Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 230151

Area Forecast Discussion
851 PM CDT Fri Aug 22 2014


Latest surface analysis indicating what`s left of an old outflow
boundary from today`s convection located over parts of southeast
Illinois this evening with some widely scattered showers along
it. Further to the north, the synoptic frontal boundary was located
over far southern Wisconsin with a weak cold front noted across
central Iowa. Mid to upper 70 dew points were widespread from Iowa
and Illinois south into the northern half of Missouri. 00z ILX
sounding showing plenty of Cape for a parcel able to get above
800 mb later tonight with current Most Unstable Capes ranging from
2500-3500 J/kg. ILX Local objective analysis indicating a cap in
place across the north and east, probably the result of the
convection persisting into the early afternoon. Precipitable water
values off the wall on the SPC mesoanalysis page with 2.2-2.3
inches analyzed this past hour from west central Illinois southeast
into far southern Illinois. 0-6km shear values of only 15 to 20
kts compared to last night when we saw 30-35 kts.

Now the main forecast concern, where and when convection breaks out
again overnight. Models were definitely weaker with the low level
jet with more of a split in the jet indicated with one area splitting
off north into the central Plains with a much weaker jetlet forecast
into southwest Missouri. Models do show some weak 850 mb moisture
convergence/theta-e advection later tonight over parts of northeast
Missouri into southeast Iowa. Latest HRRR model trending more towards
what we saw with the 18z Nam-Wrf solution with the focus more with
the aforementioned weak outflow boundary across south central and
southeast Illinois later tonight. Any storms that do organize will
bring the threat of heavy rain due to the weaker wind fields aloft
and gusty winds, along with quite a lightning show. Storms that have
been progressing slowly across parts of central Iowa late this
afternoon and early this evening not showing a great deal of
organization at this point as they were located under the building
500 mb ridge.

Have made some minor adjustments to the grids to reflect the current
trends this evening and will be sending out an updated ZFP to freshen
up the wording for the overnight hours. Overall though, no significant
changes made to the overnight convection threat across our area.


.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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