Area Forecast Discussion
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FXUS63 KILX 160730

Area Forecast Discussion
230 AM CDT Tue Sep 16 2014

.SHORT TERM...(Today)

Quiet weather expected overall today across central and southeast
Illinois. Canadian high pressure and its associated dry air mass will
continue to build into the area today, helping to scour out the low
clouds that currently reside across much of the forecast area. The
low clouds have been clearing slowly from the north as the high
builds in, and the latest trends suggest the clearing should reach
the I-70 corridor by 10 a.m. this morning. Once the skies clear,
mostly sunny skies should be the rule, as forecast soundings and CU
rule suggest diurnal cloud development should be minimal. The
possible exception to the clearing sticking, at least until sunrise,
is the potential for fog/stratus development. This development has
been patchy thus far upstream, and do not expect it to become too
widespread. Temperatures today, as they have been more often than
not lately, will be well below normal with highs mainly in the
mid-upper 60s.


.LONG TERM...(Tonight through Monday)

High pressure building southeast into the Great Lakes to dominate
our weather through Thursday. Latest model guidance continues to
indicate some sort of MCS type activity across Missouri and Kansas
Wednesday into Thursday, sliding along the edge of the upper ridge
that will slowly inch eastward into the Plains. However, the
persistent east/northeast flow over our area should keep this
precipitation at bay.

Significant shortwave will dig as it crosses the Rockies on Friday,
settling into the Mississippi Valley by Sunday morning. Have largely
kept the forecast dry through about midday Saturday, then quickly
increased the PoP`s from west to east during the afternoon and
evening hours, as the front arrives. One significant item of concern
is with the potential for heavy rain. The remnants of tropical
cyclone Odile over Baja California will be spreading into the Desert
Southwest over the next day or so. The latest GFS model continues to
show that a piece of energy and associated moisture from this system
will be drawn northeast into the northern wave, reaching central
Illinois after midnight Saturday night with the potential for a few
inches of rain. The ECMWF and Canadian models are not quite as
generous on this point, but do draw in some of the moisture into the
wave by Saturday morning and produce weekend rainfalls of up to 1.5
inches. Have increased PoP`s Saturday night to around 60% across
most of the forecast area, as confidence increases in the general
rainfall trend over the area. Continued the PoP`s on Sunday east of
I-55, although this should be winding down during the afternoon as
the front exits the area.

A large broad upper trough will persist over the Midwest early next
week in the wake of this system. Latest ECMWF model has
significantly backed down on the amount of cool air with this
system, with 850 mb temperatures on Monday now around +5C rather
than the -2C from the previous run. The GFS lags behind the ECMWF
with bringing the cooler conditions into the area. As a result, have
kept highs in the 70s on Sunday and Monday, with the coolest
conditions just beyond the current forecast period.


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

Precip has departed well to the southeast of all terminal sites,
with clearing of the clouds expected to reach PIA by 05z and
progressing to CMI/DEC around 0730z. Still expect some fog
formation after the clearing develops, with a few areas developing
VLIFR ceilings and fog. A bit of a tough call on timing and
coverage of any dense fog, but for now BMI and CMI were the only
two locations with LIFR/VLIFR conditions between 11z-13z. The
latest HRRR keeps low vis east of CMI, with lingering low clouds
after midnight at all sites by PIA. It is possible that a stratus
layer forms instead of ground fog, but will lean toward fog
formation with this set of TAFs.

Skies should clear out quickly Tues morning with very dry air
above the shallow layer of moisture. Clear skies will continue
through the remainder of the TAF period to 06z/16.

Winds have become N behind the departing cold front, with a brief
increase in wind speeds to 10-12kt. Light winds should prevail the
remainder of the night and into Tuesday, as high pressure keeps
the pressure gradient weak.




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