Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 310457

Area Forecast Discussion
National Weather Service Lincoln IL
1157 PM CDT SAT JUL 30 2016

Issued at 859 PM CDT Sat Jul 30 2016

Diurnally induced showers/storms/clouds have just about
dissipated, leaving mostly clear skies across central and
southeast Illinois. Main concern through the overnight hours, as
in recent days, is the extent of low clouds/fog that may develop
overnight. While the overall scenario has not changed much, the
local airmass is just a little drier than the past couple nights.
Forecast soundings and some short-range guidance suggest low
cloud/fog development may be minimal tonight. However, other
short-range guidance suggests a repeat performance. Plan to keep
patchy fog in the forecast, along with an increase in clouds by
late tonight/early Sunday morning. The coverage should be most
extensive along/north of the outflow boundary that sank into the
area from northern Illinois this evening. This would mostly impact
the northern 1/3 to 1/2 of the forecast area. Remainder of
forecast looks good and only minor tweaks are needed.


.SHORT TERM...(This evening through Sunday)
ISSUED AT 244 PM CDT Sat Jul 30 2016

Isolated to scattered showers and thunderstorms north of I-72 and
in far southeast IL should dissipate after sunset as weak
instability of 800-1400 J/kg wanes. A short wave trof moving into
central IL this afternoon will exit east of IL this evening
allowing convection to diminish as well. A surface frontal
boundary over southeast parts of IN/IL/MO will track southeast of
the Ohio river on Sunday as weak 1020 mb high pressure over the
upper Great Lakes region noses southward toward IL tonight into
Sunday. This should provide fair weather overnight and Sunday with
partly sunny skies Sunday. Patchy fog expected to develop after
midnight and lift by mid morning Sunday, similar to past few
mornings. HRRR shows the fog developing/spreading from ne to sw
overnight similar to last night with light northeast flow regime
and moist boundary layer with dewpoints in the mid to upper 60s.
Lows overnight mostly in the mid 60s with highs Sunday in the low
to mid 80s.

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 244 PM CDT Sat Jul 30 2016

An upper level ridge is forecast to amplify over the central U.S.
starting on Monday as the 500 mb trof that was across the region for
the start of the weekend gradually shifts off to our east. A warm
front is expected to move closer to our area later Sunday night into
Monday bringing increasing chances for showers and thunderstorms,
especially to the western half of the forecast area. Orientation of
the low level jet on the latest ECMWF further west than what we saw
yesterday with one segment over northern Iowa into northern
Wisconsin with a second segment over central Kansas. This would put
the more favorable lift and theta-e advection to our west and north
Sunday night into Monday morning with convection developing out along
the Missouri River valley and tracking east-southeast along the warm
front. The ECMWF not showing much development in this area while the
GFS continues from its previous runs in bringing the rain chances
further east into our area by Monday morning.

Rain chances will increase from west to east on Monday with another
complex of storms possible to our west and north Monday night, which
would track southeast into parts of our area into Tuesday morning.
Low level forcing looks more favorable for our area Monday night
into Tuesday and that is when our POPs are at their highest as the
warm front edges slowly north Tuesday and Wednesday. As the upper
level heights continue to build over the central U.S. midweek, we
should start to see a gradual lowering of rain chances for later
Wednesday through Thursday, although the model consensus suggests
at least a 20-30 percent chance of rain during at least the
afternoon hours Wed/Thu. A stronger shortwave is forecast to track
across the U.S./Canadian border later Thursday which will help to
push a cool front east into the Midwest late Thursday night and
Friday with POPs starting to increase again ahead of that feature.

Warmest temperatures still appear to be from Tue thru Friday with
afternoon readings rising into the upper 80s to lower 90s.
Combining the surface temperatures with another round of high dew
points from the mid to possibly upper 90s, we are looking at heat
index values to increase into the upper 90s to around 103 degrees,
with the higher heat index readings edging into southeast Illinois
Friday afternoon. Something to keep an eye on early in the week,
the latest deterministic models, especially the NAM-WRF, were
indicating the surface dewpoints starting to pool along the slow
moving warm front Tuesday afternoon with areas over west central
Illinois seeing dewpoint temperatures approaching 80 degrees again.
That would push heat index values close to 105 degrees in some of
our western counties Tuesday afternoon, however, this solution 80
to 84 hours out appears to be an outlier at this time, especially
with as much uncertainty with the forecast position of the warm
front over our area in the Tuesday through Wednesday time period.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1156 PM CDT Sat Jul 30 2016

As it has been in recent nights, main concern revolves around
extent of stratus/fog development again overnight. With the
modest drying that occurred today, expect conditions to be better
than early this morning, but MVFR conditions are expected, with
areas of IFR possible. Conditions should quickly return to VFR
Sunday morning.


.ILX Watches/Warnings/Advisories...


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