Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 180458

Area Forecast Discussion
National Weather Service Lincoln IL
1158 PM CDT Sat Mar 17 2018

Issued at 900 PM CDT Sat Mar 17 2018

Drier air is beginning to filter in across central and southeast
Illinois from the north in the wake of the system that passed
through the Midwest earlier today. While the arrival of this drier
air and departure of forcing with the system has ended the
precipitation risk, tonight`s forecast is still a little tricky.

Skies across the forecast area have started to clear from the
north, and this clearing should continue to expand southward
overnight. The ground surface is wet across most of the area due
to precipitation that occurred over the past 24 hours. This fact,
combined with light winds and clearing skies would seem to be a
good setup for fog later tonight. Most of the high resolution
guidance suggests light fog development later tonight, with
pockets of locally dense fog. Guidance generally suggests the
thickest fog will be over southwest portion of the forecast area,
which will also be one of the last areas to clear. Would generally
expect areas that clear sooner to have thicker fog since they have
longer for radiational cooling and associated saturation to occur.

Going forecast already had mention of patchy fog and see no reason
to deviate from this right now. Plan to update forecast for the
latest hourly trends (mainly sky cover). Can`t rule out the need
for stronger fog mention or possibly an advisory before morning,
but confidence is not high enough to adjust at this time.


.SHORT TERM...(This evening through Sunday)
ISSUED AT 303 PM CDT Sat Mar 17 2018
Ridging at the surface and aloft over the Plains this afternoon,
keeping a northerly component in the winds over the forecast area.
Cooler air filtering into the region, but the stubborn stratus
associated with the low passing through the Midwest into IN this
afternoon is slow to erode. HRRR slowly progresses this shield of
clouds to the south tonight, eventually moving to mostly clear for
Central IL by morning. Concerned that the dry air is a little
further to the north than prev addition to the fact
that there is not significant llvl flow to change the airmass
quickly. Lighter winds in the overnight combined with llvl
moisture still trapped in the low levels may slow the progression
of the low clouds to the southwest. There is currently a good 30
mile or so lag of the dry air behind the clearing line to the
north/northeast. With that in mind, have put patchy fog in the
forecast after midnight for where the clearing is anticipated.
Should the delay last longer, there is a potential for some of
that fog to become dense...but guidance winds stay up just enough
to drop confidence in that for now. Tomorrow, mild temps with
highs in the 50s. The swath of low clouds holding just to the
southwest with light easterly winds attempting to advect some of
that drier air in at the surface. How far south those clouds get
will greatly impact temperatures particularly SW of a line from
Macomb to Decatur.

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 303 PM CDT Sat Mar 17 2018
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 went with a
more southerly solution this run, with the GFS and ECMWF keeping
the associated precip well south of the NAM solution. Given the
erratic nature of the solutions out of the models at this point,
keeping the slight chc/chc pops across the northern tier for now.
Rain for Monday and Monday night is expected, before briefly
changing over to snow on its departure to the east on Tuesday.
Models are relatively consistent in suggesting that this wave will
shift us back to a busier and more progressive northwesterly flow
pattern aloft. Several waves suggested in this pattern, but the
GFS is pretty robust with the development of a surface ridge,
inhibiting precip development (Wed night in particular). Low
confidence dry forecast through the middle of the week once the
northwesterly flow kicks in. The next major storm is taking shape
going into next weekend, with precip starting on Fri.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1154 PM CDT Sat Mar 17 2018

Skies continue to clear across central Illinois late this evening.
The only remaining local TAF site with CIGS is KSPI & it should be
clearing there over the next few hours. Still expect patchy MVFR
or possibly IFR fog will develop later tonight across the entire
area. Once the fog burns off Sunday morning, VFR conditions should
prevail for the rest of the period. Light winds are expected
through the entire 06Z TAF valid time.




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