Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 170207

Area Forecast Discussion
National Weather Service Lincoln IL
907 PM CDT Fri Mar 16 2018

Issued at 858 PM CDT Fri Mar 16 2018

Strong, but vertically stacked, system is slowly tracking east
from the central Plains. Fairly strong elevated WAA/isentropic
lift to the east of the system is helping to produce a band of
showers & isolated thunderstorms across central and southeast
Illinois this evening. Dry low-level air mass has been slow to
erode today, aided by dry easterly low-level flow to the north of
the approaching system. Top-down moistening is gradually taking
place, allowing some heavier rainfall to develop. However, the dry
layer is also producing enough evaporative cooling in localized
spots to produce sleet. The steadier band of rainfall will move
out of the area by shortly after midnight, although another band
is likely to spread southeast down the vicinity of the I-74
corridor as the system moves through. This precipitation should
also fall mostly in the form of rain, but a few hours of near
freezing temperatures across the far northern portion of the
forecast area late tonight/early Saturday could produce a light
glaze of freezing rain. However, the icing would most likely
impact elevated surfaces due to the milder soil/surface
temperatures that developed over the past few days.

Have adjusted forecast for the latest trends, primarily to fine-
tune PoP trends.


.SHORT TERM...(This evening through Saturday)
ISSUED AT 309 PM CDT Fri Mar 16 2018
Tonight, the developing low to the SW edges closer to the region.
The precip so far has been eroded by a couple of dry layers aloft
per the 12z ILX soundings. Showers will likely continue to spread
into the evening hours across Central IL. Cloud cover associated
with the approaching precip will limit the diurnal swing of temps
to the overnight lows. Even so, the temperatures after midnight, mainly
north of a line from Galesburg to Danville, will drop to near or
just below freezing. HRRR shows a break in the first round of
precip for a few hours after midnight, then develops a secondary
line of showers moving in around dawn. The chances for wintry mix
are mostly confined to the extreme northern portions of Stark,
Marshall, Woodford, Mclean, and Vermilion after midnight...with
another chance for some closer to the 0600-0800L time frame. Early
morning traveling/commute should use caution along and north of
these areas. As to precip type, the warm nose above the 3000-4000
ft in the sounding will melt any crystals introduced above.
However, the NAM has a deeper layer of freezing temps below that
to about 1000 ft, leaning towards sleet. GFS is a bit warmer...and
both models are right at the freezing mark at the surface. A
troublesome forecast...but cannot rule out the threat in the
northern portions of ILX. Most of Central IL will see a cold rain,
with some risk for thunder, mainly in the south and southeast.
Tomorrow, showers will likely continue through the day until the
surface low moves out of the region.

.LONG TERM...(Saturday night through Friday)
ISSUED AT 309 PM CDT Fri Mar 16 2018
Weak ridging moves back into place through the remainder of the
weekend. Temps Saturday will drop into the 40s...but rebound to
the 50s on Sunday with light SE winds. Overnight Sunday into
Monday...lows drop only to the low to mid 40s as increasing clouds
once again inhibit cooling in the overnight hours. Next system
for Monday/Monday night is still a bit problematic for track, with
the GFS now trending north of ECMWF which is holding to a more
southerly track. Rain for Monday and Monday night is expected,
before changing over to snow on its departure to the east on
Tuesday. Northerly extent of the precip will be the question mark
with the track for the most part. Models suggesting that this
wave will shift us back to a busier and more progressive
northwesterly flow pattern aloft. The next storm is taking shape
going into next weekend, with precip starting on Fri.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 652 PM CDT Fri Mar 16 2018

A band of showers will rotate northeast across the central
Illinois terminals this evening and possibly into the early
overnight hours. Then, after a lull in the precipitation threat,
additional showers may impact mainly KPIA, KBMI, and KCMI late
tonight/Saturday morning. There is a chance some of this second
round of rainfall may fall with freezing temperatures for a few
hours, posing a light icing risk.

A dry air mass has limited the rainfall reaching the ground so
far, and kept CIGS/VSBYS high. However, expect the lower levels to
eventually saturate over the next few hours with rain becoming
somewhat heavier and low-end MVFR or possibly IFR conditions
developing into Saturday.

Easterly winds will prevail for much of the night. However, they
will trend more northeast as the system producing the
precipitation shifts off to the east.




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