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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
657 am CDT Monday Apr 21 2014

305 am CDT

Main forecast concern is with rain/thunder chances and trends today
with cold front moving through the forecast area this afternoon and
early this evening. Pattern remains amplified and progressive
through the week...with another organized chance of precipitation and a
frontal passage Thursday and yet another later next weekend.

Early morning surface map depicts a trough of low pressure extending
from northwest Minnesota southwest through the Sioux Falls area and
into the Central Plains. A complex upper trough was over central
North America with at least three distinct short wave disturbances
from north to south per 00z radiosonde observation and current GOES vapor imagery. The
upper trough is prognosticated to continue to propagate east across the
area today...with surface low pressure lifting north of the County Warning Area and
deepening across the northern Great Lakes in response to the
stronger northern stream short wave digging along the US/Canadian
border. Low level moisture plume from Texas to the upper Mississippi
Valley has been limited somewhat by piecemeal nature of upper
forcing along upper trough and blocking of central/eastern Gulf by
expansive surface high pressure ridge which extends from the lower
Mississippi Valley to off the New England coast. However...starting
to see development of isolated rain showers/thunderstorms and rain into far northern
Illinois/southern WI early this morning in advance of approaching short
wave and above subtle elevated warm frontal zone which stretches
east/southeast from Sioux Falls low. Guidance runs in general
agreement that precipitation chances will slowly expand across the region
by this afternoon as surface cold front tracks east into the area as
surface low deepens to our north. Combination of large scale ascent
associated with middle-level trough and low level frontal convergence
results in steepening lapse rates by afternoon...with forecast
soundings depicting about 300-700 j/kg MLCAPE in pre-cold frontal
environment. This is expected to be sufficient to produce increasing
coverage of rain showers/thunderstorms and rain as the front moves east across the region this
afternoon and early this evening...though deep shear remains
relatively weak with main upper jet core well to our north. Precipitation
chances then diminish rapidly from the west/northwest this evening
as front pulls away to the east and drier air pushes in from the
northwest. Low level thermal ridge ahead of cold front should easily
support temperatures in the 70s again today...with southwest boundary layer
flow 10-15 knots enough to prevent any significant lake breeze

Strong short wave digs southeast across the Great Lakes region
tonight...with cool/dry low level advection in the wake of
associated surface low and cold front resulting in cooler temperatures
tonight and Tuesday...with northerly winds ahead of surface high
pressure which builds into the area Tuesday night and Wednesday.
While it will be sunny...after a couple of days well into the
70s...lower 60s appears to be about the warmest across our far
western/southern counties on Tuesday and Wednesday while northeast
winds off Lake Michigan limit Chicago metropolitan area to middle-upper
50s...with some 40s right along the lake. Lows may dip into the low
30s in some spots Tuesday night as winds diminish with the approach
of the ridge axis.

Medium range forecasts remain consistent in bringing another rather
amplified upper trough east of The Rockies on Wednesday...with
surface low pressure developing across the plains. Moisture return
looks to be a little more robust with this feature...with moistening
and saturation in isentropic warm air advection potentially yielding
some showers across parts of northwest Illinois by Wednesday afternoon...
and across much of the forecast area Wednesday night. The greatest
chance for precipitation is occluding cold front plows east
across the County Warning Area. While temperatures warm...position of triple point in
occlusion remains across/south of our area and does not portent
temperatures as warm as this past weekend with generally 60s anticipated
and perhaps only 50s along North Shore in southeast flow. Models
depict second disturbance rotating around upper low to our north
Thursday night into Friday...wrapping a secondary cold front
southeast across the area and maintaining some potential for showers
across especially northern/eastern parts of the forecast area. Some
differences arise with orientation of eastern North American trough
which results by next weekend... though looks to be another cooler
than normal period with north/northeast flow for US to the west of
the upper trough.
While run to run consistency exists in GFS and European model (ecmwf) long range
solutions...European model (ecmwf) remains faster with departing East Coast long wave
trough and brings another low/warm front into the region (but to our
south) late Saturday and Sunday. Have generally followed the European model (ecmwf)
trend which brings precipitation chance back in north of the warm front
Saturday night/Sunday.




//Ord and mdw concerns...updated 12z...

* frontal passage this evening.

* Scattered showers/thunderstorms developing this afternoon.


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

Relatively lighter south to southwest winds in place this morning
as surface trough remains well west of the middle/high
level cloud cover continue to move overhead. A trend towards a
more southwest to west direction will occur today out ahead of
this approaching trough and frontal boundary...with VFR ceilings
slowly lowering. This morning will remain dry with latest radar
imagery depicting widely scattered showers/thunderstorms still
well west in Iowa. Although...expect scattered showers to begin
moving across the terminals by early afternoon with this
development persisting into the afternoon. It still appears as if
more widespread showers and thunderstorms will be tied closer to
the front...affecting the terminals during the middle to late
afternoon time frame possibly lingering into the early evening.
Expect frontal passage by this evening...with winds turning to the northwest
while precipitation and cloud cover shift to the southeast.


//Ord and mdw confidence...updated 12z...

* medium high confidence with wind speed/direction.

* Medium high confidence with shower/thunderstorm
potential...medium confidence of timing/duration.


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

Tuesday night...VFR.

Wednesday...chance of showers...otherwise VFR.

Thursday...a period of rain showers/thunderstorms and rain likely.

Friday...slight chance of showers...otherwise VFR.

Saturday...chance of showers late...otherwise VFR.

Sunday...chance of showers...otherwise VFR.



433 am CDT

As a ridge of high pressure over the eastern Great Lakes continues
to shift to the east this morning...a trough of low pressure over
the central and northern plains will shift east to the central
Great Lakes today. With these features in place...expect generally
southerly flow across the lake for most of today. Although...this
surface trough and associated cold front will slowly approach the
lake later in the day and evening...with winds turning to the
southwest and west before turning to the northwest on the backside
of this front. Winds will ramp up quickly behind the front with to
30 knots winds expected tonight and into Tuesday morning. Then as
this low departs and high pressure begins shifting towards the
lake Tuesday...winds will diminish throughout the day. A period of
lighter flow over the lake is expected Tuesday night into
Wednesday as this high builds overhead. Expect the next system to
affect the region to begin shifting east towards the lake
Wednesday night into Thursday...with southeast winds increasing
during this period.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...10 PM Monday to 10 am Tuesday.



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