Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
902 am CDT Thursday Apr 17 2014

Discussion...
257 am CDT

Quiescent weather is set to continue into this weekend with an
enjoyable weekend shaping up if you are a fan of 60s and
potentially 70s by Sunday. These temperatures were the primary
forecast challenge this morning.

Synopsis...
the main rossby wave on this side of the Globe is retracting
northeast from eastern Canada with still low heights and a broad
but low amplified trough left in its wake across southern Canada
and the central Continental U.S.. within that feature there are multiple
perturbations with the most well-defined circulation across the
upper Great Lakes and responsible for over a foot of snow for
parts of the Twin Cities into northern WI yesterday into last
night. A narrow plume of middle-level Pacific moisture associated
with this wave is drifting over the area this morning. That is
most of the contribution to meager 0.3 to 0.4 regional precipitable waters
sampled last evening...as the low-levels remain quite dry and act as a
hindrance for much or any precipitation across the area tonight. At the
surface...the system low is entering western WI early this morning
with a surface cold front extending southward into Iowa and MO. The
baroclinicity and convergence along this front will weaken
considerably this afternoon into evening as it moves into the
area.

Today and tonight...
cloud cover should undulate some today with the Pacific moisture
aloft and some lower clouds oozing into north central Illinois behind
the surface cold front. There likely will be a sharp increase in
middle/high clouds this afternoon for areas along and east of
Interstate 55 as a shearing impulse across western Kansas this morning
approaches the area. The 850mb/925mb temperature climatology would
support upper 50s to middle 60s across the area...but that as usual
will depend on duration of some of the aforementioned cloud cover.
Offshore flow is expected across downtown Chicago so no lake
cooling today...but as winds turn northerly this evening and a lake
component likely pushes inland...a fairly quick drop to around 40
degrees could occur during the middle to late evening. Moisture and lift
support for any rain with the sheared wave tonight continues to
look less and less impressive...and really only for areas along
and east of Interstate 55 is there a low chance. Have refined the
temporal resolution some...but all in all this is a low chance and
quantitative precipitation forecast event.

Friday and beyond...
building upper heights over surface ridging will be seen on Friday.
The light surface flow should allow for a Sharp Lake breeze to
develop with around a 10 degrees contrast from downtown Chicago to
outlying areas. Most outlying areas may see similar temperatures to
that of today.

Then moderation will unfold this weekend as guidance is in good
agreement of 500mb heights above 570dm and 850mb temperatures
climbing to 6c to 10c through the weekend. The main jet stream
axis will be in southern Canada and a closed low over the
southeast U.S. Will keep the pattern less progressive at our
latitude this weekend than originally forecast. We continue the
trend of the previous shift to collaborate an increase in
temperatures...namely on Sunday with southerly flow up to the lake
front and less cloud cover. Climatology would support 70 degree
temperatures up to the northern forecast area and depending on
cloud cover could realize even warmer. The next system is forecast
to move over the area around Monday but again confidence on this
timing is low. Dew points look to be up into the 50s and possibly
Lower/Middle 60s with this passage...so as mentioned before a
possible chance for thunder. But the synoptic look and magnitude
of this system within model guidance presently does not look
impressive.

Mtf

&&

Aviation...

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

* winds becoming west this afternoon...then NE by Friday morning.

* MVFR ceilings are possible this afternoon through tonight.

Jee

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

Low pressure is slowly lifting across Wisconsin with a cold front
pushing east into north central Illinois early this morning. S to
south-southeast winds ahead of the front will be in place early...then will veer
to the SW by middle to late this morning. There is currently a broad
MVFR/IFR stratus deck in place across much of Iowa behind the front.
Expect these ceilings to push east across rfd...but for now do
anticipate it moving into the Chicago terminals as the front
continues to weaken. High pressure will build into the upper Midwest
tonight causing winds to veer to the north..and possibly
northeast....though should be fairly light...less than 5 knots much of
the late evening and overnight hours. Light winds in place tomorrow
will allow an early afternoon lake breeze to push inland Friday with
winds picking up slightly out of the northeast.

Bmd

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

* high confidence in wind trends...low-medium in timing.

* Low confidence in if MVFR ceilings move in.

Jee

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

Friday night...VFR.
Saturday...VFR.
Sunday...VFR.
Monday...chance rain. MVFR/IFR possible.
Tuesday...VFR.
Wednesday...slt chance rain.

Bmd

&&

Marine...
313 am CDT

Weakening area of low pressure will lift from Iowa into Wisconsin
today with moderate southerly flow ahead of the low. A Small Craft
Advisory remains in effect...primarily for the Illinois nearshore
waters...but expect conditions to improve by middle morning after winds
turn westerly and diminish behind a cold front. High pressure will
build across the region and should keep winds relatively light
Friday though Saturday...then winds freshen up some Saturday night
into Sunday ahead of another weak low. Otherwise...it appears the
next chance for moderate to strong winds will be from midweek on
next week as a potentially more significant low pressure system could
impact the region.

Bmd

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...Illinois nearshore waters until 10 am Thursday.

&&

$$

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