Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
953 PM CDT Sat Apr 18 2015
Update...Ely winds will continue to keep southern WI dry through sun
am. The shortwave trough currently over la will then lift northward into
lower Michigan for sun evening. This continues to put eastern WI on the western
periphery of a large rain area. Meanwhile the closed upper low
currently over eastern Colorado will become an open wave and move across MO
and Illinois sun nt and into lower Michigan on Monday. This should bring a round
of light to MDT rain to much of southern WI sun nt and Monday. Much cooler
temperatures to then prevail.
Aviation/06z tafs/...an Ely low level jet up to 45 kts will likely produce
low level wind shear until 12z sun. Otherwise VFR conditions are expected into
early sun after. By late after...rain will overspread the southeastern WI taf
sites. Ceilings will eventually drop below 600 feet with visibilities ranging
from 2-5sm. The rain may hold off until the evening over south central
WI but eventually the ceilings will lower below 1 kft with visibilities
ranging from 3-5sm.
Previous discussion... /issued 357 PM CDT Sat Apr 18 2015/
Tonight and Sunday...forecast confidence high.
A closed upper low over eastern Colorado will gradually weaken and
lift northeast through the weekend. Meanwhile... a deep middle level
trough will swing through the northern plains and eventually close
off over the upper Midwest/southern Ontario by Sunday night. This
complex weather pattern gives US an interesting forecast for this
Showers and thunderstorms over western Iowa this afternoon will lift
north with a shortwave ejecting out of the upper low. This shortwave
will combine with the northern upper trough to create a surface low
over Minnesota that will produce more showers and track northeast on
Sunday. The precipitation associated with this low pressure trough will
approach south central WI tonight into Sunday morning... but will be
up against too much dry air and weak forcing to make any headway
into the mkx forecast area until late Sunday afternoon.
Also... there are a few isolated showers/thunderstorms near the I-80
corridor across northern Illinois this afternoon. These are popping up due
to a shortwave meeting up with that surface cold front that swept
through southern WI this morning. This front will lift back
northward as a warm front tonight... but it is diurnally unfavorable
to produce precipitation for southern WI. It is also going to encounter
dry air in the low levels.
Tonight... another lobe of vorticity will break away from that
closed upper low and draw Gulf moisture up into the Midwest as a
surface low develops and tracks up the Ohio River valley. The
northwest side of this low is expected to brush southeast Wisconsin
and bring a slug of rain to the area late Sunday afternoon as that
low tracks north through Indiana into lower Michigan.
It is possible that portions of south central WI may get missed or
only get a little rain from the precipitation with this complex low
High temperatures should manage to reach the upper 60s from Madison
and west given the delayed rain on Sunday. Temperatures near the
Lakeshore will be in the 50s given onshore flow.
Short term...Sunday night through Tuesday night
Sunday night...forecast confidence high
Rain will definitely overspread all of the forecast area with
passage of surface low just to the east over lower Michigan...driven
by short wave that lifts north through the MS River Valley and
across Lake Michigan as it is absorbed into developing closed low
over southwest Ontario Sunday evening. Heavier quantitative precipitation forecast amounts with
deformation area in the east...with a second lower maximum with
forcing along cold front that reaches the far eastern County Warning Area around 12z
Monday. Low temperatures in the 40s expected minimal cold advection
into the forecast area by 12z Monday.
Monday and Monday night...forecast confidence medium
Some disagreement about how quickly rain exits eastern Wisconsin
with secondary wave running up along the cold front. Consensus probability of precipitation
leads to chance probability of precipitation in the morning...highest in the east...tapering
from south to north through the day to slight chance across the
northern half Monday night...mainly due to short wave energy
rotating around closed low and approaching secondary cold
front/trough from the north. Could see a few snow flakes mixing in
with the rain Monday night into early Tuesday morning.
In spite of early clouds and precipitation...it will turn gusty Monday with
cold air advection and drier air allowing mixing up to 25 to 30 knots
winds at 800 mb. Deep mixing also allows high temperatures to reach the low
to middle 50s.
925-850 mb cold pools drop south through the state Monday
night...moving into southern WI by 12z Tuesday...with lows in the middle-
Tuesday and Tuesday night...forecast confidence high
Steep lapse rates and a bit of cape noted on forecast soundings with
increasing instability Tuesday as southern portion of 925-850 mb
cold pools sweep through the area..then shift off to the east
Tuesday afternoon and evening. Forcing enhanced by vorticity maxima
swirling around low center pass over the region. Will have chance
probability of precipitation everywhere then lower from SW to NE during the day Tuesday as
with departing forcing and instability. Highs in the 40s north
and around 50 south with clouds and light precipitation.
Long term...Wednesday through Saturday...forecast confidence medium
This period marked by slow eastward progression of cold closed upper
low as it wobbles over the upper Great Lakes into Thursday...before
moving east across southern Quebec to the far NE U.S. By Saturday morning
as near-zonal upper jet winds remain over the southern 1/2 of the lower
48 states. Although a fairly cloudy period...precipitation will be tied
close to the center of the closed low to the north of the forecast
area...and to south along the baroclinic zone on the southern
periphery of the overall cyclonic flow. Temperatures will be 5 to
10 degrees below seasonal normals.
Gusty northeast winds in the wake of a cold front will gradually
ease through the late afternoon. VFR conditions will continue
through midday Sunday with dry air in place.
Winds just above the ground will be out of the southeast at 30 to 40
knots late tonight. Models vary on the intensity of these winds.
Winds at the surface will be southeast around 11 knots. This will lead to
marginal non-convective low level wind shear conditions. I left it
out of the tafs due to uncertainty for now.
Look for rain to spread into southeast WI by late Sunday afternoon
on brisk northeast winds. This should bring MVFR to IFR ceilings. Rain
is delayed in spreading into south central WI from the west during
this time as low pressure tracks into the area. Periods of rain and
MVFR/IFR ceilings/visibility are expected Sunday night through Monday morning.
Brisk and gusty northeast winds and resultant high waves in the wake
of a cold front will ease through late afternoon. The Small Craft
Advisory was allowed to expire. Another short period of gusty
northeast winds is expected Sunday afternoon/evening as low pressure
tracks from Indiana to lower Michigan.
Near-critical fire weather conditions are occurring over south
central Wisconsin this afternoon. Relative humidity is down near 25
percent with east winds up to 15 miles per hour with some gusts around 22 miles per hour.
Conditions will improve Sunday through early next week as low
pressure tracks through the region and brings some rain.
Lm...Small Craft Advisory until 10 PM CDT this evening for lmz644>646.
tonight/Sunday and aviation/marine...mrc
Sunday night through Saturday...rem