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 021621

Area Forecast Discussion
1121 AM CDT Thu Oct 2 2014


Have decided to issue a Flash Flood Watch for portions of the
southwest CWA which were hit harder with the rains last night.
This would include areas from Rushville southeast to Jacksonville,
then eastward to Taylorville. Observed totals were heaviest in
Christian County with 2-4 inches common, with similar totals
along the Illinois River. Moderate risk of flash flooding is
indicated in this area, with another 2 to 3 inches possible. Watch
will be out shortly.


Short break in the rain occurring over a large part of the
forecast area this morning, however a large area of showers and
thunderstorms was beginning to spread into the far western CWA.
Latest HRRR guidance has this overspreading many areas west of
I-55 into early afternoon, but the model has not been handling the
narrow axis along the I-70 corridor very well. Storms expected to
increase across the eastern CWA during the afternoon. Main cold
front is still well off to the west, and should start to move into
the western CWA during the evening hours. The rain trends should
rapidly decrease behind the front after its passage overnight.

Main question for this afternoon/evening will be with severe
weather potential. Right now only areas east of I-55 are seeing
scattered breaks in the clouds. MUCAPE values around 500 J/kg or
so across the south half of the forecast area, per SPC
mesoanalysis. RAP soundings from Springfield do bring CAPE`s up to
around 1500 J/kg during the late afternoon as the precipitation
becomes more concentrated west of the I-55 corridor. Rather moist
soundings do favor heavy rain, with precipitable water around 1.8
inches which is near the 99th percentile for early October. Any
severe weather that occurs will be more of a wind threat in our
area due to the moist profile.


.SHORT TERM...(Today)

06z/1am surface analysis shows warm front extending from central
Missouri eastward into the Ohio River Valley.  Several clusters of
convection have been tracking along/north of the front overnight,
aided by a 30-35kt 850mb jet streak oriented from the Southern
Plains northeastward across the boundary.  This activity will
continue for the next few hours, but should begin to wane by
mid-morning as front lifts further northward.  Based on current
radar trends and 00z model data, will carry likely PoPs
along/northwest of I-70 through midday, with only chance PoPs
further south.  Meanwhile, low pressure developing over Oklahoma
will track northeastward into southern Wisconsin by this evening.
Line of thunderstorms is expected to develop along/ahead of
associated cold front, with most model solutions keeping this
convection just west of the KILX CWA until late afternoon/early
evening.  Have increased PoPs to categorical across the Illinois
River Valley this afternoon in case storms get going faster than
anticipated, but think bulk of activity will hold off until
evening.  Aside from the rain chances today, it will be another
very warm day with highs ranging from the middle 70s far northwest
around Galesburg to the middle 80s along/south of I-70.


.LONG TERM...(Tonight through Wednesday)

Latest Day 1 convective outlook from SPC places all of
central/southeast Illinois under a slight risk for severe weather
tonight.  With a very warm/humid air mass surging northward ahead of
an approaching cold front, resulting CAPE values will likely
exceed 1500J/kg later today.  In addition, low-level wind shear
will be increasing with 0-6km bulk shear values generally in the
30-40kt range.  These favorable parameters combined with strong
convergence along the cold front will lead to the development of a
line of thunderstorms this evening.  Models generally develop the
storms along the Mississippi River by late afternoon, then push
them eastward to the I-55 corridor by mid-evening and into
east-central Illinois shortly after midnight.  Main severe weather
threat will be strong/damaging winds, although highest
probabilities for widespread severe will likely remain further
southwest from the St. Louis area southwestward to Texas.  With
precipitable water values climbing over 1.75, high rainfall rates
will be likely with any storms that develop.  Localized flash
flooding may develop within the strongest storms: however, recent
rainfall amounts have not been great enough over a wide enough
area to warrant a Flash Flood Watch at this time.

Front will push into Indiana by Friday morning, bringing an end to
the greatest rain chances.  Still some model discrepancy concerning
timing of frontal departure, with the ECMWF being the slowest.  Have
therefore hung on to chance PoPs early Friday morning along/east of
I-57.  A secondary upper wave currently evident on water vapor
imagery over Alberta will dive southeastward Friday
afternoon/evening, amplifying the mean trough over the Great
Lakes/Midwest.  Synoptic lift and steep mid-level lapse rates
associated with this feature will be enough to warrant a slight
chance for showers, mainly during the afternoon.  Main weather story
on Friday will be the windy and sharply cooler conditions.
Tightening pressure gradient between departing front and approaching
Canadian high pressure will lead to strong W/NW winds gusting to
between 30 and 35mph.  Temperatures will struggle to rise much at
all from morning lows, with readings remaining in the 60s.

With 850mb temps progged to drop into the -3 to -5C range, low
temperatures by Saturday morning will fall into the middle
30s...representing a nearly 50 degree temp drop from expected high
temps today! Due to a continued brisk northwesterly breeze, am
not expecting any frost to develop Friday night.  Heart of cold
air mass will be in place across Illinois on Saturday, with highs
struggling to reach the middle to upper 50s and overnight lows
once again dropping into the middle to upper 30s.  With much
lighter winds expected, a few patches of frost will be possible by
Sunday morning, especially if skies can remain clear ahead of the
next approaching short-wave trough.

After that, the Midwest will be dominated by northwesterly
upper-level flow through Tuesday before trough retreats into Canada
by the middle and end of next week.  This will mean continued below
normal temps in the 60s through Tuesday, followed by a warming trend
and a return to the 70s by Wednesday and Thursday.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)

The timing of thunderstorms over the next 24 hours will be the
main concern, and the associated IFR/LIFR conditions during
storms. Based on current radar trends, there should be a break in
the showers and storms at all terminals this morning. SPI will
see the return of some storms the soonest as a band of storms
moves out of NE Missouri and reaches SPI as soon as 16z. That
convection could reach to DEC as well, and have indicated that in
the TAFS. PIA, BMI and CMI could remain dry for a longer period,
possibly well into the afternoon or early evening. CMI looks to
be closer to the current storm track, so may see a few showers
pass close by to the south during the day.

The main threat for strong to severe storms looks to be this
evening for PIA, SPI and BMI. A 3-hour tempo for heavy rain and
storms with IFR conditions was included in the TAFs for the
current best-estimate on timing of the line of storms. We
estimated 00z for PIA and 04z for CMI, but those hours may need
adjustment as the cold frontal timing becomes more apparent.

After the cold front passes, winds will begin to shift to the SW
then W, with some gusts possible by morning. Also, MVFR clouds are
likely to persist through 12z/7am Friday.


FLASH FLOOD WATCH through late tonight FOR ILZ040-047-049>052.



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