Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 230000
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
700 PM CDT Thu Jun 22 2017

.SHORT TERM...(This evening through Friday)
ISSUED AT 255 PM CDT Thu Jun 22 2017

A warm/moist airmass is in place across central Illinois this
afternoon...with 19z/2pm temperatures in the 80s and dewpoints in
the lower 70s.  Deep-layer moisture associated with the remnants of
Tropical Storm Cindy continues to flow northward from the Gulf
Coast, allowing isolated showers to develop east of the I-55
corridor.  HRRR has been consistently showing these showers becoming
more numerous over the next few hours, especially south of I-70.
Current mesoanalysis shows SBCAPEs of less than 1000J/kg across the
SE KILX CWA, so only isolated thunder is anticipated late this
afternoon/evening.  Meanwhile further northwest, a cold front
extending from Lake Superior southwestward to Nebraska will begin to
drop into the area late tonight.  Models are in fairly good
agreement that a line of thunderstorms will develop within a narrow
corridor of favorable instability/shear across Wisconsin/northern
Iowa late this afternoon...then will slowly drop southeastward into
central Illinois tonight.  Consensus keeps this activity northwest
of the CWA until after midnight: however, the 12z WRF-ARW is
slightly faster and brings storms into the northwest counties
shortly before midnight.  Have adjusted PoPs to keep low chance
going through the evening along/east of I-57 where deep-layer
moisture will be most prevalent.  Further west, have kept the I-55
corridor dry...and have brought chance PoPs into the Illinois River
Valley after 03z.  Best rain chances will arrive overnight as the
line of storms tracks/spreads southeast.  While storms may initially
become strong to severe across Iowa, they will be losing daytime
instability as they come into the CWA overnight, with MUCAPE vales
decreasing to 1500-2000J/kg.  Wind shear will be quite weak, so
think as instability wanes...so will the storms.  Could potentially
see a few non-severe wind gusts across the N/NW between midnight and
4am, but the overall trend will be for the storms to weaken.

Cold front will sink southward through central Illinois on
Friday, bringing scattered showers and thunderstorms. Have
lingered rain chances through the morning, with the convection
dropping south of I-70 by late afternoon. High temperatures on
Friday will range from the middle to upper 70s across the SE
counties where cloud cover and rain will persist...to the lower
80s further northwest where sunshine will return during the
afternoon.

.LONG TERM...(Friday night through Thursday)
ISSUED AT 255 PM CDT Thu Jun 22 2017

Cold front will sink southward into the Ohio River Valley Friday
night, allowing a much cooler/drier airmass to settle into the
region over the weekend.  A series of short-waves dropping out of
Canada will carve a significant trough over the Great Lakes.  While
most of these weak features will remain well to the north, one may
come close enough to trigger widely scattered showers across the
north Sunday afternoon.  Have included slight chance PoPs
along/north of a Galesburg to Chenoa line accordingly.  As the flow
becomes increasingly cyclonic, showers may also be possible across
parts of central Illinois Monday afternoon: however, with surface
high pressure over the region...have opted to leave them out of the
forecast for now.  Main story Saturday through Tuesday will be the
cool temperatures, with highs remaining in the 70s and overnight
lows dropping into the lower to middle 50s.

The upper trough will lift northeastward by the middle of next
week...replaced by a zonal or perhaps even slightly southwesterly
flow aloft.  Degree of weak ridging that develops over the Midwest
in the wake of the trough remains in question, thus the timing of
the next significant rain chance is also somewhat uncertain. The 12z
GFS features a stronger ridge and thus a slower eastward progression
of the next cold front until next Thursday.  Meanwhile, the GEM is
quite a bit faster and brings rain into Illinois as early as during
the day Wednesday.  With the ECMWF/GFS both trending slower, think
this is the way to go.  As a result, have kept Wednesday dry...with
PoPs coming back into the picture Wednesday night into Thursday.
With rising upper heights, temperatures will climb back into the 80s
by Wednesday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 659 PM CDT Thu Jun 22 2017

Showers have been decreasing early this evening in the warm
sector across central Illinois. Steady southerly winds are pumping
low level moisture into the area, from the remnants of TS Cindy.
However, the coverage of precip from that system should remain SE
of the TAF sites until later in the day tomorrow. The initial
forcing for showers/storms will come with a cold front later
tonight. High resolution guidance still show a consensus that
storms will arrive after 06z/1am near PIA, and then progress to
the southeast in a line, affecting the remaining TAF sites through
13z/8am Friday morning. A narrow corridor of instability is
expected to precede the front, and a few stronger storms could
reach as far southeast as PIA. However, storm intensity should
steadily weaken as the front progresses southeast of PIA.

Cloud cover could drop from MVFR to IFR as the front moves across
Illinois, with a few storms possibly even producing brief LIFR
conditions. Have not included any tempos for those conditions yet,
and kept MVFR in the prevailing weather. Will monitor closely for
a stratus IFR layer near and just behind the front.

Winds will remain southerly the first 6-8 hours of this TAF
period, then begin to shift around to the southwest and eventually
W-NW behind the cold front. Wind speeds early this evening could
linger at 10-15kt, but prevailing winds should remain 10kt or less
for later tonight and tomorrow morning. Increased mixing Friday
afternoon will push winds back into the 10-15kt range.

&&

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

$$

SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Shimon


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