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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1100 PM CST Wed Dec 11 2019

923 PM CST

No major changes to the going forecast for tonight. Made
adjustments to hourly temperature trends. Some locations where a
half inch to one inch of snow fell earlier today dipped a bit
lower than forecast this evening with surface high pressure
ridging stretched back across the area and clear skies. For
instance, Pontiac briefly got down to 12 degrees within the past
few hours. As the surface ridging exits eastward and east to
southeast winds gradually increase, along with increasing mid and
high clouds, temperatures will rise overnight.

On Thursday, weak surface low will track just northwest and then
north of the local area. In response, brisk southerly winds
gusting to 25 to 35 mph and likelihood of just mid clouds over the
area will support a nice warm-up. Bumped up high temperatures
slightly, especially along and southeast of a Peru to Evanston
line where cloud cover should be less thick and now have highs in
the mid 40s indicated per consensus of MOS guidance. Stout short-
wave trough associated with the weak surface low will track north
of the area, but glancing blow of forcing and strong isentropic
ascent from strong low level jet and warm air advection will compensate some.
Forecast soundings indicate lower levels remaining dry with
antecedent air mass starting out quite dry, so a mix of virga and
spotty light rain or sprinkles appear possible for areas mainly
north of I-88 in northern Illinois. Can't rule out some wet
snowflakes mixing in if the spotty precip moves in while temps are
still in 30s during mid-late morning, but expect no impacts.



Short term...
238 PM CST

Through Thursday...

Surface ridge continues to build across the region this
afternoon, with remaining stratus south of I-80 slowly eroding.
Additional erosion this afternoon will allow clear skies going
into this evening. With clear skies and surface ridge in place,
expect temps to steadily fall into the evening with the night time
lows occurring during this time. Upstream warm air advection will spread across
the area tonight ahead of an approaching system, with steady or
rising temps likely during the overnight hours. Continued warm air advection and
gusty southerly winds on Thursday will support above normal temps
in the 40s. Have kept temps in the low 40s, as returning mid/high
clouds later tonight will hold in place on Thursday. Approaching
mid level vort and surface low are expected to move just north of
the cwa, with the bulk of the associated precip likely also
staying to the north. Surface front on the backside of this system
does work its way toward northwest Illinois late in the day and could
be a source for light precip development Thursday afternoon. If
precip were to develop, highest chances would be for locations
near the Illinois/WI border and would expect it to be liquid.



Long term...
309 PM CST

Friday through Wednesday...

Thursday night and friday: low pressure associated with light snow
to the north on Thursday will be centered over east-central Wisconsin
at 00z Friday. Meanwhile, an approaching cold front will stall over
or just west of the far northwest portion on the County Warning Area. Guidance
differs on the amount of lingering moisture along the front, but
there is some indication that a sufficiently deep layer of
saturation will be present in conjunction with residual frontal
convergence to possibly produce light drizzle across northwest of a
line from Waukegan to DeKalb to Dixon. Temps should stay at or above
freezing during this time, but if drizzle were to continue into the
overnight hours, some temps may begin to fall slightly below
freezing. Friday is expected to be dry under mid to upper-level
clouds ahead of an approaching system for Saturday, though some
light precip may encroach on the northwest County Warning Area by 00z Saturday.

Friday night and Saturday night: guidance continues to show
variability with the evolution of a pair of phasing waves over the
region during this time. These waves will likely phase to some
degree over the upper Mississippi River valley by Saturday morning.
The European model (ecmwf) continues to back off on the more aggressive solution from
a couple days ago and is now mostly dry, whereas the GFS has trended
stronger. While some clarity and specifics are still needed, the
overall pattern supports the potential for some light warm air advection precip of
rain/snow and possibly some freezing drizzle Friday night into
Saturday morning. A more potent bout of snow mixed with some rain
will possibly follow with the trough and associated cold front late
Saturday morning through the afternoon. If this solution does pan
out, a brief period of hazardous conditions with gusty northwest winds and
moderate snow could occur during the afternoon.

Sunday through wednesday: generally zonal flow aloft over surface
high pressure across the upper Mississippi River valley will produce
dry and quiet conditions across the area on Sunday. The larger scale
flow will then break down by Monday as troughing over the North
Pacific digs into the western Continental U.S.. a prominent southern jet will
interact with this trough by late Monday, though guidance differs on
where this will initiate. The GFS continues to show a more
progressive and east-northeast-moving mid-level trough producing a notable
shield of snow across the forecast area Monday into Monday evening.
Considerable spread exists amongst gefs/European model (ecmwf) ensemble members and
deterministic guidance, but some ensemble members in both the gefs
and European model (ecmwf) advertise a shield of enhanced precip across the County Warning Area.
Probabilistically, this appears to be a low chance event with much
of the energy likely remaining southeast of cwa, but will continue
to monitor if other guidance trends northwest with energy/higher quantitative precipitation forecast
over the coming days.

Tuesday and wednesday: dry conditions should prevail through much of
the period. The presence of a deep cyclonic flow pattern over the
northern Great Lakes suggests the potential for an embedded weak
wave to affect the area with light precip at some point during this



for the 06z tafs...

- gusty south winds near 30 kt, and low level wind shear above shallow mixed layer
are the main aviation forecast concerns through Thursday.

Surface high pressure ridge axis has drifted east of the terminals
late this evening. Light/variable or calm winds will become light
southeast overnight, gradually increasing through daybreak. High
clouds will continue to stream into the region through morning.

Low pressure taking shape across the High Plains this evening will
lift northeast across Northern Lake Michigan by Thursday evening.
In response, southeast winds will become gusty Thursday morning,
before shifting south-southwest as a warm front lifts north of the
terminals late morning. Slightly deeper mixing south of the warm
front will support wind gusts increasing to near 30 kt during the
day. Despite the mechanical mixing with the gusty surface winds,
winds above a shallow inversion aloft will increase to around 50
kts, producing a period of potential low level wind shear conditions into the
afternoon. The gradient will eventually weaken late Thursday into
Thursday night, as a weak cold frontal trough approaches from the
northwest. This will allow south-southwest winds to slowly

Dry low level air mass will result in mainly mid and high level
cloud cover through much of the day Thursday. Forecast soundings
eventually depict lowering VFR clouds late in the day/Thursday
night, though with best chance of precipitation generally north of
the terminals into WI. Guidance indicates some potential for MVFR
ceilings late evening (rfd) or after midnight (chicago terminals),
though confidence is somewhat low at this distance especially if
no precip occurs nearby.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz743-lmz744-lmz745...6 am Thursday to
midnight Friday.

Small Craft Advisory...lmz740-lmz741-lmz742...6 am Thursday to 9
PM Thursday.



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