Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 270525
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1225 AM CDT Sat May 27 2017

.UPDATE...
Issued at 905 PM CDT Fri May 26 2017

Upper level shortwave associated with severe thunderstorm activity
across central IL this afternoon and evening is now east of
Illinois with subsidence behind the feature causing diminishing
shower and thunderstorm activity. Nevertheless, a band of showers
and potentially an isolated thunderstorms extends from northern
Vermilion County westward toward Springfield along an outflow
boundary from the previous convection. This boundary is expected
to sink slowly southward promoting at least a slight chance of
thunderstorms along the I-72 corridor through late evening and
into the I-70 corridor in the early morning. Light winds and moist
conditions following rainfall will promote some potential for fog
across the area overnight. Northern Vermilion county should see
locally dense fog in areas that receive prolific hail this
evening. Updates have been sent this evening regarding the above
features.

&&

.SHORT TERM...(This evening through Saturday)
ISSUED AT 259 PM CDT Fri May 26 2017

Clusters of thunderstorms along the southern periphery of an MCV
will track across north-central Illinois this afternoon into the
early evening.  Based on radar trends and output from the HRRR and
3km NAM, it appears locations along/north of the I-74 corridor will
be impacted through about 00z/7pm.  As a result, have increased PoPs
to likely across this area through the remainder of the afternoon.
Once this system races into Indiana, it will drag a weak cold front
southward into central Illinois this evening.  With strong WAA
occurring throughout the entire depth of the atmosphere, mid-level
capping will be strong enough to prevent much convection from
developing along the trailing front. Have therefore only carried
slight chance PoPs across the southern two-thirds of the KILX CWA
from mid-evening through the overnight hours.  With the boundary in
the vicinity, winds are expected to become light/variable.  Given
increasing boundary layer moisture (both from advection and any
rainfall that occurs this afternoon), think fog will develop
tonight.  HRRR continues to show fog developing along the I-74
corridor by around midnight, then spreading further southward across
the remainder of the area as the night progresses.  Have added
patchy fog across the board accordingly.

Frontal boundary will stall along the I-70 corridor late tonight
through Saturday and will eventually serve as a focusing mechanism
for strong to potentially severe convection late Saturday.  Latest
models have continued to trend later with the next round of storms,
with most high-res solutions shifting the emphasis into the evening.
Given good consensus that Saturday will be dry, have dropped PoPs
until late in the afternoon across the far SW CWA.  Main event will
arrive Saturday evening, with models still not set on an exact
timing just yet.  NAM/GFS are the fastest models, with convection
developing upstream across Kansas during the afternoon...then
tracking eastward along the boundary and arriving in west-central
Illinois by early evening.  Meanwhile, other convective-allowing
models such as the WRF-NMM and WRF-ARW delay the arrival of the
convection until late evening into the overnight hours.  Storm mode
by the time the system arrives will be linear, so damaging wind
gusts with an isolated tornado threat would be most probable.  At
this time, have gone with likely PoPs across the western half of the
CWA during the evening, then further east to the Indiana border
after midnight.  If trends continue, the timing may eventually need
to be adjusted a bit later.

.LONG TERM...(Saturday night through Friday)
ISSUED AT 259 PM CDT Fri May 26 2017

Once the Saturday night wave of convection passes to the east, it
will give the stalled frontal boundary a push to the south on
Sunday.  Airmass destabilizes ahead of the front by afternoon,
resulting in scattered thunderstorm development primarily along and
south of I-70.  Have included 30 PoPs across this area, with little
or no precip expected further north across the remainder of the
area.  High temperatures on Sunday will reach the middle to upper
70s.

Pronounced upper low will dig southward through Ontario into the
Great Lakes early next week, resulting in a cooling trend across
central Illinois.  A weak surface trough will sink southward around
the parent low on Monday, potentially bringing a few showers.
Convergence along the boundary will be weak and with the main upper
cold core still to the north, think areal coverage of precip will
remain low.  As a result, have only mentioned slight chance PoPs for
showers/thunder along and north of I-74 Monday afternoon.  Cool/dry
conditions will prevail Monday night into Tuesday before the next
potential system spreads showers across south-central Illinois
Tuesday night.  After that, mainly dry weather is expected until a
stronger system brings the next chance for rain by Friday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1225 AM CDT Sat May 27 2017

Main challenge in the short term will be coverage of IFR
ceilings/visibilities. Area of stratus below 1,000 feet currently
extends from about KGBG-KDNV and is settling southward. HRRR
guidance suggests this should cover most of central Illinois by
about 09-10Z, with visibilities down to around a mile or two.
Improvement is expected from the northeast early Saturday morning.
Later in the period, focus shifts to a large area of
showers/storms that will be approaching from the west. Some
uncertainty with the exact track, with some of the higher-
resolution guidance suggesting a track that would bypass central
Illinois. As such, will limit convective mention to VCTS for now.

&&

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

$$

UPDATE...37
SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Geelhart


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