Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 300159

Area Forecast Discussion
National Weather Service Lincoln IL
859 PM CDT FRI JUL 29 2016

Issued at 854 PM CDT Fri Jul 29 2016

Updated area PoPs into midday Saturday earlier this evening to
reduce/remove them. Most of forecast area is dry at the moment,
and this should continue to be the case until diurnal instability
resumes Saturday. While there are isolated showers/storms across
far northern portion of forecast area, along outflow boundary from
earlier convection across northern Illinois, trends in
coverage/intensity are definitely down.

Still looks like there will be some fog/low cloud development by
late tonight, much like earlier today. This is well covered by
current forecast. Aside from PoP reduction, most forecast tweaks
were minor.


.SHORT TERM...(This evening through Saturday)
ISSUED AT 245 PM CDT Fri Jul 29 2016

Isolated to scattered showers and thunderstorms over northern IL,
much of IN and into far southeast IL has developed in more unstable
airmass where LAPS CAPES are 1500-2500 J/kg with highest CAPES
over northeast IL into west central IN. Highest bulk shear of
25-35 kts is over southern IL and highest south of I-70 where
convection is occurring. Surface map shows weak 1012 mb surface
low pressure just north of Lafayette Indiana with a weak frontal
boundary extending through southeast IL/MO where convection is
occurring. Precipitable water values are fairly high yet from
1.5-1.8 inches in southeast IL and supports locally heavy rain
with slow moving cells in Lawrence and eastern Crawford counties.
Aloft a 579 dm 500 mb low near the IA/IL border will track east
across the southern Great Lakes tonight and bring the upper level
trof axis southeast across central IL this evening. Most of the
isolated convection over northern and eastern CWA will diminish
after dusk this evening as the airmass stabilizes and the upper
level wave exits southeast of central IL later this evening.
Lingering low level moisture (dewpoints currently in the upper 60s
and lower 70s) supports patchy fog formation overnight until mid
morning Saturday along with some low stratus clouds where fog gets
denser as winds remain light. HRRR model is showing the fog
forming from ne to sw after midnight.

MET and MAV temperature guidance similar tonight and Saturday and
using a model blend for lows tonight and highs Saturday. Low
temperatures overnight will be in the mid to upper 60s with mildest
readings southeast of I-70 where dewpoints are a bit higher. Highs
Saturday of 80 to 85F with coolest readings from I-74

.LONG TERM...(Saturday night through Friday)
ISSUED AT 245 PM CDT Fri Jul 29 2016

500 mb trof axis to shift off to our east Saturday night taking any
lingering shower chances with it. Will go with low chance POPs in
the early evening, and then go dry for the remainder of the night.
Surface high pressure will move into the Midwest on Sunday bringing
quiet weather to the forecast area before upper level heights begin
to build over the region early next week. A warm front is forecast
to approach the area on Monday with an MCS forecast to develop along
the nose of the low level jet late Sunday night over the Missouri
River valley with the convection shifting east and southeast along
the warm front. Will carry low chance POPs over the western half of
our forecast area late Sunday night into Monday morning with 30-40
POPs CWA-wide on Monday as the warm front lifts just to our north.

Another overnight MCS is forecast a bit further north Monday night
into Tuesday morning over southern Minnesota/northern Iowa with the
1000-500 mb thicknesses suggesting a southeast track into most of
our area Tuesday/Tuesday night before convection pushes off to our
east and 500 mb heights amplify over the central portion of the
country shifting the ring of fire/convection chances further to our
north starting Wednesday into Thursday. Unfortunately, the blend of
guidance is weighted more towards the GFS which is quite a bit wetter
than the ECMWF, as a result, the overall forecast after Tuesday is
still showing more precip chances that what I would like to see at
this point but has trended a bit drier than what we were given
yesterday at this time with rain chances over most of the extended
period. The nocturnal MCSs the develop early next week seem to have
a more profound affect on just how amplified the 500 mb pattern will
be over the central U.S., with the GFS not as aggressive with the
ridge compared to the 12z ECMWF.

MEX guidance has trended a few degrees cooler for the middle and
latter portion of the forecast period while the extended ECMWF MOS
has been fairly consistent in indicating afternoon highs will edge
back to or just above 90 degrees. As the warm front approaches then
shifts north of our area by mid-week, we should see not only an
increase in temperatures but also dew points which will translate
to heat index values back to around 100 by mid-week.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 637 PM CDT Fri Jul 29 2016

Shower/storm chances across central Illinois terminals looking
fairly sparse over the 00Z TAF valid time. A couple areas of more
widespread convection nearby include near the Ohio River along an
old frontal boundary, and over northern Illinois with an upper
level disturbance. Best chance for convection to impact a local
terminal over the next few hours is at KPIA, KBMI, and possibly
KCMI, as outflow from northern Illinois storms pushe toward the
region. However, the already minimal threat will wane over the
next few hours as diurnal heating is lost. Chances may increase
again Saturday as the atmosphere heats up, but coverage is
expected to be too low to mention for now.

Otherwise, primary aviation forecast concern locally is low
cloud/fog threat late tonight/early Saturday. Expect a scenario
fairly similar to last night with mainly MVFR conditions, but
local IFR CIGS & VSBYS are possible, especially at KPIA & KBMI.


.ILX Watches/Warnings/Advisories...


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