Area Forecast Discussion
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FXUS63 KILX 232048

Area Forecast Discussion
248 PM CST Fri Jan 23 2015

.SHORT TERM...(Tonight)

Clearing line from the morning stratocumulus deck was unable to make
any further eastward progress, so the clouds decided to dissolve in
place instead. 2 pm visible satellite imagery showing most areas
west of I-55 now mostly sunny, with the clouds rapidly eroding on
the western flank of what`s left. The clear skies won`t last too
long though, as mid-level cloud deck ahead of the next frontal
boundary is already in southeast Iowa. The boundary itself is still
back across eastern Nebraska through the Minneapolis area, and will
be moving through our forecast area between midnight and sunrise.
However, no precipitation is expected with it. Lows across the
forecast area should fall into the upper 20s.


.LONG TERM...(Saturday through Friday)

By and large, the forecast period will be dominated by a ridge
west/trof east pattern across North America, a pattern that has been
pretty common of late. The fast moving northwest upper-level flow
that has been over the forecast area has had numerous, mainly weak,
impulses embedded within it traveling over or in our vicinity. Most
of these waves lately, and those expected over the next several
days, have been too weak and/or moisture starved to produce much
precipitation locally. This trend should continue to be the case for
most of the days of this forecast. However, a more significant
clipper system still appears on track to impact the area late
Saturday night into Sunday night. This system is our primary
forecast concern today.

A weak upper/surface trof will move across the area later
tonight/early Saturday, but it is only expected to be accompanied by
an increase in cloud cover. This cloud cover is currently evident
upstream from the Great Lakes into the central Plains.

The timing of the main clipper is pretty well agreed upon, and has
been for a few days now, with the primary precipitation risk
occurring late Saturday night/early Sunday Morning into Sunday
afternoon. However, the track of the system continues to exhibit
considerable model spread, as well as run-to-run variability. The
overall model trend had been toward a more northerly track until
about 24 hours ago, but now the trend is back toward the south. The
northern track would have resulted in minimal precipitation risk for
the forecast area, with the bulk of it falling in the form of rain.
The more southern track, most pronounced in the latest run of the
GFS, would support more widespread precipitation locally, with at
least the northern portion of the forecast area in line for some
accumulating snow. Forecast confidence in the details with this
system remains low considering the variability/inconsistency.
However, given the trends of the past 24 hours, have boosted PoPs
into the Likely category for much of the forecast area on Sunday,
and added some light snow accumulation across the north. If the
models lock into a more southern solution, one that takes the
clipper just south of the forecast area like the current GFS,
temperatures will need to be lowered, PoPs raised, and snow amounts
boosted. For now, will be conservative with the changes until the
ultimate outcome is a little more clear.

For the remainder of the period, mostly quiet weather is expected. A
weak wave is most likely to impact the area Monday and again by
Thursday. However, these disturbances appear too weak to carry
mentionable precipitation at this time. Temperatures through the end
of the next work week should be at or above normal. The coolest
readings are likely Monday/Tuesday behind the main clipper, and
again Friday behind Thursday`s disturbance.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)

Main concern in the short term will be with the timing of the
lifting of MVFR ceilings today. Clearing trend is making painfully
slow progress eastward. Have adjusted the timing of the clouds
scattering out based on the RAP model humidity level around 2000
feet, which would bring it to KPIA/KSPI around 23Z and to KCMI
around 01Z or so. This is a faster than a straight timing of the
cloud deck`s movement, as there are some thinning spots starting
to show up west of KPIA hinting at some potential afternoon
erosion, as suggested by the HRRR model. Any clearing will not
last terribly long, as mid and high clouds are already approaching
ahead of a frontal boundary that will be moving through the TAF
sites in the 10-14Z time frame on Saturday. While the front will
be dry, MVFR ceilings are likely to overspread central Illinois
Saturday morning following passage of the front.




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