Area Forecast Discussion
Issued by NWS Central Illinois

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KILX 020451

Area Forecast Discussion
1151 PM CDT Mon Sep 1 2014


Still a somewhat tricky forecast for the overnight hours. Spotty
convection, and it has been diminishing, persists to the north of
the forecast area along the cold front. Meanwhile, convection
continues to expand across Missouri and Kansas, and is starting to
spread into southwest Illinois, along an old outflow boundary.
Moisture is pooling along the outflow boundary, and precipitable
water values along this axis are near two inches. Main concern
through the night is how significant the local rainfall threat
will be, mainly with the rains near the outflow boundary.

Think the current flash flood watch in our southeast counties
looks good. While some convection is beginning to spread into our
southwest counties, the heavier rainfall is still well back to the
southwest. The storms with the heavier rainfall also seem to be
developing a bit of a cold pool with a more east to southeast
trajectory developing as opposed to the northeast trajectory near
our forecast area. This further south track of the heavier
rainfall over the next several hours is also supported by the
latest run of the HRRR. So, do not need to make significant
changes to forecast overall. Most tweaks will be to try and fine
tune the latest PoP trends.


.SHORT TERM...(This evening through Tuesday)

18z/1pm surface analysis shows outflow boundary from late night
convection across Iowa extending from near Kankakee to just west of
Taylorville.  Airmass behind the boundary has stabilized greatly,
with the special 18z KILX upper air sounding showing less than
200J/kg of CAPE and a strong cap at around 850mb.  Meanwhile ahead
of the boundary, LAPS soundings indicate CAPES of 2000-2500J/kg and
little to no capping.  Will be watching the E/SE CWA for potential
development over the next couple of hours, but window of opportunity
will be small and any convection that fires will quickly push into

Next concern will be the heavy rain threat tonight.  As has been
advertised by the last several runs, latest models continue to show
showers/thunderstorms becoming widespread along the trailing boundary
tonight as 40-45kt LLJ develops from the Southern Plains
northeastward into the Ohio River Valley.  Still some questions as
to exactly where the axis of heaviest rain will set up, with the NAM
appearing to be a bit too far north.  GFS/HRRR seem to have a better
handle on the rain given current scenario.  Think scattered
showers/storms will develop across the southern half of the CWA
early this evening, then as LLJ jet energy increases, areal coverage
will expand toward midnight.  With storms becoming parallel to upper
flow and likely training over the same locations, am concerned about
flash flood potential across the E/SE overnight.  Precipitable water
values ramp up to over 2 inches, while upper jet strengthens over
the Great Lakes providing enhanced synoptic lift.  Based on GFS/HRRR
QPF fields, have decided to issue a Flash Flood Watch for all
locations along/south of a Paris to Shelbyville line.  Rainfall
rates may exceed 2 inches per hour at times, leading to a flash
flooding risk.  Have gone with categorical PoPs across the Watch area,
tapering down to just slight chance across the far north around
Galesburg. Have lingered PoPs across the E/SE into Tuesday morning:
however, all model guidance suggests frontal boundary will push
south of the area and bring an end to the rain chances by midday.


.LONG TERM...(Tuesday night through Monday)

The cold front should clear our southern counties by 7 pm Tues,
leaving slightly drier air in its wake. That should provide a break
in rain chances Tuesday night into early Wed morning. However,
during the day Wed, the front is expected to return north as a warm
front in Missouri and W IL. Sept 01/12z guidance has trended more
aggressive with a shortwave on Wed moving across N Missouri into W
IL, especially the Canadian GEM, and to a lesser extent the NAM/GFS.
The 850mb LLJ will be mainly in MO, so as the wave moves east across
IL it will be losing some support from moisture transport. We still
expanded the chance PoPs farther northeast into IL, to include
areas southeast of a line from Havana-Decatur-Lawrenceville, with
slight chances up to Peoria-Champaign.

The storms chances evolve to areas NW of the IL river for Wed night
as the instability axis, thermal ridge, and nose of the 850mb LLJ
advance toward NW IL. The warm front will respond by advancing north
of our counties for Thursday, bringing a brief period of heat and
humidity for Thur and Friday. Highs should climb into the upper 80s
and lower 90s as dewpoints reach up into the low to mid 70s. Peak
heat indices may reach into the upper 90s depending cloud cover.

Low pressure moving along the Canadian border will drag a cold front
into Illinois on Friday. Storm chances will increase NW of the IL
river later Thurs night, with widespread chance PoPs for Friday and
Friday night in the vicinity of the cold front. A few storms may
even affect areas southeast of I-72 into Saturday morning.

A significant airmass change will follow that frontal passage, as a
1024mb Canadian high builds southward. Highs Sunday and Monday will
remain in the low to mid 70s, with lows bottoming out Sunday night
in the mid 50s.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)

A band of showers and thunderstorms will continue to brush past
KSPI, KDEC, and KCMI for the next several hours, but KPIA and KBMI
should be largely dry. Conditions will likely degrade to MVFR as the
rains continue and/or as fog develops in the case of KPIA/KBMI.
Quieter weather and VFR conditions will prevail for most of the
daytime hours Tuesday and into the evening as drier air filters
into the area behind a cold front.


FLASH FLOOD WATCH until 7 AM CDT Tuesday FOR ILZ056-057-061>063-



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