Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 020818
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
218 AM CST MON MAR 2 2015

.UPDATE...

HIGH PRESSURE IS SPREADING SOUTHEAST OVER THE CENTRAL PLAINS AND
SKIES ARE CLEARING A BIT FASTER THAN EXPECTED. TEMPS AND DEW POINTS ARE
ALSO FALLING A BIT FASTER THAN EXPECTED. AS SUCH...LOWERED
OVERNIGHT LOW TEMPS WITH SINGLE DIGITS EXPECTED OVER NORTH CENTRAL
IL AND TEMPS AROUND 10 DEGREES NEAR THE LAKE AND IN NW INDIANA.
OTHERWISE THE FORECAST LOOKS ON TRACK.

JEE

&&

.SHORT TERM...
258 PM CST

THROUGH MONDAY EVENING...

QUIET WEATHER THROUGH MONDAY EVENING AS HIGH PRESSURE TRAVERSES THE
REGION. SKIES WILL CLEAR OUT THIS EVENING AND START OUT SUNNY MONDAY
WITH JUST SOME HIGH THIN CLOUDINESS ARRIVING DURING THE AFTERNOON.
THE SUN ANGLE NOW IS AS HIGH AS IT IS IN EARLY OCTOBER AND GIVEN THE
EXPECTED SUNSHINE MONDAY ALONG WITH SIMILAR H8/H9 TEMPS SUSPECT THAT
HIGHS WILL CLIMB WELL INTO THE 20S...EXCEPT IN THE URBANIZED/HIGHLY
SUBURBANIZED CHICAGO METRO AREA WHERE I`VE GONE A BIT ABOVE GUIDANCE
DUE TO LOWER ALBEDO WHICH SHOULD ALLOW SUN TO DO ITS THING AND PUSH
TEMPS INTO THE LOWER 30S.

IZZI

&&

.LONG TERM...
258 PM CST

TUESDAY NIGHT THROUGH SUNDAY...

PRIMARY FORECAST CONCERN IS IN THE LATER MONDAY NIGHT INTO TUESDAY
TIME FRAME WHEN GFS/ECMWF HAVE REMAINED RATHER CONSISTENT AND IN
PRETTY GOOD AGREEMENT ON EJECTING A PIECE SHORTWAVE ENERGY FROM THE
DESERT SW LOW NORTHEAST INTO THE MIDWEST...WHILE A NORTHERN STREAM
SHORTWAVE DROPS SOUTH AND AMPLIFIES A TROUGH OVER THE NORTHERN
PLAINS. GUIDANCE HAS CONTINUED TO INDICATE THAT THESE WAVES WILL
REMAIN UNPHASED AND THUS LOWER AMPLITUDE/QUICKER MOVING WHICH SHOULD
RESULT IN A SHORTER DURATION OF SIGNIFICANT PRECIP AND LOWER AMOUNTS
GIVEN LESS TIME FOR SIGNIFICANT RETURN FLOW. THERMAL PROFILES IN
BOTH MODELS CONTINUE TO SHOW A SUBSTANTIAL WARM AIR ADVECTION
RESULTING IN FREEZING LINE ALOFT TO LIFT NORTHWARD ACROSS THE CWA
OVERNIGHT MONDAY NIGHT INTO TUESDAY MORNING. STILL TIME FOR MODELS
TO CHANGE THEIR TUNE AND ADJUST TRACK FARTHER SOUTH...BUT GIVEN THE
CONSISTENCY OF LATE THINKING THAT APPEARS LESS AND LESS LIKELY.

GIVEN THE CURRENT FORECAST THERMAL PROFILES WOULD EXPECT PRECIP TO
START AS SNOW OR SLEET MONDAY NIGHT AND THEN TRANSITION TO A PERIOD
OF FREEZING RAIN THEN ALL RAIN TUESDAY AS COLUMN CONTINUES TO WARM.
EXTENSIVE DEEP SNOW PACK WILL LIKELY RETARD THE WARMING NEAR THE
SURFACE BUT GIVEN 850MB/925MB TEMP CLIMO GIVEN FORECAST VALUES
TUESDAY WOULD SUPPORT HIGHS IN THE 50S...THINK WE SHOULD BE ABLE TO
WARM ABOVE FREEZING GIVEN THE FAIRLY HIGH SUN ANGLE. PAVEMENT TEMPS
THIS AFTERNOON HAVE WARMED INTO THE 40S (WITH SFC TEMPS IN 20S)...SO
SUSPECT THAT ANY SNOW/SLEET/FREEZING RAIN THAT DOES FALL TUESDAY
WILL NOT BE HIGH IMPACT ON TRAVEL AFTER SUNRISE UNLESS IT WERE TO
FALL QUITE HEAVILY AND TEMPS BE COLDER THAN FORECAST. ICE/SNOW
ACCUMS SHOULD BE CONFINED MORE TO ELEVATED SURFACE THAN TO PAVEMENT
AS TEMPS WARM...SO WHILE THERE LOOKS TO BE A QUICK SHOT OF MODERATE
WINTRY PRECIP CENTERED AROUND RUSH HOUR TUES MORNING...NOT
ANTICIPATING MUCH ICE/SNOW ACCUM ON MOST ROADWAYS. HAVING SAID THAT
EVEN RAIN CAUSES BAD TRAFFIC SO EVEN IF MOST ROADS ARE JUST WET AND
NOT ICE/SNOW COVERED WEATHER WILL LIKELY RESULT IN TRAVEL DELAYS.
DRYING ALOFT SHOULD RESULT IN STEADIER/HEAVIER PRECIP TAPERING OFF
TO MAINLY DRIZZLE AND OCNL LIGHT RAIN DURING THE AFTERNOON TUESDAY.

ANOTHER FAIRLY SIGNIFICANT BUT PROGRESSIVE SHOT OF LATE SEASON
ARCTIC AIR SPILLS INTO THE REGION MIDWEEK AS NORTHERN STREAM TROUGH
DIGS SOUTHWARD IN WAKE OF THE TUESDAY`S SLOPPY SYSTEM. FAIRLY QUIET
PATTERN THEN SETTLES IN WITH LITTLE SIGNIFICANT PRECIP THREAT MID
WEEK THROUGH NEXT WEEKEND AND ALTERNATING BOUTS OF COLD/WARM AIR
ADVECTION WITH A COUPLE CLIPPER PASSAGES. GIVEN THE HIGH SUN ANGLE
AND LESS HARSH TEMPS (OTHER THAN QUIT HIT MID WEEK) WOULD EXPECT
THAT WE WILL SLOWLY BEGIN TO EAT AWAY AT THE DEEP SNOW PACK OVER THE
REGION THE NEXT WEEK OR SO.

IZZI

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* LIGHT WEST-NORTHWEST WINDS FLOPPING TO LIGHT SOUTHEAST BY
  EVENING. MAY BE A PERIOD OF VARIABLE DIRECTION 5 KT OR LESS THIS
  AFTERNOON. SOUTH-SOUTHEAST WINDS INCREASING TOWARD TUESDAY
  MORNING.

* SNOW/MIXED PRECIP EXPECTED TO DEVELOP BY 12Z TUESDAY
  MORNING...WITH CIG/VIS QUICKLY DETERIORATING TO MVFR/IFR/LIFR
  JUST BEYOND END OF ORD 30 HOUR TAF.

RATZER

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

HIGH PRESSURE BUILDING EAST ACROSS THE TERMINALS MONDAY...AND
MOVING OFF TO THE EAST BY MONDAY EVENING. CURRENT WEST WINDS
10-12 KTS WILL DIMINISH TO LESS THAN 10 KT THIS MORNING AS
GRADIENT WEAKENS. WINDS WILL CONTINUE TO DIMINISH AND WILL LIKELY
BECOME VARIABLE IN DIRECTION AS THE HIGH PASSES OVERHEAD EARLY
THIS AFTERNOON...EVENTUALLY SETTLING TO A LIGHT SOUTHEAST THIS
EVENING AS THE HIGH PULLS AWAY TO THE EAST. SOUTH-SOUTHEAST WINDS
EXPECTED TO INCREASE ABOVE 10 KT TOWARD TUESDAY MORNING...PROBABLY
WITH GUSTS 20+ KT BEYOND 12Z.

VFR HIGH/MID CLOUDS WILL GRADUALLY INCREASE FROM THE SOUTHWEST
THROUGH THIS AFTERNOON AS MOISTURE BEGINS TO SPREAD NORTH AHEAD OF
AN AREA OF A LOW PRESSURE SYSTEM WHICH WILL DEVELOP ACROSS THE
PLAINS LATER TODAY/TONIGHT. CLOUDS WILL CONTINUE TO THICKEN AND
LOWER TONIGHT IN ADVANCE OF A WARM FRONT WHICH WILL LIFT TOWARD
THE AREA TUESDAY MORNING. FORECAST MODELS ARE IN FAIRLY GOOD
AGREEMENT IN PRECIPITATION DEVELOPING FROM THE SOUTHWEST ROUGHLY
10-12Z EARLY TUESDAY...LIKELY INITIALLY IN THE FORM OF SNOW. A
QUICK DETERIORATION FROM VFR TO MVFR/IFR/POTENTIALLY LIFR IS EXPECTED.

RATZER

//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* LOW IN WIND DIRECTION THIS AFTERNOON...BUT HIGH IN SPEEDS LESS
  THAN 10 KT.

* MEDIUM-HIGH IN TIMING ONSET OF SNOW TUESDAY MORNING.

* HIGH ALL OTHER ELEMENTS.

RATZER

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

TUESDAY...SNOW MIXING WITH FREEZING RAIN AND SLEET. IFR LIKELY.

WEDNESDAY...MVFR CIGS.

THURSDAY THROUGH SATURDAY...MAINLY VFR. POSSIBLE MVFR CIGS
SATURDAY.

BEACHLER

&&

.MARINE...
217 AM CST

HIGH PRESSURE SPREADING EAST FROM THE PLAINS EASTWARD TO THE MIDWEST
AND GREAT LAKES TODAY WILL ALLOW WESTERLY WINDS TO DIMINISH QUICKLY
THIS MORNING...AS THE GRADIENT WEAKENS WITH THE APPROACH OF THE
SURFACE RIDGE AXIS. WINDS WILL BACK TO THE SOUTH-SOUTHEAST BY
TONIGHT AS THE HIGH PULLS AWAY TO THE EAST...AND WILL INCREASE INTO
THE 25 KT RANGE BY TUESDAY MORNING AS AN AREA OF LOW PRESSURE LIFTS
NORTHEAST FROM THE PLAINS TO THE MID-MISSOURI VALLEY. THE LOW IS
EXPECTED TO TRACK NORTHEAST ACROSS WISCONSIN AND NORTHERN LAKE
MICHIGAN TUESDAY...WITH WINDS FURTHER INCREASING INTO THE 30-35 KT
RANGE ON THE SOUTHERN 2/3 OR SO OF THE LAKE DURING THE DAY. CURRENT
MODEL RUNS ARE NOT AS BULLISH IN DEEPENING THE LOW AS IT APPROACHES
AND IT APPEARS THAT A BRIEF PERIOD OF MARGINAL GALE FORCE WINDS
MIDDAY-AFTERNOON TUESDAY. WILL CONTINUE TO MENTION GALE FORCE GUSTS
BUT WILL HOLD SHORT OF A GALE WARNING AT THIS TIME. WINDS WILL THEN
EASE JUST A BIT BY TUESDAY EVENING AS THE TRAILING COLD FRONT MOVES
ACROSS THE LAKE...BUT ARE EXPECTED TO REMAIN NEAR 30 KT AS THE SHIFT
TO THE WEST-NORTHWEST TUESDAY NIGHT AS COLD AIR SPREADS ACROSS THE
LAKE AND ANOTHER LARGE HIGH PRESSURE SYSTEM BUILDS FROM CANADA
SOUTHEAST ACROSS THE PLAINS. WINDS LOOK TO HOLD IN THE 25-30 KT
RANGE WEDNESDAY AS WELL...AS THE CENTER OF THE HIGH SLOWLY TRACKS
SOUTHEAST...EVENTUALLY MOVING INTO THE LOWER OHIO VALLEY BY LATE
THURSDAY. THIS WILL ALLOW WINDS TO GRADUALLY DIMINISH AND BACK TO
THE SOUTHWEST THURSDAY NIGHT. GUSTY SOUTH-SOUTHWEST WINDS RETURN
FRIDAY AS THE HIGH MOVES OFF TO THE SOUTHEAST AND LOW PRESSURE
TRACKS NORTH OF THE LAKES.

WHILE MOST OF THE IL/IN NEAR SHORE WATERS APPEAR ICE COVERED PER
SATELLITE AND WEBCAM IMAGERY...WILL CARRY A SMALL CRAFT ADVISORY
TUESDAY/TUESDAY EVENING WITH 25-30 KT WINDS DURING THAT PERIOD.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...6 AM TUESDAY TO 10 PM TUESDAY.

&&

$$

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