Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 211753

Area Forecast Discussion
National Weather Service Lincoln IL
1253 PM CDT Mon May 21 2018

Issued at 1042 AM CDT Mon May 21 2018

A weak low pressure center is over SE Iowa this morning with a
warm front extending eastward near the I-74 corridor and cold
front extending southward toward NE Texas. Dewpoints are in the
60s in the warm sector south of the warm front and east of the
cold front. The low will slowly move east or ENE with central IL
primarily in the warm sector until the front arrives in the
Illinois River valley around 1 p.m. and crosses into Indiana
overnight. In this environment, scattered showers will continue
ahead of the cold front through the morning, becoming more
vigorous during the afternoon as surface heating takes place, with
thunderstorms becoming more prevalent. SPC continues a marginal
risk for severe thunderstorms east of I-57 mainly for damaging
wind threat with thunderstorms. Updates this morning have been
mainly for short term breaks in shower activity, but overall
forecast is in good shape with chance PoPs into the afternoon,
trimming down from the west in the afternoon as the cold front
progresses eastward. Highs around 80 most areas, but just a touch
lower north of I-74 from Peoria westward.


.SHORT TERM...(Today and tonight)
ISSUED AT 206 AM CDT Mon May 21 2018

A strong short-wave evident in GOES-16 6.95 um and 7.34 um water
vapor bands over mid-Missouri valley is providing support to
convection moving through Illinois. Although instability is
limited, synoptic forcing is stronger than with the waves over the
last few nights and there is more confidence that the
precipitation should hold together into the day. Models in good
agreement with a surface low crossing northern Illinois during the
afternoon and evening. A weak cold front in the wake of the low
should provide some additional forcing as it moves east across the
eastern portion of the state near and after peak heating. 00z
3-km NAM and HRRR suggest that any redevelopment will be
scattered, but the combination of enhanced convergence along front
and diurnal instability may be enough to produce several strong
storms, mainly from I-57 eastward, during the afternoon and
evening. Appears strong winds would be the primary threat given
the unidirectional wind profile and limited instability.

A gradual drop off in rain chances should occur from west to east
Monday as the wave axis moves slowly across the Midwest. Abundant
clouds and scattered precipitation should keep temps from rising
much Monday, particular northwest of the Illinois River, highs
should be near normal for middle/late May.

.LONG TERM...(Tuesday through Sunday)
ISSUED AT 206 AM CDT Mon May 21 2018

Once the wave moves out of the region early Tuesday, much drier air
will advect into the area as high pressure covers much of the Great
Lakes region. Mid-level heights climb significantly through
midweek with 500 mb heights increasing 80 m between Tuesday and
Thursday Morning. This should push sfc temps well into the 80s by
the end of the week.

Remnant energy from the trough currently along the west coast will
move over the ridge and slide down into the Great Lakes toward the
end of the week. Although the major synoptic models have this
general idea, they do differ significantly on the strength of the
wave and how it interacts with a weakness along the northwestern
Gulf of Mexico coast. Depending on the amount of phasing that
occurs, rainfall may be prolonged and potentially heavy over the
Holiday weekend per 18z GFS, but 00z GFS matching up better with
ECMWF which keeps the energy separated and keeps bulk of Gulf
moisture from interacting with the northern stream wave until after
it has passed the forecast area.  If this model trend continues
there will likely be a period or two of precipitation over the
weekend though probably not a complete washout.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1252 PM CDT Mon May 21 2018

Ceilings are transitioning to high-end MVFR with daytime heating,
while scattered thunderstorms are likely to develop ahead of an
approaching cold front over the early few hours of the 18Z
forecast period. As the cold front progresses eastward this
afternoon, thunderstorms will migrate east of the central IL
terminals by 22-24Z. VFR cigs expected to return at that point,
however patchy fog overnight will likely result in IFR-MVFR vsby
and cigs, possibly locally worse. Winds becoming SSW 6-10 kts for
the afternoon, then diminishing overnight and eventually turning
NW 5-8 kts Tuesday morning.




LONG TERM...Barker
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