Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 221955
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
255 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 18Z TAFS through 18Z Friday afternoon)
Issued at 1245 PM CDT Thu Jun 22 2017

MVFR ceilings rapidly developed across central Illinois this
morning as moisture from the remnants of Tropical Storm Cindy
streamed into the area. Ceilings will gradually rise to low VFR
over the next 1-2 hours...then will remain VFR until later tonight
as a cold front approaches from the northwest. Latest models are
in fairly good agreement, with a line of thunderstorms developing
along the front and pushing into the area mainly after midnight.
Based on model consensus, have included predominant showers with
VCTS for a 3-4 hour period overnight into early Friday morning.
Timing appears to be 08z-12z for KPIA...and 10-13z further
southeast at KDEC. Once the front passes, forecast soundings
suggest ceilings will once again drop back to MVFR Friday morning.
Southerly winds will initially gust to around 20kt this afternoon
and evening, then will veer to the west-northwest and will
decrease to 10kt or less late tonight.


&&

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

$$

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



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