Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 220501
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1201 AM CDT Tue Aug 22 2017

.UPDATE...
Issued at 916 PM CDT Mon Aug 21 2017

A cold front currently extends from low pressure over northern
Wisconsin into the central Plains. Numerous showers and
thunderstorms have developed along/ahead of the front and these
should push across central and southeast Illinois as the front
approaches/moves across the area. Spotty showers/storms are also
possible well before the front arrives. The more robust convection
appears likely to stay just south of the forecast area, with the
possible exception of far southeast Illinois. The outflow boundary
from storms earlier today lies from west-east just south of the
forecast area. The more stable/overturned air mass to the north of
this boundary has tended to weaken the storms that develop. Best
threat for stronger storms will not arrive until after midnight as
the stronger convection near Kansas City pushes toward the area.

Going forecast was in good shape overall, although handling the
hourly PoPs in this convective rainfall regime has proven
challenging. Most changes were minor tweaks to the hourly trends
into Tuesday.

&&

.SHORT TERM...(This evening through Tuesday)
ISSUED AT 314 PM CDT Mon Aug 21 2017

While the primary synoptic frontal boundary remains well to the
NW from Wisconsin to Nebraska, a well-defined outflow boundary
much further south has served as the primary focusing mechanism
for scattered convection this afternoon. 20z/3pm radar mosaic
continues to show scattered cells along the ouflow...primarily
from Robinson...to Effingham...to Pittsfield...then westward to
near Kansas City. This activity is moving E/NE along the outflow
boundary and is expected to continue for the remainder of the
afternoon into the early evening before gradually dissipating as
daytime instability wanes. Meanwhile further northwest, a large
complex of thunderstorms directly associated with the synoptic
front continues to drop E/SE across southern Minnesota and
northern Iowa. These storms will spread/develop further southeast
and eventually reach central Illinois later this evening into the
overnight hours. Based on radar trends and model output, have kept
PoPs in the low chance category across much of the area through
the evening, then have spread likely to categorical PoPs from west
to east overnight. Widespread showers and thunderstorms will be
ongoing early Tuesday morning, before the cold front settles
southward to the I-70 corridor by late afternoon. Have therefore
continued likely PoPs through the morning, then have tapered them
down to just chance as the day progresses. Due to extensive cloud
cover and rainfall, high temperatures will remain in the lower
80s.

.LONG TERM...(Tuesday night through Monday)
ISSUED AT 314 PM CDT Mon Aug 21 2017

Once the front pushes into the Ohio River Valley, a cooler/drier
airmass will take hold across the Midwest for the balance of the
week. Deep upper troughing anchored over the Great Lakes will
ensure northwesterly flow and below normal temperatures in the
upper 70s through Saturday. A vigorous short-wave is progged to
drop southeastward out of Canada, re-enforcing the prevailing
trough on Thursday. Both the GFS and ECMWF have a good handle on
this feature, with the GFS producing scattered showers across
north-central Illinois Thursday afternoon. ECMWF is dry for now,
but will have to keep an eye on the temps aloft and the resulting
lapse rates as a few clouds and showers are definitely not out of
the question. Given track of wave just to the N/NE of central
Illinois and the dry airmass in place, have kept the forecast dry
at this time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1201 AM CDT Tue Aug 22 2017

Line of convection between KBMI and KCMI should be near KCMI by
TAF valid time and will start them off with a either a VCTS or a
tempo ground depending on where the line sits at issue time.

Vort max over eastern Kansas evident in water vapor imagery is
main trigger for strong storms between Kansas City and Springfield
MO this evening. The storms will be moving eastward into a less
unstable airmass before they reach Illinois so expect intensity
should be on the decline before reaching central Illinois
terminals. Will have to watch KSPI as the more unstable air may
advect into the site from the southwest.

Large area of stratiform precip with embedded convection stretches
ahead of a cold front that stretches near Green Bay WI to Iowa
Falls IA at 04z will likely persist much of the night as the
associated upper wave digs quickly southeast with the
amplification of the upper trough over the eastern U.S. Upstream
IFR Cigs and Vsbys are evident in the stratiform shield and will
likely advect into the TAF sites as the front approaches through
sunrise. Will time IFR conditions into terminals from 08z to 13z
from northwest to southeast. Front should be through KDEC and
KCMI by 18z and will shift winds and rapidly improve cigs.

&&

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

$$

UPDATE...Bak
SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Barker


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