Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Chicago/Romeoville Illinois
731 PM CST Tuesday Nov 25 2014
729 PM CST
The forecast has been updated to include much cooler minimum
temperatures over north central and northeast Illinois...as well
as adjust sky cover to meet trends of current low clouds and
incoming high clouds.
Early this evening the area is under an overall long wave trough
but in-between any defined circulations. A robust 190 knots jet at
250mb observed at iln and car at 00z is roaring northeastward to
our east and downward motion on the left entrance side of this
looks to be supporting a strong subsidence inversion over the
forecast area. Beneath this some stratocu is somewhat stuck so to
say...mainly along and south of I-80 in our area. Cloud-bearing
winds are light westerly and will turn southwesterly overnight as
The Heart of the low-level ridge shifts to our south. Envision
clouds continuing to creep eastward before northeastward. This
should keep evening temperatures steady in areas from Valparaiso
Outside of this area an ideal radiational cooling setup is in
place with light/calm winds...a clear sky...and fresh snow cover.
Already a handful of sites in north central Illinois are at lower
teens and now have forecast minimums at some of the cooler
outlying spots dropping into the single digits...maybe even as low
as around 5...late this evening. A light southerly wind at the
surface should set in overnight along with high clouds spreading
in...so temperatures will likely rebound in these areas.
See no changes to make to wednesdays low snow chance forecast
with regard to the evolving clipper system over the Dakotas. Would
be surprised if 00z guidance has much change
eastward/northeastward with this system based on the somewhat weak
850-925mb thermal gradient currently in place on the 00z radiosonde observations
over the Missouri River/mid-Mississippi valleys. The tightening of
this thermal gradient where greater isentropic ascent will be
looks to be well to southwest...with more of graduated ascent and
possibly light snow/flurries extending into the forecast area
during the evening.
231 PM CST
Tonight through Wednesday...
temperature inversion has been able to hold onto the clouds...with
a stubborn stratus layer. Fortunately this wedge of clouds was
slowly sliding east this afternoon...with some clearing taking
place across western Illinois and stretching further west. Surface ridge was
pushing east into southern MO/northern Arkansas...which placed the
forecast area in the westerly flow at the surface. There is some
potential that patchy fog may develop tonight...given the slowly
eroding cloud cover and dew points not able to dry this afternoon. This
currently is a low confidence scenario...but felt it was Worth
mentioning. With some clearing likely to take place in the western
County warning forecast area...temperatures should likely be able to radiate into the low teens
there. Elsewhere overnight lows will hover around 18-22.
Surface ridge for Wednesday will stretch from the western Gulf north into
lower Michigan...and likely be slow to depart the region. This has
been impacting the placement/moisture within a weak shortwave diving
southeast across the upper Midwest for Wednesday. Guidance has continued
to demonstrate less favorable setup for light snow and also timing for
later Wednesday. Hi-res local solution even suggests flurries late Wednesday
afternoon may be the extent of any precipitation. With at least some
high/mid-lvl clouds...temperatures may again be a challenge. Current
solutions point towards highs warming into the low/middle 30s...but
could see temperatures struggling to warm beyond the low 30s.
With the middle-level trough axis pivoting east across the forecast area
Wednesday evening and some weak Omega into the moist layer...some light snow
could produce a dusting Wednesday evening but the confidence with this
scenario continues to diminish. A frontal boundary will then slide
east across the region Wednesday night...with another ridge of high pressure
slowly building in from the upper Midwest.
231 PM CST
Thanksgiving through Tuesday...
500mb trough will be departing the region for Thanksgiving...with
broad surface ridge building in from the west during the day. This should
provide a gradual erosion to the cloud cover. Temperatures will hover in
the middle/upper 20s...but if clouds can erode earlier then temperatures may be
able to warm a degree or two higher.
Middle-level flow begins to transition into a semi-zonal setup...with the
potential for another low pressure system bringing a chance for light
snow Friday. Ensembles continue to suggest that Sat could warm well
into the 40s...perhaps touching 50 degrees in the southern County warning forecast area. For
the second half of the weekend guidance differs on timing of a
frontal boundary. If a slower solutions occurs...sun could also be a
mild day with highs in the middle/upper 40s.
Guidance has been suggesting a narrow baroclinic zone may develop
along the Ohio Valley early next week...which could keep some light
precipitation in the southeast forecast area.
//Ord and mdw concerns...updated 00z...
* brief period of MVFR ceilings 1600-2000 feet possible early this
Weak surface high pressure drifting east across Illinois and
western Indiana this evening. West winds 5-10 knots early will
diminish to 5 knots or less overnight...gradually backing southwest
this evening and eventually to southeast early Wednesday in
response to an area of low pressure which will move from the
Dakotas to the lower Ohio Valley. Winds...which should remain less
than 10 knots through Wednesday...will then shift northerly Wednesday
Chicago terminals have been within western periphery of low level
moisture trapped beneath an inversion based around 2500 feet above ground level...
with MVFR stratocu deck in the 1600-2000 feet range lingering mainly
south/east of Ord. This deck will continue to drift slowly east
this evening mainly affecting mdw/gyy through the early evening
hours. Erosion of this deck as occurred farther north across dpa-
Ord so not expecting a ceiling at this time. With light wind
regime there is a low-probability potential for some MVFR shallow
fog to develop mainly away from Chicago metropolitan terminals late
tonight...though confidence is low en0ugh to leave out of tafs at
dpa and rfd at this time.
VFR conditions expected overnight and Wednesday...with only
middle/high clouds associated with disturbance passing to the
southwest of the forecast area.
//Ord and mdw confidence...updated 00z...
* medium-high confidence in no persistent MVFR ceiling this evening.
* High confidence all other elements.
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
Thursday...slight chance of MVFR light snow early...otherwise VFR.
Friday...slight chance of light snow. MVFR possible.
Saturday through Monday...primarily VFR...though a period of MVFR
or IFR ceilings possible sataurday into early Sunday.
147 PM CST
A high pressure ridge continues to push east causing winds to
diminish across the region. Meanwhile a low currently over the
Dakotas reaches the middle Mississippi Valley tomorrow morning. Light
winds become east over the southern half of the lake and variable
over the northern half tomorrow afternoon. The low weakens as it
passes over central Illinois tomorrow and high pressure then builds over
the plains tomorrow night. North to northwest winds increase to 30
knots Thursday as the pressure gradient increases between the high over
the plains and large low moving up the East Coast. Winds diminish
as the high moves over the lake Thursday night. Winds then become
south behind the high Friday. Guidance differs on how quickly the
next low moves over Lake Superior...but the pressure gradient
increases ahead of the low resulting in south winds around 30 knots
late this week. Have the winds forecasted to come up Friday
afternoon...but they could come up as late as Saturday morning. 20
knots winds persist through the weekend with southwest winds becoming
north Sunday afternoon. Another high pressure system moves over the
lake early next week with strong south winds behind it.
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