Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 220417
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1117 PM CDT THU JUL 21 2016

.UPDATE...
Issued at 833 PM CDT Thu Jul 21 2016

Next convective system tracking south through northern Illinois
early this evening looks like it will affect parts, if not all
of our area overnight if you believe that last several runs of
the HRRR. Looks as if the short term model latched on to these
storms while they were in south central Wisconsin and seem to
be doing well, albeit a bit on the slow side compared to reality.
Looks like a well developed MCS with bowing segments will track
south along the strong instability over our area this evening with
the primary threat beging from damaging wind gusts and even the
possibility for some hail based on the steep mid level lapse rates
seen on the 00z ILX sounding. In addition, with the atmosphere
loaded with moisture, PWAT values of around 1.8 inches, if these
storms tend to train over a particular area, rainfall rates of 2
to 3 inches per hour will be possible which will produce flash
flooding.

Have made several changes to the grids with respect to the overall
coverage and timing of storms in our area and it still may not be
enough as the HRRR has been suggesting that the MCS will track
south and affect most of our area through the early morning hours.
Looks like the leading edge will push into our far northern areas
over the next hour or so and continue to push south. Will be a bit
more conservative at this point but may need to adjust the
southward trends in the precip coverage over the next several
hours along with the possibility for a new weather watch that may
include a good portion of our area. Will continue to monitor that.
Will have the updated zones out by 900 pm.

&&

.SHORT TERM...(This evening through Friday)
ISSUED AT 300 PM CDT Thu Jul 21 2016

A warm and muggy night is in store across central Illinois tonight.
After high temperatures in the lower to middle 90s this afternoon,
lows tonight will only drop into the middle to upper 70s.  Aside
from an isolated thunderstorm along/east of the I-55 corridor this
afternoon, dry conditions will be the rule into this evening.  As is
typically the case this time of year, models are struggling to
accurately forecast convective evolution.  Solutions vary
greatly...with some models going completely dry tonight and others
suggesting a potential MCS across northern Illinois late.  Current
analysis shows an outflow boundary from storms that rolled through
the Great Lakes earlier today extending from southern Wisconsin into
northwest Indiana.  With a weak 20-25kt 850mb jet flowing into the
boundary, think clusters of thunderstorms may develop along/north of
I-80 after midnight.  These storms will tend to sink S/SE toward
dawn, potentially spilling into the N/NW KILX CWA.  Have tailored
PoPs accordingly...with dry conditions everywhere through the
evening followed by low chance along/north of I-74 well after
midnight.

.LONG TERM...(Friday night through Thursday)
ISSUED AT 300 PM CDT Thu Jul 21 2016

A few showers and thunderstorms may be ongoing across the northern
KILX CWA Friday morning, but these will likely dissipate after
sunrise.  Additional convection may develop along outflow boundaries
left behind by the morning storms:however, with warm mid-level
capping in place, do not think convection will become widespread.
Most models suggest widely scattered thunderstorms everywhere except
the SW CWA where cap will be strongest.  Main story on Friday
will be the continued heat and humidity, as high temperatures once
again reach the middle 90s and heat index values climb into the
105-110 degree range.

As upper-level ridging begins to weaken and mid-level temps cool
slightly, areal coverage of thunderstorms may become a bit more
robust on Saturday.  Synoptic frontal boundary will remain well to
the N/NW, so main forcing mechanism will be mesoscale outflow
boundaries left behind by previous convection.  Have carried low
chance PoPs everywhere with highs topping out in the lower to
middle 90s.

Best rain chances still appear to be setting up Sunday/Sunday night
as cold front approaches from the northwest.  12z Jul 21 models have
now delayed FROPA, so rain chances will likely linger across at
least the southern half of the CWA into Monday.  After that, a
cooler/drier airmass will settle into central Illinois on Tuesday
and Wednesday with highs dropping back into the middle to upper 80s.
As such, have maintained the Excessive Heat Warning everywhere
through Saturday evening...and along/south of a Quincy to Danville
line through Sunday evening.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1106 PM CDT Thu Jul 21 2016

Complex of storms affecting the PIA and BMI TAF sites will
gradually shift south and affect our further south sites over the
next 2 to 4 hours. Wind gusts to 35 kts will be possible with
some of the stronger cells along with torrential rainfall and
brief MVFR and possible IFR cigs/vsbys. We look for the complex of
storms to settle south of the southern TAF sites by 09z. After
that, there may be some fog that develops and brings vsbys down to
MVFR category til 13z, and then VFR conditions are expected during
the day Friday along with some more oppressive heat and humidity.
Chances for additional storms continue on Friday but coverage
looks to be quite spotty again so will not include anything with
this forecast issuance for tomorrow afternoon/eve. Surface winds
will be light southerly tonight, but variable in direction and
quite gusty in and near the thunderstorms. Look for south to
southwest winds of 8 to 13 kts on Friday.

&&

.ILX Watches/Warnings/Advisories...
Excessive Heat Warning until 7 PM CDT Sunday for ILZ040>057-
061>063-066>068-071>073.

Excessive Heat Warning until 7 PM CDT Saturday for ILZ027>031-
036>038.

&&

$$

UPDATE...Smith
SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Smith



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