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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1013 am CST Monday Dec 22 2014

Short term...
314 am CST

Today through Tuesday...

The main concerns during this period continue to be focused on
rainfall amounts and timing.

Water vapor imagery early this morning indicates several smaller
scale middle level vorticity impulses moving within the larger scale trough
encompassing much of the central Continental U.S. From The Rockies eastward to
the Mississippi Valley. One such impulse is current shifting across
the area...and this has acted to spark off some light precipitation
across portions of the area over the past couple hours. Most surface
observations under the radar echos have reported rain or drizzle.
However...with surface temperatures hovering right at freezing
across much of northern Illinois...some areas could be experiencing
some minor glazing. This initial disturbance should move northeast
of the area by there will likely be a lull in the
precipitation for several hours this morning after around
daybreak...through middle to late morning. Thereafter...a second middle
level impulse...currently noted across eastern portions of
Oklahoma...will shift north-northeastward towards the area by this

This second disturbance will be accompanied by a better influx of
moisture...allowing for more widespread rain development across
central portions of Illinois and Indiana by late morning...before
shifting northward all of northern Illinois/northwestern Indiana by
early this afternoon. Therefore...a very wet afternoon is in store
for the area. It is still possibility that as this Bach of precipitation
begins to enter far northern sections of Illinois later this
morning...that it could onset as a mix of sleet...snow and rain.
Model soundings indicate profiles sportive of such for a small window
of time through around midday. A surge of a warmer and moist airmass
will end this by early afternoon as all rain will be favored

As we head into the evening another Bach of rainfall is expected to
develop northward across the area. This additional rainfall will be
driven by yet another influx of an even warmer more moist airmass
out ahead of a closing off middle level circulation over the middle
Missouri Valley. This process will also drive additional middle level
impulses northeastward along its southeastern periphery right across
the area. At the surface...low pressure will shift from the northern
plains to the Iowa...Missouri state line by this evening...then lift
a warm/occluded frontal boundary northward across the area by
daybreak Tuesday. Once this frontal boundary lifts over the accompanying middle level dry slot is expected to shift over
the area...bringing an end to much of the rainfall either very late
tonight or early Tuesday morning. Therefore...much of the day
Tuesday looks to be cloudy...mild and aside from the possibility for
some lingering drizzle...mainly dry.

Total rainfall amounts with this activity this afternoon into
tonight could range between a half inch and one some
periods of moderate rain are possible into this evening as precipitable waters
creep up in excess of 3/4 of an inch. I have attempted to focus the
best chances for rainfall from middle to late morning through later
tonight across my central Illinois counties...and from around midday
through tonight for my northern areas.



Long term...

314 am CST

Tuesday night through Sunday...

During this period the primary focus will be with the second storm
system expected to develop and impact the Great Lakes region
Christmas evening.

A very impressive jet excess of 150 knots...extends from the
eastern Pacific...onshore over the northwestern Continental U.S. At this
time. This jet maximum is expected to be the main player in the
development of a significant storm system by middle week. Model
solutions (nam aside) have come into some better agreement with the
surface low. appears that cyclogenesis will be induced
near the central Gulf Coast by late Tuesday...then race northward
across the Ohio Valley region and into the central Great Lakes
region by Christmas evening night. It still appears that the surface low
will deepen considerably late Wednesday and into Christmas morning
as the upper level impulse looks to take on a negative tilt. Most
operational guidance hints at healthy 15 to 20 mb deepening between
12 UTC Wednesday and 12 UTC Thursday.

Although tweaks to the track and timing of the system are still a appears the main uncertainties at this point will be
the mesoscale details...and hence where the best deformation banding
will occur later in the day Christmas evening. Both the GFS and the
par GFS indicate a slightly farther north progression of the
deformation band across mush of northeastern Illinois and
northwestern Indian Wednesday afternoon and evening. In contrast the
European model (ecmwf)...Gem and UKMET are a bit farther south...mainly impacting my
southeastern County Warning Area. Overall...there is typically a strong northwestern
gradient to these deformation this could lead to a sharp
contrast in precipitation amounts across my County Warning Area.

It appears that precipitation type within this deformation band would
likely be in the form of snow. Model soundings suggest that
boundary layer temperatures may remain a bit warm late Wednesday and
evening...but typically dynamic cooling from strong forcing for
ascent within these deformation bands leads to transition to a
period of moderate to even heavy snowfall. At the current time...I
have remained very conservative on snow amounts over the area...due
to uncertainties on where this potential band of heavier precipitation sets up.
However...I would not be surprised for some areas...particularly in my
southern and eastern County Warning Area to end up with several inches of
snow by late Christmas evening. As such...changes to snow amounts is a
good likelihood.




//Ord and mdw concerns...updated 16z...

* VFR lowering to IFR and possibly LIFR this evening.

* Rain developing over next couple hours continuing into tonight.

* Southeast winds around 10 knots becoming east-southeast 10-15 knots
by afternoon.


//discussion...updated 12z...

Low pressure moving southeast from the Dakotas to the Midwest
today...with rain and low ceilings/visibility expected to develop from the
southwest by midday/early afternoon as moisture spreads into the
region ahead of the low. Some spotty light precipitation mainly west and
northwest of Chicago terminals early this morning...with marginal
thermal profiles supporting light rain/light snow/light sleet.
Precipitation to become all rain by afternoon with temperatures rising
well above freezing. Rain continues into tonight and tapers to
drizzle at times overnight.

VFR ceilings with some MVFR visibility in haze early this morning...with
ceilings/visibility expected to deteriorate to IFR this afternoon after rain
develops and likely to LIFR overnight. Some improvement expected
Tuesday morning as warm front lifts north of terminals.

Southeast winds near 10 knots this morning to become east-southeast
today and tonight 10-15 kts. Winds turn back to the south and
eventually southwest Tuesday morning with warm front passage.


//Ord and mdw confidence...updated 16z...

* medium in ceilings through period...though medium low in timing
shift to IFR and LIFR.

* Medium-high in general precipitation timing and wind trends.


//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...

Tuesday night...dry. Mainly VFR...though MVFR possible late.

Wednesday...chance of rain/snow changing to snow. IFR likely.

Thursday...chance of MVFR ceilings...mainly early.

Friday...chance of MVFR ceilings. Chance of -ra/-sn

Saturday and Sunday...mainly VFR.



233 am CST

Forecast for Lake Michigan remains complicated this morning...with
two areas of low pressure expected to bring periods of strong winds
across the lake over the next several days.

The first low was moving southeast across the Dakotas early this
morning...with a tightening pressure gradient extending east into
the western lakes. Observation platforms along the west and
northwest shores of the lake were reporting south winds in the 25-30
knots range...with winds to 30 knots expected to back to the east as the
low moves to the Iowa-Missouri border tonight. The low will then
begin to fill as it turns northeast...lifting across Northern Lake
Michigan late Tuesday.

Expectations are that a second low will develop to the south across
the Tennessee and Ohio River valleys by early Wednesday morning...
deepening as it lifts northward into the central Great Lakes region
by Wednesday evening. Models continue to show differences in the
exact track and strength of the low as it passes east of Lake well as north and northwest wind speeds Wednesday
afternoon into Thursday morning. At this appears that
winds of 25-30 knots are likely...and possibly still gales for a time
if the low deepens a little faster. Winds then look to diminish and
back to the west and then southwest Christmas day. Models are in
better agreement in another low approaching and lifting across the
northern lakes Friday and Friday night.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...nearshore Winthrop to Wilmette harbors until 9 am Tuesday.

Small Craft Advisory...lmz741-lmz742 until 9 am Tuesday.



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