Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 190144

Area Forecast Discussion
National Weather Service Lincoln IL
844 PM CDT Sun Aug 18 2019

Issued at 843 PM CDT Sun Aug 18 2019

Very warm and humid conditions are on tap for the next couple
days, as heat index values return to triple digits by Tuesday.
Scattered thunderstorms are possible again on Monday, primarily in
east central and southeast Illinois. Storm chances will increase
on Tuesday, and a few may be strong to severe.


Issued at 843 PM CDT Sun Aug 18 2019

Storm activity has pushed east of the forecast area this evening,
with water vapor imagery showing the upper wave along the Indiana
border. Have largely eliminated much of the PoP`s overnight,
though some isolated showers/storms may still linger across the
eastern CWA. Concern overnight will be more with fog potential,
especially across the northeast CWA where the heavy rains occurred
a few hours ago. Skies have already largely cleared there, though
some narrow bands of stratocumulus linger from near Decatur to
Danville. Latest HRRR/LAMP guidance has been hinting at this for
the last few runs, though varying in the overall coverage. For
now, will only mention the fog from about Decatur eastward.


.SHORT TERM...(This evening through Monday)
ISSUED AT 330 PM CDT Sun Aug 18 2019

A broken line of strong to severe thunderstorms was over far
southeast IL into se MO and ne AR at mid afternoon. These storms
formed along an outflow boundary the was pushing southeast through
southeast IL and strongest storms currently south of CWA in
Edwards and southern Wayne counties. Unstable CAPEs of 2500-3600
j/kg were east of I-57, highest along the Wabash river. A
thunderstorm was over Champaign county from Savoy/CMI airport
south and tracking eastward toward Vermilion county. Surface
analysis shows the main synoptic front extended from 1001 mb low
pressure over eastern Lake Superior thru central WI/IA and into
central KS. Aloft a broad/weak upper level trof was over the
Great Lakes, much of the Midwest and over the northern
Plains/Rockies, while 594 dm 500 mb subtropical ridge was near
the southeast atlantic coast and sprawled back into TX/NM.

Diurnally driven scattered convection in eastern IL should
diminish toward early evening as sunsets, and only carried slight
chances or dry rest of tonight into mid Mon morning over CWA. Lows
overnight in the upper 60s to around 70 degrees. The latest models
show the frontal boundary pushing se into IL river valley around
midnight tonight and into southeast IL near I-70 by 18Z/1 pm Mon.
This boundary to help develop more isolated to scattered
convection by midday Monday and continue into early Mon evening
mainly over areas from I-72 south. Airmass gets unstable again
over areas south of I-72 by Mon afternoon and could see a few more
strong thunderstorms especially in southeast IL. Highs Mon of
85-90F warmest in southeast IL where heat indices reach the upper
90s to near 100 Mon afternoon.

.LONG TERM...(Monday night through Sunday)
ISSUED AT 311 PM CDT Sun Aug 18 2019

Isolated to scattered convection over southern CWA early Mon
evening to diminish after sunset, with another lull expected into
mid Tue morning. A disturbance and possible MCS to affect area by
Tue afternoon and evening bringing next chance of thunderstorms.
SPC has much of area in a slight risk of severe storms Tue
afternoon/evening as very unstable airmass develops with CAPES
rising to 3000-4000 j/kg during Tue afternoon over southern CWA.
Highs in the upper 80s/lower 90s on Tue with afternoon heat
indices in the upper 90s to around 105F, highest in sw CWA where
heat advisory may eventually be needed Tue afternoon.

The main frontal boundary to push sse into nw CWA late Tue night
and thru rest of CWA during the day Wed. This will likely continue
chances of showers and thunderstorms on Wed especially central and
southern CWA. Highs Wed range from lower 80s by Galesburg, to the
upper 80s to near 90F in southeast IL where heat indices peak
around 100F in southeast IL Wed afternoon.

Frontal boundary pushes south of area during Wed night and Thu
while upper level trof over the Great Lakes brings in cooler and
less humid air into central IL. Still have 20-30% chance of
convection over areas south of I-74 Wed night into Thu as Ecmwf
model is keeping qpf over southern half of CWA thru Thu afternoon,
and not as far south with frontal boundary as the GEM and GFS
models which are dry over our CWA overnight Wed night and Thu.
Stayed close to model consensus for convection chances here but
confidence of convection chances is low during Wed night and Thu.
Highs Thu and Fri of 79 to 83 degrees, warmest in southeast IL.
Lows Thu and Fri nights in the upper 50s and lower 60s.

Generally dry conditions expected much of the time from Thu night
thru next Sunday, with weak high pressure over the Great Lakes.
But the subtropical ridge over the sw and southern states and
upper level trof moving into the eastern Great Lakes and New
England puts IL in a WNW upper level flow, and could see some
disturbances tracking ESE and possibly affecting the area, though
timing and placement of these are difficult to project that far
out. Consensus of models has slight chance of convection over much
of CWA on Fri afternoon, Sat and again Sunday afternoon. Highs in
the low to mid 80s next weekend and humidity levels gradually


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 645 PM CDT Sun Aug 18 2019

A broken band of convection was well se of the central IL airports
and will continue to pull away from central IL during this
evening. Clearing wedge pushed into areas from I-55 west early
this evening and will move into DEC and CMI during next hour or
two, but still VFR conditions prevailing there with mainly
scattered to broken mid/high clouds present early this evening. A
frontal boundary over southeast parts of WI/IA and far nw MO has
4-7k ft cloud decks in central and southern IA into far northern
IL with isolated showers in central IA. This front to move toward
the IL river valley late tonight and be over central IL much of
Monday. This should increase clouds during overnight with ceilings
of 3-6k ft possible and some fog to develop especially by CMI
which had half inch rain this afternoon. The HRRR and GFSlamp even
brings some patchy dense fog to east central IL around CMI by
sunrise, while RAP keeps dense fog patches just se of DEC and CMI
late tonight. Will need to watch for fog development and possibly
lower vsbys especially at CMI and maybe DEC too. Have VCSH at PIA
and BMI after 15Z/Mon and VCTS along I-72 after 19Z/Mon where
instability will become higher Monday afternoon. SW winds 5-10 kts
early this evening stay fairly light next 24 hours, and become
light and variable during tonight.




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