Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 200447

Area Forecast Discussion
1147 PM CDT Fri Sep 19 2014


Shortwave over the Dakotas this evening will arrive in the Midwest
Saturday morning. Ahead of this feature moist southwesterly flow
will bring precipitable water up to around 1.5 to 1.75 inches and
surface dewpoints well into the 60s. Lows overnight will be up
5-10 degrees from Friday morning as a result of the increased
moisture and steady surface winds developing. Current forecast
incorporates a few locations with patchy fog overnight in SE
Illinois where lighter winds are anticipated earlier in the
night, but will consider pulling this out based on later evening
trends. As the shortwave arrives Saturday morning, lift and low
level convergence along with increasing shear will cause a line of
thunderstorms to form and propagate into the central IL forecast
area through the day Saturday, with some potential for severe hail
and wind gusts. Current forecast appears on track with this and no
major updates anticipated this evening.


.SHORT TERM...(Tonight)

1030 mb high pressure over New England and ridging southwest into
the mid MS river valley, will drift off the New England coast by
this evening, but still ridge back into se IL tonight. This should
continue fair weather across central and southeast IL through
tonight. Followed similar MAV/MET lows tonight with lows overnight
ranging from the mid 50s over east central IL to around 60F over the
IL river valley where better return southerly flow sets up. A short
wave over eastern MT and ND will track east into the upper MS river
valley by daybreak Sat. This will drive a cold from from the eastern
Dakotas se into NW IA by early Saturday morning. Chances of showers
and thunderstorms with this front should stay northwest of Galesburg
through sunrise Saturday.


.LONG TERM...(Saturday through Friday)

Main concern this period will be with the threat for severe weather
Saturday afternoon and evening as a cold front approaches the area.

Shortwave currently over eastern Montana forecast to dig southeast
as a 80 kt speed max at 500 mb tracks across the Northern Plains
on Saturday. Fairly significant 500 mb height falls forecast across
the lower Great Lakes late Saturday in response to the digging jet
with correspondent pressure falls noted to the southeast of the cold
front. Persistent south to southwest surface flow will help bring
dew points (Precipitable Water values of 1.5-1.75 inches) well
into the 60s just ahead of the cold front Saturday afternoon with
Mixed Layer Capes of around 2500 J/KG over west central through
north central Illinois. 0-6km shear values off the 12z NAM-WRF
were higher than what was forecast at this time yesterday over
the northern third of the forecast area with values of 35-45 kts
now forecast. Storms that do develop across the area will tap
into the shear and instability with forecast soundings depicting a
mostly unidirectional shear profile, suggesting more of a threat
from multicellular storms that translate into a line by late
afternoon. Initially, some of the storms may be supercellular,
especially with the deep layer shear now forecast across our north.
In addition, if temperatures warm into the low and mid 80s by
afternoon, surface base Cape values of 2500-3000 J/Kg indicated on
the NAM may be realized with the threat for large hail and
damaging wind gusts. The storms will then track quickly to the
southeast during the evening with an overall weakening trend
expected as we lose daytime heating and the stronger 500 mb
pressure falls track to our north over the lower Great Lakes.

The threat for showers will linger into southeast Illinois Sunday
morning before the amplifying trof across the lower Great Lakes
is able to shunt the cold front southeast of our forecast area by
late morning. After that, the remainder of the forecast period
will be quiet, as a surface high slowly drifts from the Midwest
into New England. An upper low that will be affecting areas along
the California coast for a couple days is forecast to open up and
eject northeast early next week into the Plains. The large area
of high pressure that will be shifting off to our east by the
middle of next week will have a lot of dry air associated with it.
As the weakening upper trof tracks across the Missouri Valley, it
appears the bulk of the precipitation associated with it will
remain to our west and north with the eastern edge of the precip
shield drying up as it encounters the dry and stable atmosphere
over the Mississippi Valley. As a result, the remainder of the
forecast will remain dry with temperatures, after a cool start early
in the period, returning to near or slightly above normal levels by
the end of next week.


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

VFR conditions will prevail across the central IL TAF sites until
around 15Z with sct-bkn mid/high cloud cover above 10 kft AGL.
Winds S7-10 kts through nighttime hours. From around 15Z-24Z a
line of thunderstorms is expected to spread across central IL from
the NW. As the line approaches...SW winds will increase to
12-16G20-25 kts. Exact timing of thunderstorms remains somewhat
uncertain but have included VCTS in TAFS to account for general
expected timing and have included 4-hour TEMPO groups for most
likely reductions in vsby/cigs with thunderstorms.




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