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Area forecast discussion 
National Weather Service Lincoln Illinois
1150 PM CDT Sat may 23 2015

issued at 849 PM CDT Sat may 23 2015

High pressure centered off the middle-Atlantic coast continues to
bring dry low level air and light southeast winds into central Illinois
with dewpoints primarily in the 40s...while low pressure moving
eastward into the Central Plains brings moist southerly flow into
the Southern Plains. As the low edges closer
overnight...increasing moisture and remnant disturbances from
convection in the Southern Plains will start to bring chances for
showers late tonight over west central Illinois. Lows will be up
several degrees from last night with temperatures reaching the low
60s. Current forecasts indicate these features and no significant
updates anticipated at this time.


Short term...(tonight)
issued at 255 PM CDT Sat may 23 2015

High pressure centered over West Virginia will control the weather
across central Illinois tonight...keeping rain chances largely at
Bay until Sunday. 19z/2pm observations continue to show an easterly
component to the low-level flow around the high...which is keeping a
dry airmass in place. Surface dewpoints are mainly in the 40s this
afternoon...and will remain low until winds veer to the
south/southwest and allow a more humid airmass currently poised
upstream across Missouri to surge northward on Sunday. Until that
time...a pronounced dry layer will persist below 850mb. As a
result...any showers that develop to the west/SW this afternoon and try
to lift into west-central Illinois will likely evaporate. Hrrr
shows this trend quite well...with little more than sprinkles
spreading into the Illinois River valley from late evening into the
overnight hours. Have therefore maintained a dry forecast through
midnight...with just a slight chance for showers west of I-55 late
tonight toward dawn Sunday. Due to the cloud cover and a
southeasterly breeze...overnight low temperatures will only drop
into the upper 50s and lower 60s.

Long term...(sunday through saturday)
issued at 255 PM CDT Sat may 23 2015

Rain chances will begin to increase on western Continental U.S.
Upper low gradually lifts toward the northern plains. Widespread
convection ahead of this system across Oklahoma/Texas tonight into
early Sunday may tend to block the northward flow of deep moisture
into the Midwest. As a result...only scattered convection is
expected to develop across central Illinois. Will raise probability of precipitation to
likely across the Illinois River valley by afternoon...but will only
feature slight chance probability of precipitation along/east of I-57.

Best bet at widespread rainfall is still on track for Sunday night
into moisture plume ahead of the advancing low shifts
eastward into Illinois. Showers and thunderstorms will become
likely across the board Sunday night...but will shift eastward a bit
more quickly on Monday than previously per 12z may 23
models. NAM/GFS/ECMWF all show precipitation across the area Monday
morning...then quickly push it eastward into Indiana during the
afternoon. With precipitation ending from west to east and skies becoming
partly sunny...atmosphere will become moderately unstable by Monday
afternoon/evening. NAM SBCAPE values climb above 2000j/kg: however,
weak 0-6km bulk shear of only 20-25kt and a lack of strong synoptic
forcing should lead to a mostly dry afternoon and evening. Still
need to watch out for any potential mesoscale boundaries left behind
by early day additional storms could certainly fire
in the unstable environment. At this model is showing
this will go with a dry period Monday

Vigorous short-wave trough evident on latest water vapor imagery
over northern California will round the base of the western Continental U.S.
Trough and lift into Illinois on Tuesday. Models are in relatively
good agreement with the speed/timing of this feature...with the GFS
being slightly faster than the European model (ecmwf). Given the arrival of stronger
lift associated with the wave...have brought probability of precipitation back into the
picture late Monday night...then have gone with likely probability of precipitation
everywhere on Tuesday.

Once the wave passes...the remainder of the week will be very warm
and humid with no organized convection. Will maintain slight/low
chance probability of precipitation Wednesday through Friday in case a subtle wave not
currently picked up by synoptic models ripples through the area
within the broad southwesterly flow pattern. Better chances for
rain will likely hold off until next weekend...when both the GFS and
European model (ecmwf) show a more significant northern-stream wave pushing a frontal
boundary into Illinois by next Saturday.


Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1150 PM CDT Sat may 23 2015

VFR conditions expected until around 18z...then scattered showers
and isolated thunderstorms will spread eastward across central Illinois
producing potential for isolated MVFR or worse cigs/vsby.
Initially...light and widely scattered showers spread into areas
from kpia-kspi westward prior to 18z with VFR ceilings continuing.
Spatio-temporal details of sub-VFR conditions after 18z are
uncertain at this time and coverage looks to be non- have incorporated vcsh/thunderstorms in the vicinity with scattered cumulonimbus at
2.5-3 kft above ground level in tafs to account for convective activity. Winds
southeast 5-7 kts overnight...increasing to 15-20 kts and becoming
southerly late morning Sunday. Slight decrease in wind speeds
expected after 00z.


Ilx watches/warnings/advisories...


short term...Barnes
long term...Barnes

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