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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1241 am CDT sun Mar 29 2015

Short term...
242 PM CDT

Through Monday...

Rest of today and tonight broad midlevel height rises/subsidence
will continue to provide quiet weather conditions although
unseasonably cold conditions persist at the surface with a deep
upper level trough centered over the eastern Great Lakes driving
cold air as far as the deep south. On the back edge of the this
system...potent vorticity maximum will drop into the northern plains this
evening and tonight with attendant surface cold front pushing into
northern Illinois late Sunday afternoon. Out ahead of the
front...strong low level jet around 50-60 knots will transport a narrow
corridor of 0.8 inch precipitable waters and a warm nose with layer maximum temperatures of
2-5c which will result in primarily rain for this system. The
main concern for any mixed/wintry precipitation will be confined to the
first couple hours at onset of precipitation where antecedent dry air may
result in diabatic cooling of the column which will allow for
sleet/freezing rain and possibly a few flakes. Warming of the
column should be fairly rapid though which will help mitigate any
brief impacts from wintry p-type. Very modest upright instability
rooted between 600-700mb will move across the region midday and
could result in a few briefly heavier showers. Precipitation will
move into the western portion of the forecast area by middle morning
and exiting east by late afternoon. Behind the front...expect most
precipitation to end...however as the main upper wave moves overhead and
cold advection results in steep low level lapse rates and shallow
saturation mainly below 850 mb...cannot rule out a few sprinkles.

Also of concern Sunday will be the magnitude of winds. A tight
pressure gradient coupled with a ribbon of 6-7mb/3hr pressure falls
will result in windy conditions across the region. Both NAM and GFS
soundings show the potential for at least 30-40 miles per hour wind gusts and
possibly a few isolated gusts to 45 miles per hour. A little uncertain about
low level lapse rates and the efficiency of mixing with the expected
cloud cover and warming middle-levels so will hold off a Wind Advisory
for the time being...but will go ahead and issue an Special Weather Statement to highlight
the concern.

High pressure builds back over the middle Mississippi Valley Sunday
night with winds diminishing and a return to dry conditions. Ridge
axis shifts east across the forecast area during the day Monday.
Temperatures should be near normal with morning lows in the low to
middle 30s and afternoon highs in the low to middle 50s.

Deubelbeiss

&&

Long term...
242 PM CDT

Monday night through Saturday...

A clipper system will dig across the upper Midwest and western
Great Lakes Monday night. The GFS and European model (ecmwf) have come in much
better agreement on surface low track from 24 hours ago with the
GFS shifting south more in line with the European. The NAM is a
northern outlier. With the expected surface track...most likely
expect to see precipitation stay north of the forecast area Monday
night...and anything forming along the cold front during the day
Monday should develop to our south/southeast. We do get a brief
reinforcing shot of colder air that will slow our warning
trend...but 60s still appear favorable for especially Wednesday
and possibly Thursday depending on frontal passage timing. Broad upper level
ridge will build across the middle section of the country Wednesday
while low pressure moves east across the Canadian prairies. 850mb
temperatures are prognosticated to warm to around 10c which should translate to
surface temperatures in the 60s...and if anything feel forecast temperatures
for Wednesday may be too low at the moment. A cold front is
prognosticated to push across the region Thursday though there are
significant timing differences between the models which would
impact temperatures. Another cooldown looks possible late in the week
into next weekend with both the GFS and European model (ecmwf) tracking a deep low
across the Ohio Valley pulling colder air into the upper Midwest
and bringing another chance of snow to northern Illinois.

Deubelbeiss

&&

Aviation...

//Ord and mdw concerns...updated 06z...

* strong southerly winds developing soon after sunrise.
* Chance of low level wind shear between 09z-13z.
* One to possibly two hour period of graupel/sleet or snow
possible at onset of precipitation late this morning.
* Period of rain through early afternoon Sunday with MVFR
conditions. Chance of more brief rain showers toward sunset.



//Discussion...updated 06z...

High pressure will continue to scoot away from the region with
southerly winds increasing through daybreak. As robust pressure
falls envelop the region by mid-morning...wind speeds and gusts
will escalate with gusts to around 35 knots favored given strong
winds just off the deck. The direction is very likely to be
slightly west of due south...but not by much.

Precipitation is expected to fill in across Iowa during the pre-
sunrise hours and move into and across northern Illinois during
the middle-late morning. This precipitation looks to last about 3-4
hours in any one place and be somewhat showery...so fluctuations
in visibility and even ceilings are likely. At onset...observational
trends and model guidance do support a mix of rain with ice
pellets or possibly even snow. This could come down at a fairly
high rate...although is favored to last as a mix/frozen precipitation
for only a short period of time if it were to come to fruition.
Rain showers should continue into early-middle afternoon before an
end...with possibly some brief light showers as the cold front
moves into the area near/just prior to sunset. This front will
shift winds west which will remain gusty through this evening.



//Ord and mdw confidence...updated 06z...

* high in wind direction today. Medium-high in speeds.
* Low-medium in snow/sleet/graupel potential at beginning with
high that it would last no more than two hours at the most.
* High in precipitation timing and medium in visibility during.
* Low in true low level wind shear early this morning but high in 40-45 knots winds
above 1000 feet.



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

Monday...primarily VFR.

Tuesday...primarily VFR. Lake breeze/NE winds developing.

Wednesday...primarily VFR. Moderate-strong south winds possible.

Thursday...chance rain showers/thunderstorms and rain in morning...otherwise VFR.

Friday...low confidence. Rain/sub-VFR conditions possible.

Izzi

&&

Marine...
149 PM CDT

High pressure over the lake has been helping for quiet conditions
with light winds...but expect this to change tonight and
especially Sunday morning as this high shifts to the east and a
trough of low pressure approaches from the west. Lighter southerly
winds this evening will increase rapidly over the entire lake
overnight with south/southwest gales to 45 knots still expected.
However southerly flow is appearing to likely approach storm force
winds for a period Sunday morning through early Sunday afternoon.
With a high likelihood of this occurrence...have maintained the
current Gale Warning in effect for the entire lake but transition
the northern half to a Storm Warning by 14z and continue it
through 20z. Gales may linger a couple of hours beyond this end
time but should be diminishing as well. Gale Warning for the south
half remains the some...but with some possibility that the gales
could continue a couple of hours beyond the end time tomorrow
evening.

Made some minor changes to the current headlines for the
nearshore...primarily extending the Gale Warning to 00z as well.
Also...winds should pick up prior to the Gale Warning for the
Indiana nearshore and so did issue an Small Craft Advisory tonight into early
Sunday.

Rodriguez

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...4 am Sunday to 10 am Sunday.

Gale Warning...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...10 am
Sunday to 7 PM Sunday.

Gale Warning...lmz777-lmz779...4 am Sunday to 7 PM Sunday.

&&

$$

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