Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 201800

Area Forecast Discussion
National Weather Service Lincoln IL
100 PM CDT Sun Aug 20 2017

Issued at 1045 AM CDT Sun Aug 20 2017

Updated the forecast to increase cloud cover today and spread
chances of showers and isolated thunderstorms further east to
areas west of I-57 this afternoon. A weakening MCS over MO and
southern IA was drifting slowly eastward with a few showers moving
into west central IL. An extensive area of mid and high clouds
along with scattered cumulus clouds have spread eastward across
much of CWA by late morning. Most of the CAM models continue to
weaken the scattered convection as it moves east into central IL
this afternoon so kept pops low (20-30%) with 40% pops over far
western CWA. A very warm and humid day with highs in the upper 80s
to near 90F (coolest areas in cloudier areas of west central IL).
Dewpoints in the upper 60s and lower 70s will give peak heat
indices in the low to mid 90s during mid and late afternoon.


.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 vigorous 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 18Z TAFS through 18Z Monday afternoon)
Issued at 1259 PM CDT Sun Aug 20 2017

A weakening leading edge of MCS was spreading a band of showers
eastward toward the IL river early this afternoon. The outflow
boundary from this MCS was approaching I-57. HRRR model is showing
isolated convection developing during mid/late afternoon over
central IL with this MCS as airmass is getting quite unstable with
CAPES rising above 2000 J/kg along and east of the IL river where
more heating occurring. Have VCSH/VCTS this afternoon especially
over northern taf sites. Then an MCV over central IA will track
east into northern IL late this evening and early overnight and
reintroduced VCTS at northern TAF sites for a few hours.
Convection Monday morning appears to be focused best by PIA. Winds
to stay under 10 kts next 24 hours (except if an isolated
thunderstorms occurs) with wind direction mainly south today, se
tonight and sw on Monday morning.





LONG TERM...Barker
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