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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1237 PM CDT Thursday Oct 23 2014

303 am CDT

A ridge of high pressure stretches from central Quebec southwest
through lower Michigan into the Ohio Valley. Early this morning this
ridge has been able to keep cloud cover displaced west of the
region. Based on latest surface observation the pressure falls are not very
large...which could be an indicator that later today the ability for
the atmosphere to produce abundant moisture/precipitation may be
diminished. Infrared imagery early this morning shows a moist channel that
extends from the Canadian border of the upper Midwest...south to
the Mexico border/western Gulf. The progression to the east has been
very slow...and latest hi-res guidance indicates that as this
feature pushes east it will encounter much drier air and diminish
the coverage and intensity of precipitation.

With the light winds and minimal cloud cover early this morning...some
locations are beginning to see patchy fog develop...mainly across
Northwest Indiana. Temperatures have been able to radiate into the middle
30s...with the exception of the far western County warning forecast area where a thin
cirrus deck has arrived and held temperatures in the upper 30s/near 40.

As the moist channel slowly slides east today...expect a steady
thickening to the cloud cover. Winds will turn southerly with some
better mixing developing and perhaps bringing winds up to 10-20mph
by late afternoon. Guidance has become more pessimistic with precipitation
coverage/intensity later today...and as a result have backed off on
probability of precipitation along with quantity of precipitation. The near surface environment still
looks to remain relatively dry...although some moistening will
occur. Expect the best precipitation coverage to occur further
north...however will continue the mention of light rain across entire
forecast area late this afternoon/early evening. Trough axis will be sliding
east across east-central wisc by 3z. Temperatures today will warm into the
50s for much of the area...and south of I-80 a few 60s.

For tonight on the back side of the trough...and departing
precipitation/cloud cover...there is some concern that patchy fog will
develop. Upstream this has already been noted...and expect this
similar setup to occur over a portion of the County warning forecast area overnight tonight
but will hinge upon how quickly clouds will depart. Lows will
generally be in the middle/upper 40s.

Friday through Sunday...
a weak lobe of vorticity is prognosticated to slide east across the Central
Plains...arriving over the forecast area Friday aftn/eve. Local arw8km
guidance and NAM suggest some light precipitation may develop with this
system...but not expecting much to the coverage at this time. Have
opted to hold onto the dry forecast...but may need to adjust probability of precipitation
slightly to reflect a few sprinkles/light rain showers. Then by early
Sat morning anti-cyclonic flow builds into the region and dries the
atmosphere out. Surface ridge will remain in place through sun...then the
middle-level ridge overhead flattens and the flow becomes quasi-zonal for
late in the weekend into early next week.

Temperatures Friday-sun will be slowly moderating...with highs in the 60s to
perhaps a few low 70s Fri/Sat. A weak frontal boundary will slide
across the region sun and could bring temperatures back into the upper
50s/low 60s.

ensembles are showing the western Continental U.S. Trough pushing east by early
next week...helping to setup the potential for periodic chances for
showers and perhaps a few thunderstorms in the Monday/Tuesday timeframe.
Temperatures Monday should rebound with a southwest flow and push into the upper
60s/low 70s. Then increased cloud cover Tuesday/Wednesday will keep temperatures in
the upper 50s to low 60s.




//Ord and mdw concerns...updated 18z...

* SW winds gusting to around 17kt this afternoon.

* Scattered -shra this evening.

* MVFR ceilings expected after the showers...IFR possible.


//discussion...updated 18z...

An upper level wave is approaching from the west with a line of
showers extending into northwest Illinois. The band of showers will
continue east and expecting it to remain ragged with hit or miss
showers. Rfd has the best chance of seeing an organized shower
this afternoon. The eastern terminals will most likely see scattered
showers or drizzle this evening.

Upstream observation indicate that ceilings will fall to MVFR or lower behind
the showers. Have the most confidence in the rfd taf with IFR
ceilings. Have high confidence in fog at rfd and medium in 1/4sm.

Clouds should last longer over the eastern terminals so kept ceilings
in the MVFR range with fog only at dpa. Conditions will improve to
VFR quickly tomorrow as the clouds shift east. Light winds
overnight become SW between 5 and 8 knots.


//Ord and mdw confidence...updated 18z...

* medium confidence in coverage and occurrence of -shra...medium-
high confidence in timing.

* Medium confidence in MVFR ceilings and timing...low confidence in
how low the ceilings will be.

* High confidence in all other forecast elements.


//outlook for Ord/mdw for 00z Saturday-12z Thursday...updated 12z...

* Friday night...dry. MVFR/fog possible.
* Saturday...dry/VFR. Gusty west/northwest winds.
* Sunday...dry/VFR. West winds becoming southeast.
* Monday...dry/VFR. Gusty south winds.
* Tuesday...chance of rain showers. Strong/gusty westerly winds.
* Wednesday...chance of rain showers. West winds.



245 am...fairly active pattern is expected into early next week.
High pressure extending from Quebec southwest through the Central
Lakes region will split today...with the high weakening and moving
south into the Ohio Valley tonight. Low pressure over Manitoba
will move north today as a weak cold front approaches the western
lakes. The gradient will tighten some today ahead of this front...
but the front is expected to dissipate over the western lakes
tonight. Thus winds may shift westerly for a time but will quickly
go back southwest by Friday morning.

The low lifting north over Manitoba will be absorbed by a stronger
low moving across central Canada tonight and Friday. This low will
then move across Ontario reaching James Bay Saturday afternoon as
it deepens. Another cold front trailing this low will move across
Lake Michigan late Friday night into Saturday morning. Ahead of
this front...southerly winds will increase again. Behind the front
winds will shift northwest 20-25kts and may approach 30 kts for a
time Saturday afternoon into Saturday evening.

A weak ridge will move from the plains Saturday to the western
lakes Sunday...with winds quickly shifting from west Sunday
morning to south/southeast Sunday evening. The gradient will then
tighten with increasing southerly winds as a large area of low
pressure develops over the plains Sunday into Monday. Cms


Lot watches/warnings/advisories...


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