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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
538 am CST Thursday Dec 18 2014

Short term...
445 am CST

Through Friday night...

Apologies for the late afd this morning due to equipment problems.

Surface ridging is in place over the area and as has been the case
with recent highs this one remains littered with clouds. Confluent
upper flow atop the area today will help to stifle the northward
progression of an upper wave centered near Kansas City early this
morning. A high to middle cloud deck will still spread over the area
bringing mostly cloudy conditions to some locations that are
fortunate enough to have scattered low clouds early this morning.
Light snow should remain to the south of the forecast area where
700-600mb frontogenesis and deeper moisture are depicted...however
light radar echoes will likely traverse southern counties for a
good part of the day. With cloud cover and a light wind regime
limiting mixing...and already one of the shortest daylight amounts
of the year...another small diurnal range is on tap. Highs should
maximum out mainly in the upper 20s.

Tonight through Friday remains challenging with cloud cover as the
low-level flow will remain light. Have leaned more toward cloudy
than scattering given the recent bullishness of clouds and null
advection. As early this morning showed across the
region...places that clear during the night with drop well into
the teens in this cool air mass. Have leaned a little closer to
that tonight but again overall confidence on cloud and temperature
specifics is low.

Mtf

&&

Long term...
441 am CST

Saturday through Wednesday...

The weekend continues to look quiet and seasonal with attention
more on next week. A strong North Pacific jet will move into the
northwest U.S. Early next week and dive into the plains states by
Tuesday...amplifying the pattern. As such...an upper trough is
forecast to take shape across the northern U.S and then likely
into a closed low across the upper Midwest or Great Lakes region
by around Tuesday or so. Despite obvious differences in specifics
and placement on long range model guidance...there is solid
consistency in a deepening trend of the upper low through Tuesday
and phasing beyond that with a low developing up the eastern
Seaboard midweek. With rapidly strengthening baroclinicty in the
models...they have consistently forecast a deep low to develop.
Four consecutive runs of the GFS and ec have shown 975mb or deeper
lows by 12z on Christmas morning across the Great Lakes/southern
Ontario/northeast U.S. Region. Both of these model ensembles also
are depicting good consistency in that deepening trend Tuesday
through Wednesday. How exactly we get to that deep low forecast on
Christmas morning...and its notable impacts over our region...if
any...still are highly uncertain.

For the forecast have increased probability of precipitation during the Monday night period
in particular...as guidance shows good overlap /even accounting
for a slowing trend/ in precipitable water and moisture flux anomalies and
overspreading forcing. The forcing may be enough to keep
temperatures down too. Have opted not to get fancy and kept
rain/snow for a good part of the area at first and trend to rain
in the likely strong southerly flow. Into Tuesday and Tuesday
night we trend back to snow in the cold advection part of the
system. Have followed a slower trend in the details closer to the
18.00 ec as often with a closed/closing wound up system in the
extended portion of the forecast it can end up slower. Certainly a
system to keep tabs on...especially for Holiday travelers.

Mtf

&&

Aviation...

//Ord and mdw concerns...updated 12z...

* MVFR ceilings this morning and again tonight. Cms

//discussion...updated 12z...

Edge of MVFR ceilings extends from just south of rfd to dpa to between
Ord and mdw. Confidence regarding how quickly or if these will
continue south is low and they could persist through much of the
day but for now have continued with clearing at dpa/mdw/gyy this
morning. High/middle clouds will continue to spread northeast across
the area today as another deck of MVFR ceilings over central WI
slowly moves south. Forecast soundings do show this cloud cover
spreading back across the terminals later this evening and
overnight and added low clouds back into the forecast...but
confidence here is low as low clouds could remain just to north
and northeast of the terminals tonight. Cms

//Ord and mdw confidence...updated 12z...

* low for ceilings through the period. Cms

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

* Friday night...dry. MVFR possible.
* Saturday and Sunday...dry/VFR. Light south winds.
* Monday...chance of rain/snow. MVFR likely. Gusty southerly winds.
* Tuesday...chance of rain/snow. IFR likely. Southwest winds.
* Wednesday...chance of snow. MVFR likely. Strong/gust west winds.

&&

Marine...

209 am...a large area of high pressure extending from Ontario
south to Illinois will very slowly move east across the Great
Lakes today and Friday...then across the eastern lakes and into
Quebec Saturday and Sunday. Northwesterly winds will gradually
diminish today...with a 36-48 hour period of light winds...5-10
kts...expected as the high moves east. Winds will shift southerly
Saturday on the back side of the high but still remain light. The
southerly winds will begin to increase Sunday as the gradient
begins to tighten ahead of developing low pressure in the plains.
This low will track northeast toward the western lakes region by
early/middle next week...but still quite a bit of uncertainty
regarding its track/strengthen/timing. Cms

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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