Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 200755
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
255 AM CDT Sun Aug 20 2017

.SHORT TERM...(Today and tonight)
ISSUED AT 255 AM CDT Sun Aug 20 2017

Closely monitoring MCS just north of a stationary front
stretching east-west across northern Missouri and central
Illinois this morning. Forecast for today focuses on how far into
the dry air above 850 mb that is evident in KILX and KDVN 00z
soundings. Latest CAMs are moving it into the western forecast
area this morning prior to dissipation and will generally follow.
Impacts on temps due to cloud cover blow-off will likely be
significant over western half of region leading to slightly cooler
temps in the Illinois River Valley east to I-55. Eastern forecast
areas will likely see plenty of sun and heat index values
reaching into the lower 90s.

Subtle waves continue to move through the broad cyclonic flow in
place over the northern half of the CONUS. Despite the weakness of
the next wave in the train, models are consistent in developing
the next nocturnal MCS a bit further east tomorrow night. There is
some differences in how that MCS is forecast to evolve however.
NAM and GFS take the convective system southeast into western
Illinois and eastern Missouri, while ECMWF suggest a more eastward
movement. Will bring chance PoPs into the Illinois River valley
tonight, but be aware that there is substantial uncertainty.

&&

.LONG TERM...(Monday through Saturday)
ISSUED AT 255 AM CDT Sun Aug 20 2017

Models consistent in building ridge over western U.S. Monday and
digging a 500mb trough southward into the midwest. A cold front
associated with the main thrust of energy is expected to push
southeast into the region Monday and through the area Tuesday.
Ahead of the front, moist south flow will push dew points into the
70s leading to CAPEs well above 1500 J/Kg and heat index values
pushing 100. One possible negating factor for vigourous convection is
the potential for cloud cover remnants from Sunday Night MCS, but
as was discussed in short-term discussion there is high
uncertainty to how that system will play out. Either way, cloud
cover should be on the relative high side north of I-70 Monday
afternoon due in part to the remnant cloud combined with diurnal
cu in the very moist environment.

Precipitation ahead of and along the front should move into the
forecast area Monday Night and push through the area Tuesday. The
precipitation should limit instability and keep high temps in the
lower to middle 80s. Precipitable water is forecast to be around 2
inches over much of the forecast area by 18z, so there will be a
threat of heavy rainfall. Potential for localized flooding will
likely depend on how much and where rain occurs in the previous 24
hours to saturate the soil. Most areas are currently on the dry
side and can handle quite a bit of precip.

Once the front clears the area late Tuesday, much drier and
cooler conditions advect into the state. Temperatures into next
weekend should average 5-10 degrees below seasonal normals.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1148 PM CDT Sat Aug 19 2017

Watching MCS over mid-Missouri Valley this evening as it moves
east southeast overnight. Last night the models seriously
underestimated the staying power of a similar system. Not much is
different tonight with a boundary in place across northern
Missouri, across central Illinois, and into central Indiana which
may provide a convergent focus and lift to keep the system
advancing. Only significant difference from last night is the
drier air in place above 850mb. At the very least, some BKN VFR
cigs will be possible from I-55 west during the afternoon. Winds
will become light southerly from south to north as the boundary
begins to lift north ahead of subtle wave approaching from the
Central Plains. Wildcard will be impacts any convection on winds
and boundary position.

LAMP quidance suggests an hour or two of shallow fog will be
possible at terminals toward morning, but it is not handling
clouds in advance of the MCS so will hit fog a bit harder at
eastern terminals though even there will keep vsbys above 3sm.


&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Barker
LONG TERM...Barker
AVIATION...Barker



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