Area Forecast Discussion
Issued by NWS Central Illinois

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KILX 212304

Area Forecast Discussion
604 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 00Z TAFS through 00Z Friday evening)

Once again, the main forecast concern will be timing and coverage
of convection overnight. Cluster of thunderstorms over Chicago`s
area may clip KBMI and KCMI in the 00z-02z time frame, but it
appears the main threat for more widespread coverage looks to be
in the 05z-08z time frame. Frontal boundary drapped to our north
will be the primary focus for storms overnight into Friday but
may dip a bit further south thanks in part to the thunderstorms
tonight. It appears areas along and north of the I-74 corridor
stand the best chance of seeing these storms but not very confident
at this point as to the coverage so will continue to carry a VCTS
with the expected timing into the individual TAF site. Any
thunderstorm activity is expected to wane across the north after
14z Friday.

Surface winds tonight will be from the south at 4 to 9 kts and
then from a southwest direction on Friday at 10 to 15 kts.


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



AVIATION...SMITH is the U.S. government's official web portal to all federal, state and local government web resources and services.