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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
900 am CDT Thursday may 28 2015

Short term...
312 am CDT

Through Thursday night...

For the remainder of the overnight hours...quiet conditions will
prevail as high pressure centered over southern Ontario bridges an
old frontal trough across the Ohio Valley to merge with the
subtropical ridge over the southeastern Continental U.S.. mostly clear skies and light
and variable winds will persist through the night as temperatures drop into
the upper 50s to lower 60s.

Through the morning hours...the high pressure will shift to the east
as a multi-centered trough develops over the plains. A northern stream
shortwave will lift out of the Dakotas this morning...lifting
quickly to southwestern Ontario by early this evening. The surface reflection
of a southern stream shortwave developing over the western Central Plains
will be much less progressive...remaining in western Kansas by this evening.
The trend for the day today will be for increasing southeasterly
winds...though speeds will remain under 10mph. The subtropical
ridge extending from the southeastern Continental U.S. Through the Texas Gulf Coast
along with ongoing convection over the Southern Plains will prevent any
significant return flow of low level moisture today...with dewpoints
remaining in the low to middle 50s under mostly sunny skies. Ample
sunshine and relatively dry air should allow temperatures to reach
into the middle 80s across the area. The possible exception to the
warming will be the Lake Michigan coastal area. A weak pressure
gradient and light southeasterly synoptic scale winds...combined with
differential heating between the still Cool Lake Michigan water and
the warmer land will likely allow for a lake breeze to form. There
is still some question as to the timing of the lake breeze
development and how far inland the boundary will penetrate...but
current thinking is that surface winds should become Ely along the Illinois
shore and northerly-northwesterly along the Indiana shore...bringing cooler air
inland. Also anticipate that the boundary may push inland into
northestern Illinois...but this will likely not until just before the period of
maximum heating. So...while the inland locations will likely see temperatures
in the middle 80s...the cooling influence of the developing lake breeze
should limit temperatures along the Lakefront to the middle 70s before
dropping off through the late afternoon and early evening.

For Thursday night...the forecast focus will shift to increasing
precipitation chances. The larger scale pattern will be slowly progressive
through the evening...though the northern stream shortwave will track
east quicker than the southern stream shortwave that will be hindered
somewhat by the subtropical ridge extending across the southeastern Continental U.S..
however...there will be some progression to the pattern...with the
subtropical ridge retreating to the east...opening up the Gulf of Mexico and
introducing a return flow of moisture across the region. Latest
guidance suggests that dewpoints will rebound into the lower to
middle 60s by early Friday morning over much of the County Warning Area. The northern
stream system will drag a cold front through the upper Mississippi
Valley overnight tonight while the southern stream system moves into the
upper Ohio Valley. Isentropic lift in the warm sector of the system
should be adequate to generate showers and thunderstorms over the
middle and upper Mississippi Valley tonight and spreading into the
western portions of the County Warning Area...generally along and west of the I-39
corridor into the pre-dawn hours. Given some timing differences of
the main shortwave paths and how quickly the subtropical ridge will
retreat will impact the timing of precipitation spreading east across the
County Warning Area. So...while there is high confidence in precipitation reaching the I-39
corridor during the pre-dawn hours...there is a chance that some
activity could spread as far east as the Fox Valley and the I-55
corridor by daybreak...so have spreaded some low probability of precipitation a little
farther east than previously anticipated. As the warm/moist
advection ramps up tonight...overnight lows will be a bit higher
than the past couple days...with lows in the lower to middle 60s.

Krein

&&

Long term...
247 am CDT

Friday through Wednesday...

Friday...
main challenges for Friday will be on precipitation timing and coverage.
Guidance has indicated some slowing to the departing surface
ridge...with middle-level heights not beginning to fall until after
daybreak Friday. The more potent lobe of vorticity will be rounding the
trough axis early Friday...then forming a surface wave feature across the
Southern Plains. The quasi-stationary boundary will be oriented from
the Southern Plains northeast through the upper Midwest. But with
the minor delay in departing ridge...this may provide a weak buckle
to the greater precipitation coverage only temporarily Friday morning. By
midday the boundary will be drifting east...with abundant low level
moisture advecting north into this feature. Dew points will hover in
the low/middle 60s...as precipitable water values approach 1.5 inches. 850mb v-wind
component suggests winds will not be too strong...so at this time
would not suspect as the surface wave lifts northeast along the boundary
growth in convection to be very large. However...it does appear to
be an ideal setup for embedded thunder. With the upstream middle-level
vorticity strengthening over central Canada...this will likely allow the
quasi-stationary boundary to further enhance. This could aid in
precipitation production with the boundary sliding overhead Friday
night.

Temperatures will equally be a challenge Friday...given the thick solar
shielding poised to limit surface heating. The abundant low level
moisture will absorb any heat...making it difficult to warm and
perhaps keeping temperatures a few degrees lower than currently prognosticated.
Have nudged temperatures in the far northern County warning forecast area down a couple degrees to
reflect this into the upper 70s...meanwhile further south have held
onto the 80 to low 80s range.

Saturday through middle of next week... ensembles have continued to
forecast a high amplitude ridge across western Canada at the start of
the weekend...then slowly weakening this feature as the central
Continental U.S. Transitions into a relatively flat west-east flow by Tuesday/Wednesday
of next week. The main story for the weekend will be on the temperatures
abruptly changing from early Summer to mid-Spring...with a Stout
frontal boundary sweeping across the forecast area Sat midday.
This will bring much cooler air as winds quickly veer
north/northeast and bleed the much cooler marine air from Lake
Michigan over northeast Illinois/northwest in Sat/sun. Highs Sat may
struggle to warm beyond the upper 50s across northeast Illinois/northwest
in...with only low 60s elsewhere.

With flow becoming somewhat flat towards Monday/Tuesday across the
central Continental U.S....this will help to flush the surface ridge east from
the Great Lakes region. This will likely allow temperatures to slowly
return to seasonal conds by middle of next week...however
currently guidance indicates a dry stretch as middle-level heights may
begin to nudge up closer to Tue/Wed. But confidence in the final
few periods of the extended are currently low.

Beachler

&&

Aviation...

//Ord and mdw concerns...updated 14z...

* east-southeast winds around 10 knots this afternoon following lake breeze
passage.

* Chance of rain showers/thunderstorms and rain early Friday morning.

Krein/mm

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

As weak high pressure settles over northern Illinois/in/srn Lake
Michigan...winds have dropped of to calm/light and variable. Some
patchy ground fog has been observed in sheltered locations...but
is not expected at the terminals. Later this morning...a
deepening trough over the central/northern plains will help turn winds to
prevailing southeasterly...with speeds less than 10 knots. The weak southeasterly
gradient will remain in place through the day...and should be weak
enough to allow for a lake breeze to develop. The latest high res
model guidance agrees with conventional Wisdom that a lake breeze
should develop under the expected pattern...with winds shifting to
just south of east and increasing to around 10kt.

As the surface trough begins to move east across the northern plains
and toward the upper Mississippi Valley lake Thursday night...
winds will veer to southerly. Warm/moist advection under the
southerly flow will bring increasing low and middle clouds and a
chance of rain showers/thunderstorms and rain by the early morning hours.

Krein

//Ord and mdw confidence...updated 14z...

* medium confidence in lake breeze wind shift timing and
speed/direction following lake breeze passage.

* Medium confidence in precipitation timing late tonight.

* High confidence in all other forecast elements.

Krein/mm

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

Friday night...periodic shra/tsra. MVFR possible. Southwest winds.

Saturday....chance rain showers early. MVFR possible. Strong north-
northeast winds.

Sunday...VFR/wx none. Northeast winds.

Monday...VFR/wx none. East winds.

Tuesday...VFR/wx none. Southeast winds.

Wednesday...slight chance rain showers. South winds.

Krein

&&

Marine...
247 am CDT

Weak surface ridge of high pressure centered over eastern
Ontario...with some lingering influence of dry air across Lake
Michigan for the bulk of today. The western Great Lakes will begin
to see the southerly flow this afternoon/evening...as winds
increase. A slow moving frontal boundary oriented over the
Southern Plains stretches north to just west of Lake Superior.
This boundary will begin to inch east towards the northern
portions of Lake Michigan Friday morning...allowing the gradient to
tighten and southerly winds to increase. Then the frontal boundary
will slide southeast slowly across Lake Michigan...along with the
gradient tightening by Friday night into Saturday. A strong ridge
of high pressure will drop south from central Canada along the
back edge of the front...helping to increase a period of north gales
Saturday for the central/southern portions of Lake Michigan.
Guidance has backed off a touch on the wind speeds...however a
gale watch may still need to be issued. The strong winds coupled
with much cooler air will easily build waves for the southern half
of the lake Saturday. Cool air and north/northeast winds will
continue to occur Sat night through sun...likely maintaining larger
waves along the Illinois/in nearshore waters.

Beachler

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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