Scientific Forecaster Discussion
fxus63 kmkx 131225
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
625 am CST Wed Dec 13 2017
Model guidance continues to keep the heaviest band of snow mainly
east central Wisconsin. Still think that many areas still have a
good shot at seeing snow, especially with the comma head of snow
band as the low moves southeast. Therefore will continue the
advisories on the southwest fringe. In addition to the main band
during the day, will need to watch for lake effect later this
afternoon and evening. Snow amounts will vary greatly from
southwest to northeast across the area, with the heaviest still in
the Sheboygan area.
Model guidance continues to keep the heaviest band of snow north
and east of the terminals today. Still think that all sites
still have a good shot at seeing some snow, especially with the
comma head of snow band. In addition to the main band during the
day, will need to watch for lake effect later this afternoon and
evening. Ceilings and vsbys will vary greatly from southwest to
northeast across the area, with mainly VFR southwest areas, and
LIFR in snow in northeast areas vicinity Sheboygan. Will need to
watch for a band of MVFR/IFR with the comma head as it pushes
southeast. Also IFR in lake effect snow off Lake Michigan later
this afternoon and evening.
A Small Craft Advisory is in effect for the nearshore waters until
mid morning Thursday. Initially gusty southeast winds will become
light for a brief period of time as the surface low moves
overhead, before winds increase again from the northwest. Waves
will build to 3 to 6 feet early in the day, then build to 5 to 8
feet by Wednesday evening. Winds may also briefly approach gales
late Wednesday night.
Previous discussion... (issued 247 am CST Wed Dec 13 2017)
Today and tonight...forecast confidence is medium...
An upper shortwave moves across the upper Mississippi Valley and
across Wisconsin today. Moderate upper level divergence with
moderate to strong 700 mb upward motion.
Tight baroclinic zone, with a lot happening rapidly dynamically
Short term guidance in good agreement on carrying low level
circulation and surface low pressure from southeast Minnesota
across southern WI/northern Illinois border area today. GFS/NAM/ECMWF
showing fairly good consistency with this feature. Main problem
will be a sharp line to the band of moderate precipitation on the
south end. Models are trending a bit farther north with this. Will
therefore trim snow amounts a little, but keep headlines as is,
since a slight deviation back south is certainly possible.
One worrisome snafu is potential for losing ice crystals for a
time in the southwest County Warning Area as drier air wraps around low pressure
circulation, hence slightly lower confidence. Did include small
chance for -zl/-zr in the southwest. Low level drying remains
more limited in the eastern County Warning Area during the day. Cross sections
show impressive lift across northeast County Warning Area in the dendritic growth
zone close to an area of sloping frontogenetical forcing. In
addition, an inverted trof extending northwest from the low level
circulation may increase the convergence along the Lakeshore areas
from Milwaukee to Sheboygan in the late afternoon into the early
evening. Delta-T around 14-15c in the early evening with lake
induced cape over 250j. Part of the low level conditionally
unstable layer reaches the snow growth region as well. With snow
liquid ratios around 15 to 1, expecting synoptic snowfall in the 3
to 6 inch range over far northeast near Sheboygan. Continued
Winter Weather Advisory for all of the northeast, with eastern
areas continuing into the evening.
Thursday through Saturday...forecast confidence is medium to high:
A series of weak shortwaves will roll through the area late week
into the weekend. This will bring flurries to a dusting of snow at
times through this period. Right now it looks like the best chance
for light accumulations is Friday night into Saturday across the
northeast half of the forecast area. Even the snowiest models
still keep accumulations generally less than an inch.
Sunday through Tuesday...forecast confidence is medium:
Big question Marks remain with the low pressure system progged to
move through on Sunday. The previous European model (ecmwf) looked similar to the
current Canadian, painting a couple inches of snow across
southern Wisconsin. The latest European model (ecmwf) is weaker and farther north
with the low, bringing light snow in the northwest forecast area
and a mix of rain/snow in the southeast. The GFS on the other hand
is progressive with the wave and keeps the better moisture/forcing
to the south, resulting in dry weather here. Split the
differences among the solutions for now.
Models are in decent agreement for Monday, showing dry weather
between waves. Another wave Monday night and/or Tuesday should
bring light snow chances back to southern Wisconsin.
There are some moderate differences in temperatures among models
for early next week. Overall though, a blend of solutions results
in temps near to a couple degrees above normal for mid-December.
Model guidance is coming into somewhat better agreement tonight in
keeping the heaviest band of snow north and east of the terminals
Wednesday morning. The latest tafs reflect this, with higher
visibilities than the 00z or 03z issuances. Still think that all
sites except msn still have a good shot at seeing some snow
tomorrow, and any slight southward shift in the snow band could
result in much more impactful conditions than currently forecast.
In addition to the main band during the day, will need to watch
for lake effect later Wednesday afternoon and evening, especially
Another Small Craft Advisory is in effect for the nearshore waters
from mid morning Wednesday to mid morning Thursday. Initially
gusty southeast winds will become light for a brief period of time
as the surface low moves overhead, before winds increase again
from the northwest. Waves will build to 3 to 6 feet early in the
day, then build to 5 to 8 feet by Wednesday evening. Winds may
also briefly approach gales late Wednesday night.
WI...Winter Weather Advisory until 9 PM CST this evening for wiz051-
Winter Weather Advisory until 8 PM CST this evening for wiz065-
Winter Weather Advisory until 6 PM CST this evening for wiz046-
Lm...Small Craft Advisory until 9 am CST Thursday for lmz643>646.