Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 170210

Area Forecast Discussion
910 PM CDT Sat Aug 16 2014


Low pressure centered over west-central IL-east-central MO will
move southeastward over the next day, reaching central KY Sunday
evening. Along/south of the low, lines of thunderstorms have
formed while ahead of the low along a warm frontal boundary
stretching from west-central to southeast IL another band of
scattered moderate-to-heavy showers and isolated thunderstorms
have developed. Areas of heavy rain are possible through portions
of central IL overnight associated with the cold frontal band
moving in from the west, although most of the heavy rainfall activity
looks like it will remain south of the central IL forecast area.
Have made adjustments to temperatures/humidity/winds based on
recent trends, and will be sending a round of adjustments to
precip chances tonight as it looks like POPs can be trimmed down
from around I-74 northward.


.SHORT TERM...(This evening through Sunday)

Remnants of earlier convective complex continue to diminish this
afternoon, with most of the rain extending from near Springfield
southeast to Evansville. Vort max evident on radar and satellite
imagery over far northeast Missouri, with the surface low a bit
further southwest over north central Missouri. Some enhancement seen
on the visible satellite near the vort, where some sunshine has been
occurring at times.

Main forecast concern in the short term is with the rain chances and
heavy rain potential. Copious moisture in the atmosphere is
accompanying this vort max, as evidenced by 4-5 inch rains that
occurred over far northern Missouri last night. Precipitable water
values over 2 inches will be dropping southeast over the area
through Sunday, with warm cloud depths of 12kft to 14kft. Low level
jet not expected to be especially impressive, but will provide a
supply of moisture to the area. Heavy rain guidance from the Weather
Prediction Center shows the south half of the forecast area under a
slight risk of flash flooding through Sunday. 3-hour flash flood
guidance is around 2.8 to 3 inches. Higher resolution models are
concentrating the heaviest totals along the Mississippi River closer
to the track of the vort max, but some heavier totals are possible
northeast toward Springfield and Decatur. Will need to watch this
closely as the storms form this evening, but no flood watches are
anticipated at this time.

On Sunday, have concentrated the highest rain chances over the
southeast half of the forecast area closer to the surface low track.
Have kept rain chances across areas northwest of the Illinois River
to only around 20%.


.LONG TERM...(Sunday night through Saturday)

Sunday night will see the surface and upper low become more
progressive and move east away from Illinois. At 00z/7pm the low
center is projected to be in far southern Indiana, and should reach
far southern Ohio by 12z/7am Monday. In the wake of the low...wrap
around moisture is expected to linger in at least the eastern half
of the area. Rain chances will continue at least through Sunday
evening southeast of Taylorville to Champaign, and diminish from the
west after midnight. A shallow layer of moisture is forecast to
linger in much of the area on Monday. Dry air in the mid levels will
try to erode the low level clouds, but a filtered sunshine is more
likely through the day on Monday. We cant rule out a stray shower
east of I-57 on Monday, mainly in the morning, but most areas should
remain dry.

An extended period of warm and humid conditions is projected to
begin on Tuesday, but a low pressure system moving from the upper
midwest across northern IL may keep enough clouds and scattered
showers across the area to trim a few degrees from MOS guidance
highs. Southerly winds flowing into IL toward that low pressure
system will still increase high temps and moisture content, with
highs climbing into the 80 to 85 degree range. Slight chances of
showers will linger into Tuesday evening, but dry conditions should
return after midnight and into Wednesday morning.

On Wednesday, a lingering surface trough across IL behind that
departing low could provide a focus for moisture convergence and
isolated showers, but there are better chances for a dry day in the
majority of Illinois. Any sunshine will help to push highs into the
upper 80s, with heat index readings in the mid to upper 90s.

Similar heat and humidity is expected to continue through Saturday
as upper level ridging builds into the Mississippi River Valley,
bringing the hot airmass from the southwest states with it. Heat
index readings south of I-70 may climb over 100F during the
afternoons. Illinois should remain on the NW flow side of the upper
ridge, so any stray shortwaves drifting over the ridge could trigger
a few showers/storms, especially from Wed night through Thursday
night. Diminishing storm chances are expected on Friday and Saturday
as 700mb temps climb to near 12C creating a capping layer to keep
updrafts in-check. Heat index readings through the extended period
are expected to remain just below heat advisory levels of 105F, but
heat precautions will still be necessary each day.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)

Low pressure centered over west-central IL-east-central MO
forecast to reach central KY by 00Z Monday. Along/south of the low
lines of thunderstorms forming while ahead of the low along a warm
frontal boundary stretching from KLWV-KMQB scattered showers are
in progress. Expecting scattered MVFR cigs until around 03z mainly
along the KLWV-KMQB line. After 03z...expecting isold-sct MVFR
cigs and vsby along with TSRA from around KSPI-KPRG southward,
while areas to the north are more likely to see less intense
showers but with lowering cigs/vsby in BR/-SHRA. From 05Z-12Z
onward central/SE IL more likely to see areas MVFR/IFR cigs/vsby
in BR/-SHRA than with TSRA, with some potential improvement likely
after 15Z as the low progresses southeastward.




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