Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
357 am CDT sun Apr 20 2014

319 am CDT

Main forecast challenges/concerns are with maximum temperatures today with a
possibility of lake cooling...and then with increasing chances for
showers and thunderstorms on Monday.

Quiet conditions continue this morning as high pressure remains
situated across the region...while surface trough/boundary are
situated well northwest of the County Warning Area. Middle/upper level pattern is
fairly active across the Continental U.S. This morning with exiting vorticity maximum
to the north and approaching weaker/low amplitude upper level
trough lifting northeast to the Southern Plains...and with another
vorticity maxima shifting east through the northwest Continental U.S.. these
features have provided a fair amount of middle and high level
moisture over the central Continental U.S....but with the majority of this
moisture staying west of the County Warning Area. With the frontal boundary held
up to the northwest today...expect dry conditions once again today
and for most of tonight but with a slow increase in cloud cover
throughout the day. Latest satellite imagery depicting middle/high
level cloud cover trying to inch its way closer to the west and
northwest County Warning Area and although I do anticipate a portion of this cloud
cover to clip the far northwest and northern County Warning Area...bulk of this
cloud cover should remain outside of the County Warning Area through early
afternoon. Could see some of this cloud cover spill further east
for some periods today...but still think it wont be enough to
provide anything more than partly cloudy skies. Even with
potential for clouds to push across a portion of the County Warning Area...feel
that today will be a rather warm day with well above normal temperatures
in the middle to upper 70s across the County Warning Area. As warm air advection steers thermal axis
overhead today...did raise temperatures a couple of degrees with most
locations observing these middle/upper 70 degree temperatures with locations
across the eastern County Warning Area likely more in the low 70 Range. Lake
cooling could become an issue later this afternoon as surface
pattern relaxes and low level flow weakens. This could relax the
surface winds enough to allow for some onshore turning of the
winds mainly for Chicago northward into Lake County...with a
slight possibility for this to occur over Northwest Indiana.
Although did account for this in the maximum T grid...feel that most
of the warming will occur prior to this turning of the winds and
resultant cooling. So locations near the lake will likely observe
upper 60s or lower 70s before this possible cooling occurs later
this afternoon.

Expect tonight to remain on the dry side but with some possibility
for a stray shower or two more to move across the far northwest
County Warning Area more towards early Monday morning. As several waves of energy
lift northeast out ahead of approaching trough to the southwest
and the stronger and more defined middle level trough drops southeast
through the northern plains and upper Midwest Monday...expect
surface trough to become more organized and then deepen as it
begins working east towards the County Warning Area by middle day Monday. does appear as if a slower trend is showing up
within model guidance and have reflected this trend in the grids
keeping only chance probability of precipitation mainly for the northwest half of the County Warning Area
through middle day Monday. Large scale support increases as well as
low/middle level convergence strengthening through the afternoon. As
this does appear that some better moisture return does
finally occur out ahead of this trough/frontal boundary. Expect
scattered shower development to persist through the afternoon with
the best chances for precipitation across the entire County Warning Area expected during
this time frame. Guidance has continued the increasing instability
trend...with instability axis now anticipated well into Wisconsin
and out over the lake. Did increase mention of thunder...with
chance thunder for the entire County Warning Area on Monday. With flow aloft
rather weak as well as the instability to be on the weaker this time expect any thunderstorm development to be
weak/unorganized. Trough/front will push through Monday evening
with confidence still low with regards to continued chances for
thunder...especially with guidance showing this weak instability
quickly being shunted to the south of the County Warning Area. So have left out
any mention of thunder for Monday evening over the southeast third
of the County Warning Area...but could possibly be included with later forecasts.




//Ord and mdw concerns...updated 09z...

* none.


//discussion...updated 06z...

Southeast winds near 10 knots will veer south Sunday morning and
increase slightly...with a few occasional gusts 15-20 knots possible
during the afternoon hours. Dry VFR conditions will continue...
with scattered middle/high clouds increasing and eventually forming a more
solid middle-level deck Sunday night.


//Ord and mdw confidence...updated 09z...

* high confidence all forecast elements.


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

Monday...afternoon rain showers likely. Brief MVFR possible.

Tuesday and Wednesday...VFR.

Thursday...chance shra/tsra...mainly at night.

Friday...VFR probable.



217 am CDT

High pressure ridge axis has moved east of the lake this morning...
while low pressure well to the north was moving from Manitoba into
Ontario. This low will continue east to near James Bay by this
evening...with a cold frontal trough trailing southwest across the
upper Midwest and Central Plains to another low developing over the
southern High Plains. This front will begin to sag across Northern
Lake Michigan later a weak low pressure wave lifts
northeast into the Great Lakes region along the trailing cold front.
Low pressure will consolidate and deepen across the central and
eastern lakes region by late Monday as an upper disturbance digs
into the region...with the cold front moving to the east and
southeast of Southern Lake Michigan Monday evening. With the low
deepening off to the east...and a broad area of high pressure
building across the upper Midwest...northerly winds will briefly
increase into the 20-25 knots range across Lake Michigan Monday
night and Tuesday. The surface high pressure ridge will then
spread east across the western lakes region Tuesday night into
Wednesday. Another fairly deep low pressure system will then move
across the northern plains and upper Midwest Wednesday night into
Thursday...with increasing south winds in the 25-30 knots range
developing across Lake Michigan. The low will move across the
northern lakes Thursday with a cold front moving east across the
lake by Thursday night.



Lot watches/warnings/advisories...


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