Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 171724

Area Forecast Discussion
National Weather Service Lincoln IL
1124 AM CST Tue Jan 17 2017

Issued at 919 AM CST Tue Jan 17 2017

Bulk of the dense fog has lifted over the northern CWA this
morning. Some lower visibilities hanging on around Bloomington,
but visibility there has recently risen to 1 mile and should
continue to trend upward. Low clouds aren`t going anywhere anytime
soon though, with a pretty widespread shield of stratus extending
as far west as eastern Kansas.

The highs for the day have already occurred, and temperatures the
rest of the day should fluctuate only slightly. Updates to the
hourly temperature trends primarily impacted the southeast CWA,
lowering them several degrees from the lower 50s into the mid-
upper 40s.


.SHORT TERM...(Today and tonight)
ISSUED AT 329 AM CST Tue Jan 17 2017

08z/2am surface analysis shows 1008mb low near Chicago, with cold
front extending southward along the I-57 corridor.  A few showers
are ongoing ahead of the front near the Indiana border: however,
most of the precipitation has come to an end across central
Illinois.  The widespread dense fog from last evening has also
mostly dissipated, except for locations along/north of a Canton to
Bloomington line where visibilities are 1 mile or less.  HRRR has
been consistently showing dense fog over eastern Iowa spilling E/SE
along the I-74 corridor this morning: however, based on prevailing
W/SW boundary layer flow, think its forecast takes the fog a bit too
far south.  Based on current upstream obs over northern Missouri,
have decided to trim the southern row of counties from the Dense Fog
Advisory to only include Knox, Stark, Marshall, Peoria, and
Woodford.  Still some question as to how quickly the fog will
dissipate later this morning, with the Rapid Refresh showing it
clearing the area as soon as 15z, while the HRRR lingers it until
closer to 18z.  Given lack of strong dry advection in the wake of
the departing low, have opted to go more pessimistic and have
therefore lingered areas of fog across the north through midday.
Once the fog dissipates, overcast conditions will prevail for the
balance of the day and through tonight as well.  Highs today will
range from the upper 30s around the lower 50s near
the Indiana border.  Lows tonight will generally be in the lower to
middle 30s.

.LONG TERM...(Wednesday through Monday)
ISSUED AT 329 AM CST Tue Jan 17 2017

A weak area of high pressure will build into the Midwest on
Wednesday, leading to a gradual clearing trend.  NAM forecast
soundings show a shallow layer of moisture remaining trapped beneath
a strengthening subsidence inversion, which would lead to continued
cloudy skies through Wednesday.  The GFS on the other hand shows the
moisture mixing out and skies clearing.  Think the NAM may be
suffering from its usual low-level moisture bias, so have decided to
go with skies becoming mostly sunny on Wednesday with highs in the
lower to middle 40s.

NAM/GFS/ECMWF are all in relatively good agreement with the next
system approaching from the southwest on Thursday.  Long-wave trough
currently over northwest Mexico will lift northeastward into the
Plains on Thursday, with a slug of deep-layer moisture pushing
northward ahead of the system into the Ohio River Valley.  As
forcing increases upon approach of the wave, showers will become
likely from Thursday afternoon through Friday morning.  Rainfall
amounts will range from around one tenth of an inch near as much as three quarters of an inch in the Wabash
River Valley.

Once the wave lifts into the Great Lakes, warm and dry weather will
be on tap for Friday and Saturday.  Thanks to building upper heights
and strong southerly flow, temperatures will climb well into the 50s
by Saturday.  In fact, both the GFS and ECMWF numeric guidance
suggests temps could potentially climb into the 60s on Saturday.
After that, another strong wave will approach from the southwest
early next week...bringing a round of rain from Sunday into Monday.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1124 AM CST Tue Jan 17 2017

Main issue much of the forecast period will be persistence of
IFR/LIFR type ceilings. Large area of 500 foot ceilings covers
much of central Illinois at midday, although some MVFR conditions
not too far south of the KSPI-KCMI corridor. Forecast soundings
showing some potential for ceilings here to lift just above 1,000
feet at times later this afternoon, but with the uncertainty, will
keep the ceilings on the lower end for now. Visibilities have
lifted in most places after the earlier fog, but some potential
for it dropping back down into the 1-3 mile range late tonight.
Some more wholesale improvement in ceilings is possible late in
the period.




LONG TERM...Barnes
AVIATION...Geelhart is the U.S. government's official web portal to all federal, state and local government web resources and services.