Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 200500

Area Forecast Discussion
1200 AM CDT Wed Aug 20 2014


Weak frontal boundary currently extending from northeast Illinois
through the Galesburg area to far northern Missouri. Earlier
convection has pushed well to the east and skies are now mostly
clear in our area, although some rogue thunderstorms have recently
developed along an outflow boundary across Clark County. Not any
appreciable change in temperature on either side of the front, but
dew points are slightly lower to the north. Front is not expected
to make much additional headway overnight. HRRR and RAP showing
some additional convection developing overnight on the tail of the
front across northern Missouri southeast to near St Louis.
Currently think the precip should stay to our southwest, but will
see an increase in clouds after midnight across the southwest CWA.


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

19z/2pm surface analysis shows slow-moving cold front along the
Mississippi River. Isolated showers and thunderstorms are beginning
to develop immediately along the boundary within a narrow uncapped
instability axis.  Further east, the air mass across much of central
Illinois remains capped in the wake of convection that moved across
the area earlier in the day.  Will hold on to slight chance PoPs
through the early evening as the front makes its way slowly eastward
into the KILX CWA.  Once daytime instability is lost, will go with a
dry forecast across the board overnight.  With the boundary in the
vicinity, winds will become light/variable tonight, setting up the
possibility of fog.  Forecast soundings suggest low-level moisture
will remain high, with both the MET and MAV guidance indicating
reduced visbys.  Based on this guidance and a persistence forecast
from the past couple of nights, have included patchy fog after
midnight. Weak frontal boundary will remain in the area on
Wednesday, resulting in a continued slight chance for
showers/thunder, with afternoon highs climbing into the middle to
upper 80s.


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

Wednesday night after midnight had previously looked like our best
chances of storms over the next few days, but the 12z guidance has
sped up the shortwave/MCV by 6-12 hours and weakened it. Our area
will still remain along the storm track over the building upper
level ridge, so despite lowering PoPs below the likely category,
chances will remain in most areas, with lowest chances southwest of
Springfield to Effingham.

The theme of higher storm chances in N-NE Illinois will continue for
the balance of the extended forecast, as the upper ridge and dome of
hot air build from southwest to northeast. The storm track appears
to generally follow the 588dm 500mb height line into this weekend.
We kept a narrow channel of chance PoPs across our N-NE counties
from Thursday to Saturday, with minor shifts east and west during
that time. Slight chances could extend as far south as Rushville to
Lincoln to Mattoon, but the building dome of heat should provide
enough mid-level capping to limit the southward extent of storms.

Model differences in the Sunday to Tuesday time frame have prompted
a continuation of periodic slight chance PoPs as a cold frontal
boundary tries to push into the dome of hot air. The 12z runs have
limited the forward progression of the front into IL on Sunday night
and Monday, when previous runs indicated an air mass change for less
hot conditions on Monday. Now the heat may linger as long as Tuesday
with the only hints of relief coming for areas north of I-74.

Heat index readings from Wednesday to Sunday will climb to into the
95F to 102F range at peak heating, which could be locally limited by
any pop-up thunderstorms each afternoon. Heat advisory levels are
105F+, but conditions will still be very uncomfortable, and heat
precautions will be necessary each day.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)

Skies clear across the TAF sites late this evening, but will see
some increase in clouds from the west late. Main concern is with
potential for fog. Latest short-range guidance suggesting the
KSPI-KCMI corridor most at risk for significant fog, as a weak
frontal boundary is draped in that area and dew points are pooling
along it. However, the increase in clouds near KSPI may help
mitigate the fog from getting out of control. The RAP model is
fairly robust with widespread dense fog affecting all the TAF
sites, but the HRRR has it more patchy within the KPIA/KSPI/KBMI
triangle. Right now, will go with a TEMPO period of 1SM at the
southern TAF sites and keep it a little higher further north,
where the air is a bit drier. Will also need to watch for
convection developing along the front in northern Missouri over
the next few hours. General thought is that this should stay
southwest of KSPI and will not mention at this time.

In the longer range of the TAF, concern is with timing of
convection late Wednesday afternoon and evening. NAM is faster and
much further south with storms moving through as early as 19-21Z,
while the latest GFS favors the evening hours, with other guidance
much more widely scattered but in the evening. Will favor the
later time frames with VCTS mention in the TAFs, although
confidence is somewhat uncertain at this time.




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