Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 280913

Area Forecast Discussion
National Weather Service Lincoln IL
413 AM CDT Wed Jul 28 2021

Issued at 218 AM CDT Wed Jul 28 2021

Hot and humid conditions will yield dangerous heat across central
Illinois today and Thursday. Heat index values will reach into the
triple digits each day, triggering a Heat Advisory for much of the
region. Strong-to-severe thunderstorms will be possible Thursday
afternoon, mainly east of Interstate 55, but forecast confidence
remains on the lower end of the spectrum at this time. A stretch
of cooler and drier weather can then be expected this weekend
into early next week.


.SHORT TERM...(Today through Thursday)
ISSUED AT 218 AM CDT Wed Jul 28 2021

Dangerous Heat:

The periphery of a 594mb heat ridge continues to build across the
Mid-Mississippi Valley. Temperatures are expected to warm into
the lower 90s this afternoon juxtaposed to surface dewpoints
climbing into the lower 70s in response to a gentle westerly wind.
This combination of heat and humidity has the potential to
produce triple digit heat index values (99F - 106F) for locations
west of I-55 this afternoon. This has prompted a Heat Advisory,
but there is some bust potential with our forecast as convective
debris from decaying convection across northern Illinois casts a
thin deck of cirrostratus across our outlook area this morning.
Nevertheless, hi-res models are in decent agreement that clouds
will break for sun later this morning, at least for a couple
hours. With steep lapse rates and very warm temperatures aloft, it
shouldn`t take much solar insolation for temperatures to surge
into the 90s by early afternoon, even if additional clouds filter
in shortly after.

Temperatures will be equally hot on Thursday, but the humidity
will be even higher. Heat index values are forecast to range
between 100F - 109F for locations along and south of I-74. This
will require an eastward expansion of our current Heat Advisory.
Look for that update to come out later today.

Severe Weather Potential:
Thunderstorm chances increase on Thursday as a weakly forced cold
front sags south across Illinois. The boundary layer will become
very unstable ahead of this front, and will be enhanced by an
elevated mixed layer that advects in from the northwest. Mid 70s
surface dewpoints and steep mid-level lapse rates will yield
highly buoyant parcels (3500-4500 J/kg SBCAPE) ahead of the front
in a mostly uncapped environment. Insufficient deep layer shear
(<30 kts) could limit storm organization as the upper-level jet
core remains displaced northward across the Great Lakes region
Thursday afternoon, but that may be enough given the degree of

There are still questions on when and where convective initiation
may take place Thursday afternoon across central Illinois. Based
on the synoptic pattern, there`s a strong signal for a nocturnal
MCS to clip our far east late Wednesday night into Thursday
morning as a robust mid-level shortwave trough and 30-40kt LLJ
move across the region. If this feature lays out an outflow
boundary across our northeast counties, and several of the 00z
CAMs suggest it will, then the outflow will become the focus for
strong- to- severe thunderstorms Thursday afternoon, mainly east
of I-55, ahead of the incoming cold front. SPC has maintained a
Marginal Risk (1/5 threat level) for Thursday across central
Illinois, and this seems reasonable given the current level of
forecast uncertainty.

At this point, hail and wind are the primary severe weather
threats, but the storm relative hodograph (different from what you
see on Bufkit and SPC sounding page) suggests a tornado or two
will also be possible despite the unidirectional shear profiles.


.LONG TERM...(Friday through Tuesday)
ISSUED AT 218 AM CDT Wed Jul 28 2021

Depending on how quickly the cold front gets flushed out of
region, drier and cooler conditions may return by Friday. The NBM
knocks temperatures down into the mid- to-lower 80s for the
weekend. Ensemble guidance maintains the 594mb heat ridge through
the weekend, but model trends continue to exhibit a southern push
as a strong upper- level low develops over the Hudson Bay and
helps squeeze the center of that heat ridge into the Southern
Plains. Showers and thunderstorms will remain a possibility on
Saturday, mainly south of a Galesburg to Lawrenceville line, as
subtle shortwave energy rotates through deep northwest flow. A
stretch of drier but continued cool weather looks likely by Sunday
and extends into the middle of next week.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1146 PM CDT Tue Jul 27 2021

High pressure over the region will provide clear skies and
light/calm winds overnight. Patchy fog is possible closest to the
ridge axis, mainly at SPI/DEC/CMI. Light SW winds will set up
during the day Wednesday as the high settles to our south.


Heat Advisory from 1 PM this afternoon to 8 PM CDT Thursday for



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