Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 210141

Area Forecast Discussion
841 PM CDT Wed Aug 20 2014


Latest surface analysis indicating the warm front has shifted into
northern Illinois early this evening. Local objective meso-analysis
depicting the highest Mixed Layer Capes over extreme western Illinois
with readings around 2500 J/KG along the Mississippi River with very
little convective inhibition, while further east, a much more stable
air mass was in place thanks in large part to the clusters of storms
that tracked southeast across parts of central Illinois during the
morning and into the early afternoon hours. Small cluster of storms
that tracked northeast from central Missouri late this afternoon to
near St. Louis over the past hour, was showing a slow weakening trend
over the past hour. but we will need to watch our far southwest counties
to see if any of that activity has enough staying power to make it this
far northeast.

Main forecast concern overnight will be with the threat for more
elevated storms to fire just after midnight. With the front a bit
further north than expected, convection will fire further north
than last night and then track southeast into parts of north central
Illinois, with the better chances tonight across the north with
progressively lower POPs as you head south. Both the RAP and HRRR
models have been very consistent in indicating the better coverage
of showers and storms will be across our northern counties, closer
to the warm frontal bounday.


.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 00Z TAFS through 00Z Thursday evening)

Two concerns this forecast period which include TSRA chances and
coverage later this evening, and fog development after midnight.
Warm front currently analyzed north of all our TAF sites late this
afternoon with a slow track north expected overnight. Radar and
surface observations indicating some light shower activity
occurring north of Lincoln, tracking to the east at 35 mph so may
have to include some light showers in the CMI TAF for the first
hour or so. Convection expected to be more widespread along and
to the north of the warm front, so will continue to carry VCTS
across our PIA, BMI and CMI sites for the potential for convection
to affect areas along and mainly north of I74 after 05z. With the
abundant low level moisture in place from the rainfall of today,
expect some fog to form after midnight as well so will include
some MVFR vsbys for that potential at all of our sites late tonight
thru 12z Thu. Surface winds will be light southerly tonight at 4
to 9 kts, and southwest at 10 to 15 kts on Thursday.




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