
upper 20s for mint.
Aviation(00z tafs)...axis of low level jet will pass across
southern WI overnight. Above 1k feet, southwest winds will be
gusting up to 45 to 50 kts resulting in low level wind shear
criteria. Some short term guidance is trying to bring lower clouds
and MVFR ceilings into the area Sunday morning, but not sold on
this scenario. May hold off until the afternoon when upstream cold
front and low level trof approach southern WI. Satellite imagery
does reveal some low clouds over the southern MS valley region. Do
expect this area to start spreading northward, but this area of
low clouds may remain to the south of southern WI. Some light
rain or drizzle will be possible later Sunday night.
Previous discussion... (issued 312 PM CST Sat Dec 7 2019)
Tonight through Sunday...forecast confidence is high...
A system exiting the northern rockies will track across the
US/Canada border tonight through Sunday. Much of the forcing is
going to be focused along northern WI and the Lake Superior region
and with limited available moisture the forecast will remain dry
through Sunday morning. Stronger S/SW winds via a low level jet will
transport enough moisture in the lower levels to allow for low end
pops near the Illinois border Sunday afternoon as a front stalls out.
However, there might still be enough dry air aloft to keep things
dry. In either case though, temps are warm enough for precip to
remain liquid. Overnight the winds keep things mixed and lows may
not fall much below the freezing mark. The warm air advection associated with
the stronger southerly flow will allow temps to warm into the
lower 40s for Sunday.
Monday through Wednesday...forecast confidence is high.
In the mid levels on Monday there is a very broad trough that will
be digging through most of the Midwest. There will also be an
embedded shortwave that moves through the region on Monday. The
embedded shortwave will help drive surface cyclogenesis of the
low and drive it out of the plains toward the Midwest. There is
still some uncertainty in the track of the surface low Monday
afternoon. These slight differences in track bring uncertainty in
when the rain to snow transition begins Monday evening. Overall
the system is rather weak and could produce of 1 to 2 inches of
snow or less. The bulk of the snow will be falling mainly north of
Milwaukee and Madison.
By Tuesday and Wednesday, there will be an Arctic air mass over
the upper Midwest. A high pressure system will be keeping it dry
and winds around 10 mph. This should keep wind chills around -10
to -15 Tuesday night into early Wednesday morning.
Thursday through Saturday...forecast confidence is medium.
There will be high pressure in place on Thursday, with
temperatures warming into the mid 20s. Things then become more
uncertain for Friday and Saturday. In the mid levels there is
zonal flow and temperatures will begin to climb towards normal. A
short wave will come through the Midwest, but the timing is still
uncertain. The short wave could help produce some light
precipitation on Friday, but it will most likely hold off until
Saturday. There could be some light snow Saturday from the short
wave and the presence of an elongated trough out the the west.
Overall nothing of much concern at this point.
VFR conditions are forecast through Sunday morning. A system
passing to the north of the region tonight will bring an increase
in southerly winds this afternoon through Sunday. These southerly
winds bring better low level moisture towards the end of the taf
period and MVFR cigs spreading into the region are forecast.
Stronger winds stream in overnight and could result in wind shear
issues at all terminals.
A low pressure system will move across Lake Superior tonight
through Sunday morning. Strong west/SW winds aloft overspread the
area and these winds should be able to mix down and gale force
gusts are expected over the open waters. The strongest gusts will
be focused across the northern half of Lake Michigan but a Gale
Warning is in effect for the entire open waters of Lake Michigan.
For the nearshore waters believe there is enough of an inversion
to prevent the strongest winds from making it down to the surface
and a Small Craft Advisory is in effect. Waves could range from 10
to 14 feet across the far northern waters. Winds and waves begin
to diminish Sunday afternoon. We remain under a breezy north to
northwesterly flow through the week and gusts could approach small
craft conditions at times through the week.
Lm...Gale Warning from 9 PM this evening to 2 PM CST Sunday for
Small Craft Advisory until 3 PM CST Sunday for lmz643>646.