Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 220401

Area Forecast Discussion
1101 PM CDT Thu Aug 21 2014


Cluster of storms that developed to our north late this afternoon
have increased in intensity and areal coverage and were tracking
southeast into parts of north central Indiana early this evening.
00z ILX sounding indicating cape values of close to 4700 J/kg
with the latest objective mesoanalysis depicting mixed layer capes
down to between 2000 to 3000 J/kg. 0-6km shear values around 30 kts
this evening so certainly enough instability and deep layer shear
for organized storms. However, a couple of things going against
widespread thunderstorms overnight. One, the closest frontal
boundary to our area was well north and not expected to make much
southward push until Friday morning, and second, upper level heights
continue to slowly build north tonight and Friday. Models do show
the low level jet focused over parts of Iowa late tonight and a
few of the hi-res forecasts develop convection west of the Mississippi
River around 05z and bring scattered convection into primarily the
northern third of the forecast area overnight.

Surface dew points have risen into the mid and in some cases upper
70s, so needless to say, its going to be a rather warm night with
lows in the low to mid 70s and not much wind to help out. No change
to the Heat Advisory starting Friday afternoon as it looks like an
extended period of hot and humid weather into at least early next
week for our area. Made some minor tweaks to the grids for the
rest of the evening, but not enough to warrant an updated ZFP at
this time.


.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 06Z TAFS through 06Z Friday night)

The main forecast concerns this period include timing and coverage
of convection overnight, along with the potential for some patchy
fog across our northern TAF sites. Little overall change in thinking
from the previous discussion with some scattered showers and
thunderstorms occurring to our north and trying to make their way
southeast into central Illinois. Expect the coverage to gradually
increase over the next several hours but still not certain enough
across our northern TAF sites to include a tempo group just yet.
Will keep an eye on the radar up until issuance time and change
if need be. Further south, will continue to hold off mentioning
anything at SPI and DEC as it appears the higher probabilities
for better coverage to the rain will be to the north. Any storm
that does impact a TAF site has the capability of dropping cigs
and vsbys very briefly to MVFR or even IFR conditions before
quickly improving.

Some MVFR vsbys possible with fog as our temp-dew point spreads
were down to a few degrees, especially across the north. The
shower and thunderstorm threat looks to diminish by 15z across the
north with any fog quickly dissipating by 13z, with mainly VFR
conditions all areas thru tomorrow evening. Southerly winds of
4 to 8 kts will prevail tonight and south to southwest winds of
10 to 15 kts are expected on Friday.


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



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