Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 220224

Area Forecast Discussion
National Weather Service Lincoln IL
924 PM CDT Mon Aug 21 2017

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 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

.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 00Z TAFS through 00Z Tuesday evening)
Issued at 702 PM CDT Mon Aug 21 2017

A cold front will cross the central Illinois terminals by midday
Tuesday. Numerous showers and thunderstorms are likely
along/just ahead of the front, although more scattered storms are
possible well ahead of the boundary. Carried a few hours of
predominant showers and VCTS closer to FROPA, but amendments may
be needed to capture the more scattered convection before the
front. VFR conditions should prevail for much of the period, but
guidance continues to suggest a period of MVFR CIGs behind the
front Tuesday. Southerly winds ahead of the front will shift
northwest behind the front.




LONG TERM...Barnes
AVIATION...Bak is the U.S. government's official web portal to all federal, state and local government web resources and services.