Area Forecast Discussion
Issued by NWS Central Illinois

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291
FXUS63 KILX 221752
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1252 PM CDT THU SEP 22 2016

.UPDATE...
Issued at 1100 AM CDT Thu Sep 22 2016

Only made minor tweaks to the forecast for sky cover and
temperatures in the northern part of the forecast area. High
clouds spreading east-southeast from the convection in Iowa should
mainly be thin, so the impacts to temperatures should be minimal
in central IL this afternoon. Another unseasonably warm day is in
store for the area with mid-afternoon highs ranging from 87-92
across the whole forecast area.

A couple of the short range models are trying to develop some
convection in central Illinois by early this evening ahead of an
outflow boundary projected to be in north central IL. Current
trends indicate this should stay far enough north of our area to
keep the rain out of the area...but will continue to keep an eye
on this through the afternoon.

&&

.SHORT TERM...(Today and tonight)
ISSUED AT 321 AM CDT Thu Sep 22 2016

Well above normal temperatures and generally quiet weather is
expected across central and southeast Illinois through the period.
Southerly low-level flow between an area of high pressure centered
over the Ohio Valley and low pressure centered over the central
Plains will persist, with flat ridging building in aloft. Given this
scenario, would expect temperatures similar to yesterday with highs
near 90 and lows in the 60s. While dry weather is expected to
persist, we`ll still have to keep an eye on the ongoing convection
over the upper Midwest. A similar setup yesterday ended up kicking a
couple outflow boundaries, with spotty showers/storms, into the
forecast area. Nearly all current guidance keeps any convective
remnants north of the area today, which is better agreement than was
seen yesterday.

.LONG TERM...(Friday through Wednesday)
ISSUED AT 321 AM CDT Thu Sep 22 2016

As an overview, the forecast remains warm through the weekend as a
ridge keeps control over our area.  An upper low moving into the
west coast is resulting in increasingly southwesterly flow aloft,
and this is expected to continue through the first half of the
weekend.  Unseasonably warm and dry should continue, with the series
of small waves bringing precip mainly north of ILX.

The issues with the forecast are centered on the low/trof as it
moves across the country.  The GFS and the ECMWF have both been
inconsistent in handling the system aloft at 500mb with respect to
phasing or splitting it as an open wave in the northern stream...and
a cutoff low in the SW/Southern Plains.  Each run is different in
handling the synoptic system, regardless of the sfc fields
apparent relative consistency. The few things that seem to remain
a trend is a slower approach overall, and have no problems staying
dry through the weekend. However, with the difficulty in the
modeling, the low chance pops from Monday into Tuesday is actually
a preferred way to handle the forecast for now. Cooler air
accompanying the trof will drop the max temps in the first part of
next weak to more seasonable expectations.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1251 PM CDT Thu Sep 22 2016

Mainly thin cirrus clouds were moving east-southeast into central
and eastern Illinois early this afternoon from the remnants of
early morning thunderstorms back in northern Iowa. At the surface
the main synoptic front extended from the central Plains across
central Iowa into southern WI, with an outflow boundary from
southwest WI into eastern IA. These boundaries are expected to
stay north of the region today so a southwest wind should prevail.
A little more mixing than previously anticipated will allow winds
to stay at or a bit above 10kts in central/eastern IL the rest of
the afternoon.

Weak wind fields and very few clouds this evening should lead to
rapid decoupling of the low levels of the atmosphere and pretty
strong inversion. The RAP and CONSShort point to light fog
developing toward daybreak, so included MVFR visibility for the
TAF sites along the I-74 corridor. A lower crossover temperature
at SPI and DEC should keep light fog from forming.

&&

.ILX Watches/Warnings/Advisories...
NONE.
&&

$$

UPDATE...Miller
SHORT TERM...Bak
LONG TERM...HJS
AVIATION...Miller



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