Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181941
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
241 PM CDT SAT APR 18 2015

.UPDATE...
1003 AM CDT

MINOR CHANGES THIS MORNING TO TEMP AND SKY TRENDS THROUGH THE
AFTERNOON.

MAIN CHALLENGE TODAY WILL BE WITH HOURLY TEMPS...WITH QUICKLY
MOVING COLD FRONT PUSHING SOUTH THROUGH THE CWA AT THIS TIME. THIS
HAS LOWERED TEMPS FROM THE MID TO UPPER 60S TO THE LOW 60S
INLAND...AND UPPER 40S TO 50S FOR AREAS NEAR THE LAKE.
HOWEVER...MAIN COLD ADVECTION PUSH REMAINS WELL NORTH OF THE CWA
AT THIS TIME. WITH RELATIVELY HIGHER 950/850MB TEMPS STILL IN
PLACE TEMPS SHOULD RECOVER SLIGHTLY THROUGH THE AFTERNOON...WITH
70 DEGREE TEMPS STILL LIKELY OUTSIDE OF LOCATIONS IN FAR
NORTHEAST ILLINOIS AND FAR NORTHWEST INDIANA. INCREASED CLOUD
COVER THROUGH THE AFTERNOON WITH FAIRLY DENSE CIRRUS MOVING
OVERHEAD PER IR IMAGERY AND WITH FAIRLY CLOUDY CONDITIONS HERE AT
THE OFFICE...WITH PARTLY CLOUDY TO PARTLY SUNNY SKIES LIKELY
BECOMING MORE OVERCAST LATE THIS AFTERNOON INTO THE EVENING.

RODRIGUEZ

&&

.SHORT TERM...
337 AM CDT

THROUGH SUNDAY...

SOME CHANGES ARE IN THE AIR FOR THE NORTHERN HALF OF THE AREA TODAY
AND LOCATIONS CLOSER TO THE LAKESHORE...WHILE SOUTHERN AREAS WILL
ENJOY SUMMER LIKE CONDITIONS THAT PREVAILED ON FRIDAY.

THE CLOSED UPPER LOW THAT WAS OVER THE FOUR CORNERS AS OF THIS TIME
YESTERDAY IS NOW ONLY CENTERED OVER SOUTH CENTRAL COLORADO. THE
CLOUD SHIELD IN THE WARM MOIST ADVECTION SECTOR AHEAD OF THE LOW IS
GETTING CLOSER TO THE AREA...BUT STILL LARGELY SOME MID AND HIGH
CLOUDS DRIFTING THROUGH. VERY MILD CONDITIONS ARE IN PLACE WITH
DEWPOINTS HOLDING IN THE MID 40S NORTH TO LOWER 50S SOUTH KEEPING
TEMPS IN THE 50S OUTLYING AREAS TO NEAR 65 DOWNTOWN IN THE HEAT
ISLAND DUE TO THE MILD SW OFFSHORE FLOW.

TODAY AND TONIGHT...THE FIRST CHANGES COME IN THE FORM OF A SHALLOW
COLD FRONTAL BOUNDARY THAT WILL SINK SOUTHWARD DOWN THE LAKE IN THE
COMING HOURS...WITH NORTHERLY WINDS SHIFTING NE THIS MORNING. EXPECT
A RAPID TEMPERATURE DROP DOWNTOWN JUST AFTER DAYBREAK AS THIS OCCURS
WITH TEMPS HOLDING STEADY IN THE 50S MUCH OF THE DAY. THIS BOUNDARY
WILL CONTINUE TO OOZE SOUTHWARD BUT WILL BE HALTED BY THE INCOMING
SYSTEM AND WARM SSE FLOW.  THEREFORE EXPECT QUITE A CONTRAST IN
TEMPS TODAY AS HIGHS IN THE TIER OF COUNTIES CLOSE TO THE LAKE PEAK
THIS MORNING IN THE 60S THEN COOL...WHILE AREAS SOUTH LIKE PONTIAC
REALIZE FULL MIXING AGAIN AND PUSH THE UPPER 70S. EXPECT MORE CLOUDS
TO FILTER IN THROUGH THE DAY...NOT ONLY ALONG THE FRONT BUT IN WARM
SECTOR AS WELL. SOME HI-RES GUIDANCE TRIES TO BREAK OUT SOME SHOWERS
ALONG THE BOUNDARY AS WELL...BUT HAVE MAINTAINED A DAY FCST AT THIS
TIME WITH THE UPPER LEVEL RIDGE IN PLACE LIKELY KEEPING CLOUDS
SHALLOW IN NATURE. WITH THE SYSTEM CONTINUING ITS SLOW MOVEMENT
TONIGHT..THE SURFACE LOW WELL TO THE SOUTH AND MOISTURE ADVECTION
CONFINED THERE AS WELL...HAVE CONTINUED PREVIOUS SHIFTS TREND TO
SLOW POPS AND GONE DRY OVERNIGHT AS WELL.

SUNDAY...SURFACE LOW PRESSURE DOWNSTATE INTENSIFIES AND LIFTS NE
AHEAD OF THE INCOMING COLORADO UPPER LEVEL LOW. MODELS DISAGREE ON
THE STRENGTH OF THE INDIVIDUAL LOWS THAT FORM...BUT MANY DO LIFT THE
FIRST ONE THROUGH EASTERN IL/WESTERN IN. AS THIS OCCURS...A WARM
FRONT WILL LIFT NORTHWARD AND BECOME THE FOCUS FOR SHOWER ACTIVITY
WITH A 30-40 KT LOW LEVEL SE FLOW DRIVING INTO THE FRONT. WITH
PRECIPITABLE WATER VALUES UP AT 1.25 INCHES OR SO...SOME 2 S.D.
ABOVE NORMAL AS NOTED IN THE SPC PRECIPITATION DISCUSSION...ALONG
WITH A DEEP MOIST COLUMN AND HIGH FREEZING LEVELS SHOULD PROMOTE
EFFICIENT RAIN PRODUCERS. THE FOCUS OF THIS ACTIVITY AT THIS TIME
WILL BE ALONG THE EASTERN HALF OF OUR AREA AS IT LIFTS
NORTH...THOUGH THIS A COMPLICATED EVOLUTION WITH SEVERAL SMALL SCALE
DISTURBANCES IN PLACE.  INSTABILITY IS WEAK AND VERY HIGH
BASED...BUT DECENT FORCING AND PARCELS LIKELY UNDERGOING CHARGE
SEPARATION...ISOLATED THUNDER MENTION SEEMS A REASONABLE COURSE OF
ACTION.

KMD

&&

.LONG TERM...
352 AM CDT

SUNDAY NIGHT THROUGH FRIDAY...

PRECIPITATION WILL CONTINUE LIFTING NORTH AND EAST AHEAD OF THIS
FIRST WAVE. WITH A MODEST CYCLONE...EXPECT SOME DEFORMATION
PRECIPITATION TO CONTINUE DURING THE EVENING HOURS.  SOME REMNANT
ENERGY ASSOCIATED WITH THE COLORADO LOW WILL LARGELY DRAG BY TO THE
SOUTH...BUT OUR FOCUS WILL BE ON THE NORTHERN STREAM UPPER LOW THAT
WILL DOMINATE OUR WEATHER PATTERN FOR A FEW DAYS. A COLD FRONT ON
THE BACK SIDE OF WHAT APPEARS TO BE A SECONDARY LOW WILL ALLOW RAIN
SHOWERS TO CONTINUE THROUGH THE NIGHT AND INTO MONDAY MORNING BEFORE
DIMINISHING IN THE AFTERNOON.

THE UPPER LEVEL PATTERN BECOME MORE AMPLIFIED NEXT WEEK AS RIDGING
REBUILDS IN THE WEST AND A STRONG UPPER RIDGE IN THE ATLANTIC WITH
THE UPPER LOW STUCK WITH NOWHERE TO GO.  EXPECT SEVERAL COOL DAYS
WITH SMALL CHANCES FOR RAIN SHOWERS AS WE REMAIN ON THE SOUTH SIDE
OF THE NOW SLOW MOVING NORTHERN STREAM LOW AND ELOGNATED VORT MAXES
PIVOT SE ACROSS NE IL/NW IN. WITH THE GROWING SEASON
UNDERWAY...EXPECT FROST CONDITIONS MAY BECOME A CONCERN IN
OUTLYING AREAS.

KMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* EAST NORTHEAST WINDS ABOVE 10 KT.

* MVFR CEILINGS LIKELY BY MIDDAY SUNDAY...WITH IFR/LIFR POSSIBLE
  SUNDAY AFTERNOON.

* SHOWERS LIKELY SUNDAY AFTERNOON.

RODRIGUEZ

//DISCUSSION...UPDATED 18Z...

OVERALL QUIET CONDITIONS ACROSS THE TERMINALS EARLY THIS
AFTERNOON...WITH GUSTY EAST NORTHEAST WINDS AND VFR CONDITIONS IN
PLACE. VFR CONDITIONS WILL PREVAIL FOR THE REMAINDER OF THE DAY
AND TONIGHT...BUT WITH CONDITIONS LIKELY DETERIORATING SUNDAY
MORNING AS A LOW PRESSURE SYSTEM MOVES NORTHEAST TOWARDS THE
TERMINALS. EXPECT CEILINGS TO LOWER THROUGH MIDDAY WITH MVFR
CONDITIONS BECOMING LIKELY...AND THEN FURTHER LOWERING THROUGH THE
AFTERNOON. STILL SOME SLIGHT UNCERTAINTY WITH THE TRACK OF THIS
SYSTEM AND RESULTANT PRECIP/CEILINGS...BUT IT DOES APPEAR POSSIBLE
FOR IFR AND EVEN LIFR CEILINGS TO BE POSSIBLE VERY LATE IN THE
FORECAST PERIOD. DUE TO LACK OF CONFIDENCE...HAVE EXCLUDED ANY
MENTION OF THESE LOWER CEILINGS IN THE TAFS. SHOWERS WILL SPREAD
NORTHWARD OUT AHEAD OF THIS SYSTEM DURING THE DAY SUNDAY WITH
HEAVIER PRECIP BECOMING MORE LIKELY BY MID AFTERNOON. AT THIS
TIME...NOT ANTICIPATING THUNDER ACROSS ANY OF THE TERMINALS WITH
THE HIGHER CHANCES OF THIS OCCURRING JUST TO THE SOUTHEAST. THIS
COULD CHANGE THOUGH IF A MORE NORTHWARD TRACK OF THE SYSTEM WERE
OBSERVED.

RODRIGUEZ

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH CONFIDENCE WITH WIND SPEED/DIRECTION.

* HIGH CONFIDENCE WITH MVFR CEILINGS SUNDAY...LOW CONFIDENCE WITH
  IFR/LIFR.

* HIGH CONFIDENCE WITH SHOWERS SUNDAY.

RODRIGUEZ

//OUTLOOK FOR ORD/MDW FOR 00Z MONDAY-12Z SATURDAY...UPDATED 12Z...

SUNDAY NIGHT...SHRA/ISOLATED TSRA SOUTH OF TAF SITES. MVFR
LIKELY/IFR POSSIBLE. EAST WINDS BECOMING NORTHEAST.

MONDAY...CHANCE OF SHRA/MVFR EARLY...IMPROVING TO VFR. GUSTY WEST
WINDS.

TUESDAY...SLGT CHANCE SHRA...BUT MAINLY VFR. GUSTY WEST WINDS.

WEDNESDAY...SLGT CHANCE SHRA. MAINLY VFR. WEST-NORTHWEST WINDS.

THURSDAY...VFR. NORTH-NORTHEAST WINDS.

FRIDAY...MAINLY VFR. CHC SHRA.

BEACHLER

&&

.MARINE...
239 PM CDT

GUSTY NORTHERLY WINDS IN NEARSHORE AREAS THIS AFTERNOON HAVE BEEN
DRIVEN BY A TIGHT PRESSURE GRADIENT THAT WILL ONLY SLOWLY EASE INTO
TONIGHT. THE WIND DIRECTION SHOULD REORIENT MORE EASTERLY BY LATE
EVENING WHILE GUSTS ARE LIKELY TO FALL BELOW SMALL CRAFT CRITERIA
ALONG THE IL SHORE...AND POSSIBLY AS EARLY AS SUNDOWN OR SO. FEEL
MOST COMFORTABLE EXTENDING THE SMALL CRAFT ADVISORY INTO EARLY
EVENING GIVEN ALSO THE PERSISTENT FETCH TODAY HAVING BUILT THE
WAVES.

SUNDAY WILL SEE A LOW PRESSURE TROUGH OVER THE PLAINS IN THE MORNING
BASICALLY SPLIT INTO SEPARATE LOW PRESSURES DURING THE DAY. THE
STRONGEST AND MORE IMPACTING ONE WILL BE THE SOUTHERN LOW THAT
WILL TRACK FROM THE LOWER OHIO VALLEY MIDDAY SUNDAY TO JUST EAST
OF LAKE MICHIGAN SUNDAY NIGHT. THIS WILL DRIVE NORTH TO NORTHWEST
WINDS BEHIND IT...WITH A NOTEWORTHY INCREASE IN THE INDIANA
NEARSHORE AREA. IT IS POSSIBLE WIND GUSTS END UP HIGHER THAN
FORECAST...WARRANTING A SMALL CRAFT ADVISORY DURING THE LATE DAY
SUNDAY AND SUNDAY EVENING. AS THE LOW PRESSURE PHASES AND PIVOTS
IN EASTERN ONTARIO...NORTHWEST TO WEST WINDS WILL CONTINUE OVER
THE LAKE INTO MIDWEEK.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 10 PM SATURDAY.

&&

$$

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