Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 280926
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
426 AM CDT SAT MAR 28 2015

.SHORT TERM...
306 AM CDT

THROUGH TONIGHT...

SURFACE HIGH PRESSURE UNDER DRY NORTHWEST UPPER FLOW TODAY WILL
OFFER A SUNNY SKY AND LITTLE IN THE WAY OF SENSIBLE WEATHER. A LAKE
BREEZE IS EXPECTED TO DEVELOP AND SPREAD INLAND A COUPLE TIERS OF
COUNTIES EARLY TO MID AFTERNOON. THIS WILL STUNT WARMING SOME AND
MAYBE EVEN LEAD TO A DROP BY A COUPLE DEGREES FOR PLACES NEAR THE
LAKE SHORE.

THE LOW-LEVEL AIR MASS WILL MODIFY SOME TODAY AND WITHOUT AS STRONG
OF A LAKE ADVECTIVE COMPONENT IN THE WIND SPEED...TEMPERATURES
SHOULD RESULT AROUND 5F HIGHER. THE LIGHT ONSHORE FLOW DURING LATE
MARCH ON AVERAGE RESULTS IN ABOUT A 6-7 DEGREE DIFFERENCE BETWEEN
THE FAR DOWNTOWN CHICAGO AND THE FAR OUTLYING METRO AND AM NEAR THAT
IN THE FORECAST FOR TODAY...ABOUT 38-39 OUTLYING TO 34 DOWNTOWN.
TAGGING 40 IS EXPECTED TOWARD THE I-39 CORRIDOR.

THE DRY AIR MASS TODAY AWAY FROM THE LAKE MODIFIED AIR COULD RESULT
IN DEW POINTS FALLING INTO THE SINGLE DIGITS DURING PEAK MIXING
EARLY TO MID AFTERNOON.

MTF

&&

.LONG TERM...

SUNDAY THROUGH SUNDAY NIGHT...

MAIN CONCERNS ARE WITH STRONG SOUTHWEST WINDS AND RAIN SHOWERS
AHEAD OF AN APPROACHING COLD FRONT. PART OF STRONG UPPER WAVE/VORT
MAX MOVING ONTO PAC NW COAST AS NOTED ON W/V IMAGERY THIS MORNING
WILL DIVE SOUTHEASTWARD TOWARD THE AREA. IN ADDITION...ROBUST LOW
LEVEL JET OF 50+ KT WILL FOCUS A NARROW AXIS OF STRONG MOISTURE
TRANSPORT...WITH WAA/ISENTROPIC ASCENT ALSO AIDING IN PRECIP
BLOSSOMING. FAIRLY STEEP LAPSE RATES ALOFT COULD RESULT IN BRIEF
PERIODS OF MODERATE INTENSITY SHOWERS. THERE IS SOME CONCERN THAT
PRECIP COULD START AS LIGHT SNOW OR A MIX AT ONSET IN PARTS OF THE
NORTHERN CWA...AIDED BY WET BULBING OF INITIALLY DRY COLUMN...BUT
RAPID WARMING OF THE BOUNDARY LAYER SHOULD CAUSE ANY INITIAL
FROZEN PTYPE TO GO OVER TO RAIN. DEPARTURE OF EXPANSIVE HIGH
PRESSURE TO THE SOUTHEAST AND APPROACH OF COLD FRONTAL TROUGH
TRAILING FROM LOW PRESSURE NEAR OR NORTH OF LAKE SUPERIOR WILL
RESULT IN FAIRLY IMPRESSIVE ISALLOBARIC PRESSURE FALLS. THUS
SOUTH-SOUTHWEST WINDS WILL BECOME QUITE STRONG...WITH GUST SPEEDS
UP TO ~40 MPH SUPPORTED BY MODEL SOUNDINGS. THE STRONG WINDS WILL
COUNTERACT HIGHS IN THE MID TO UPPER 40S TO MAKE FOR A RAW DAY.

HAVE BUMPED UP POPS A BIT MORE WITHIN THE CATEGORICAL RANGE FROM
THE LATE MORNING THROUGH EARLY AFTERNOON...THEN A TRAILING UPPER
WAVE SHOULD KEEP SOME SHOWER ACTIVITY GOING INTO THE NORTHEAST
HALF ESPECIALLY OF THE CWA IN THE LATE AFTERNOON AND POSSIBLY
EARLY EVENING DEPENDING ON PATH OF THE SHORT WAVE. WINDS WILL TURN
WEST AND THEN DIMINISH BEHIND THE COLD FRONTAL PASSAGE...WITH
CLEARING SKIES AND LOWS FALLING INTO THE LOWER 30S.

MONDAY THROUGH FRIDAY...

UPPER LEVEL PATTERN WILL DEAMPLIFY SOME...WHICH WILL KEEP THE COLD
AIR LOCKED TO THE NORTH IN CANADA. OVERALL...THIS WILL RESULT IN A
MUCH MILDER WEEK...THOUGH THERE IS UNCERTAINTY IN THE GUIDANCE IN
HOW WARM CERTAIN DAYS WILL BECOME. THUS STAYED CLOSE TO A GUIDANCE
SMART BLEND FOR TEMPERATURES. MONDAY SHOULD BE A FAIRLY NICE DAY
IN THE MID TO UPPER 50S AND NO LAKE COOLING ANTICIPATED.
DIFFERENCES ALSO CONTINUE ON HANDLING OF A CLIPPER TYPE SYSTEM
MONDAY NIGHT...THOUGH 00Z ECMWF DID TREND NORTH AND 00Z
OPERATIONAL GFS ALSO REMAIN WELL NORTH. MAINTAINED LOWER END
CHANCE AND SLIGHT POPS FOR NOW NORTH OF THE KANKAKEE RIVER. WARMER
END OF GUIDANCE WOULD YIELD AT LEAST MID TO UPPER 60S BY
WEDNESDAY...THEN NEXT FRONTAL SYSTEM COULD ARRIVE WEDNESDAY
NIGHT...WITH A SLOWING TREND ON LAST FEW MODEL CYCLES. ADDED
SLIGHT CHANCE THUNDER WORDING IN SOUTH...AS GFS AND SOME ENSEMBLE
MEMBERS DO BRING AN IMPRESSIVE SURGE OF HIGHER DEWPOINTS
NORTHWARD. UNCERTAINTY THEN REALLY GROWS LATER IN THE WEEK...WITH
SOME POTENTIAL FOR A STRONGER LOW PRESSURE AREA AS PER 00Z
OPERATIONAL ECMWF.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* LAKE BREEZE POSSIBLY TOPPING 9 KT THIS AFTERNOON.
* 40-45 KT SW WINDS BETWEEN 1000-2000 FT NEAR DAYBREAK SUNDAY
  POSSIBLY CREATING LLWS.

//DISCUSSION...UPDATED 06Z...

HIGH PRESSURE WILL REMAIN DIRECTLY OVER THE AREA THROUGH TODAY
BEFORE SHIFTING EAST TONIGHT. THIS WILL ALLOW FLOW FROM THE
NORTHEAST OR EAST-NORTHEAST TO INCREASE DURING THE EARLY
AFTERNOON...WITH SPEEDS OF 9 KT OR SLIGHTLY OVER POSSIBLE FOR AT
LEAST A COUPLE HOUR WINDOW DURING THE AFTERNOON. SURFACE WINDS
WILL TURN SOUTHERLY TONIGHT AS LOW PRESSURE APPROACHES FROM THE
NORTHWEST. SURFACE SPEEDS SHOULD GRADUALLY INCREASE LATE SATURDAY
NIGHT WHILE LOW-LEVEL JET WINDS INCREASE MARKEDLY ABOVE 1000 FT
OR SO...POSSIBLY RESULTING IN LLWS NEAR DAYBREAK SUNDAY.

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* MEDIUM IN TIMING AND MAGNITUDE OF LAKE BREEZE. LOW IN DURATION
  OF SPEEDS AT 9+ KT.
* MEDIUM-HIGH IN 40-45KT SW LOW-LEVEL JET WINDS NEAR DAYBREAK
  SUNDAY. LOW IN TRUE LLWS.

//OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z FRIDAY...UPDATED 00Z...

SUNDAY...MVFR CIGS DEVELOPING WITH A PERIOD OF RAIN.

MONDAY...PRIMARILY VFR DURING DAY. CHANCE OF SHRA AT NIGHT.

TUESDAY...CHANCE OF MVFR CIGS EARLY.

WEDNESDAY...VFR.

THURSDAY...CHANCE OF SHRA...OTHERWISE VFR.

MTF/IZZI

&&

.MARINE...
425 AM CDT

MAIN MARINE CONCERN CONTINUES TO BE WITH SOUTH-SOUTHWEST GALES ON
SUNDAY. HAVE ISSUED A GALE WARNING FOR THE ENTIRE LAKE...INCLUDING
THE NEARSHORE WATERS. STRONG LOW PRESSURE WILL TRACK NEAR OR NORTH
OF LAKE SUPERIOR AND EXPANSIVE HIGH PRESSURE WILL DEPART TO THE
SOUTHEAST US...WITH TIGHT PRESSURE GRADIENT AND STRONG PRESSURE
FALLS AHEAD OF COLD FRONT TRAILING FROM THE LOW. SOUTH-SOUTHWEST
WINDS WILL QUICKLY RAMP UP AND BECOME STRONGEST ON THE NORTHERN
PORTION OF THE LAKE...WHERE THERE REMAINS SOME POTENTIAL FOR GUSTS
APPROACHING STORM FORCE FOR A FEW HOUR WINDOW MID TO LATE SUNDAY
MORNING. HAVE STAYED WITH 45 KT SPEED MENTION IN GLF...BUT MAY
NEED TO ADD OCCASIONAL STORM GUSTS IN SUBSEQUENT ISSUANCES. IN THE
NEARSHORE AND SOUTH HALF...EXPECTING SPEEDS OF 35 TO 40 KT. WINDS
WILL SHIFT WEST BEHIND THE COLD FRONT SUNDAY EVENING...WITH SPEEDS
REMAINING ELEVATED AROUND 30 KT ACROSS THE OPEN WATERS. WILL NEED
TO WATCH FOR A RENEWED PERIOD OF GALES ON THE NORTH HALF CLOSEST
TO THE DEPARTING LOW PRESSURE AREA.

A CLIPPER TYPE SYSTEM WILL THEN MOVE ACROSS THE LAKE MONDAY
NIGHT...WITH UNCERTAINTY IN THE PATH OF THIS SYSTEM...FOLLOWED BY
HIGH PRESSURE MID WEEK AND THEN A COLD FRONTAL PASSAGE WEDNESDAY
NIGHT. THERE IS SOME POTENTIAL FOR A STRONGER SYSTEM APPROACHING
THE LAKE LATER IN THE WEEK...BUT PLENTY OF UNCERTAINTY DUE TO
SIGNIFICANT DIFFERENCES IN FORECAST GUIDANCE.

RC/MTF

&&

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

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...4 AM SUNDAY TO 10
     AM SUNDAY.

     GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...10 AM
     SUNDAY TO 4 PM SUNDAY.

     GALE WARNING...LMZ777-LMZ779...4 AM SUNDAY TO 7 PM SUNDAY.

&&

$$

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