Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Milwaukee/Sullivan WI
910 PM CDT Sat Apr 25 2015
Clouds continue to slowly scatter out in subsidence behind short
wave trough and with drier air working south through the region.
Winds will lighten but remain mixy enough overnight to prevent
lows from dipping down below the expected low to middle 30s.
Expecting VFR conditions through the forecast. Could see some
diurnal cumulus Sunday afternoon...especially over eastern
portions of the forecast area...with lift from a weak short wave
rotating around the back side of elongated upper low centered over
the Canadian Maritimes but dry air in place should limit coverage
to no more than scattered.
Northeast winds ease tonight to around/just under/ 10 knots. The
northeast winds will increase slightly to again with mixing but
without the gusts like Saturday.
Winds are easing to the lower end of Small Craft Advisory
criteria...and should continue to lower overnight. Will keep
current end time to Small Craft Advisory to allow waves enough
time to lower below small craft levels.
Previous discussion... /issued 320 PM CDT Sat Apr 25 2015/
Tonight and Sunday...forecast confidence - high.
Last of the rain showers will move off to the southeast of the area
by late afternoon...along with 700 mb deformation/frontogenesis
response. Clouds are expected to gradually clear the area from north
to south later this afternoon and evening. Already seeing sunshine
in the far northern counties...as drier air moves southward into the
Models in good agreement with high pressure lingering north of Lake
Superior tonight and Sunday...keeping the drier flow of air into the
region. Quiet weather is expected. Cool temperatures tonight into
the lower to middle 30s are expected...with highs Sunday into the
middle to upper 50s well inland. Onshore winds will keep cooler 40s
near the lake.
Sunday night through Tuesday...forecast confidence - high.
Weak push of low to middle-level cold air advection and middle level
moisture may bring scattered to briefly broken clouds to eastern County Warning Area Sun night as
lobe of middle-level vorticity rotates around eastern Continental U.S. Upper low. Any
precipitation with this feature to remain to the east. Otherwise...high
pressure and subsidence will increase across the region Sunday night
and Monday as large upper low over gtlakes and northeast Continental U.S.
Begins to shrink and spin out into the North Atlantic. Lighter
winds and nearby high pressure ridge axis will result in winds
remaining mostly onshore during the daytime near Lake Michigan.
Modis imagery from Friday revealed lake sea surface temperature in the low
40s. Lakeshore areas likely to remain in the upper 40s to low 50s
on Monday but get a few degrees warmer on Tuesday due to lighter surface
winds...beginning the day northwest before veering onshore. Sunshine
and warming low level temperatures will allow inland locations to reach the
60s both days...perhaps nearing 70 in the west on Tuesday.
Keeping an eye on approaching weak trough in northwest flow. This
trough will affect northwest WI on Tuesday and some middle-high clouds will
likely spread into southern WI later in the day from this feature.
Questionable available moisture and weak forcing so will continue dry
forecast for Tuesday.
Tuesday night through Saturday...forecast confidence - medium.
Last several runs of the European model (ecmwf) show consistent signal in carrying
middle-level short wave trough across WI Tuesday and Wednesday. All medium range
guidance showing some form of northwest flow short wave moving
across WI during this period. While GFS much weaker with this
feature and Gem keeps upper circulation sliding south into Iowa...only
European model (ecmwf) shows this system strengthening over the Lake Michigan area Tuesday night
into Wednesday. Not ready to buy into this scenario as European model (ecmwf) also shows
upstream short wave kicker already moving across southern can north of
the plains on Wednesday. Also...Gem...GFS and dgex showing more
progressive...weaker situation. Will probably keep very small chances
for -shra in parts of the area for Tuesday night/Wednesday to acknowledge weak
trough passage. Split flow continues to bottle up deeper moisture
well to the south across southeast Continental U.S..
This weak trough does eventually phase with southern stream system to
result in amplifying long wave trough over eastern Continental U.S. On Thursday and Friday.
However descrepancy remains as slow GFS shows deepening upper low
over eastern gtlakes and northeast Continental U.S. During this period while European model (ecmwf)
more progressive and takes southeast Continental U.S. Low into western Atlantic by
Friday. These different solutions have a direct affect on expected
temperatures upstream across central and western Continental U.S.. more
amplified GFS solution keeps deeper ridging and colder northwest
flow lingering longer across western gtlakes into next weekend. More
progressive European model (ecmwf) shows upstream ridging breaking down faster...with
milder more zonal flow settling into WI on Sat...while GFS remains
cooler. Dgex and wpc leaning toward model blend. Either
way...warming trend to continue into next weekend. Approaching
upstream trough and increasing moisture may eventually return thunder
to the area around Monday...may 4th. CPC 8-14 day temperature
outlook has greater liklihood for above normal temperatures spreading into
WI for the period may 2-8.
Last of the rain showers will move southeast of Kenosha by late
afternoon...as a trough of low pressure passes to the south of the
region. Gusty northeast winds are expected until sunset...with VFR
Northeast winds will weaken somewhat tonight into early Sunday...as
high pressure axis moves into the region. Northeast winds at 1500
feet above ground level may linger around 30 knots tonight...but not
enough of a wind speed difference from the surface to require low
level wind shear mention in tafs. Northeast winds will become gusty
at the eastern sites again Sunday afternoon.
Clouds will gradually diminish from north to south across the area
late this afternoon and this evening...with mostly clear skies
expected later tonight into Sunday.
Tight pressure gradient and some mixing near the shoreline resulting
in northeast gusts in the 20 to 25 knot range at marine observation
sites. This should continue into the early evening hours...before
the pressure gradient weakens and mixing subsides.
These winds will continue to generate waves of 3 to 5 feet for most
of tonight. Waves should subside later tonight...perhaps a couple
hours earlier than the 12z expiration time of the Small Craft
Advisory. Will let later shifts evaluate need for an earlier ending
Lm...Small Craft Advisory until 7 am CDT Sunday for lmz643>646.
tonight/Sunday and aviation/marine...wood
Sunday night through Saturday...mbk