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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1023 PM CDT sun may 24 2015

Mesoscale discussion...
722 PM CDT

Continue to monitor what at present are widely scattered storms
across north central and northeast Illinois. Analysis of
observations indicate the surface warm front draped from near
Clinton Iowa to Rochelle to near Joliet and Lansing. The current
Tornado Watch area overlaps the better deep and low-level shear
for storm updraft persistence...along this dew points appeared
pooled of 65 to 70...aiding in low-level cape that should enable
surface-based potential for at least a couple hours after dark.
This is verified by southerly winds across central Illinois into
the southern County Warning Area still gusting at 25-30 kts as of 715 PM.
However...uncertainty exists on how widespread coverage of storms
will be...especially with somewhat of a downtick the past 30-45
minutes. High-resolution guidance shows spread in
solutions...although an agreement that east of Illinois Highway 47
/much of the Chicago Metro/...that coverage will be very limited
to none through this evening. Recent radar and satellite trends would
tend to support the attention continues to be on north
central Illinois.

A few storms have exhibited rotation...including one in the dvn
County Warning Area that did produce a tornado near Eliza per local storm report.
Earlier a storm that tracked from Lee County into Ogle County from
610 to 650 PM exhibited rotation as it neared and overtopped the
boundary. This verifies the northern end of the warm sector to the
boundary being a helicity rich environment. As long as storms can
track along it /I.E. Conditional threat/...and the boundary layer
can remain mixed with parcels lifting from the surface or near
it...then a brief tornado threat will continue this evening.



Short term...
209 PM CDT

Through tonight...

Very sharp warm front has been gradually lifting northward through
the afternoon hours. Temperatures have soared into the 80s behind
the front...with muggy middle 60s dewpoints as well. Meanwhile far
northern Illinois is holding onto the 70s with temperatures holding around
60 at the lake with onshore winds. Aloft... a deep and closed upper
low is spinning over northern Colorado. A negatively tilted trough
axis out ahead of the low into northern Texas with a modestly strong
shortwave along and ahead of it. Showers and thunderstorms continue
ahead of this feature. Closer to home...upper level ridging is in
place across the eastern half of the area...and no notable
shortwaves other than some weaker waves are found in SW flow aloft
across far western Illinois. More organized shower activity from
I-39 westward associated with this...but much of the area remains
in broad and moist SW flow. Therefore scattered showers continue
to fester farther eastward though the are struggling a bit more in
the less favorable environment.

Expect as the afternoon continues that showers remain likely in our
western areas...while much of Illinois will be susceptible to
scattered showers through the afternoon period as well. The showers
put down nearly two tenths of an inch in an hour at Peoria...and
with precipitable water values on the rise...some brief downpours remain possible.
Instability is modest and focused along and well south of I-80 where
an isolated mention has been held...but we do have some strikes in
LaSalle County as of this writing.

Looking farther downstate behind this batch of showers cumulus field
is fairly tempered with more agitated clouds in Missouri...and more
so farther south in the arklatex region. Have tried to show somewhat
of a downward trend in chances for precipitation across NE Illinois
tonight in between the current activity before the trough axis
begins pivoting northward. Moist isentropic lift remains a bit more
robust along and west of I-39 where occasional showers will continue
even this evening and held onto likely probability of precipitation longer in these areas.

Overnight is when things will ramp up across our local area. The
negatively tilted trough axis gets shunted northeastward on a
potent middle/upper level jet. Low level moisture advection increases
which will bring the most organized period of showers and
isolated/scattered thunderstorms across the area...after midnight
and into the morning of Memorial Day. With a very strong low level
wind field and and closed surface low passing across eastern
Iowa...southerly winds will be ramping up considerably late
tonight into Memorial Day morning.

More on the winds tomorrow and the long term discussion soon...



Long term...
302 PM CDT

Monday through Sunday...

Scattered showers and storms will likely start the Memorial Day
Holiday but should be moving quickly northeast in tandem with
deepening low pressure advancing from northeast Iowa early in the
morning to northern WI by midday. Showers should end from southwest
to northeast between 8 am and 11 am for most locations. The bigger
story on the Holiday will be the gusty winds associated with this well as warmth...with some locations possibly reaching
upper 80s.

The synoptic setup is in place for a three-four hour period of
strong winds Monday morning...with global and high-res models
indicating 1.) System dry slot nose advancing over the
area...2.)Strong surface pressure fall/rise couplet quickly moving
through in the morning...and 3.) Steep low/level lapse rates and q-
vector divergence indicating a portion of the low-level jet winds
will be mixed down in gusts. The strongest southwest winds gusting
to 40 to 45 miles per hour with a possible 1-2 hour time of 50+ miles per hour gusts look
to be between 9 am and 2 PM. This is in the immediate wake of the
showers...though some of the last showers/storms could tap some
of these higher winds with isolated strong gusts before ending.
Right now the wind forecast is a little below Wind Advisory
criteria...but evening and overnight shifts will have to consider
this possibility looking at further guidance regarding magnitude and
duration of these well as taking into account the Holiday
and amount of outdoor activities/parades.

These winds will transport a strong thermal ridge into the area.
Subsidence within the dry slot will likely aid in scattering clouds
by early afternoon. So sun should further heating on top of the low-
level warm advection. Given a mild low tonight...expecting highs to
easily be able to get into the 80s areawide. With some central Illinois
locations knocking on the door of upper 80s today...could envision
these type of readings and even a 90 or two spreading over
much of northern Illinois and northwest in on Memorial Day...although the
increased dew points are expected to limit that upper threshold.

With a very broad longwave trough across the western and central
U.S. Gradually deamplifying midweek...a generally unsettled weather
pattern continues through Tuesday and Tuesday night. Height falls
and just more favorable jet support for lift will spread over on
Tuesday. A short wave trough with a slight negative tilt will be in
line with this...and support for showers and at least scattered
storms is agreed well upon in guidance. Have continued the upward
trend with probability of precipitation. With the rain around...temperatures on Tuesday
look to be around 10 degrees cooler than Monday.

Beyond...the main refinements were to try to indicate any times
where probability of precipitation may not need to be mentioned. Wednesday afternoon
through Thursday...and possibly a little longer...look to be a
period of building heights for much of the area and would be the
window where chances of rain would seem small. The chances for
precipitation increase in global ensembles as we enter next
weekend...with the 12z GFS particularly wet. At this time...wpc
does have around an inch in their deterministic day 6-7 guidance.




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

* scattered showers and isolated thunderstorms and rain overnight.

* Winds...more slowly than expected...turning south and

* Strong and gusty south winds Monday morning and early afternoon.


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

Timing is uncertain...but a warm front lifting north through the
area this evening will turn winds from east to southeast and
eventually to south. Scattered showers and isolated thunderstorms
remain possible at the terminals this evening...but more
favorable area is to the west. With southerly winds bringing
increased moisture overnight...additional rain showers/thunderstorms and rain is expected
to develop late. By morning these southerly winds strengthen
considerably and become quite gusty as well.


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

* medium to low confidence on wind trends overnight but high

* Medium confidence on convective trends.


//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...

Tuesday...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/slight chance thunderstorms and rain. Southeast winds.

Friday...chance rain showers/slight chance thunderstorms and rain. Southwest winds.

Saturday...chance rain showers. East winds.



330 PM CDT

A warm front will lift northward into the southern part of the
lake this evening...or possibly not until overnight...but either
way winds will turn southerly during the night. Higher dew point
air will advect northward on these southerly winds allowing for
potential fog development. As low pressure lifts north through
Wisconsin on Memorial Day morning....south to southwest winds
will encompass the entire lake. Also...wind speeds will rapidly
increase in the Illinois and Indiana nearshore waters...with
offshore winds reaching gale criteria during the morning and
lasting into at least early afternoon. Over much of the open
waters...stable air will likely keep the strongest winds from the
surface...although still 20 to 30 knots winds are expected and
lasting into Monday night.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...4 am Monday to 9 am Monday.

Gale Warning...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...9 am
Monday to 3 PM Monday.

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



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