Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 211957

Area Forecast Discussion
257 PM CDT Thu Aug 21 2014

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

18Z/1pm surface analysis shows stationary frontal boundary extending
from northern Iowa to southern Lake Michigan.  Scattered convection
continues to develop and track along this boundary, mainly impacting
locations north of the I-80 corridor.  An outflow boundary is
evident on visible satellite imagery dropping southeastward across
eastern Iowa and this may trigger isolated showers/thunder across
the far northwest KILX CWA this afternoon into the early evening.
Aside from this possible convection, the next round of significant
precip will likely hold off until late tonight when 30-35kt LLJ
develops from the southern Plains northeastward into Iowa.  As this
jet interacts with the frontal boundary, a cluster of thunderstorms
will develop across eastern Iowa/northwest Illinois toward midnight,
then will track E/SE across north-central Illinois overnight.
Models continue to struggle with exact placement and track of
convection, with the 12z NAM once again appearing to be too
aggressive with its southward push of precip into the CWA tonight.
Prefer the Rapid Refresh model solution which keeps most of the
convection confined to locations along/northeast of the I-74
corridor.  As a result, will carry likely PoPs after midnight
northeast of I-74, tapering down to just slight chances further
south along a Springfield to Paris line.  Thunderstorm complex will
track into Indiana Friday morning, resulting in decreasing PoPs
across the board.  With frontal boundary well to the north and upper
heights rising across the area, mainly dry weather will be on tap
Friday afternoon.  High temperatures will reach the upper 80s and
lower 90s.  These readings combined with dewpoints in the lower to
middle 70s will produce heat index values of around 100 degrees,
with readings approaching 105 degrees across the S/SE CWA, where a
Heat Advisory will be issued beginning Friday afternoon.


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

12Z forecast models continue to build a strong upper level ridge
over IL this weekend and lingers over the Ohio River valley early
next work week. This will bring the longest and strongest heat wave
of the summer to central and southeast IL with highs in the low to
mid 90s from Friday through Tue and dewpoints peaking in the mid 70s
from late morning into early evening. This giving afternoon heat
indices of 98-106F and highest in sw counties where a heat
advisory will be issued from Friday afternoon through Monday
afternoon. Heat indices could even peak around 105F sw counties
this weekend. Southeast IL will continue to have heat indices of
100-105F Tuesday afternoon so heat advisory may need to be
extended with future updates. Due to uncertainty of convection
chances and possibly more cloud cover at times over ne counties
Fri and Sat, especially from I-74 ne, kept them out of heat advisory
but will issue SPS product to ahead heat indices reaching 100F or
even a few degree higher at time through early next week.

Another short wave riding se over top off upper level ridge across
northern/ne IL could bring 20-30% chance of showers and
thunderstorms to ne CWA especially from I-74 ne Friday night and
then shifting further ne during the day Saturday with just slight
pops ne areas. Upper level ridge appears strong enough over
central/se IL to keep convection chances ne of area from Saturday
night through Monday.

Models have trended slower with bringing a frontal boundary se into
NW IL Monday night/Tue and trimmed pops and confined them further nw
during that time. Also went a bit warmer with highs Monday and
Tuesday due to slower frontal movement and upper level ridge holding
longer over central/se IL. Better chance of showers and
thunderstorms appears to now be Wed/Thu time frame with upper level
trof starting to dig more into the upper Midwest and driving front
further southeast into central IL Wed and toward the Ohio river
valley on Thursday.


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

Scattered convection continues to track along a nearly stationary
frontal boundary draped across far northern Illinois/southern
Wisconsin early this afternoon. This activity will remain north of
the central Illinois terminals through this evening. Main aviation
question will be when/where additional nocturnal convection will
develop later tonight. Once again the NAM seems too aggressive in
driving precip southward into the area tonight, while the HRRR is
completely dry through 04z. Prefer a compromise solution as seen
on the latest Rapid Refresh model, which features a cluster of
showers/storms developing across eastern Iowa by around midnight,
then tracking E/SE across north-central Illinois overnight. Based
on this model, have introduced VCTS at the I-74 terminals between
07z and 09z. Further south, have gone completely dry at both KSPI
and KDEC through the night. Once any early morning convection
clears the area, dry weather is anticipated after 14z.


HEAT ADVISORY from 1 PM Friday to 7 PM CDT Monday FOR ILZ042-



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