Area Forecast Discussion
Issued by NWS Central Illinois

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978
FXUS63 KILX 201738
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1238 PM CDT Thu Jul 20 2017

.UPDATE...
Issued at 1002 AM CDT Thu Jul 20 2017

The trailing end of a decaying MCS is currently dropping into
central Illinois...with 15z/10am radar imagery showing scattered
showers/thunder along and north of a Springfield to Danville line.
All models show this initial surge of convection dissipating over
the next 2-3 hours...followed by partial clearing. Latest radar
shows an outflow boundary extending from just southwest of
Lincoln...to near Decatur...to north of Paris. This boundary will
tend to mix back northward this afternoon and will serve as the
focusing mechanism for additional storms later today. Based on
expected location of the boundary, have included low chance PoPs
along/north of the I-74 corridor late this afternoon and evening.
Due to extensive cloud cover this morning, have adjusted high
temps down several degrees across the N/NE KILX CWA. Afternoon
highs will range from around 90 degrees from Bloomington eastward
to Danville...to the middle 90s across the S/SW CWA. Despite the
slightly cooler temps across the N/NE, dewpoints well into the 70s
will still create heat index values in excess of 100 degrees.

&&

.SHORT TERM...(Today and tonight)
ISSUED AT 330 AM CDT Thu Jul 20 2017

Not much of a shift in the forecast overall as Central IL sits on
the edge of a dome of very hot air and a more active pattern just
to the north for the upper Midwest and the northern tier of IL.
The main message remains the heat, with advisories and warnings
active across the region. Periodic chances for showers and
thunderstorms... particularly for areas north of I-74 as remnant
outflows from the northern storm systems drift a bit further south
before they weaken. This morning, for example, the HRRR continues
to develop some showers on the SWrn flank of the convection moving
through northern IL in the next few hours. As a result...have
pulled some chance pops in for areas mainly north of a line from
Peoria to Danville. May see little more than an increase in clouds
however, as the storms will be weakening this far south.

Hot air continues to push into the Midwest and by this afternoon
h85 temps will climb to 21-23C and surface temps respond up to the
mid to upper 90s. With the muggy airmass in place with dwpts in
the 70s, afternoon heat indices will climb into the 105-110F range
for a large portion of the state. Please exercise extreme caution
if outside in the heat of the day. Tonights temps will not provide
much in the way of cooling off, only dropping into the mid 70s in
a stagnant and muggy airmass.

.LONG TERM...(Friday through Wednesday)
ISSUED AT 330 AM CDT Thu Jul 20 2017

The forecast for Friday and Saturday will be much the same as the
previous discussion. Hot and humid, and more thunderstorms passing
to the north. The strength of the upper ridge is once again the
deflecting force to keep the storms to the north...and is the
question mark with the abundant mesoscale influences. Friday has
shifted to a better chance to see some relief from the heat,
albeit briefly, with a stronger wave moving precip into the center
of the state. Either way, heat continues through Saturday.
Saturday night, the front that has been developing for the last
few days finally makes its way to the south and spreads precip
chances through the state...and cooler temps on the back side of
it for Sunday. Forecast expected to be dry for the start of next
week, with temperatures far more moderate in the low to mid 80s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1238 PM CDT Thu Jul 20 2017

VFR conditions will prevail through the entire 18z TAF period.
High-res models continue to suggest widely scattered thunderstorms
will develop along a lingering outflow/differential heating
boundary later this afternoon and evening. Based on latest obs, it
appears this boundary will lie roughly along or just north of the
I-74 corridor at peak heating. Given uncertainty regarding exact
position of boundary and expected low areal coverage of
convection, opted not to carry VCTS at the I-74 terminals...but
this may need to be added as radar trends become more apparent
later this afternoon.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 8 PM CDT Saturday for ILZ027>029-
036-037-040>042-047>054-061-062-066>068-071>073.

Heat Advisory until 8 PM CDT Saturday for ILZ030-031-038-043>046-
055>057-063.

&&

$$

UPDATE...Barnes
SHORT TERM...HJS
LONG TERM...HJS
AVIATION...Barnes



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