Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 260919
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
319 AM CST MON JAN 26 2015

.SHORT TERM...
315 AM CST

THROUGH TONIGHT...

AN IMPRESSIVE WATER VAPOR LOOP THIS MORNING AS ROARING SUBTROPICAL
JET ACROSS THE SOUTHEAST IS DEEPENING A SIGNIFICANT SYSTEM THAT WILL
IMPACT THE NORTHEAST U.S. OVER THE NEXT 24 TO 36 HOURS. THE NORTHERN
STREAM JET WHICH IS ALSO HELPING TO CARVE OUT THE DEVELOPING TROUGH
IS DIVING SOUTHWARD OVER OUR REGION TODAY. ON THE CYCLONIC SIDE OF
THIS...FROM MANITOBA TO THE LOWER OHIO VALLEY...AN ELONGATED AREA OF
VORTICITY GENERATION WILL SUPPORT FORCING FOR ASCENT TODAY AND
TONIGHT...WITH THE MOST FOCUSED ZONE BEING A BETTER DEFINED WAVE
SEEN OVER CENTRAL MN EARLY THIS MORNING. THE RUN-TO-RUN TREND ON THE
RAP MODEL EARLY THIS MORNING IS FURTHER
WEST/SOUTHWEST...SUPPORTING UPSTREAM RADAR TRENDS AND FAVORING
NORTH CENTRAL IL TO GET IN A PERIOD OF SNOW BEGINNING LATER THIS
MORNING FOLLOWED BY A GRADUAL EXPANSION EAST. WHILE MOST OF THE
SNOW WILL BE LIGHT...THERE IS A WINDOW OF TIME LATE THIS MORNING
AND EARLY THIS AFTERNOON OVER NORTH CENTRAL ILLINOIS WHERE
850-700MB CONVERGENCE AND A F-GEN INDUCED CIRCULATION MAY ALLOW
FOR SPOTTY BURSTS OF MODERATE SNOW...WHICH HIGH-RESOLUTION
GUIDANCE TRIES TO INDICATE TOO. WITH ADJACENT OFFICES AND WPC
COLLABORATION...HAVE AROUND AN INCH FORECAST IN THE ROCKFORD
AREA...WITH AROUND A HALF INCH FURTHER EAST OF THERE. SOUTHWARD IN
THE FORECAST AREA THE DRY AIR SEEMS MORE PRONOUNCED...SO MAY END
UP JUST BEING FLURRIES AT TIMES.

THE FORCING ALOFT WEAKENS LATER THIS AFTERNOON AND EVENING SO
EXPECTING LIGHT SNOW TO BECOME MORE PATCHY IN COVERAGE. THE MOISTURE
IN THE ICE LAYER GRADUALLY DIMINISHES FROM WEST TO EAST THROUGH THE
EVENING...WITH STILL SOME LIGHT ADIABATIC ASCENT IN THE LOWEST
COUPLE THOUSAND FEET. IF THAT LOWEST LAYER IS SATURATED OR ABLE TO
THERE COULD BE A FREEZING DRIZZLE CONCERN...SO MAINTAIN THE GOING
CHANCE IN THE FORECAST.

FOR TEMPERATURES TODAY...DID BUMP DOWN SEVERAL DEGREES AS CLEARING
LAST NIGHT ALLOWED PLACES TO DROP QUICKLY AND NOW THE CLOUD CANOPY
WILL HINDER THEM FROM WARMING MUCH. FORECAST HIGHS ARE MAINLY IN THE
MID 20S WITH A FEW UPPER 20S POSSIBLE. WHILE THAT DOES NOT MAKE
HEADLINES IN JANUARY...IF THE FORECAST PANS OUT THIS WOULD BE THE
FIRST DAY WITH HIGHS BELOW 30 SINCE JANUARY 14TH.

MTF

&&

.LONG TERM...
315 AM CST

TUESDAY THROUGH SUNDAY...

HIGH PRESSURE WILL MOVE OVER THE AREA TUESDAY AFTERNOON AND NIGHT
BEFORE RETURN FLOW ON WEDNESDAY. THE CHALLENGE OF CLOUD COVER
PRESENTS ITSELF AGAIN ON TUESDAY AND WHILE HAVE SLOWED THE
CLEARING TREND TUESDAY MORNING...STILL THINK THE AFTERNOON SHOULD
SEE SOME BREAKS OF SUN AREAWIDE. SOUTHERLY FLOW ON WEDNESDAY IS
FORECAST TO BRING 925MB TEMPERATURES TO AROUND 0C WHICH USING
CLIMATOLOGY WITH LIMITED SNOW COVER SHOULD SUPPORT WIDESPREAD MID
TO UPPER 30S WITH A FEW LOWER 40S POSSIBLE IN THE SOUTHWEST CWA
OR CHICAGO METRO.

THE LARGE CLOSED LOW OFF THE BAJA OF CALIFORNIA THIS MORNING IS
FORECAST BY THE SUITE OF GUIDANCE TO DRIFT NORTHWARD AND GET CAUGHT
UP IN THE WESTERLIES BECOMING AN OPEN WAVE ACROSS THE MIDDLE OF THE
COUNTRY. WITH THE MASSIVE SYSTEM OFF THE NORTHEAST LIKELY TO SLOW
THINGS A BIT...CONTINUE TO LEAN WITH THE SLOWER EC AND GEM MODELS
WHICH WOULD FAVOR PRECIPITATION CHANCES MORE SO LATE WEDNESDAY NIGHT
AND THURSDAY...AROUND 12H SLOWER THAN THE GFS. FORECAST VERTICAL
PROFILES ON BOTH MODELS WOULD SUPPORT MORE OF RAIN AHEAD OF THIS
SYSTEM...ESPECIALLY CONSIDERING LOW-LEVEL TEMPERATURES SHOULD AT THE
VERY LEAST HOLD STEADY IF NOT MODERATE WEDNESDAY NIGHT...POTENTIALLY
TO 35 TO 38 DEGREES GIVEN EC AND GEM SOLUTIONS. ON THE BACKSIDE OF
THIS SYSTEM IT WOULD SEEM THERE WOULD BE AT LEAST A PERIOD OF SNOW
CHANCES. OBVIOUSLY A FEW DAYS OUT AND A CUTOFF LOW INVOLVED...A
LOT COULD CHANGE WITH SPECIFICS...BUT RIGHT NOW THIS IS NOT
LOOKING LIKE A HIGH IMPACT EVENT.

LATE THIS WEEK INTO THE WEEKEND THE NORTHERN STREAM FLOW PATTERN
IS FORECAST TO REMAIN QUITE PROGRESSIVE WHILE ANOTHER CUTOFF LOW
IS PROGGED TO DEVELOP OVER THE FAR SOUTHWEST US. THE 00Z EC HAS
FORECAST A STRONG SYSTEM IN THE NORTHWEST FLOW TO WIN OUT ABOUT A
WEEK FROM NOW AND DRAG DOWN -15C TO -20C 850MB TEMPERATURES.
SOMETHING TO MONITOR AS WE HEAD TOWARD THE FIRST DAYS OF FEBRUARY.

MTF

&&

.AVIATION...

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

* BRIEF HIGH-END MVFR CIGS OFF LAKE MICHIGAN SHOULD SCATTER BY
  SUNRISE.

* PERIOD OF LIGHT SNOW POSSIBLE THIS AFTERNOON/EVENING WITH
  REDEVELOPMENT OF MVFR CIGS WHICH LINGER TONIGHT.

RATZER

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

WEAK HIGH PRESSURE RIDGE QUICKLY MOVING INTO FORECAST AREA FROM
THE NORTHWEST OVERNIGHT...THOUGH NORTHEAST WINDS HAVE RESULTED IN
A LAKE-INDUCED PLUME OF MVFR STRATOCU FROM ABOUT ORD EAST...WITH
BASES AROUND 2500 FT. THESE CIGS WILL LIKELY AFFECT ORD/MDW/GYY
UNTIL ABOUT SUNRISE...WHEN MODELS DEPICT NORTHEAST FLOW QUICKLY
BECOMING DISORGANIZED WITH APPROACH OF SFC RIDGE. SFC WINDS TO
BECOME LIGHT/VARIABLE FOR A TIME THIS MORNING AS THE SURFACE HIGH
PRESSURE RIDGE MOVES ACROSS THE TERMINALS...GENERALLY FLOPPING
FROM LIGHT NORTHERLY TO LIGHT SOUTHERLY BY MID-DAY.

IR SATELLITE PICS ALREADY SHOWING HIGH CLOUDS SPILLING INTO THE
REGION FROM THE NORTHWEST AHEAD OF THE NEXT WEAK CLIPPER SYSTEM
WHICH WILL AFFECT THE AREA MONDAY AFTERNOON/EVENING. LOW LEVELS
ARE INITIALLY DRY AND WILL TAKE SOME WORK TO SATURATE FROM ABOVE
AS MID-LEVEL FORCING INCREASES MID-DAY INTO EARLY AFTERNOON. MOST
GUIDANCE DOES INDICATE SOME LIGHT SNOW DEVELOPING DURING THE
PERIOD...THOUGH A COMBINATION OF WEAKENING FORCING AND FORCING
FOCUSED BELOW TEMP LAYER CONDUCIVE TO LARGER SNOWFLAKE GROWTH
SUGGESTS THIS WILL BE RATHER LIGHT. WHILE A FEW INCONSEQUENTIAL
FLURRIES COULD BEGIN SOONER...THE TIMING OF THE PERIOD MOST
LIKELY TO SEE SOME LIGHT SNOW AND MVFR CIG/POSSIBLE IFR VSBY LOOKS
TO BE DURING THE AFTERNOON AT RFD...AND DURING THE LATE AFTN/EARLY
EVENING FOR CHICAGO TERMINALS. SNOW SHOULD TAPER OFF DURING THE
EVENING AT RFD AND BY LATE EVENING FOR CHICAGO TERMINALS...WITH
LOSS OF STRONGER LIFT AND DRYING IN MID-LEVELS. SOME INDICATION
THAT THERE COULD BE A BRIEF PERIOD OF FREEZING DRIZZLE AS THE SNOW
ENDS ESPECIALLY AT RFD WHERE MID-LEVELS DRY A LITTLE MORE
QUICKLY...THOUGH MODELS MAY BE OVERDOING THE DEGREE OF SATURATION
IN THE BOUNDARY LAYER AND CONFIDENCE IS LOW THAT FZDZ WOULD OCCUR.
IT DOES HOWEVER APPEAR THAT SOME SORT OF HIGH-IFR OR LOW-MVFR CIG
MAY LINGER DURING THE NIGHT.

RATZER

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

* HIGH IN WIND TRENDS.

* MEDIUM-LOW IN SNOW TIMING/INTENSITY THIS AFTERNOON AND EVENING.

RATZER

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

TUESDAY...MVFR CIGS POSSIBLE.

WEDNESDAY...VFR DURING THE DAY THEN CHANCE OF -RASN AND MVFR/IFR
AT NIGHT.

THURSDAY...CHANCE OF RAIN/SNOW. MVFR LIKELY/IFR POSSIBLE.

FRIDAY...CHANCE OF MVFR CIGS...ESPECIALLY EARLY.

SATURDAY...CHANCE OF LIGHT SNOW/MVFR CIGS.

IZZI

&&

.MARINE...
143 AM CST

ONLY MARINE FORECAST CONCERNS ARE A BRIEF PERIOD OF ENHANCED
SOUTH-SOUTHEAST WINDS THIS AFTERNOON AND EVENING ESPECIALLY ON THE
NORTHERN PORTION OF THE LAKE...AND AGAIN WEDNESDAY AFTERNOON. GUSTY
NORTH WINDS NEAR 30 KT ARE ALSO LIKELY THURSDAY AND THURSDAY
NIGHT.

WINDS CONTINUE TO DIMINISH QUICKLY ACROSS LAKE MICHIGAN EARLY THIS
MORNING...AS WEAK HIGH PRESSURE RIDING SLIDES SOUTHEAST ACROSS THE
LAKE. THIS RIDGE WILL ALSO PASS QUICKLY OFF TO THE SOUTHEAST THIS
AFTERNOON IN WHAT REMAINS A PROGRESSIVE WEATHER PATTERN...WITH
ANOTHER LOW PRESSURE SYSTEM CURRENTLY NEAR LAKE WINNIPEG MOVING
QUICKLY SOUTHEAST INTO ILLINOIS BY MIDNIGHT TONIGHT WHILE FILLING.
THIS WILL RESULT IN WINDS BECOMING SOUTH AND INCREASING TO AROUND 20
KTS ON THE LAKE THIS AFTERNOON...THEN DECREASING AND BACKING TO THE
NORTHEAST INTO TUESDAY MORNING AS THE LOW PASSES AND WEAKENS.
ANOTHER HIGH PRESSURE RIDGE WILL SETTLE ACROSS THE WESTERN LAKES BY
TUESDAY EVENING.

ANOTHER MORE DEVELOPED LOW WILL FORM ACROSS THE LAKES TUESDAY
NIGHT...AND WILL DEEPEN SLIGHTLY AS IT APPROACHES FROM THE WEST
WEDNESDAY AND PASSES JUST SOUTH OF LAKE MICHIGAN EARLY THURSDAY
MORNING. THIS WILL AGAIN RESULT IN WINDS INITIALLY BECOMING SOUTH
AND INCREASING TO 20-25 KT WEDNESDAY AFTERNOON AND EVENING...THEN
BACKING TO THE NORTH EARLY THURSDAY AND GUSTING NEAR 30 KT AS THE
LOW PULLS AWAY ACROSS THE EASTERN LAKES AND UPPER OHIO VALLEY
THROUGH THURSDAY AFTERNOON AND NIGHT. HIGH PRESSURE THEN SPREADS
ACROSS THE REGION BY FRIDAY AFTERNOON.

FOR THE ILLINOIS AND NORTHWEST INDIANA NEARSHORE WATERS...AS SMALL
CRAFT ADVISORY REMAINS IN EFFECT UNTIL MID-MORNING TODAY...AS WAVES
CONTINUE TO SUBSIDE FROM YESTERDAY/LAST EVENINGS STRONG NORTHEAST
WINDS.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 9 AM MONDAY.

&&

$$

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