Area Forecast Discussion
Issued by NWS Central Illinois

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123
FXUS63 KILX 281759
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1259 PM CDT Fri Apr 28 2017

.UPDATE...
Issued at 1020 AM CDT Fri Apr 28 2017

Updated the forecast today to address chances of showers along
with thunderstorms chances appearing along and south of I-72
during this afternoon. A quasi-stationary front was just south of
a Pontiac to Peoria to Macomb line, while a warm front was south
of IL from central TN and southern AR. Aloft a 546 dm 500 mb low
was developing over ne Utah and sw WY. Latest radar mosaic shows
mainly light rain showers along and south of I-72 while another
smaller band of light rain showers was along and north of a
Monmouth to Galesburg to Henry line and lifting ne. More
widespread and heavier showers with some thunderstorms were
upstream over central and northern MO and sw IA.

Latest hi-resolution models like the NAM, HRRR and RAP show warm
lift lifting northward into southern IL by late afternoon/early
evening with rain showers spreading ne over rest of central IL
into early afternoon with thunderstorms possible this afternoon in
southeast IL. SPC day1 outlook continues slight risk of severe
thunderstorms late this afternoon and evening along and southeast
of a Paris to Shelbyville line, while enhanced risk of severe
storms southeast of a Robinson to Flora line where 30% risk of
very large hail of 2 inch diameter or larger. Southeast IL also
has 5-15% risk of damaging wind gusts, while 2% risk of tornadoes
in southeast Lawrence county. Temps currently range from mid 40s
by Galesburg and Lacon, to the mid 60s along highway 50 from Flora
to Lawrenceville. Highs to contrast from the upper 50s by
Galesburg and Lacon, to around 70F along highway 50, with warmer
mid 70s-lower 80s south of CWA, along and south of I-64 which
gets into the warm sector later today.

&&

.SHORT TERM...(Today and tonight)
ISSUED AT 415 AM CDT Fri Apr 28 2017

Initial batch of rain showers is pushing eastward across Missouri
early this morning, as a frontal boundary starts to set up over
the mid- Mississippi Valley. Most impact of this batch will be
across the northwest half of the forecast area this morning, with
a secondary area of showers tracking northeast toward the I-70
corridor early in the afternoon. This southeastern area will see
the beginnings of the frontal boundary which will plague the area
this weekend. May see a few strong to severe thunderstorms this
afternoon and evening in that area, although latest runs of the
high-resolution models suggest the line of storms may form a
little closer to the Ohio River early this evening. However, this
line should start to lift northward after midnight, and have
included categorical PoP`s around 80% from about the I-70 corridor
southward. Northern part of the CWA may actually remain dry for a
good part of the night until the front lifts northward.

.LONG TERM...(Saturday through Thursday)
ISSUED AT 415 AM CDT Fri Apr 28 2017

The seeds of a strong upper low are starting to take shape over
Colorado early this morning, and will strengthen over the
4-corners region tonight. Copious moisture advection will take
place off the Gulf into this frontal zone, with an extended period
of precipitable water values of 1.5-2 inches through the weekend.
The front will slowly lift northward Saturday as the low moves to
the Texas panhandle, with a formidable surface cyclone forming
Saturday night and Sunday as the low ejects northeast into the
Plains. The front will make more northward progress on Sunday as
the low lifts northeast, with a cold front sweeping eastward
Sunday evening.

Have made no changes to the flood watch at the present time,
although recent model trends suggest an expansion at least to the
Illinois River may need to be considered soon. Rainfall totals of
4 to 6 inches are possible between I-55 and I-57, with 4 inches as
far northwest as the Illinois River. The heaviest rain is likely
Saturday afternoon and night, as a 50-60 knot low level jet sets
up around 850 mb to pump in the moisture. Once the cold front
sweeps through, only scattered showers are expected through Monday
evening, due to the wraparound from the exiting low.

The front is also going to cause a significant headache with
temperatures. A very large temperature range is expected over the
CWA on Saturday, ranging from near 50 degrees around Galesburg to
the lower 80s near Lawrenceville. Potential exists for major
temperature busts if the front does not set up near the I-70
corridor as expected. More of the CWA will be in the warm sector
on Sunday, with 70s more likely as far northwest as the Illinois
River. The deep cold air advection behind the low will result in
highs only in the 50s on Monday, but temperatures should moderate
back into the 60s for the remainder of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1259 PM CDT Fri Apr 28 2017

A large area of light to moderate rain showers had spread ne
across central IL by midday. MVFR ceilings with IFR to MVFR vsbys
are occuring at PIA and models show the lowest ceilings this
afternoon from 500-1500 ft at along and nw of the IL river at PIA.
HRRR and RAP models show the showers diminish from sw to ne late
this afternoon and early evening with a lull expected much of this
evening and into early overnight. Then a 2nd wave of showers and
isolated thunderstorms to spread ne over central IL later tonight
into Sat morning. Carried VCTS along I-72 at SPI, DEC and CMI late
tonight and Sat morning while keeping just showers at PIA and BMI.
One frontal boundary has lifted back just nw of PIA early this
afternoon with ENE winds shifting SE. SE winds 7-14 kts and gusts
of 15-20 kts early this afternoon will shift NE during this
evening and diminish some before increasing again later tonight
into Sat morning. A warm front will lift northward into southern
IL later this afternoon toward I-64, then lift up toward I-70 by
early Sat afternoon.



&&

.HYDROLOGY...
Issued at 415 AM CDT Fri Apr 28 2017

A large area of 3 to 6 inches of rain is expected this weekend
over central and southeast Illinois, with the heaviest totals
between the I-55 and I-57 corridors. Contingency river forecasts
using 3 to 4 days of QPF suggest flooding would likely develop
Sunday or Monday on the Embarras, Sangamon, and Illinois Rivers,
and a bit earlier on the Little Wabash River where Wednesday`s
rainfall totals were heavier and still causing rises. Some of the
model trends have been shifting the heaviest rain axis a bit
further northwest, which could cause a bigger impact on the
Illinois River. People with interests on area rivers need to
monitor the forecasts closely over the next couple days.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 7 PM CDT this evening through Monday
morning for ILZ038-042>046-048-051>057-061>063-066>068-071>073.

&&

$$

UPDATE...07
SHORT TERM...Geelhart
LONG TERM...Geelhart
AVIATION...07
HYDROLOGY...Geelhart



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