Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 280252
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
852 PM CST Mon Feb 27 2017

.UPDATE...
Issued at 846 PM CST Mon Feb 27 2017

Current forecast basically on track through the night, with
precipitation chances/coverage increasing over the next few hours.
00Z KILX sounding was still quite dry, and this was suggested by
earlier forecast soundings as well. Strong WAA/isentropic ascent
is ongoing and will persist through the night. This forcing and
associated moistening of the atmosphere will have shower coverage
increasing significantly for the rest of the night. Mid-level
lapse rates are forecast to become steeper at the same time, and
is supportive of thunderstorm development. However, thunderstorms
that occur into early Tuesday are not expected to become severe.

Plan to tweak forecast for the latest hourly trends, but minimal
change is required at this time.

&&

.SHORT TERM...(This evening through Tuesday)
ISSUED AT 259 PM CST Mon Feb 27 2017

Low pressure currently organizing in the lee of the Rockies across
eastern Colorado will track northeastward into the Great Lakes over
the next 24-36 hours...setting the stage for a round of potentially
severe thunderstorms late Tuesday afternoon and evening.  As the low
begins to approach, a warm front will lift northward into central
Illinois tonight.  Scattered thunderstorms will develop ahead of the
boundary, particularly from late evening into the overnight hours.
Modest elevated instability is expected...with the 18z NAM showing
MUCAPE values increasing to 400-600J/kg.  Given decent synoptic lift
and relatively low freezing levels, a few of the storms late tonight
into early Tuesday morning could produce small hail.

Most model guidance continues to suggest the early morning
convection will shift north and east of the area, leading to a lull
in the precip from mid-morning through mid-afternoon.  The low and
its associated cold front will not arrive across Illinois until
Tuesday night: however, one or more broken lines of storms may form
well ahead of the front beginning late Tuesday afternoon.  Have
tailored PoPs to keep them in the slight chance to chance category
for much of the day, then ramping up to likely in the Illinois River
Valley after 21z/3pm.  Any storms that fire Tuesday afternoon will
tap into a highly unstable/highly sheared airmass.  MUCAPE values
across the western KILX CWA are progged to reach the 1500-2500J/kg
range while 0-6km bulk shear increases to impressive values of 60-
70kt.  As a result, damaging wind gusts, large hail, and isolated
tornadoes will be possible with the cells.

.LONG TERM...(Tuesday night through Monday)
ISSUED AT 259 PM CST Mon Feb 27 2017

Updated SPC outlook for late tomorrow and especially tomorrow night
has expanded the slight and enhanced risk areas for severe weather
further north. The higher probability for stronger wind gusts and
very large hail is located along the I-70 corridor, but with as much
deep layer shear and instability present over all of our forecast
area, all modes of severe weather will be likely. The main concern
will be timing with most of the convective allowing models
suggesting development west of the Illinois River Valley in the 21z-
00z time frame, with a gradual transition from discrete supercells
into more of a QLCS with embedded circulations along the line
Tuesday night into early Wednesday. Still at this point confidence
not that high with respect to timing of the thunderstorm
development to our west along the front with several models
suggesting storms may develop by mid afternoon tomorrow. However,
forecast soundings continue to indicate a rather stubborn capping
inversion in place through at least 21z before low level
convergence increases along the cold front out to our west.
Highest POPs will be Tuesday night before the shower and storm
chances gradually shift southeast with the front Wednesday
morning, with windy and colder weather moving in Wednesday for a
brief stay through Friday.

A northwesterly flow will be in place across the region during the
second half of the week which will draw down seasonably cold air
into central Illinois. Later in the week, a clipper system is still
progged to track southeast into the Midwest Thursday afternoon. The
speed of the system and the fact the moisture will be limited with
this system, POPs and QPF values will remain low across mainly the
east late in the day and early Thursday evening.

As far as temperatures this week are concerned, the warmest day will
be tomorrow, with some lower 70s possible over the southwest CWA and
mid-upper 60s elsewhere. The colder weather, as mentioned above,
still appears to be short lived, as the upper flow quickly becomes
zonal during the weekend with temperatures back to the 60s by Sunday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 557 PM CST Mon Feb 27 2017

A weather disturbance will spread across the Midwest tonight. This
will result in MVFR CIGS developing, along with showers and a few
thunderstorms. The precipitation will be most widespread from
around Midnight to around sunrise. MVFR CIGS should linger for
most of the day Tuesday. Southerly winds will prevail through the
period.


&&

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

$$

UPDATE...Bak
SHORT TERM...Barnes
LONG TERM...Smith
AVIATION...Bak



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