Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 270915

Area Forecast Discussion
National Weather Service Lincoln IL
415 AM CDT Tue Jul 27 2021

Issued at 225 AM CDT Tue Jul 27 2021

Heat index values near 105 degrees are possible both Wednesday and
Thursday for portions of western and central Illinois. Hot
temperatures and high humidity will threaten heat- related
illness for people and pets that experience prolonged exposure to
these dangerous outdoor conditions. In addition, a boom-or-bust
severe weather day is possible on Thursday before a cold front
comes and flushes the high heat and storm potential to the south.
A stretch of cooler and drier weather is then likely later this
weekend into early next week.


.SHORT TERM...(Today and tonight)
ISSUED AT 225 AM CDT Tue Jul 27 2021

Patchy morning fog should quickly mix away as low-level lapse
rates respond to intense solar radiation. Skies will remain
mostly clear today as the periphery of an upper-level ridge and
center of surface high pressure build across central Illinois.
Only a little bit of elevated haze is expected today as wildfires
continue to burn out West. Afternoon temperatures will once again
surge to around 90 degrees, and heat index values will hover
around 95 as surface dewpoints begin to respond to a gentle
southerly wind.

Temperatures will fall to near 70 degrees overnight beneath a thin
deck of broken cirrostratus.


.LONG TERM...(Wednesday through Monday)
ISSUED AT 225 AM CDT Tue Jul 27 2021

Impactful weather is expected for both Wednesday and Thursday as
a dual threat bears down on portions of the Midwest. Dangerous
heat will trigger a Heat Advisory for both Wednesday and Thursday,
while a potential severe weather threat looms for Thursday.

Dry weather is anticipated on Wednesday as a 594mb heat ridge
expands eastward across the Mid-Mississippi Valley. Temperatures
should climb into the lower 90s on Wednesday as winds veer to the
west ahead of an incoming cold front. These westerly winds will
help usher in a more humid air mass as surface dewpoints creep
into the lower 70s. This will yield oppressive afternoon heat
index values for portions of western and central Illinois, and
has prompted a Heat Advisory (Heat Index > 105 F).

Temperatures will be equally hot on Thursday, but the humidity
will be even higher. Heat index values are forecast to range
between 103-109 F, which may require another heat-related headline
on Thursday if cloud cover doesn`t cause the forecast to bust.

Thunderstorm chances increase on Thursday, but it is still unclear
on whether storms may become severe. One limiting factor could be
sufficient deep-layer shear, with the upper-level jet core likely
displaced too far northward across the Great Lakes region. On the
other hand, the boundary layer will be supremely unstable as mid-
70s dewpoints pool along a sagging cold front. More clarity is
anticipated later today as the CAMs (convective allowing models)
attempt to resolve the mesoscale pattern. But from a pure
synoptic stand-point, there`s a strong signal for a nocturnal MCS
to clip our far east late Wednesday night into Thursday morning
as a strong mid-level shortwave and 30-40kt LLJ move across the
region. If this feature lays out an outflow boundary across our
northern counties, it would become the focus for strong-to-severe
thunderstorms later in the day on Thursday ahead of the incoming
cold front. SPC has hedged toward a Marginal Risk (1/5 threat
level) for Thursday across central Illinois, and this seems
reasonable given the current level of forecast uncertainty.

Depending on how quickly the cold front gets flushed out of
region, drier and cooler conditions are possible by Friday. The
latest NBM knocks temperatures down into the mid- to-lower 80s
for the weekend, and we see no reason to stray from that. 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 throughout the
weekend, mainly south of a Galesburg to Lawrenceville line, as
subtle shortwave energy rotates through deep northwest flow.
Drier but continued cool weather looks more likely by Monday.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1132 PM CDT Mon Jul 26 2021

High pressure over the region will provide fair weather across
the central Illinois terminals. Winds will be light and variable
or calm overnight, then set up out of the SW but remain less than
10 kt during the day Tuesday. VFR conditions will prevail through
the period.


Heat Advisory from 1 PM to 10 PM CDT Wednesday for ILZ027>031-



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