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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1012 PM CDT Wednesday Apr 23 2014

Discussion...
931 PM CDT

Showers continue to move/develop eastward across northern Illinois
early this evening within a zone of increasing isentropic ascent
along the western periphery of a surface ridge of high pressure. This
activity is expected to continue moving eastward across northern
Illinois late this evening...and likely ending prior to daybreak
Thursday as the better forcing and moistening shifts to the north
and east of the region. Therefore...no big changes are needed to the
going forecast for tonight.

Thursday is shaping up to be a breezy and a milder day across the
area as southeasterly flow ramps up in response to an area of low
pressure approaching western Illinois during the afternoon. It
appears that in spite of the south-southeasterly wind
component...that winds could be very gusty...up in the 30 to 35 knots
range...during the afternoon. Also...it appears that these
southeasterly winds will transport a drier lower level air mass in
across the area during the day...and this could lead to lower
surface dew points than currently forecast...especially across the
eastern half of the area. With this in mind...I have backed off on
probability of precipitation during the afternoon Thursday...with a likely slower evolution
of the precipitation. It appears the main forcing and moisture will
not arrive across the area until Thursday evening. At this time I
have left some low end probability of precipitation in during the afternoon...but if these
trends continue...we may be able to go dry all afternoon on
Thursday...especially across the eastern half of the area. Periods
of rain still appear to be a good bet Thursday night.

Kjb

//prev discussion...
310 PM CDT

Synopsis...another pleasant day today...but showers move in from the
west this evening. Showers and storms are expected tomorrow
afternoon and evening with a dry and warm end to the work week.
This weekend looks to have below normal temperatures with showers
and perhaps some thunderstorms along and south of I-80 for the first
half of next week.

Rest of today and tonight...
a large upper level ridge is over the Central Plains while a surface
high is centered over Lake Michigan. The ridge passes overhead
tonight while the surface high pushes east to the Northern Ohio
valley through eastern Quebec.

A weak shortwave is over eastern Nebraska and western Iowa with
showers over western Iowa. The showers will continue to push
east...but with dewpoint depressions at 20+ degrees...thinking the
showers will dissipate a bit before moving into central Illinois. As such
thinking the first showers will enter the far eastern County Warning Area after 5pm
CST. Used the local arw...rap...and hrrr to create the probability of precipitation for the
rest of today and tonight. Thinking a band of scattered showers will
slowly cross northern Illinois this evening in association with an upper
level vorticity streamer. The best coverage will be between midnight
and 4 am CST with a chance of light rain showers north of I-80. The
band then lifts north early Thursday morning as the ridge moves
directly overhead. Not expecting any significant precipitation
accumulations.

Cloud cover will increase through this afternoon and looking at min
temperatures in the upper 30s to middle 40s across the County Warning Area.

Tomorrow and tomorrow night...
an upper level trough moves over the plains tomorrow morning and the
trough passes over the region tomorrow night. The upper level low
associated with the trough passes farther north...along the
US/Canada border. The surface low also begins over the plains
Thursday morning and then moves over Illinois Thursday evening. Guidance
has the low progressing a bit slower and made the necessary
adjustments.

Rain and some storms move in from the west through the morning and
early afternoon. Instability is not extremely impressive with the
system so kept only a chance of thunder. Quantitative precipitation forecast amounts on the other
hand could be impressive with totals around a half inch of rain.

Winds will also be shifting from southeast in the morning to out of
the west overnight behind the low. Looking at a breezy afternoon
with south winds gusting to 35 miles per hour. The ample warm air advection will lead to maximum
temperatures from around 60 along the Illinois/WI border to the upper 60s if not
70 in the southern County Warning Area.

Friday and Saturday...
the upper level low continues to shift east reaching New England by
Saturday afternoon/evening. The surface low shifts east and
expecting precipitation to come to an end early Friday morning. West winds
will be gusting to around 30 miles per hour Friday and am thinking there could be
some dry air mixing down to the surface so lowered dewpoints a bit.

The air behind the cold front is not really all that cold since maximum
temperatures on Friday look to be around 70 degrees. A back door cold
front slides down from the north in the evening. Some guidance
members feature a band of precipitation forming ahead of the front over far
northern Illinois and Northwest Indiana. Put in a chance of precipitation since forcing
looks good...but have low confidence in the band of precipitation actually
forming. As such kept probability of precipitation in the chance range. Winds turn
northeast behind the front and overnight lows will be in the upper
30s to middle 40s.

Some guidance members keep a band of precipitation over the region Saturday
morning...but with high pressure moving over the region have low
confidence and went with a dry forecast Saturday. Skies will clear
but the cooler air off the lake will result in highs in the upper
40s to middle 50s along the lake...and then in the low to middle 60s in
the outlying areas.

Guidance features precipitation moving in ahead of the next low Saturday
night. Have low confidence in this round of precipitation as well...but
left a chance of rain in the forecast. Overnight lows will range
from the upper 30s to the middle 40s.

Extended...Sunday through Tuesday...
an upper level closed low shifts over the Great Plains Sunday
evening/night. Guidance differs on its exact path early next week
but does have the upper level low over the region through at least
middle week. Its associated surface low moves over the plains late
Sunday night/Monday morning and slides south of the County Warning Area Monday
night. The 12z guidance agreed a bit more about the surface lows
path...and have it moving through southern Illinois Tuesday.

In two words early next week looks chilly and wet. A boundary over
northern Illinois brings the lift for the first wave of precipitation early on
Sunday...but then multiple vorticity streamers pass through the
region as the low slowly shifts east. This will result in a chance
of precipitation throughout the extended. Will include a slight chance of
thunder along and south of I-80 where there will be limited elevated
instability but enough to perhaps support a couple of thunderstorms
Sunday night and Monday. The upper level jet dips far south through
Texas allowing cool air to spread across the region. As such highs
will vary between the low 50s north to middle 60s south with a cooling
trend through Tuesday. Luckily overnight lows look like they will
remain above freezing.

Jee

&&

Aviation...

//Ord and mdw concerns...updated 03z...

* chance of light rain late this evening/overnight.
* Strong/gusty south/southeast winds Thursday.
* Chance of rain showers late Thursday afternoon.
* Showers with MVFR visible/ceilings Thursday evening.
* Isolated thunderstorms and rain Thursday evening. Cms

//discussion...updated 00z...

A large area of VFR light showers over eastern Iowa and western Illinois
will continue to diminish as it moves east this evening with very
dry low levels ahead of this rain aiding its erosion and this
trend is handled fairly well with short term/hi res guidance.
Additional showers then redevelop in the vicinity of a warm front
late this evening and overnight...mainly over northern Illinois and this
will be the best timing for showers and may need a tempo but
confidence is too low both coverage and timing to include anything
with this forecast. Ceilings will remain VFR lowering into the 6-9kft
range.

East/northeast winds around 10kts will diminish through the
evening and turn more east/southeasterly toward morning. Speeds
will gradually increase toward sunrise as well but shortly after
sunrise the gradient will tighten and combined with low level
mixing...winds will rapidly increase to 20kt with gusts to 30kt
with the potential for stronger gusts across northwest Illinois
including rfd. Wind direction will remain mainly southeasterly
much of the day...slowly turning more southerly into the
afternoon. Winds will begin to diminish by middle/late afternoon...
first at rfd and then spreading east across the Chicago area
terminals by evening.

Low pressure responsible for the stronger winds will move across
the area Thursday evening and this appears to be the best time
period for showers. Have delayed shower arrival until 20z at rfd
and 01z at Ord. Once showers arrive...winds will likely quickly
diminish and ceilings could quickly lower through MVFR with a brief
period of IFR possible late Thursday evening or early Friday
morning before winds shift west/northwest. Thunder potential also
appears to be tied more to the cold front/wind shift which will be
later in the evening and while possible...confidence too low to
include in this forecast. Cms

//Ord and mdw confidence...updated 03z...

* medium for light rain late this evening/overnight.
* High for winds through the period...medium for timing changes.
* Medium for showers Thursday evening with associated cigs/vis.
* Low for thunderstorms and rain Thursday evening. Cms

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

* Friday and Saturday...dry/VFR.
* Sunday through Tuesday...chance of rain showers. MVFR possible.

&&

Marine...
219 PM CDT

Progressive flow will continue through Friday before a much more
amplified and locked pattern will set up for the weekend through the
middle of next week. The high pressure presently over the lake will
be scooted eastward early Friday as low pressure from the Central
Plains books eastward into northern Indiana. During Thursday this
will provide strong southerly flow. A fairly Stout inversion should
be present over the lake...but strong offshore winds in nearshore
areas should lead to 25 to possible 30 knots gusts. As a deep trough
sets up across the western U.S. And slowly moves east through the
weekend and early next week...surface high pressure will get
locked over the central Great Lakes. A tightening pressure
gradient on its southern and western sides will provide continuous
east to northeast surface winds. This looks like a potential
prolonged period of Small Craft Advisory criteria.

Mtf

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...10 am Thursday to 10 PM Thursday.

&&

$$

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