Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
921 PM CST Sat Dec 20 2014
904 PM CST
We still have the possibility of patchy freezing drizzle
overnight with the best chance in our western areas. That
said...temperatures are around freezing and not expected to drop
much given the cloud cover and warm advection pattern setting up.
At this point impacts do look minimal...especially farther east. Have
kept temperatures nearly steady overnight. Some precipitation may make it
closer to Chicago late tonight into Sunday morning once the warm
front gets closer...but it would be largely in the form of patchy
drizzle. Fog may be possible as well overnight.
247 PM CST
Through Sunday night...
The streak of cloudy days continues...with the stubborn pattern to
continue. As a result of the thick cloud cover...and lack of
organized advection of any warmer air at the surface...temperatures have held
mainly around 30 to 35 across the bulk of the County warning forecast area. Surface ridge
continues to drift east this afternoon...with a weak southerly flow
developing. There was a weak middle-level vorticity lifting northeast across
southern wisc/western lower Michigan...which was able to produce a few
flurries across the far northern Illinois areas.
This afternoon/early evening looks to remain quiet/dry...with a light
south/southeast wind. Infrared imagery shows the next wave across the
Central Plains...which is prognosticated to arrive across northwest Illinois
after midnight. There appears to be some weak advection of moisture
in the low-levels overnight. Soundings suggest a narrow warm wedge
aloft which could transition any precipitation overnight to liquid. The
forcing/Omega is rather weak...which may limit droplet size and
result in just patchy freezing drizzle. Temperatures will likely not move
much overnight...so have held onto middle/upper 20s overnight.
Local arw8km solution indicates the moisture to be rather marginal
with the system arriving overnight/Sun morning...but progressive and
lifting north of the County warning forecast area by midday. Weak thermal ridge at 850mb
would suggest temperatures may actually warm Sun afternoon...however given the
stubborn pattern we have been in would lean towards nudging temperatures
down a degree or two. Moisture layer will remain locked in across
the region through sun/Sun night. With a continued southerly flow at the
surface...temperatures will likely only fall to around 30/low 30s.
247 PM CST
Monday through Saturday...
500mb trough begins to dig into the central Continental U.S. Monday morning...with
warm air steadily advecting north across the forecast area. While
the thick cloud cover will persist...the push of warm/moist air will
bring temperatures into the 40s the far southern County warning forecast area could see a few
locations touch the upper 40s Monday afternoon. Precipitation will steadily push east
as the upper level system progresses towards the region...but with
the warm air in place precipitation should fall as rain.
Monday night the 500mb vorticity will slide into the upper Midwest...but the
trough will dig south across the Southern Plains. Guidance has been
indicating a secondary shortwave ejecting from the trough axis...and
lifting it northeast across the tenn valley Christmas evening. The
northern surface low will be wrapped up across central wisc Tuesday...with
the moist channel feeding north across the forecast area through Tuesday
afternoon. The southern low will be quickly deepening as it lifts north
across the Ohio Valley Wednesday morning...allowing winds to be on the
increase and helping to steadily transition precipitation over from rain to
snow Post frontal passage Wednesday morning.
The last several cycles of guidance have demonstrated differences in
placement/track of the surface low Wednesday aftn/eve. Latest solution is
prognosticated to lift north across northern in and then slightly
retrograde across the southern tip of Lake Michigan Wednesday evening. Then by
Christmas day the trough will be pivoting east and helping to lift
the surface low northeast away from the region. Some wrap around
moisture/snow looks favorable Wednesday night/Thursday morning...before coming
to an end just after daybreak Christmas morning. This system will
continue to be closely monitored.
Temperatures will be returning to more seasonal conds...with highs
Christmas day in the low/middle 30s. Flow remains rather progressive
with another weak push of warmer air returning Friday and temperatures back to
the upper 30s/low 40s. Then for Friday ensembles are indicating yet
another trough beginning to dig into the central Continental U.S....with a surface
low lifting northeast across the western Great Lakes.
//Ord and mdw concerns...updated 03z...
* ceilings lower to MVFR late tonight and potentially to IFR Sunday
* slight chance of -fzdz or -dz Sunday morning
Have largely followed the trends from previous set of tafs with
lowering ceilings tonight/Sunday...though have sped up the arrival of
MVFR ceilings tonight and lowered ceilings a bit Sunday. Expansive area of
IFR/br/-dz blankets Iowa and should slowly spread eastward through
the period. Only bringing ceilings down to lower end MVFR Sunday could
end up being too optimistic for Ord/mdw/gyy...will monitor trends
in the eastward expansion of IFR/LIFR tonight and we may need to
add IFR into the tafs for Chicago terminals in later forecasts. As
well...if ceilings do drop lower then potential would be greater for
some -fzdz transitioning to -dz as temperatures inch upward. Not
anticipating ceilings to lift much from morning lows Sunday...so ife
conditions develop they could persist through the day. Certainly
could see some occasional gusts of 15-20kt Sunday afternoon from the
south-southeast...but thinking the expected stratus deck should limit mixing
enough to keep gustiness sporadic to occasional at best.
//Ord and mdw confidence...updated 03z...
* medium confidence in ceiling trends through Sunday morning...low
confidence in timing and how low ceilings get
* low confidence in ceilings Sunday afternoon
* low confidence in dz/fzdz potential Sunday morning
* medium-high confidence in winds
//outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...
Monday...MVFR/IFR early lowering to IFR/LIFR. Rain developing
Tuesday...oncl rain showers. IFR probable.
Wednesday...chance shra/shsn. MVFR likely. IFR possible.
Thursday...MVFR ceilings possible early.
Friday...chance of rain/snow late. MVFR ceilings possible again.
203 PM CST
Light southeast to east winds will increase to 10-25 knots tomorrow
morning then to 30 knots in the afternoon as the gradient tightens
between the departing high and the next low. The low moves over the
Dakotas Sunday night and models agree a bit more in the pattern
early next week. Have medium confidence in the wind forecast with
this next low. The low weakens as it moves over Wisconsin Tuesday
and a second low forms over the southern Mississippi Valley Tuesday
afternoon. The second low will race north and deepen reaching
Michigan Wednesday. Models differ on how quickly the low will reach
Michigan. Both models continue to feature westerly high end gales
behind the low Wednesday afternoon into Thursday. West winds
diminish Thursday as the low continues off to the north. The next
low moves over the Southern Plains Thursday afternoon. Guidance
differs on this lows path but it should impact the lake late next
Lm...Small Craft Advisory...lmz740...10 PM Sunday to 3 am Tuesday.
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