Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 260541
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1241 AM CDT Wed Apr 26 2017

.UPDATE...
Issued at 845 PM CDT Tue Apr 25 2017

Latest surface map indicates a frontal boundary over east central
Iowa with several waves of low pressure riding along it. The
better moisture and instability this evening is located over parts
of southwest Missouri south through Oklahoma where strong convection
was ongoing this evening. The 00z ILX sounding continues to show
quite a bit of dry air in place across central Illinois with surface
dew points as of 8 pm in the upper 40s to lower 50s. As the evening
wears on persistent low and mid level warm advection and moisture
transport coupled with shortwave energy pushing out of the Southern
Plains will lead to an increasing chance for showers and thunderstorms
over extreme west central Illinois late tonight. Last several runs
of the HRRR model indicates the first wave of showers to affect mainly
our far western counties around Midnight, with a second cluster of
storms forecast to track northeast out of Missouri towards dawn
Wednesday.

Looks like an active weather day over central Illinois tomorrow
as the front slowly edges its way east during the day. The
combination of strong deep layered shear and decent instability
may bring about strong to locally severe storms, especially tomorrow
afternoon and evening. Precipitable water values of well over an
inch will bring about the additional threat for heavy rainfall to
central Illinois where 1 to 2 inches of rain is not out of the
question. The storms will tend to track from southwest to northeast
along the slow moving frontal boundary enhancing the threat for
heavy rainfall, especially tomorrow evening.

No major changes were made to the grids this evening with respect
to timing of the showers into the west towards Midnight. Southerly
winds and cloud cover will keep it rather warm overnight with most
areas not dropping below 60 degrees which is reflected in the current
ZFP. As a result, no evening update will be needed.

&&

.SHORT TERM...(This evening through Wednesday)
ISSUED AT 303 PM CDT Tue Apr 25 2017

Gusty south winds will continue into this evening, as a cold front
approaches from the west. Low level moisture will be on a steady
increase tonight, with the cold front expected to reach near the
Mississippi River by 12z/7am Wednesday. Scattered showers and a
few thunderstorms will be possible late tonight, mainly west of
the Illinois river, but possible as far east as the I-55 corridor.

Better chances of rain will begin Wednesday afternoon as the cold
front pushes to near I-55. Instability and wind shear parameters
look favorable for some severe storms to develop late Wed
afternoon, with damaging winds the primary threat, and large hail
also possible.

.LONG TERM...(Wednesday night through Tuesday)
ISSUED AT 303 PM CDT Tue Apr 25 2017

The extended forecast will start out Wednesday evening with a
squall line moving from SW to NE across central Illinois, roughly
along the I-72 corridor. The NAM is even hinting at multiple
squalls training over the same areas, which would ramp up the
potential for flash flooding. Instability appears to drop off by
03z, so severe threat should be mainly from late Wed afternoon to
mid evening.

The cold front will push into Indiana by sunrise on Thursday, but
showers are expected to linger Thursday morning. Dry conditions
should return for Thursday afternoon as cooler air arrives on NW
winds. Highs will be limited to the upper 50s and low 60s, or
about 15 to 20 degrees cooler than Wednesday`s highs.

Attention then turns to an extended period of wet weather from
Friday through Sunday night. The first wave or rain looks to
arrive Friday morning and continue through the day. There are some
model differences as to the location of the warm front as a
shortwave moves across southern Illinois. The better chances of
heavy rainfall appears to set up after midnight Friday night and
continue through Saturday night. The system will be driven by a
deepening upper level trough in the Plains on Friday, with a
surface low eventually ejecting into far NW Illinois by Sunday
afternoon. Copious amounts of moisture are projected to surge into
Illinois ahead of the system as it occludes and slowly drifts
northeast toward the western Great Lakes this weekend. While the
potential for severe storms in Illinois will be marginal, several
periods of heavy rainfall could push rainfall totals between 2 to
4 inches in much of central and southeast Illinois. The highest
totals look to be along and south of I-70, but model adjustments
over the next few days should provide better clarity on the axis
of heavy rainfall.

We continued the Hydrologic Outlook for the potential for heavy
rainfall and flooding with the combination of locally heavy rains
Wednesday/Wed night in addition to the heavy rains Friday through
Saturday night. While showers continue on Sunday under the
occluding low, the potential for heavy rains should diminish by
Sunday morning as dry air wraps into the upper low, leaving a more
shallow saturated layer.

Dry conditions should close out the extended forecast for Monday
and Tuesday. Monday will be noticeably cooler, with highs similar
to Thursday, in the upper 50s to low 60s. A warming trend will
develop for Tuesday, as increasing sunshine helps boost high temps
into the upper 60s to around 70.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1240 AM CDT Wed Apr 26 2017

Line of showers/storms taking shape across Missouri and now
brushing the Iowa/Illinois border at 0530Z. Concern initially will
be with how far east this makes it, as the line will be running
into some dry air this far east. In the short term, only have VCTS
mention at KPIA/KSPI toward 11Z and have kept the thunder mention
out further east through the morning. Second surge of showers and
thunderstorms will move in from the southwest after 18Z. By 00Z,
main threat of thunder will be east of a KBMI-KDEC line and will
likely be east of the TAF sites by 03Z.

Ceilings are high so far, but initial surge of showers will bring
MVFR conditions to KBMI/KPIA after 12Z. Ceilings dropping
elsewhere into MVFR range by midday or early afternoon, and likely
will be near IFR levels by mid-late afternoon as the storms
arrive. These lower ceilings will linger into Thursday evening.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...Smith
SHORT TERM...Shimon
LONG TERM...Shimon
AVIATION...Geelhart



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