Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 011749

Area Forecast Discussion
1249 PM CDT Mon Sep 1 2014


Line of strong to severe thunderstorms tracked across Iowa last
night, then rapidly dissipated as it pushed toward the
Mississippi River early this morning. Outflow boundary from this
dying convection is currently approaching the I-55 corridor
as evidenced by westerly winds across all sites in the Illinois
River Valley. Think this outflow will eventually serve as the
primary focus for additional thunderstorm development later today.
Still some question as to exactly where the boundary will lay out
and when convection will fire, but the latest thinking from SPC
has pushed the best severe weather risk much further southeast,
generally along/south of a Danville to Jacksonville line. HRRR is
performing very poorly this morning and its solution has been
completely disregarded. Meanwhile, the 12z NAM seems to have at
least a reasonable handle on the situation, showing storms
developing by early afternoon near I-55, then shifting further
east and south later in the day. Will continue to monitor
stability parameters to see if projected 1500 to 2500J/kg CAPEs
can be realized. If the airmass destabilizes as predicted,
increasing shear values will help storms organize and potentially
produce large hail and damaging winds this afternoon into this
evening. Am still concerned about the threat for heavy rainfall
across the E/SE CWA tonight as favorable jet dynamics produce
enhanced synoptic lift along/ahead of approaching cold front. As
storms become parallel to upper flow, training appears likely
tonight along/south of a Danville to Jacksonville line. Latest NAM
continues to put down some hefty rainfall totals across this area.


.SHORT TERM...(Today and tonight)

Shortwave trough axis approaching Illinois from the west this
morning bringing a line of thunderstorms to the region. Trends this
morning are for the line splitting into to intense
grazing Knox, Peoria, Stark county and likely Marshall and Woodford
as it continues this morning. Have reports of pea-sized hail and
wind gusts 30-40 mph with the more intense storm cells. The intense
activity looks like it will have a break through central IL in the
early morning once this initial activity shifts east-northeastward.
Isolated thunderstorms will likely develop throughout the CWA during
the morning...however daytime heating will likely be necessary to
bring CAPE values up to 1500-2500 J/KG range to bring a better
chance for severe storms when combined with 30-50 kt bulk shear
values also shifting into central Illinois. Large hail and severe
wind gusts will be a threat with the stronger storms this
afternoon...and have continued mention of severe hail and heavy rain
in forecast grids beginning at 3 pm this afternoon...continuing
through around midnight. Heavy rain will continue through the night
as a potent and moist low-level jet continues to push through the
CWA...along with lift associated with the progressive trough and
cold frontal boundary.


.LONG TERM...(Tuesday through Sunday)

00Z models have trended slower with frontal boundary pushing into
southeast IL Tue especially the NAM model and have increase pops to
likely se of I-70 Tue morning. NAM model may be overdoing qpf
amounts Tue morning over southeast IL but models have trended higher
with qpf amounts with precipitable water values of 1.5-2 inches
from 12-15Z Tue and have increased qpf amounts to around a quarter
inch in southeast IL. IL river valley should be dry Tue and chances
of showers and thunderstorms will diminish from nw to se during
Tue afternoon. Highs 82-88F Tue and still rather humid especially
in southeast IL with dewpoints 70-75F while dewpoints lower into
mid 60s over IL river valley. Dry conditions return across area
Tue night with frontal boundary dropping south of area. Lows in
the low to mid 60s Tue night.

Models show frontal boundary starting to lift back north especially
over MS river valley and MO on Wed and have kept 20-30% chance of
showers and thunderstorms sw areas by Wed afternoon with ne areas
staying dry Wed. Warm highs in the mid to upper 80s Wed and model
guidance has trended upward in high temps Wed. Dewpoints elevate
during day Wed into upper 60s and lower 70s Wed afternoon.

Convection chances appear highest over northern IL Wed night closer
to short waves tracking into WI/northern IL and eventually eastward
into lower MI and northern IN. Have areas south of I-72 dry Wed

Thu and Fri still appear to be back into the heat and humidity with
highs in the upper 80s and lower 90s and dewpoints peaking in the
low to mid 70s giving afternoon heat indices of upper 90s to lower 100s
especially Thu afternoon and in southeast IL Friday afternoon. A 593
dm 500 mb high pressure ridge over the southeast states builds the
heat into central/southeast IL late this work week. Most of our area
should be dry Thu and Thu night with just slight chance of
convection Thu afternoon mainly over the Wabash river valley and
nw of IL river Thu night. Models have trended faster with cold
front dropping se through central IL Friday and thru southeast IL
Fri evening. Have increased pops Friday and also trending cooler
with temps and dewpoints during the weekend as 1025 mb high
pressure settles into the Great Lakes region.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)

Convective development/evolution remains in question, as previous
outflow boundary has pushed east of the I-55 corridor early this
afternoon. Most model solutions are keeping central Illinois
largely dry through the afternoon, with thunderstorms
developing/increasing after dark across the south. Based on 12z
NAM and latest HRRR, it appears the axis of heavy rain will be
focused along the I-70 corridor between 05z and 12z. As such, have
kept both KPIA and KBMI dry through the entire 18z TAF period.
Further south, have introduced -RA with VCTS at the remaining
terminals after 05/06z, thinking they may be on the northern
fringe of the low-level jet enhanced precip area overnight.




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