Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 070809

Area Forecast Discussion
National Weather Service Lincoln IL
309 AM CDT Tue Apr 7 2020

Issued at 309 AM CDT Tue Apr 7 2020

Southwest winds will bring a taste of summer to central and
southeast Illinois today, as highs reach near 80 degrees. A cold
front will push into the area this evening, with scattered
thunderstorms possible. Some of the storms could be severe with
damaging wind gusts and hail.


.SHORT TERM...(Today and tonight)
ISSUED AT 309 AM CDT Tue Apr 7 2020

Latest surface analysis indicated 1002 mb low pressure over
eastern South Dakota. A warm front extended southeast of the low
into far northern IL. South winds and mostly cloudy skies were
holding overnight temps up near typical early April highs, in the
upper 50s to mid 60s. A 30 kt low level jet impinging on the
boundary produced scattered convection just to our north earlier
this evening but has now shifted east into Indiana.

Southwest boundary layer flow to increase through this afternoon
in response to a tightening pressure gradient as the low tracks
due east towards the Great Lakes. This will bring strong warm
advection and much above normal temperatures. Current satellite
image and RAP/HRRR soundings suggest skies to turn mostly sunny
through midday and as a result have trended highs towards the
warmer guidance near 80 degrees.

The strong warming will extend into the mid levels, as 850 mb
temps peak around +15 to +17C. This introduces a strong cap at
that level which will inhibit any convection through this
afternoon. All eyes will be on the approaching cold front which
will move into the northwest CWA during the early evening. If lift
is strong enough to overcome the cap, scattered convection will
develop. Moderate instability will be present with 1500-2000 J/kg
MUCAPE and sufficient 30-40 kt deep layer shear for organized
updrafts. This would promote damaging wind gusts and large hail if
convection can develop, thus we have conditional SPC slight risk
for eastern IL with marginal risk for the bulk of the CWA. Latest
hi- res CAM output and HREF suggest it will be a close call on
whether or not any convection does form, with the primary timing
from 6 PM (northwest CWA) to 1-2 AM (south of I-70).


.LONG TERM...(Wednesday through Monday)
ISSUED AT 309 AM CDT Tue Apr 7 2020

A stronger cold front will track across the area Wednesday
afternoon and evening, associated with a post-frontal band of
showers and thunderstorms. The lower levels dry nicely behind
Tuesday night`s system, resulting in large temp/dew point spreads
and inverted-V type soundings as the convection moves through.
This could promote gusty winds with any storms and SPC maintains a
marginal severe risk.

The frontal passage will introduce a major pattern change, as
longwave trofing is forecast to deepen over the eastern half of
the CONUS through the end of the week. This will plunge
temperatures well below normal, with highs probably struggling to
get out of the 40s for at least the northern half of the CWA both
Thursday and Friday. In addition, winds look quite blustery
especially on Thursday, as we remain in a tight MSLP gradient
between deepening low pressure over the northeast U.S. and
Canadian high building down through the Plains. Forecast soundings
suggest deep mixing amid strong cold advection, resulting in
30-40+ mph northwest gusts. Coldest overnight lows look to occur
Friday morning, when a widespread freeze is looking more likely.

Still much uncertainty in an evolving pattern over the weekend,
but the model consensus shows overrunning rains may develop on
Saturday with the potential for a more phased/significant system
late in the weekend tracking near the Ohio Valley.  Thus chance
pops remain Saturday through Sunday. This to be followed by even
stronger below normal temperature anomalies in the early to mid
portions of next week, which is supported by the 6-10 day CPC


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1136 PM CDT Mon Apr 6 2020

VFR ceilings prevail at the central Illinois terminals late this
evening. Based on satellite trends and latest HRRR/RAP forecasts,
it appears these clouds will remain in place through the night. As
the nocturnal low-level jet streak strengthens, widely scattered
showers/thunder will develop in closer proximity to a warm front
along/north of I-80. Will include VCSH at KPIA/KBMI in case a few
showers graze this area. These showers will track into north-
central Indiana by Tuesday morning...with dry conditions expected
for the remainder of the day. A period of MVFR ceilings will be
possible during the morning at KPIA/KBMI before increasing SW
winds scatter the clouds. Winds will initially be S/SE at 5-10kt,
then will veer to SW and gust 20-24kt by Tuesday afternoon.




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