Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 130200
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
900 PM CDT Sat Apr 12 2014

.DISCUSSION...
ISSUED 900 PM CDT Sat Apr 12 2014

Latest hourly runs of the HRRR and RAP models suggest that
forecast area will remain dry through the night, with the
shower/storm threat increasing from the northwest after sunrise
Sunday. This is a drier solution than some of the earlier runs, as
well as most of the synoptic model runs. Feel this drier trend is
the way to go.

The local airmass, per 00Z KILX and forecast soundings, is quite
dry, and there is not expected to be significant moisture
advection into the area overnight. In fact, the low level flow in
our area is expected to diminish for a time later tonight, with the
best low level inflow apt to remain into the frontal zone to our
northwest. This is where significant convection has blown up over
the past few hours. The main threat for any precipitation reaching
the forecast area before daybreak is if a significant cold pool
can develop with the storms to the northwest and help them
propagate to the southeast. Even if this were to occur, the storms
would be moving into an increasingly hostile environment and
should weaken significantly with time.

Plan to update to remove PoPs from this evening, and reduce them
overnight, restricting them to locations west of the Illinois
River Valley. Otherwise, only minor tweaks are needed for the
expected hourly trends.

Bak
&&

.AVIATION...
ISSUED 647 PM CDT Sat Apr 12 2014

The 00Z TAF forecast becomes rather tricky by late tonight across
the central Illinois terminals. Southerly winds, frequently quite
gusty, should prevail through the period ahead of a slowly
approaching storm system. However, the forecast models continue to
struggle with the details regarding when the precipitation threat
arrives locally as the system draws closer. This also has a
significant impact on how soon conditions fall below VFR.

The low pressure center and frontal system with the storm still
lie well west and north of central Illinois this evening. While
some of the guidance has precipitation chances arriving tonight as
convective storm complex remnants to our northwest sink toward the
area, confidence in this occurring is low. The better chance of
showers and storms will come later Sunday morning into midday as
the main system approaches the Illinois border from the west.
Tried to time in arrival of the more widespread pre-frontal
precipitation with the best model consensus, and carried no more
than a VCSH mention before then.

Bak
&&

.PREV DISCUSSION...
ISSUED 215 PM CDT Sat Apr 12 2014

SHORT TERM...Tonight through Tuesday

Warm weather to hold over our area through most of Sunday before a
change to much colder weather on Monday. Main forecast concerns will
be with timing of showers and thunderstorms into west central IL
later this evening and over the remainder of the forecast area by
Sunday night with light snow chances rearing its ugly head Monday
evening as the last of the southern stream shortwaves pushes across
the Ohio Valley.

Warm front that raced through our area early this morning became
active just after sunrise with severe storms across northern IL.
That has effectively put a stop to the northward push of the
boundary and that area from far northern IL west-southwest thru
central Iowa will be the focus for additional thunderstorm
development later this afternoon. Further south in our area,
forecast soundings continue to show a capping inversion in place
between 800-700 mb and short term forecast models suggest this
will hold into the evening hours. So with any focusing mechanism
remaining well to our north and the cap in place, POPs late this
afternoon thru the mid evening hours will be low.

Further west and north, with some impressive CAPE and shear values
forecast across Iowa, supercells will likely be the initial storm
mode before transitioning to more of quasi-linear system late
this evening. Question becomes if it does become organized and a
cold pool develops, we will see a shift southeast into our far
northwest counties late this evening. This is supported by the SPC
WRF and HRW-ARW and NMM simulations which quickly consolidates
from any discrete storms in west central Iowa into a fairly decent
line of storms before the cold pool becomes dominant and storms
dissipate as they translate southeast into our area by midnight.
Still the chance that if the storms remain rather un-organized and
do not develop any cold pool, we may not see much if any rain
across our north overnight as the storm movement will remain
mostly east-northeast across Iowa into northern IL. Will continue
to keep the highest POPs across the northwest for tonight with no
precip expected east of I-55.

It still appears we will be able to enjoy one more warm day on Sunday
with our east and southeast counties probably rain free until Sunday
night. The latest ECMWF and NAM-WRF eject a southern stream wave
northeast into the Midwest by late in the day with an increase in
shower and thunderstorm chances from west to east with the highest
during the day mainly west of I55. Moisture transport really picks
up ahead of this wave late in the day with precipitable water values
of 1.25-1.40 inches, well above climatological norms for our area
for the middle of April. All areas should see a decent rain event
Sunday night as the surface wave slowly moves northeast along the
boundary, which will be right across our forecast area Sunday
night into Monday morning. Better Cape and Shear values will be south
of our area with forecast soundings pretty much saturated from the
ground up Sunday night, so the main threat will be with the threat
for heavy rainfall as the heavier bands of rain with isolated storms
move from southwest to northeast. Rainfall totals still look to be
in the 1 to 2 inch range with some isolated higher amounts possible
Sunday night into early Monday as the surface low enhanced the low
level convergence along the slow moving frontal boundary.

Much colder weather will slowly filter in on Monday as the wave
shifts off to our northeast allowing the boundary to make a bit
more progress to our east during the day. May have a tough time
getting temps to budge out of the 40s north, with morning highs
expected in the southeast before temps fall off in the afternoon.
The NAM-WRF and to a certain extent the GFS model suggests additional
energy to push across our area on Monday with temp profiles trending
colder and more supportive of at least a rain/snow mixture by late
afternoon, and especially after dark. However, the better lift looks
to shift away from our area in the evening so precip amounts look to
be very light, but cold enough to support a mix before the precip
shuts down by midnight.

With the slow moving 500 mb trof axis finally shifting off to our
east late Monday night, skies should begin to clear as high
pressure settles in bringing frost/freezing temperatures to most of
our area by Tuesday morning with lows from the mid 20s far north
to the lower 30s southeast. Quiet weather expected the remainder
of the day on Tuesday as the fair weather system drifts across our
area keeping it on the dry and  chilly side.

LONG TERM...Wednesday through Saturday

Several weather systems to affect our area this period but moisture
will be rather limited with the first wave pushing across the Great
Lakes on Thursday. Next wave embedded in the zonal flow arrives
on Saturday with a better supply of moisture ahead of it. As a result,
will add some mentionable POPs into the grids for the start of the
weekend. After the cool start on Wednesday, our upper flow flattens
out enough to allow milder temperatures back into the area for the
end of the week with temperatures closer to normal for the middle
of April.

Smith

&&

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

$$







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