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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
639 am CDT Monday may 25 2015

Short term...
345 am CDT

Through Monday night...
for the remainder of the night and into Monday morning...precipitation
chances continue to wane as latest radar trends only show some scattered
rain showers which are decreasing in intensity. In spite of dewpoints in
the low 60s...and a strong low level jet...50 knots at 925 mb...there
is an element lacking which is seems to be preventing the
maintenance of rain showers/thunderstorms and rain over northestern Illinois/northwestern in. The main missing
elements is forcing. The surface and upper level low are way to the
west...over the northern plains. The shortwave climbing up the northwestern
periphery of the high amplitude ridge is being slowed by the high
amplitude...and is being forced to a more westerly track through the
upper Mississippi Valley than previously anticipated. So...for the
remainder of the overnight hours and the daytime hours Monday...have
back of to isolated-scattered rain showers. Will keep isolated ts mention for the late
morning hours...through around noon...with the daytime heating...but
confidence in any ts is rather low at this time. By this
afternoon...a middle level dry slot is expected to move across the
region...further limiting precipitation chances and it is quite likely that
the region will remain dry through the day. However...while the
precipitation chances are diminishing...the likelihood of strong winds is
still a concern. With the diminishing precipitation chances...less sky cover
than previously anticipated is also possible. A strong southwesterly
pressure gradient is setting up across the region between surface
ridging across the southeastern Continental U.S. And and low pressure from the upper
Mississippi Valley to the Western Plains. The strong gradient will
help drive the surface winds and the diminishing cloud cover could help
enhance deep layer mixing...tapping stronger winds aloft. Sustained
25-25 miles per hour winds are likely with gusts of 30-35 miles per hour.
Also...persistent warm/moist advection at the surface will bring
temperatures into the low to middle 80s with dewpoints in the lower
60s.

For Monday night...the breezy conditions will persist into the as
there should be little change to the surface and upper level patterns.
The upper ridge will lose a little amplitude as short wave energy
overtops the ridge...but general troughing will persist across the
upper Mississippi Valley through the Central Plains. The next
chance for precipitation will hold off until late tonight/early Tuesday
morning and the next shortwave lifts out of the Southern Plains and into
the middle Mississippi Valley. Ample low level moisture will
already be in place so it will just a matter of when the forcing
will get here. Again...there is some concern that the forcing will
be to the west...with the shortwave tracking through the middle
Mississippi Valley and into WI. So...will carry chance probability of precipitation for the
region...with likely probability of precipitation approaching the Pontiac area by early
Tuesday morning. The persistent warm/moist advective pattern and
breezy conditions should keep temperatures from dropping off
significantly overnight and lows should be in the middle to upper
60s.

Krein

&&

Long term...
239 am CDT

Tuesday through Sunday...

Tuesday through Wednesday...
500mb trough axis will slowly pivot east across the Central Plains
into the middle-Missouri Valley Tuesday...with a surface wave sliding north
across Iowa into central wisc late Tuesday. The surface feature is not
expected to be strong...however the gradient does tighten with
modest height falls in the lowest layers of the atmosphere. Some
instability does remain along with relatively moist surface conds as
dew points will remain in the upper 50s/low 60s. Precipitable water values will also
hover just over 1 inch...which is seasonal. Will hold onto mention
of rain and thunder...but could see this mainly focused in the Tuesday
morning timeframe.

With considerable solar shielding prognosticated for Tuesday...temperatures may
struggle to warm beyond the middle/upper 70s. But with guidance hinting
at a dry slot lifting northeast across the region Tuesday evening...this
could introduce some erosion to the cloud cover in the afternoon hours
Tuesday and allow some surface heating to occur and push temperatures to around 80.

Guidance lifts the dry slot overhead late Tuesday night...with the 500mb
trough axis arriving around daybreak Wednesday and lifting east across lower
Michigan. This will bring dry only slight chance probability of precipitation as weak surface
ridging begins to nudge east across the Missouri Valley towards the
forecast area midday Wednesday. It is possible that enough dry air
influence could allow for removal of the slight chance probability of precipitation Wednesday
afternoon. Thermal trough lags behind the frontal boundary Wednesday...so temperatures
could still warm back to around 80 and perhaps the low 80s in the
southern County warning forecast area.

Thursday through Sunday...
anti-cyclonic flow arrives Thursday with at least one period of dry
weather expected for northern Illinois/northwest in. Only a couple
ensemble members continue to forecast probability of precipitation along the southern forecast
area Thursday...but could see this trending dry in the next few
solutions. With warm temperatures aloft expected Thursday and thin cloud
cover...Thursday highs could peak in the low 80s.

Then focus turns towards the upcoming weekend and a pattern shift
away from the warm/humid conditions. Ensembles show minimal spread
in the late week periods...with the upstream ridge over
Alaska/western Canada and a downstream middle-level ridge across the
southeast Continental U.S.. both of these features in time begin to
weaken...with the longwave pattern transitioning into a semi-zonal
orientation by the second half of the weekend.

Oper guidance has been consistent with progging a robust 500mb
trough to slide south from central Canada Friday...along with a strong
Canadian surface ridge on it/S heels. With a channel of moisture
remaining across the Southern Plains stretching northeast through
the upper Midwest...this boundary will become active with a surface wave
developing across the Southern Plains. This low will then lift
northeast towards the County warning forecast area Friday with a cold front arriving across the
forecast area Sat. Winds look to shift to the north...with much
cooler air sliding south and likely knocking temperatures back to sub-
seasonal conds in the upper 60s/near 70 degree range Sat/sun. If
northeast winds materialize...next weekend areas adjacent to Lake
Michigan along the Illinois/in shoreline could be locked in the upper 50s to
low 60s for highs with a cold flow off of Lake Michigan.

Beachler

&&

Aviation...

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

* strong and gusty south winds through early evening.

* Chance rain showers/thunderstorms and rain late tonight through tomorrow.

Krein

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

Main concern for today will be strong...gusty south winds. As of
11z this morning...winds are already gusting to 25-30kt and expect
that with deeper mixing later this morning...should be able to tap
into a 50 knots low level jet to mix down gusts to 35 knots or even
slightly higher. The strong low level jet should move off to the
east by early afternoon...so while winds should still be strong
and gusty...peak gusts should lower to around 25kt.

With the persistent fetch of low level moisture into the
region...a layer of MVFR stratocu has overspread the region.
Expect that bases should not lower much less that 1800ft above ground level.
There is some uncertainty as to the duration of the MVFR cloud
layer...but with little change to the overall pattern through the
day...there is a chance that the MVFR ceilings could hang in through
much of the day. However...there is also a chance that stronger
mixing in the afternoon hours could allow for the MVFR deck to
scatter out.

Latest radar trends indicate that an area of rain showers lifting northeastward
from central Illinois into Indiana. The trajectory of the precipitation area should
keep the bulk of the precipitation east and south of gyy...though a few scattered
rain showers area not out of the question. Also...some rain showers associated
with a middle level impulse lifting north through Iowa and into western
WI is generating scattered rain showers as well. It is also possible that rfd
could see a few rain showers this morning...but the probability is too low
to include in the taf as the latest radar trends indicate a
weakening trend there.

A secondary low is expected to develop over the upper Missouri
Valley this evening...keeping a modest pressure gradient over the
region. So moderate...gusty winds should persist well into
tonight. Another strong middle level impulse is expected to track
through the middle Mississippi Valley and into Wisconsin late
tonight...bringing a renewed chance for rain showers/thunderstorms and rain to the region
late tonight and tomorrow.

Krein



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

* medium to high confidence in wind trends.

* Medium to high confidence that the terminals should remain
dry...though there is a small chance for some rain showers in vicinity of the
terminals through the morning hours.

* Medium confidence in ceiling trends.

* Medium confidence in precipitation timing late tonight.

Krein

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

Tuesday night...rain showers likely/chance thunderstorms and rain. Gusty southwest winds.

Wednesday...chance of rain showers/thunderstorms and rain early. West winds turning east.

Thursday...chance rain showers/thunderstorms and rain late. Southeast winds.

Friday...likely rain showers/chance thunderstorms and rain. MVFR possible. Southwest winds
becoming northeast overnight.

Saturday...likely rain showers/chance thunderstorms and rain. MVFR possible. Northeast
winds.

Sunday...chance thunderstorms and rain. MVFR possible. Northeast winds.

Krein

&&

Marine...
304 am CDT

Low pressure will be steadily lifting north across the Missouri
Valley this morning...reaching Lake Superior by this evening and
continuing to slide east/northeast towards Quebec by Tuesday
morning. A frontal boundary will remain over the far northern
portions of Lake Michigan. Meanwhile gusty southerly winds are
expected to increase this morning...with occasional gales for the
nearshore waters of IL/in. Across the open waters...expect the
warm/moist air to struggle to reach the surface of the lake with some
degree of lake stability. Waves will steadily build across the
northern half of the lake as a result of the long fetch. Still
feel that gusts will reach 30kt across the lake...however
confidence has diminished slightly as new guidance has become a
little less with the wind fields. Nonetheless...waves for the
northern half of the lake will likely top out in the 4 to 7 feet
range with occasional waves reaching 9 feet this afternoon/evening.

The gradient quickly diminishes this evening...but expect
southerly winds to persist. This will allow waves to subside to 3
to 5 feet. Another low will arrive Tuesday...with winds nudging back up
however warm air will continue to flow across the lake and could
further enhance the lake stability and keep waves from building to
large.

Beachler

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...nearshore waters
until 9 am Monday.

Gale Warning...nearshore waters until 3
PM Monday.

Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...3 PM Monday to 4 am Tuesday.

&&

$$

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