Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 211043

Area Forecast Discussion
National Weather Service Lincoln IL
543 AM CDT Fri Jul 21 2017

.SHORT TERM...(Today and tonight)
ISSUED AT 325 AM CDT Fri Jul 21 2017

Weak boundary draped across the region, not exactly a traditional
representation of a front, is the boundary between an area of more
active convection, and a dome of hot air that is dominating the
mid Mississippi River Valley this morning. In fact, the southern
tier of the country is dominated by an upper ridge and plenty of
hot air that has moved into place aloft over the past few days.
Friday lands ILX right in the middle of an extended period of heat
and the appropriate warnings and advisories. Very little in the
forecast has changed, including the nearby convection shunted off
mostly to the northwest through northeast, as waves move along a
more active flow/tighter height gradient aloft. Models have
continued to struggle with the evolution of the MCS/storms in the
past week, as much as handling the boundary layer RH and as a
result, convective parameterization is understandably off. Much
of the shorter term is a blend of best model output and
persistence. Until the upper ridge breaks down considerably, or a
larger trof moves into the Upper Midwest later in the weekend, the
trend will continue of hot, humid, and best chances for showers/TS
confined to areas north of Interstate 74 from the outflow of the
northern storms and the perimeter of that convection.

Today will remain hot...with h85 temps successfully reaching 23C +
by the 00z sounding...and any convection may make small impacts
to locally reduce max temps...but confidence in expansive coverage
is not high enough to reduce going forecast highs...and heat
indices will remain in the 100-110F range. Late in the
afternoon/evening, models show some consistence in passing through
some shower/ts chances a little further south.
Tonight...remaining hot and muggy with the overnight lows in the
mid to upper 70s.

.LONG TERM...(Saturday through Thursday)
ISSUED AT 325 AM CDT Fri Jul 21 2017

Same discussion applies for Saturday...Hot and humid, and better
chances for more thunderstorms passing to the north. The strength
of the upper ridge is holding the bulk of the storms to the
north...but local outflows and mesoscale influences result in at
least a slight chance of storms north of Interstate 74. Saturday
night, the front/boundary that has been developing for the last
few days finally makes its way to the south with a wave moving
into the Great Lakes aloft. Said wave is starting to dig out a
trof aloft and push through a slightly different airmass. With the
movement of the boundary, precip chances spread through the
state. Slightly 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. It should
also be mentioned that the previously dry Mon-Wed period is
starting to pick up some major differences in the GFS/ECMWF
solutions. Expect some changes to the forecast on the other side
of the weekend in subsequent forecasts.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 541 AM CDT Fri Jul 21 2017

Largely unchanged forecast, with convection remaining mainly north
of ILX terminals. However, convection passing to the north and
northeast will keep a VCSH in PIA and VCTS in BMI this morning.
Another round of showers/ts possible for the afternoon hours again
in the north closer to the stalled frontal boundary. Next issue
for the forecast may be the development of a llvl jet in the
predawn hours tomorrow morning lending to a potential for LLWS at
PIA. Confidence not high enough to go with it just yet.


Excessive Heat Warning until 8 PM CDT Saturday for ILZ027>029-

Heat Advisory until 8 PM CDT Saturday for ILZ030-031-038-043>046-



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