Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 222256

Area Forecast Discussion
556 PM CDT Fri Aug 22 2014

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

Main short-term concern is potential convective re-development
tonight. As has been the case for the past few days, forecast models
once again are having difficulty predicting the exact placement
and track of nocturnal storm complex. Synoptic boundary remains
draped well to the north from far northern Iowa to southern Lake
Michigan: however, convection from earlier today was tied to an
outflow boundary that dropped further southward into central
Illinois. Think storms will develop along mesoscale boundaries
later this evening as the low-level jet strengthens into the
region. Radar trends are already showing scattered thunderstorms
forming across central Iowa and these should develop/spread
eastward into west-central Illinois later this evening. Mean flow
would take the convection E/SE across the area overnight into
Saturday morning. Based on radar trends and latest model data,
have gone with high chance PoPs along/northeast of I-74, tapering
down to just slight chances across the far S/SE CWA tonight. Given
the very moist airmass as characterized by precipitable water
values of over 2 inches, locally heavy rainfall will be possible
late tonight into early Saturday morning. Convection will tend to
move further eastward into Indiana and diminish toward midday
Saturday, resulting in a mainly dry afternoon. Given increasing
amounts of sunshine, high temperatures on Saturday will reach the
upper 80s and lower 90s. These readings, combined with dewpoints
in the middle 70s, will produce heat index values of 100 to 105F.
Have therefore continued the current Heat Advisory with no areal


.LONG TERM...(Saturday night through Friday)

Will continue the heat advisory for areas from Lincoln ssw through
Monday afternoon for heat indices peaking from 100-107F and highest
on Sunday and Monday afternoon over sw counties where several
areas will be near 105F heat index. Highs in the low to mid 90s and
dewpoints in the 70s to support these heat indices. Will need to
keep alert to the possibility of expanding the heat advisory a bit
further nne Sunday through Tuesday if heat indices reach closer to

Strong upper level ridge building over the mid ms river valley
Sun/Mon to cap atmosphere over central/sw IL and generally went dry
with just ne IL have isolated convection. Upper level ridge to slowly
break down during middle of next week allowing a frontal boundary to
press se into IL. ECMWF is slower with front on Wed night and Thu.
GFS is Wed afternoon into Wed night while GEM has front passing se
into the IL river valley by overnight Wed night. With models
trending slower past few days, favor the slower ECMWF and GEM models
with frontal boundary pushing through central IL Wed night and
southeast IL Thu. Have 20-30% chances of showers and thunderstorms
returning Tue and Tue night, with best chances Wed/Thu. Still very
warm and humid Wed with highs in the mid to upper 80s while
southeast IL near 90F where afternoon heat indices peak in upper 90s
to near 100F. More cooler air filters in on Thu with highs around
80F central IL and lower 80s southeast IL. Dewpoints slip into the
mid to upper 50s during Thu night. Have dry conditions returning by
Friday with more comfortable temperature and humidity levels, with
highs next Friday in the upper 70s central IL and near 80F
southeast IL with dewpoints in the upper 50s and lower 60s.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)

Main concern tonight will once again be timing and coverage of
showers and thunderstorms across the forecast area. Main synoptic
scale frontal boundary was positioned well north of the area late
this afternoon, however, there were several outflow boundaries
noted in the latest analysis across parts of central Illinois.
These features may be the focus for convection tonight but
confidence on just when and coverage of storms is still very much
in question. Any thunderstorm that does develop will have the
capability of producing brief MVFR or IFR conditions with a quick
recovery expected once the storm passes. Best timing appears to
be in the 04z-08z time frame when convection spreads into parts of
central IL.

Whatever shower and thunderstorm activity we see into Saturday
morning is expected to diminish, similar to what occurred today,
by early to mid afternoon. Surface winds will be light southerly
tonight at 3 to 8 kts, and then into the southwest at around 10kts
by late Saturday morning or early afternoon.


HEAT ADVISORY until 7 PM CDT Monday FOR ILZ042-047>054-056-



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