Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI
FXUS63 KMKX 241609 AAA
Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1009 AM CST Tue Jan 24 2017
12Z guidance is rolling in this morning and first impression is
that precip arrival will probably need to be slowed a bit in the
forecast. New model QPF seems fairly close to what`s in the
forecast. Models seem to be trending a bit colder though, so could
end up with more snow southeast half than what`s in there now.
Will continue to evaluate the data is the remainder of the 12Z
models come in.
Will probably need to slow down the arrival of the precip this
evening a bit based on latest model data. Also need to consider
the possibility of more snow vs rain, as models seem to be coming
in a little colder. This would be most noticeable in the southeast
half of the forecast area where the current forecast has an inch
or less of snow.
Overall, expecting mainly snow for the this event northwest half
of the forecast area, with rain mixing in with snow at times
southeast. Concerned with a loss of ice crystals for a time
tomorrow too, which might result in some more rain mix. Snow in
the southeast will be very wet...probably 4-8 to 1 ratios...with
closer to 10 to 1 ratios northwest.
Precip will eventually wind down Wednesday evening into early
Low clouds will hang around through this event, with lower
visibilities at times with the snow.
West winds will become variable today and then turn easterly by
evening as a weak high pressure ridge passes through. Approaching
low pressure from the central Plains will cause the east winds to
increase overnight, a few gusts may reach Small Craft criteria.
The center of the low pressure system will pass across southern
Lake Michigan on Wednesday. At this time, north to northwest winds
on the backside of the departing low are expected to gust to 25
to 30 knots for a time Wednesday night into Thursday.
.PREV DISCUSSION... /Issued 323 AM CST Tue Jan 24 2017/
TODAY AND TONIGHT - Confidence...Low to Medium
Dense fog has remained patchy early this morning due to widespread
low stratus and low level boundary layer mixing. However mixing
will continue to decrease through the early morning as upstream high
pressure ridge continues to edge our way. Hence wl keep fog mention
through the morning with areas of fog and localized dense fog
Short term guidance in reasonable agreement on mid-level short wave
trof over southwest CONUS tracking northeast into the western
GrLakes by late tonight. Lee-side cyclogenesis expected to develop
in the central plains tonight and then track toward southern WI.
Southern WI will be in the enhanced convergence area overnight as
low level jet intensifies and brushes the area later in the night.
In addition, enhanced synoptic forcing from left exit region of
upper jet spreading across the area overnight. As a result,
impressive layer Q-vector convergence reaches 30 units at multiple
levels. One change is that short term guidance a little slower in
warm air advection spreading into southern WI ahead of plains
cyclogenesis, so was able to delay precip start time until late aftn
thru mid-evening. Will continue small thunder threat in the south
tonight due to weak elevated CAPE spreading into the region on nose
of low level jet.
Main concern is ptype. Trend has been slightly colder and forecast
soundings and critical thicknesses show rapid cooling in the low
levels taking place once precipitation intensity increases during
the evening. With path of mid-level circulation taking farther
south route as well, thinking mix precip will change to mostly snow
over most of south central into east central WI during the evening
and overnight. Hence bumped up snowfall totals in these areas but
kept more mix and lower totals in the far southeast including MKE,
ENW and JVL. Posting Winter Weather Advisory for most of the area,
starting this evening and running through Wednesday for snowfall and
wintery mix. If cooling trend continues, future shifts may need to
bump up snow amounts even more and add more of southeast WI to
advisory, and possibly upgrade to a Winter Storm Warning. Also
concern that frontogenetical banding and thunder threat could result
in heavier snow band across parts of south.
WEDNESDAY... Forecast confidence is medium.
The surface low will track along the WI/IL border during the day
Wednesday. This is a slower track than previous forecasts. There is
fair agreement between the models of the track, although the ECMWF
is farther south and therefore colder over southern WI than the
other models. So there is a little uncertainty about where the
rain/snow line is going to track on Wednesday.
Most of southern WI will change over to rain for Wed afternoon as
temperatures warm into the mid to upper 30s. Southeast WI could
reach 40. A track farther south would keep northern portions of the
forecast area with wet snow all day. Right now areas north of
Madison to Sheboygan are forecast to get up to 2 inches of snow
during the day.
Strong subsidence at 500mb in the wake of the upper low will dry out
the snow growth layer late Wed night. This could lead to freezing
rain/drizzle instead of light snow. We will keep an eye on this for
the potential to extend the advisory.
THURSDAY THROUGH SUNDAY... Forecast confidence is medium.
A series of shortwave troughs embedded within cyclonic flow over the
Great Lakes will bring occasional snow showers and flurries to
southern WI through the weekend, along with brisk northwest winds
and persistent low clouds. Highs will be closer to the seasonal
normal in the mid to upper 20s.
MONDAY THROUGH TUESDAY... Forecast confidence is medium.
Another low is expected to drop down from south central Canada and
bring a chance for light snow and gusty winds to WI Monday night
into Tuesday. It looks like northern WI would have the heavier snow
with this one. This system should also bring a decent surge of cold
AVIATION/12Z TAFS/...Patchy fog will likely linger through the
morning as boundary layer winds diminish. A few spots will
experience dense fog this morning. Otherwise, IFR cigs this morning
should increase to MVFR later today. Approaching low pressure will
bring a mix of rain and snow to the area tonight, with some
locations changing to or starting as snow. Possible snow could
become heavy for a period later this evening into the overnight.
MARINE...West winds will become variable today and then turn
easterly by 00z as a weak high pressure ridge passes through.
Approaching low pressure from the central Plains will cause the east
winds to increase overnight, however gusts are expected to remain
below Small Craft criteria. The center of the low pressure system
will pass across southern Lake Michigan on Wednesday. At this time,
north to northwest winds on the backside of the departing low are
expected to gust to 25 to 30 knots for a time Wednesday night into
WI...WINTER WEATHER ADVISORY from 6 PM this evening to 6 PM CST
Wednesday for WIZ046-047-056-057-062-063-067-068.
WINTER WEATHER ADVISORY from 9 PM this evening to 6 PM CST
Wednesday for WIZ051-052-058-059-064.
TODAY/TONIGHT AND AVIATION/MARINE...MBK
Wednesday THROUGH Monday...MRC