Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 111910
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
210 PM CDT Fri Apr 11 2014

.DISCUSSION...
ISSUED 200 PM CDT Fri Apr 11 2014

SHORT TERM...Tonight through Monday

Slow moving storm system to bring "warm" air north into our area
for the next couple of days followed by the threat for heavy
rainfall over parts of central Illinois, especially late Sunday
afternoon through early Monday.

The main forecast concerns this period will be with temperatures
and timing of precip east into our area starting Saturday evening
far northwest and over the remainder of the area by Sunday night.
Models in decent agreement with respect to frontal position this
weekend but are having their usual problems with moisture advection
north into the Midwest. Both the NAM-WRF and operational GFS were
bringing in low 60 degree dew points north into our area tomorrow
afternoon and evening ahead of a slow moving frontal boundary
approaching from the northwest. Looking at upstream surface obs over
parts of Arkansas and southern Missouri, it appears the dew points
were forecast about 5 to 7 degrees too high for early this afternoon
compared to reality. So once again, will side more with the ARW
solution as it seems to mix out the lower levels of the atmosphere
more efficiently with afternoon dew points forecast mostly in the
mid 50s. In addition, forecast soundings suggest a much better
cumulus field across our area with the returning moisture as well
but that won`t hold temps back as 850 temps suggest afternoon
readings in the 75 to 80 degree range accompanied by south winds
gusting as high as 35 mph at times in the afternoon.

Aforementioned front will be settling southeast into central
Wisconsin southwest through west central Iowa by evening. Surface
base capes off the ARW soundings ranging from 1200-1500 j/kg
across far west central Illinois by late afternoon. With the main
forcing mechanism still well west and northwest of our area, feel
the better chances for scattered thunderstorms will be over eastern
Iowa northeast through northwest Illinois Saturday afternoon and
early evening. What those storms do with respect to an associated
outflow boundary and eventual propagation is still uncertain for
our northwest counties but it still warrants low chance POPs over
our far northwest areas by late afternoon with the better chances
spreading ever so slowly east and southeast during the late evening
hours of Saturday. Radar simulations off the SPC and HRW-WRF NMM
delay any significant convection until the 01z-04z time frame well
to our northwest as soundings suggest the cap will be slow to
break late in the afternoon. Even the NAM-WRF solution indicating
the better K indices remaining well to our northwest until late
Saturday night, and even that is confined to our far northwest
counties as the better 925-850 mb jet and favorable theta-e advection
during the evening remains well to our west. With the slower model
trends will keep the better chances for rain confined to our
northwest Saturday night into Sunday with the widespread rains
associated with the stronger upper wave and surface front will be
late Sunday afternoon into Monday morning.

Latest ECMWF has continued to trend slower with the overall speed
of the large 500 mb inching its way across the central U.S. into
early next week, with a secondary piece of energy near the base
of the trof seen on the latest run, which the 12z NAM-WRF was
hinting at as well. This has resulted in a much slower ending of
precip and actually shows a second wave of precip pushing northeast
ahead of the southern stream shortwave into an air mass supportive
of at least a rain snow mix by the end of the day Monday into
Monday night. Not going to jump on that bandwagon quite yet but it
will need to be watched.

LONG TERM...Tuesday through Friday

500 mb trof axis to shift to our east Tuesday afternoon with the
upper flow quickly transitioning back to zonal by mid-week which
should make for a quick recovery in temperatures. After some very
chilly readings Tuesday and Wednesday mornings with frost/freezing
temperatures possible, we should see readings return close to normal
by Thursday and Friday when afternoon temperatures climb back into
the 60s. If the ECMWF solution is correct, we may need to bump up
POPs Monday night across the east with most of the precip just off
to our east Tuesday morning. After that, not very confident on our
next chance for precip with the model blend plastering precip all
over the place later in the week. Next significant wave moving well
to our north late in the week, but with all the deeper moisture
shunted away with our early week trof, not much support for any
meaningful rain chances in the extended.

Smith
&&


.AVIATION...
ISSUED 1259 PM CDT Fri Apr 11 2014

VFR conditions expected to continue across the central IL airports
through 18Z/1 pm Saturday. Scattered cirrus clouds this afternoon
and then scattered-broken mid/high clouds increase during tonight.
4-6k ft broken clouds to lift ne toward SPI around 12Z/Sat and I-74
TAF sites around 15Z/Sat. NAM brings in MVFR celings around 2.5k ft
during Saturday morning but feel this is too low considering the
dry low levels in place initially over central IL. NW winds less than
10 kts to become sw during the afternoon and south tonight as weak
high pressure over IL drifts toward the mid Atlantic states. South
winds to increase to 15-20 kts with gusts of 25-30 kts after 15Z/Sat.
1002 mb low pressure to move into the Midwest Sat with increasing
pressure gradient over central IL and increasing the southerly
winds.

07

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$




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