Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
900 am CDT Wednesday Apr 23 2014

402 am CDT

Main forecast concerns this morning are with relatively low chance
probability of precipitation for showers/sprinkles late this afternoon/tonight...then better
potential for rain and a few thunderstorms Thursday/Thursday
evening with approaching surface low pressure wave Pacific cold
front. Temperatures look mild for Friday behind Pacific front...but
cooler/potentially wet weather still appears to dominate extended
periods of forecast from the weekend into early next week.

In the near term...surface high pressure ridge was over Wisconsin
and Illinois early this morning...providing cool/dry and mainly
clear conditions with light north-northeast winds. A few spots
dipping to/below the freezing mark...especially across the Fox and
Rock River valley regions this morning. Frost advisory remains in
effect through 7 am CDT for most of far northern/northeast Illinois and
northwest ind. Looking ahead...early morning GOES imagery already
shows high clouds increasing across the plains in region of warm
advection in advance of western Continental U.S. Upper trough. Short-term
models remain consistent in bringing a weak middle-level short wave
from the Central Plains into our region through this evening...with
middle/upper level moist advection and convergence/frontogenetic
forcing along an elevated warm frontal zone expected to produce some
high based light showers/sprinkles into north central Illinois by later
this afternoon...and across much of the north/northwest County Warning Area tonight
as forcing continues ahead of a second weak short wave. Have
generally maintained highest probability of precipitation northwest of I-88 corridor across
Illinois although a few showers/sprinkles could occur just about anywhere
tonight. Precipitation potential increases significantly Thursday as main
upper trough approaches from the west...and low level winds shift
more southerly as a surface low pressure wave lifts from the
middle-Missouri Valley northeast into Wisconsin by evening. Narrow
moist axis...combined with increasing deep forcing with approach of
surface cold front and large scale ascent ahead of upper trough
should be sufficient to produce fairly decent coverage of
rain/showers and a few thunderstorms especially by afternoon.
Overall trend of guidance has been to slow advance of cold front response to slower progression of closed low along the
Canadian border. Thus have held onto higher probability of precipitation into Thursday
evening especially across the eastern County Warning cold front completes
its transit.

Temperature wise...east-northeast winds today continue our cooler than
normal weather particularly along Lake Michigan. Have generally gone
with/slightly above warmer mav numbers west/southwest well away from
the lake...though shore locations will likely not get out of the 40s
again today. Cool conditions continue tonight across far northeast
Illinois where continued easterly component and later arrival of thicker
cloud cover will allow mins to dip into the middle-upper 30s. Gradual
veering of low level wind to southeast and slow increase in low
level moisture from the south will help keep far southern County Warning Area above
40. Milder southeast flow Thursday will be offset by clouds/precipitation
and a continued southeast wind component through the morning...
though should see 60s all but immediate North Shore and likely close
to 70 south of I-80. Breezy/mild behind Pacific cold front Thursday
night into Friday...with perhaps the warmest temperatures of this forecast
period with good mixing and west winds. Low level thermal forecasts
support guidance highs from middle-upper 60s north to the low 70s

Models have shown some variation/adjustment with handling of large
scale flow pattern and resulting synoptic details from Friday night
Onward. Overall...slower Canadian border short wave passes north of
the region Friday...and trails a second cold front south into the
area by early evening. Some potential for showers along the front
Friday night...though guidance inconsistent with timing/placement of
precipitation at this time and feel slight chance probability of precipitation sufficient at this
time. Cool/dry high pressure builds across Ontario and the lakes
into Saturday as upper trough continues off to the east. Highly
amplified trough moves across western Continental U.S....with closed low
developing and eventually moving out across the plains
Sunday-Tuesday. Gulf moisture is eventually drawn north across low
level baroclinic zone associated with west-east stationary front to
our south...with increasing potential for rain from the southwest
during the period. Cool/dry east-northeast flow from the retreating
high to our northeast should slow things a bit...and keep probability of precipitation
graduated from lower northeast to higher southwest. Model
differences in details/timing of evolution of upper and surface lows
late in the period diminish confidence in finer details at this




//Ord and mdw concerns...updated 14z...

* northeast winds with gusts to around 15 knots today.

* Scattered light showers possible late tonight.


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

With high pressure building overhead today...expect quiet weather
to persist. Although...the terminals remain on the eastern fringe
of this high with some stronger winds still in place. Northeast
winds will continue with gusts around 15 knots likely within the next
couple of hours. These gusts will likely be observed through middle
day...with a diminishing trend anticipated this afternoon. Winds
will turn more east southeast tonight into Thursday while
increasing as this high exits to the east and a trough of low
pressure approaches from the west. VFR ceilings will develop this
evening and overnight...with showers developing more towards late
tonight. Although...expect these showers to remain rather light
with no visible restriction


//Ord and mdw confidence...updated 14z...

* high confidence with wind speed/direction.

* High confidence with VFR conditions.


//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...

Thursday night...shra/MVFR likely with a chance of thunderstorms and rain.

Friday...slight chance of showers late...otherwise VFR.

Saturday...slight chance of showers late...otherwise VFR.

Sunday...chance of showers...otherwise VFR.

Monday...chance of showers...otherwise VFR.

Tuesday...chance of showers...otherwise VFR.



310 am CDT

As the lake remains on the eastern fringe of building high
pressure from the west...northwest to north winds of 10 to 20 knots
remain. Although...this high will move over the lake today with
speeds diminishing and with winds likely becoming more light and
variable by the afternoon. Then as this high continues east
tonight into Thursday and a trough of low pressure over the
central and northern plains moves east towards the western Great
Lakes...expect east to southeast winds to increase. Southeast
winds will further increase on Thursday before winds turn to the
southwest Thursday night into Friday as this low progresses
through the region.



Lot watches/warnings/advisories...


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