Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 201101
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
601 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 12Z TAFS through 12Z Monday morning)
Issued at 600 AM CDT Sun Aug 20 2017

Warming cloud top temperatures in the northern portion of the
linear MCS in Iowa suggest weakening trend with the eastward
advancing portion of the line. Area is moving into weaker
instability as it moves east and further from warm front. The
southern portion of the MCS remains strong as it moves southeast
along the CAPE axis over Missouri. Still some question as to how
far east storms will make it before dissipating. Latest HRRR (08z)
suggests a widely scattered storm or shower may reach KPIA and
KSPI late this morning. Will introduce a brief period of VCTS/VCSH
at those sites. Otherwise...terminals will be influenced by
remnant cloud from the system today. There remains significant
uncertainty with next nocturnal MCS expected to develop over Iowa
this evening and as it pushes east or southeast may impact KPIA
after 06z. Will add a VCTS to reflect current thinking.

High clouds associated with this mornings MCS has limited BR
formation and current temperature/dew point spreads are 3-6
degrees at central Illinois terminals. We will remove the tempo
group that was put in at 06z.

&&

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

$$

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



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