Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
542 PM CST sun Jan 25 2015
206 PM CST
Tonight through Monday night...
Cool air has struggled to slide south this afternoon...however temperatures
are beginning to fall into the upper 20s across far northeast Illinois.
Across the central/southern County warning forecast area temperatures remain in the low 30s with
dew point depressions not much more than 1 to 2 degrees. North of
I-80 precipitation is mainly flurries...with a rain/snow mix slowly
changing to all snow later this afternoon across the southern County warning forecast area.
The shortwave that brought the light snow continues to slide east
across southern Illinois...and will steadily bring an end to the precipitation
across the forecast area this evening from northwest to southeast.
Winds will gradually diminish as well as the pressure gradient
Surface ridge centered to the north this afternoon...will sag south and
expand to cover a portion of the western Great Lakes/Missouri Valley
overnight. While some erosion to the clouds is probable north of the
forecast area...expect some decrease in clouds closer to daybreak
Monday. Currently expect overnight lows to dip into the middle/upper
teens...however this will likely not occur until closer to daybreak
The brief break in clouds Monday morning will quickly give way to the
next clipper system arriving Monday aftn/eve. Guidance has maintained a
progressive look to this next wave...however operational solutions
are equally bullish on erosion of moisture within the lowest few
layers of the atmos late Monday night. Have some concern about p-type Monday
aftn/eve. At the onset moisture will quickly saturate through the -10
degree c layer and touch the favored dgz...however the depth of the
moisture becomes more shallow Monday evening. Along with the lack of good
lift and shallow moisture...it is looking more likely that precipitation
may be a mix of freezing drizzle/lgt-snow.
A weak thermal ridge tries to push overhead Monday afternoon...as low level flow
turns southwesterly. Temperatures should hover around freezing...however this
will hinge on cloud cover arrival and development of precipitation. If this
is delayed...temperatures could push a few degrees warmer Monday early afternoon.
Heights slowly increase Monday night...with the moisture peeling east
along with the weakening shortwave. At this time have maintained a
mention of light snow or patchy freezing drizzle Monday night...but could
end up with a dry period.
206 PM CST
Tuesday through Sunday...
towards the middle of the week ensembles maintain a high confidence
in the trough along the southeast Continental U.S. Weakening with ridging
expanding east across the Central Plains. This should place the bulk
of the region into a weak surface ridge Tue/Wed. Wednesday afternoon the surface ridge
quickly slides southeast...which could allow some warm air to advect
northeast into the County warning forecast area and push surface temperatures close to 40 degree.
With a quasi-zonal setup Wednesday ngt/thur...guidance suggests a robust
middle-level vorticity will quickly slide east from The Rockies towards the
plains Wednesday evening. Guidance then further strengthens this wave and
lifts northeast across the forecast area Thursday. Soundings suggests
that precipitation could start as mainly rain with this system...then
dynamically cool overnight into Thursday and transition precipitation over to
A relatively active pattern continues towards the end of the week
into early next weekend...with temperatures generally in the upper 20s/low
//Ord and mdw concerns...updated 00z...
* light snow possible Monday afternoon/early evening
System responsible for today's weather is quickly moving away and
anticipate winds to quickly subside this evening and skies to
clear out. Middle-high level cloudiness will arrive later tonight in
advance of the next fast moving weak system slated to affect the
area Monday afternoon and evening. Still looks like a several hour
period of light snow could occur Monday afternoon and
evening...but amounts would be light and confidence not terribly
high so have just maintained prob30 for now. Guidance continues to
appear overly pessimistic with respect to low cloud development
Monday and have leaned toward the recently more superiorly
performing mav guidance with just some MVFR ceilings in the afternoon.
Light winds overnight into Monday morning will trend more light
south to southeast Monday afternoon and evening.
//Ord and mdw confidence...updated 00z...
* medium confidence in timing and low confidence in occurrence of
light snow Monday
* high confidence in remaining forecast elements
//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...
Tuesday...MVFR ceilings possible.
Wednesday...VFR during the day then chance of -rasn and MVFR/IFR
Thursday...chance of rain/snow. MVFR likely/IFR possible.
Friday...chance of MVFR ceilings...especially early.
Saturday...chance of light snow/MVFR ceilings.
131 PM CST
A quick moving clipper system...currently centered across southern
Illinois...will rapidly shift eastward towards the Atlantic coast
by Monday morning. Northeasterly winds of 30-35 knots continue across
the southern 1/3rd of Lake Michigan...and these winds look to
continue for the next few hours...before a diminishing trend begins
early this evening. Another weakening low will shift southward just
west of the lake late Monday and Monday night. This will result in a
period of southerly winds over the lake during this period. Later in
the week another low is expected to impact the Great Lakes region.
This system still has the potential to produce a period of strong
southerly winds late Wednesday and Wednesday night...followed by a
period of strong northerly winds Thursday into early Friday.
lmz744-lmz745-lmz777-lmz779-lmz874-lmz876-lmz878 until 6 PM
Small Craft Advisory...nearshore waters
until 9 am Monday.
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