Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 300210
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
910 PM CDT Fri Aug 29 2014

.UPDATE...
ISSUED AT 900 PM CDT FRI AUG 29 2014

Weakening line of convection continues to push across west central
Illinois this evening. Not too much precipitation has filled in
between this line/weak trof and main cold front, which extends
from upper Midwest into central Plains. Expect this should
continue to be the case as the best low level jet/upper dynamics
will remain closer to the front. So, plan to adjust PoPs downward
a bit for the rest of the night (away from and behind current
line). Remainder of forecast looks pretty good.

&&

.SHORT TERM...(This evening through Saturday)
ISSUED AT 302 PM CDT FRI AUG 29 2014

19z/2pm surface analysis shows warm front extending from southern
Minnesota to southern Indiana, generally along the I-74 corridor
across central Illinois.  Widely scattered showers and thunderstorms
have developed along the front and will continue to do so for the
balance of the afternoon.  Meanwhile further upstream, a trough of
low pressure is evident across central Missouri where thunderstorms
are becoming more numerous.  As this trough shifts northeastward,
storm chances will begin to increase across southwest Illinois early
this evening, then across the entire KILX CWA by mid to late
evening.  May see a gradual decrease in areal coverage as daytime
instability is lost, so will keep PoPs in the 50-60 range tonight.
As has been the case for the past several days, any storms that
occur will be capable of frequent lightning and very high rainfall
rates of 1.5 to 2 inches per hour.

The trough axis will lift northeast of the area late tonight into
Saturday morning, with only scattered showers/storms lingering early
in the day.  As a cold front currently across the Plains pushes
eastward and interacts with the very moist airmass in place across
the region, a renewed round of convection will be likely Saturday
afternoon/evening.  Will carry high chance PoPs in the Illinois
River Valley and likely further east across the remainder of the CWA
accordingly.

&&

.LONG TERM...(Saturday night through Friday)
ISSUED AT 302 PM CDT FRI AUG 29 2014

In the wake of Saturdays precip, the boundary at the sfc slows and
becomes a bit more diffuse overall and models are starting to speed
up the clearing as the boundary slips to the southeast into the Ohio
River Valley.  Though precip chances linger in southern and east
central Illinois on Saturday night, Sunday may be a break in the
widespread chances for rain.  Next wave slowing in the 12z runs of
the ECMWF and the NAM...though the wave aloft is slowing, the GFS
qpf is slower to respond.  Postponing the pops resulting in a dry
Sunday evening for the eastern half of the state with pops
increasing in the overnight hours along and west of Interstate 55,
with precip spreading and dominating much of the day on Monday.
Given the high PW values in the atmosphere, heavy rains will remain
a threat for the forecast pd.  Delaying the onset of precip from
Sunday night into Monday also resulting in a prolonging into Monday
night. Question for the models beyond Monday night really revolving
around the boundary stalling in quasi zonal flow aloft not really
supporting any major progression of the sfc boundary and yet another
pd of being just on the edge of the unsettled weather with a signif
lack of 500mb height gradient over the southern half of the CONUS.
Whereas the GFS is pushing a series of MCSs on the edge of the weak
flow, the ECMWF is far more conservative...though both models are
tempering the amplification significantly over the latter half of
the work week.  Thermal ridging in the south keeps above normal
temps over the region.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
ISSUED AT 654 PM CDT FRI AUG 29 2014

A weakening band of showers and thunderstorms occurring along a
weak surface trof is pushing into the central Illinois terminal
area from the west. This weakening trend is expected to persist
with the loss of diurnal heating. That being said, a brief period
of precipitation is likely this evening at the terminals. Have
included a two hour TEMPO group to cover the arrival of this
convective line or its remnants. Then, the best chance of
showers/storms this TAF period is expected from the midday hours
Saturday, continuing through the afternoon. This precipitation
will occur along a slow moving cold front that is currently
resides in the Plains. However, given the uncertainty in explicit
timing and/or coverage along/ahead of the front, have only
included a VCTS mention for now. While predominantly VFR conditions
are expected through the period, MVFR or lower is briefly possible
with thunderstorms.


&&

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

$$

UPDATE...BAK
SHORT TERM...BARNES
LONG TERM...HJS
AVIATION...BAK






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