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Area forecast discussion...updated 
National Weather Service Milwaukee/Sullivan WI
1159 am CDT Tuesday Apr 21 2015

Update...

Scattered wintry mix will continue this afternoon with steep low
level lapse rates in place. Temperature profiles are struggling
to warm up due to little diurnal heating due to overcast stratus
deck across the area. Westerly winds will remain gusty this
afternoon as a tight surface pressure gradient is in place behind a
large scale low pressure system. Boundary layer mixing near the
800 mb level will support windy conditions this afternoon.

&&

Aviation/18z tafs/...

Low end VFR ceilings will persist through the afternoon Tuesday as a
low level stratus deck is in place over the area. Ceilings should be
between 3.5-5.0 kft. There is some uncertainty regarding a wintry
mix this afternoon as temperature profiles are not increasing due
to the lack of diurnal heating. A brief wintry mix is possible
this afternoon with these cooler temperatures...which could
briefly reduce ceilings and visibilities to MVFR conditions.
Otherwise...expect the low end VFR ceilings to continue. West winds
will be gusty this afternoon as a tight surface pressure gradient is
in place...along with boundary layer mixing up to the 800 mb
level. Gusts could reach up to 25 to 30 kts across the taf site
area.

&&

Previous discussion... /issued 329 am CDT Tuesday Apr 21 2015/

Today and tonight...forecast confidence is high.

The large upper low will slowly move across the northern Great Lakes
and Ontario Canada through the period. The strong surface low over NE
Ontario Canada will slowly fill during this time. The pressure
gradient and steep low level lapse will maintain brisk westerly winds
that will turn northwesterly tonight with the passage of another cold front.
Isolated to scattered showers will continue within the cyclonic flow and
instability that is present.

Short term...Wednesday through Thursday night...forecast
confidence is medium.

Models are in decent agreement with the closed 500 mb low
shifting slowly eastward across east central Ontario Canada
Wednesday...then opening up as it shifts further to the east
Wednesday night into Thursday night. Sheared 500 mb vorticity
maximum shifts through most of the area Wednesday...then eastward
through the area Wednesday night into Thursday.

Area forecast soundings suggest enough 850 mb moisture with steep
low level lapse rates Wednesday to keep low probability of precipitation in the far
northern counties. Pressure gradient will remain tight Wednesday
for another day of gusty west winds.

Weak high pressure then slides into the region Wednesday night
into Thursday night...which combined with a dry airmass should
bring quiet weather to the area. Temperatures will remain below
seasonal normals.

Long term...Friday through Monday...forecast confidence is low.

European model (ecmwf) and GFS continue to show big differences with features
during this period. The GFS keeps the area dry Friday into
Sunday...with high pressure to the northwest of the region and
northwest flow at 500 mb. It keeps the low pressure system and its
associated precipitation to the south of the region during this
time.

The European model (ecmwf) brings the low and associated 500 mb shortwave trough
further north...and brushes the area with precipitation later
Friday into Saturday. For now...will use consensus model probability of precipitation and
temperatures during this time...given the differences in the
models.

They have weak high pressure sliding through the region
Sunday...with a shortwave trough at 500 mb sliding east into the
region Monday. There are more differences with timing with this
system...so continued the use of consensus model probability of precipitation and
temperatures.

Aviation/12z tafs/...

Ceilings today and tonight will mostly range from 3.5-5.0 kft. Isolated
light showers will continue as a large upper low slowly moves
across the northern Great Lakes and southern Ontario Canada.

Marine...

A Small Craft Advisory is in effect today into the evening for
brisk westerly winds as the region remains under the influence of
strong low pressure that is located over northeast Ontario Canada.
Wind gusts up to 30 kts are forecast.

&&

Mkx watches/warnings/advisories...
WI...none.
Lm...Small Craft Advisory until 10 PM CDT this evening for lmz643>646.

&&

$$
Update...jts
today/tonight and aviation/marine...gehring
Wednesday through Monday...wood

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