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Area forecast discussion 
National Weather Service Lincoln Illinois
904 PM CDT Monday Apr 20 2015

issued at 857 PM CDT Monday Apr 20 2015
some scattered echoes on radar remaining from diurnally enhanced
showers late this afternoon. Mostly virga still...but a little
slow to clear out. Clearing trend expected to continue and
temperatures will continue to drop through the overnight. West
winds with the surface low to the north. No updates anticipated to the
forecast at this time.


Short term...(tonight)
issued at 306 PM CDT Monday Apr 20 2015

Broad cyclonic circulation continuing over the Midwest this
afternoon...with surface low pressure centered over the east tip of
Lake Superior and an upper low over northwest Minnesota. Have been
seeing some fast moving light showers/sprinkles moving through the
forecast area...but as the showers and clouds are highly cellular in
nature...these should fade with loss of daytime heating. West
portion of the County Warning Area should be mainly clear by sunset...but it may
take until middle evening for the east to scatter out. Gusty winds of
30-40 miles per hour this afternoon will be diminishing with sunset as well...
but winds should still average around 10 miles per hour much of the night. Made
little change to forecast low temperatures...which are mainly a
couple degrees either side of 40.

Long term...(tuesday through monday)
issued at 306 PM CDT Monday Apr 20 2015

Windy conditions will be the highlight for Tuesday as upper and
lower level jets intensify across Illinois. Unusually deep mixing
heights forecast by the NAM/GFS up to 9-11k feet appear to be a bit
overdone. However, even mixing heights half that deep will cause
strong winds to mix down to the ground during the day. It appears
that gusts of 30-40mph will be common again tomorrow. There is a
chance that a Wind Advisory may be needed across our northern
counties, but current indications are the sustained winds should
prevail just below the criteria of sustained 30-39 miles per hour winds for
at least an hour.

The upper trough will remain entrenched across the Great Lakes for
the rest of the week, providing sustained northwest flow and cooler than
normal conditions. A shortwave and cold front will be pushed across
Illinois on Tuesday night...triggering scattered showers. The
NAM/GFS/ECMWF are indicating a mesoscale-scale band of enhanced lift
across our northern counties overnight, so we increased probability of precipitation into
the chance category there, with slight chances across the south.
Forcing for precipitation wanes quickly Wednesday morning, so we went with
dry conditions Wednesday. Reinforcing cold air behind the cold front will
keep highs for the remainder of the week in the upper 50s to near
60, with overnight lows in the 30s. Frost conditions still look
possible mainly north of I-74 Wednesday night and Thursday night, as winds
become light and skies clear out. Lows may dip to 34-35 in spots. A
frost advisory may be needed for one or both of those nights for our
northern counties.

The extended models have come into a little better agreement on how
fast the next system may trigger rain showers ahead of the
approaching southern stream low. The European model (ecmwf) has joined the GFS in
advancing showers into central Illinois on Friday, while the Canadian is
not far off, showing rain in our far western counties Friday
afternoon. We introduced slight chance probability of precipitation SW of a line from
Rushville to Lawrenceville to trend precipitation into the forecast.

Despite differences in the track of the surface low from the plains
into Illinois Friday night into Saturday, there remains some
agreement on the forcing for precipitation across Illinois ahead of the low. Rain
now appears likely across our S-SW counties, with chance probability of precipitation
included in the remainder of the area Friday night. Chance probability of precipitation were
included across the board on Saturday, with dry conditions
returning Sat night as the low departs to the east.

As the upper trough finally moves toward the East Coast next
weekend, a zonal westerly flow will push additional shortwaves
toward Illinois. We could see a few showers develop Sunday night and
Monday in our western counties as the next wave arrives.


Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 639 PM CDT Monday Apr 20 2015
windy and still gusty since some of the wind is pressure gradient
driven. Gusts should begin to decrease over the next couple hours.
That being said...winds will stay up a bit in the 8-12kt range
through the overnight. Losing the diurnal enhancement will also
clear the skies out after sunset. Drier air pushing into the
region and low pressure moving off to the NE will maintain the
clearing through the overnight and into the morning. Some cirrus
possible...and although some midlevel moisture is reflected in the
model time heights...the cumulus rule is signaling a bit of a break
through central Illinois. Winds gusty and out of the west again
tomorrow...potentially gusting to more than 30kts.


Ilx watches/warnings/advisories...


short term...geelhart
long term...shimon

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