Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1229 am CDT Tuesday Sep 16 2014
221 PM CDT
frontal boundary this afternoon continues to slide southeast across
the forecast area. Visible imagery along and just ahead of the
boundary indicated some vertical growth to the clouds...suggestive
of some instability. There has been some spotty convection across
northeast MO stretching east through Illinois/central in but south of the
County warning forecast area. Despite considerable solar shielding has been able to keep
temperatures in the 50s...with a few locations being able to nudge to around
60. Winds will shift northwest/north behind the boundary with much
drier/cooler air steadily filtering into the region overnight.
Precipitation will come to an end later this afternoon/early evening...with just a
few lingering showers along and east of Gibson City to Fowler. Temperatures
overnight will cool into the low/middle 40s...with a few of the
traditionally cool spots possibly dipping into the upper 30s before
daybreak Tuesday. Meanwhile closer to downtown Chicago...the urban heat
island should aid in keeping temperatures in the upper 40s to around 50. The only
wildcard for overnight lows will be sky cover/winds...which with
high pressure building in should erode/diminish.
There is also some concern that with the lack mixing this afternoon and
expected temperatures being cooler than afternoon dew points...coupled with light
winds...could point towards patchy fog developing away from the
Tuesday through Thursday...
rather benign pattern for a good portion of the week. Broad
anti-cyclonic flow will build into the region Tuesday...with the
middle-level trough pivoting east. The longwave pattern will likely keep
the region in a continued northwest flow as middle-level ridging returns
to the western Continental U.S.. guidance suggests the surface ridge that arrives
Tuesday will shift east...however another dome of high pressure from
Canada is then poised to slide south and continue the dry/cool
weather Wed/thur. Temperatures will generally remain in the middle/upper
60s...but possibly reach 70 degree in several areas for Thursday. Abundant
sunshine is expected each of the next several days as well.
Overnight lows will also dip into the 40s Tuesday night/Wednesday night...then
with the surface ridge beginning to push east Thursday night temperatures could hold
Friday through Sunday...
with the surface ridge poised to push east of the region Friday...flow will
be turning southerly and the middle-level wave starting to flatten.
Ensembles continue to show considerable spread on timing of
precipitation/moisture returning...with most members pointing towards Friday
night. The atmosphere will moderate by Friday/Sat with temperatures in the
middle/upper 70s...or more seasonal. Sat appears to be the most favorable
period to see spotty showers/thunderstorms as a middle-level trough
develops over the Central Plains and nears the region Sat aftn/eve.
Then for the second half of the weekend surface ridging is prognosticated to
return to the region.
//Ord and mdw concerns...updated 06z...
* fog potential early Tuesday.
* Lake breeze potential Tuesday afternoon.
High pressure will build to the south of the terminals tonight
keeping winds very light and allowing conditions to remain
generally VFR outside of some fog this morning. Patchy fog has
already developed in some areas and will most likely impact
rfd/dpa with confidence too low at Ord/mdw/gyy at this time to
include any mention in the taf. Winds will remain light generally
4-7 knots through the day Tuesday out of the northwest though there
may be some variability in direction due to the light magnitude.
Will also have to monitor a lake breeze Monday afternoon. For now
expect a lake breeze to develop but stay near the shore.
//Ord and mdw confidence...updated 06z...
* low confidence in fog impacting terminals.
* Low confidence that a lake breeze will push far inland.
* High confidence in VFR conditions and winds less than 10 knots
//outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z...
Wednesday...VFR. Light west winds. Lake breeze possible.
Thursday...VFR. East winds.
Friday...VFR. Breezy south winds.
Saturday...chance thunderstorms and rain. MVFR possible. Breezy southwest winds.
Sunday...slight chance thunderstorms and rain. VFR. West winds.
246 PM CDT
Weak low pressure will push east through the Ohio Valley and
dissipate this evening as high pressure moves into the middle
Missouri Valley tonight. Ahead of the high a weak cold front will
spread down the remainder of the lake this evening and briefly
increase north winds to near 20 knots the high will weaken and move
over Lake Michigan on Tuesday. Low pressure over Hudson Bay will
spread into Quebec on Wednesday as high pressure pushes south out
of northern Manitoba. Ahead of the high the southwest gradient
increases with winds 15 to 25 knots for the north half of the lake. The
high reaches Lake Superior Thursday evening and will push a cold
front down the lake Thursday night. This high will then move to
the eastern lakes by late Thursday into Friday as several lows
organize in the Central Plains and Canadian prairies.
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