Sat Dec 12 Meteorology for Chicago
Brett Crapser
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1243 PM CST SAT DEC 12 2009
DISCUSSION...
335 AM CST
DIFFERENT DAY BUT SAME FORECAST CHALLENGES...PRECIP CHANCES/TYPE
TONIGHT AND TEMPERATURES THE NEXT SEVERAL DAYS.
WATER VAPOR IMAGERY DOES A NICE JOB THIS MORNING IN SHOWING THE
DE-AMPLIFICATION OF THE POLAR JET ALONG THE CANADIAN BORDER AS THE
POLAR VORTEX CONTINUES TO RETREAT NORTHWARDS. MEANWHILE...THE
SUBTROPICAL JET IS BEGINNING TO BUCKLE A BIT WITH A COUPLE
SHORTWAVES EVIDENT IN WATER VAPOR IMAGERY THIS MORNING...THE FIRST A
LEAD SHORTWAVE TROUGH EMERGING OUT OVER THE HIGH PLAINS OF COLORADO
WITH A SECOND SHORTWAVE TROUGH MOVING ACROSS THE FOUR CORNERS
REGION. THE LEAD SHORTWAVE IS SYNOPTICALLY ENHANCING THE NOCTURNAL
LOW LEVEL WIND MAXIMUM WHICH IS ALREADY RESULTING IN A NORTHWARD
SURGE IN MODIFIED GULF MOISTURE.
MODELS SUGGEST THAT MOISTURE WILL BE TOO LACKING FOR THIS LEAD WAVE
TO HAVE ANY FANFARE AS IT MOVES THRU THIS AFTERNOON...HOWEVER AS
COLUMN WORKS TOWARD SATURATION DO EXPECT CLOUD COVER TO REALLY
RAMP UP TOWARD MIDDAY ONWARD FOLLOWING A FAIRLY SUNNY START TO
THE DAY. THE SECOND SHORTWAVE MOVING INTO THE FOUR CORNERS REGION
ATTM WILL BE OUR WEATHER MAKER TONIGHT. STRONG LOW LEVEL JET WILL
LEAD TO FAIRLY IMPRESSIVE WARM AIR ADVECTION/ISENTROPIC ASCENT
WITH ATMOSPHERE FORECAST TO SATURATE FROM TOP DOWNWARD. DEEPEST
MOISTURE AND THEREFORE HIGHEST QPF/PRECIP PROBS APPEAR TO BE
LINING UP OVER OUR EASTERN/SOUTHEASTERN COUNTIES. HAVING SAID
THAT...IT DOES APPEAR AS THOUGH MUCH OF THE CWA WILL SEE A HIGH
PROBABILITY OF A LOW QPF EVENT TONIGHT AND HAVE CONTINUED TO TREND
POPS UPWARD ACROSS THE CWA.
PRECIP TYPE MAY BE TRICKY...THOUGH THICKENING/LOWERING CLOUD
COVER THIS EVENING ALONG WITH MODERATE SOUTHERLY WINDS SHOULD
PREVENT ANY DIURNAL FALL IN TEMPERATURES TONIGHT WITH ANY VERY
MODEST TEMP DROP TONIGHT LIKELY ASSOCIATED WITH WET BULB PROCESSES
AS THE ATMOSPHERE SATURATES. WITH MAJORITY OF THE CWA EXPECTED TO
START OUT AND GENERALLY REMAIN ABOVE FREEZING EXPECT TO SEE MOSTLY
LIGHT RAIN...THOUGH PRECIP COULD START OUT AS SOME GRAUPEL OR
SLEET AS COLUMN SATURATES FROM TOP DOWN. OVER AREAS WITH DEEPER
SNOWPACK COULD SEE TEMPS STRUGGLE MORE TO GET ABOVE FREEZING AND
COULD SEE SOME LIGHT FREEZING RAIN FOR A TIME...THOUGH LUCKILY IT
APPEARS AS THOUGH THE BRUNT OF THE QPF WILL REMAIN EAST OF THE
AREA MOST PRONE TO FREEZING RAIN. IT TAKES VERY LITTLE FREEZING
RAIN TO PRODUCE TRAVEL PROBLEMS AND GIVEN SUB-FREEZING GROUND
TEMPS ANY FRZ RAIN THAT DOES OCCUR COULD REALLY TURN ROADWAYS INTO
SKATING RINKS...SO NOT OUT OF THE QUESTION THAT SOME AREAS MAY
NEED A FREEZING RAIN ADVISORY TONIGHT...BUT THAT WILL DEFINITELY
BE SOMETHING THAT WILL NEED TO BE MORE OF A LAST MINUTE TYPE
DECISION FOR SUBSEQUENT SHIFTS TO CONSIDER.
AS THIS SHORTWAVE PASSES EAST OF THE AREA BY EARLY SUNDAY MORNING
FORECAST SOUNDINGS SUGGEST THAT A FAIRLY DEEP LAYER OF LOW LEVEL
MOISTURE WILL BE LEFT BEHIND AND TRAPPED BENEATH AN INVERSION.
MODELS DO TEND TO MOISTEN LOW LEVELS TOO RAPIDLY IN THESE TYPE OF
SCENARIOS...HOWEVER WITH PRECIPITATION LIKELY HELPING SATURATE
THINGS SUSPECT THAT A LOW STRATUS DECK MAY INDEED EVENTUALLY
DEVELOP. GIVEN THE PRESENCE OF A LOW STRATUS DECK AS FORECAST
SOUNDINGS SUGGEST THEN DRIZZLE (POSSIBLE FREEZING DRIZZLE OVER
DEEPEST SNOW PACKED AREAS) WOULD BE POSSIBLE SUNDAY MORNING.
FAST ON THE HEELS OF THESE WAVES WILL BE A NORTHERN STREAM TROUGH
WHICH WILL ALREADY BE INDUCING LEE CYCLOGENESIS DURING THE DAY
SUNDAY. MODELS IN PRETTY GOOD AGREEMENT ON TAKING THIS SURFACE LOW
RATHER EXPEDITIOUSLY EASTWARDS ACROSS THE REGION WITH MOST OF OUR
CWA LIKELY MAKING IT INTO THE WARM SECTOR MONDAY AFTERNOON...WITH
THE ONE POSSIBLE EXCEPTION BEING AREAS NEAREST TO THE WI BORDER
OVER NORTH CENTRAL ILLINOIS. WEEKEND SYSTEM SHOULD TAKE ITS
MOISTURE WITH IT...SO ONLY EXPECTING A SMALL CHANCE OF SOME SCT
LIGHT SHOWERS IN THE WARM SECTOR...WITH BETTER PRECIP CHANCES
CLOSER TO THE TIGHTER THERMAL GRADIENT AND SFC LOW TRACK NEAR THE
WI STATE LINE.
SOUTHERLY WINDS IN ADVANCE OF MONDAYS SYSTEM SHOULD SEND TEMPS TO
NOTICEABLY ABOVE AVERAGE LEVELS...WITH FAR SOUTHERN CWA CLIMBING
WELL INTO THE 40S. SNOWPACK UP NORTH WILL LIKELY KEEP TEMPS MOSTLY
IN THE 30S...THOUGH SOUTHERN EXTENT OF THE SNOW COVER WHICH WILL BE
MORE INFLUENCED BY ACTUAL WINDS SHOULD CRACK 40F. MONDAY`S WARM-UP
LOOKS TO BE SHORT-LIVED WITH A SHOT OF COLD AIR PROGGED TO ARRIVE
TUESDAY. MODELS HAVE BEEN FLIP-FLOPPING A BIT WITH RESPECT TO THE
INTENSITY OF THIS COLD SNAP...THOUGH LAST SEVERAL GFS RUNS ARE NOW
IN LINE WITH 11/00Z ECMWF RUN AS WELL AS THE 12/00Z RUN OF THE ECMWF
WITH A MUCH MORE TEMPERATE SHOT OF COLDER AIR. HAVE NUDGED TEMPS
UPWARD JUST A BIT TUES THRU WED TO REFLECT THE LATEST MODEL
TRENDS...THOUGH DID REMAIN BELOW OBJECTIVE GUIDANCE
NUMBERS...ESPECIALLY OVER THE SNOW COVERED AREAS.
IZZI
&&
AVIATION...
1237 PM CST
1800 UTC TAFS...PRECIPITATION TYPE/CIG TRENDS WITH APPROACHING
SYSTEM TONIGHT AND INCREASING GUSTINESS TO WINDS WILL CONTINUE TO
BE THE MAIN AVIATION FORECAST ISSUES.
STRATOCU DECK SLOWLY CONTINUES TO APPROACH NORTHERN
ILLINOIS/NORTHWEST INDIANA FROM THE SOUTH WITH VISIBLE SATELLITE
IMAGERY INDICATING NORTHERN EXTENT OF THESE CLOUDS MOVING INTO
NORTHWEST ILLINOIS. CIGS ARE VFR ON THE NORTHERN EXTENT OF
STRATOCU DECK GENERALLY IN THE 5-7K FT RANGE. AT 18Z...THE NEAREST
UPSTREAM MVFR CIGS ARE LOCATED ACROSS NORTHERN MISSOURI. A COUPLE
OF FAST MOVING UPPER LEVEL DISTURBANCES ACROSS THE ROCKIES WILL
RAPIDLY EJECT EASTWARD INTO THE PLAINS THIS AFTERNOON WITH
SOUTHERLY LOW LEVEL FLOW EXPECTED TO STRENGTHEN INTO THIS EVENING.
FAIRLY RESPECTABLE SURGE OF LOW LEVEL MOISTURE ADVECTION WILL
ACCOMPANY THIS LOW LEVEL JET LATE AFTERNOON/EARLY EVENING. VFR
CIGS SHOULD OVERSPREAD TERMINALS THIS AFTERNOON...AND BECOME MVFR
THIS EVENING IN ASSOCIATION WITH STRONGEST LOW LEVEL MOISTURE
ADVECTION. AT THIS TIME IT APPEARS THE MOST ORGANIZED AREA OF
PRECIPITATION WILL FOCUS SOUTH OF THE TERMINALS WHERE LOW LEVEL
MOISTURE CONVERGENCE ASSOCIATED WITH LOW LEVEL JET WILL BE
MAXIMIZED. NONETHELESS...STILL EXPECTING LIGHT PRECIPITATION TO
REACH TERMINALS MID TO LATE EVENING BUT TRYING TO RESOLVE EXACTLY
IN WHAT FORM THIS WILL FALL IS STILL IN QUESTION. LOW LEVELS ARE
INITIALLY QUITE DRY WHICH COULD SUPPORT NEAR SFC TEMPS REMAINING
BELOW FREEZING FOR SOME TIME INTO LATE EVENING HOURS BUT WARM
ADVECTION LATE THIS EVENING MAY ALLOW FOR TRANSITION TO LIQUID
PRECIP BEFORE COLDER AIR ADVECTS BACK IN FROM THE WEST VERY LATE
TONIGHT THAT MAY PROMOTE SOME LIGHT DZ OR FZDZ BEFORE PRECIP ENDS
SUNDAY MORNING. HAVE TRENDED TAFS TO LIGHT RAIN/SLEET MIX WITH
MORE OF SLEET/SNOW MIX AT RFD...ALTHOUGH CANNOT RULE OUT SOME
LIGHT FZRA BASED ON EXPECTED STRENGTH OF LOW LEVEL WARM LAYER. GYY
ACTUALLY MAY SEE MAINLY LIQUID WITH THIS EVENT GIVEN LESSER SNOW
COVER AND CLOSER PROXIMITY TO MAIN LOW LEVEL THERMAL RIDGE
ADVECTING IN FROM THE SOUTHWEST. DID MAINTAIN IDEA FROM PREVIOUS
TAFS OF TRENDING TO IFR CIGS FOR THE OVERNIGHT HOURS INTO EARLY
SUNDAY MORNING.
IN TERMS OF WINDS...STRENGTHENING GRADIENT SHOULD SUPPORT SOME
GUSTS TO AROUND 20 KNOTS THIS AFTERNOON INTO THIS EVENING AND WIND
SHIFT STILL EXPECTED SUNDAY MORNING AS NORTHERN STREAM THROUGH
TRACKS ACROSS SOUTHERN CANADA TAKING COLD FRONT NORTHERN ILLINOIS.
NO SIGNIFICANT CHANGES TO WIND SHIFT TO THE WEST MADE FOR 18Z
TAFS...GENERALLY IN THE 15Z-18Z TIME FRAME SUNDAY.
MARSILI
&&
MARINE...
308 AM CST
AFTER A PERIOD OF WEAKER WINDS UNDER A RIDGE OF HIGH PRESSURE
OVERNIGHT...WINDS ARE EXPECTED TO INCREASE AGAIN THIS AFTERNOON
AND TONIGHT AS A TROUGH OF LOW PRESSURE DEVELOPS TO THE NORTHWEST
OF THE GREAT LAKES AND BEGINS TO DROP SOUTHEAST AS A COLD FRONT.
LIGHT SOUTHWEST FLOW EARLY TODAY WILL TURN MORE SOUTHERLY AS
WINDS PICK UP...THEN VEER WEST BEHIND THE FRONTAL PASSAGE ON
SUNDAY. WINDS CONTINUE VEERING AHEAD OF THE NEXT LOW PRESSURE
SYSTEM WHICH APPROACHES THE CHICAGO AREA FROM THE SOUTHWEST AND
CROSSES THE LAKE ON MONDAY NIGHT.
LENNING
&&
LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
UNTIL 3 AM SUNDAY.
&&
$$