Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 201958

Area Forecast Discussion
258 PM CDT Wed Aug 20 2014

.SHORT TERM...(This evening through Thursday)

Upper ridge continues to build over the southern CONUS, providing
hot/dry weather as far north as much of Missouri into western
Kentucky.  Meanwhile further north, central Illinois remains on the
unsettled periphery of the ridge, with several weak impulses noted
within the flow.  One wave brought widespread showers/thunder to the
area today, while a second wave further upstream over South
Dakota/Nebraska will bring another round of convection late tonight
into Thursday morning.  Models remain in poor agreement concerning
the exact placement/timing of the overnight storms: however, based
on expected orientation of the LLJ and a stationary frontal boundary
draped across north-central Illinois, think the main focus for
precip will be across northern Illinois.  12z NAM is the most
aggressive in developing precip southward across much of central
Illinois, while the higher-resolution Rapid Refresh keeps the area
mostly dry overnight.  Have decided upon a compromise between the
two solutions, with highest PoPs along and north of I-74 after
midnight through Thursday morning.  Further south, will only carry
slight chance PoPs.  As upper heights rise and the front shifts
further northward, rain chances will decrease from south to north by
Thursday afternoon.  Given southerly flow and increasing amounts of
sunshine, high temps on Thursday will reach the upper 80s and lower


.LONG TERM...(Thursday night through Wednesday night)

The oppressive heat and humidity will be the main weather story
for IL through early next work week, as central and southeast IL
experiences the longest heat wave of the summer. Upper level ridge
to build over IL late this week with 500 mb heights rising to
around or above 591 dm late this week and continue through at
least early next work week. 12Z models have trended longer with
keeping subtropical ridge in place near IL into next work week,
which would keep the heat wave around longer. Currently we are
forecasting highs of 89-95F from Friday through Tuesday and
combined with dewpoints peaking in the low to mid 70s. This to
give afternoon heat indices of 98-106F from Friday afternoon
through Sunday afternoon and lingering south of I-72 Monday and
Tuesday. A special weather statement (SPS) product was issued to
address this heat wave. Heat advisory criteria is heat indices of
105F or higher or they also can be issued for heat indices of
100-105F for four or more consecutive days. This may be reached in
southern counties. Heat headlines have already been posted to
counties just to our south and sw through this weekend.

As for convection chances, looks like isolated convection will be
possible at times during the next week in the tropical airmass
especially northern and ne areas. Increased convection chances ne
counties especially from I-74 northeast Thursday night to chance
category with another short wave ridging se overtop of upper level
ridge across ne IL into IN. Best chances of showers and
thunderstorms then shifts ne of central IL this weekend as upper
level ridge strengthens over central/se IL and atmosphere becomes
more capped. Have chance of showers and thunderstorms into areas nw
of IL river later Monday night and north of Lincoln Tue and Tue
night, then chance pops across area Wed and Wed night. This due to a
frontal boundary slowly pressing se into nw IL Tue and central IL
Wed. This to gradually cool temps down by next Wed and Thu and
diminish the oppressive heat wave.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)

Most of the widespread convection from this morning has now moved
east of the central Illinois terminals, except for KCMI where rain
will linger through 19z. Elsewhere around the area, only widely
scattered light showers are noted on radar. The showers will
become less numerous in coverage as the day progresses and some
partial clearing will even be likely at both KSPI and KDEC. Next
big aviation question will be where/when the next round of
convection will develop. Models are showing a warm front lifting
northward through the region tonight into Thursday, with
showers/storms likely firing along/north of the boundary late
tonight. Still some uncertainty concerning exact placement of
convection, but general consensus focuses it across northern
Illinois between 08z and 14z. Will carry VCTS at the I-74
terminals during that time frame, but will keep both KSPI and KDEC
completely dry tonight into Thursday morning. Will also need to
keep an eye on fog potential tonight, as low-level moisture
remains high and winds are light.





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