Scientific Forecaster Discussion
fxus63 kmkx 210850
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
350 am CDT Wed Mar 21 2018
confidence...medium to high.
Low level northeast winds and Delta-T around 10 degrees continues to
generate patchy low clouds over the eastern areas early this
morning. Dewpoints remain slightly higher in this area due to
onshore flow as well. Cutoff low pressure over the Ohio Valley
expected to get nudged eastward today to the East Coast. This will
allow low level winds to back to the north ending threat of lake
clouds. Widespread cirrus shield associated with cutoff low should
also thin through the day allowing sunshine to return most areas.
Daytime temps should be a couple degrees warmer than yesterday.
Upstream weak short wave over the northern plains will weaken as it
moves southeast, but may bring a period of mid-high clouds to the
area this afternoon and evening. Weak mid-level convergence may
bring more mid-clouds to the area later tonight.
Thursday through Friday - confidence...high.
Surface high pressure will build into the region Thursday and
dominate our weather regime into Friday, leading to a period of
clearing. Temperatures will be around average with highs in the
40s and lows in the 20s.
Friday night and Saturday - confidence...medium.
00z suite of model output has come into a little better agreement.
A wave traversing the central rockies will instigate Lee
cyclogenesis, which will move east in a fast flow. Unlike the
past several days of model runs, the 00z guidance has honed in on
the location/timing of the surface low, leading to an overall
increase in confidence. Strong frontogenetical forcing will be
associated with this system, and it will be the primary focus for
precipitation production beginning Friday night and persisting
into Saturday. The big question remains, just how far east into
southern Wisconsin will precipitation reach?
There is strong consensus for stiff southeasterly low level flow
across much of our area. This persistent fetch of dry, Continental
air will be quite formidable to overcome. As we have observed
over the past few runs, extensive mid/high level moisture will
stream over the region, but the low level moisture availability
looks to be focused closer to the lower level frontogenesis
signal. At this time it appears the best chances for precipitation
will be (roughly) along/southwest of a line from Lone Rock to
Madison to Lake Geneva.
Given the strength of both the dry air fetch and forcing involved
with the system, there should be an abrupt cutoff to precipitation
to the northeast. Model quantitative precipitation forecast is in remarkable agreement over
Lafayette County, with the GFS, Gem and European model (ecmwf) all coming in at
0.5" of liquid. Likewise, quantitative precipitation forecast is on the order of hundredths of an
inch on the Lone Rock/Madison/Lake Geneva line (a mere 45 miles to
Forecast soundings are supportive of snow, with perhaps a
changeover to light rain before ending Saturday afternoon. Given
the time of year and Thermo profile, snow to liquid ratios have
been kept at, just below 10:1. This results in the potential of
2-4" of snowfall, though compaction and some melting will eat away
at this a bit. Again, the best potential for accumulating snow at
this point appears to be Iowa/Lafayette/green/portions of
southwestern Dane County. Without a doubt, additional tweaks will
There is one outlier solution to mention for this event and that's
the NAM. In spite of agreeing with the dry low level flow, strong
frontogenesis and a bonus 700mb vort have really kicked up quantitative precipitation forecast
amounts. Just not getting such signals from any other model and
have considered this an outlier.
Saturday night Onward - confidence...medium.
Quiet and seasonably mild conditions are expected Sunday/Monday.
Another wave moves through the region Monday night into Tuesday
bringing a chance for showers.
patchy MVFR low clouds will continue through mid-morning but then
should decrease as the northeast low level flow backs more to the
north. Low clouds will only affect eastern areas this morning.
Otherwise VFR conditions are expected with gradually thinning
cirrus shield today. Possible some lower VFR mid clouds move into
parts of the area later tonight.
gradually weakening pressure gradient will result in gusty onshore
winds continuing to diminish through the morning, and backing to
the north in the late morning and afternoon. High wave heights
will gradually diminish through the day, to below 5 feet by the
afternoon. Hence the current Small Craft Advisory will remain in
effect this morning.
Lm...Small Craft Advisory until 1 PM CDT this afternoon for