Tue Dec 8 Meteorology for Chicago
Brett Crapser
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1245 PM CST TUE DEC 8 2009
DISCUSSION...
1242 PM CST
NOT PLANNING ANY CHANGES TO GOING HEADLINES AT THIS TIME...THOUGH
AS TRANSITION FROM SNOW/SLEET TO RAIN OCCURS EXPECT THAT THE
EVENING SHIFT CAN START TRIMMING COUNTIES OUT OF THE ADVISORY AND
POSSIBLY EVEN THE WARNING DEPENDING ON JUST HOW FAR NORTHWEST THE
WARM AIR WRAPS. STORM TOTAL SNOWFALL WILL BE COMPLICATED SINCE IT
WILL LIKELY COME IN TWO WAVES...SEPARATED IN MANY AREAS BY PARTIAL
IF NOT COMPLETE MELTING TONIGHT AS TEMPERATURES RISE ABOVE
FREEZING AND RAIN FALLS.
ONE OTHER CHANGE MADE TO GOING FORECAST WAS TO INTRODUCE A SLIGHT
CHANCE OF THUNDERSTORMS TONIGHT. BOTH GFS AND WRF-NAM SHOW MID-
LEVEL LAPSE RATES INCREASING TO 7.5-8.0C/KM FROM 700-500MB WITH
MOST UNSTABLE LIFTED INDICES ACTUALLY DROPPING TO NEAR
ZERO...SUGGESTING THAT THE ENVIRONMENT WILL BECOME FAVORABLE FOR
NOT ONLY SLANTWISE BUT ALSO UPRIGHT CONVECTION. FORCING IS REALLY
PROGGED TO JUST BE OFF THE CHARTS WITH 500MB 12 HOUR HEIGHT
CHANGES OVER 300M. BEST CHANCE OF THUNDER OVER THE SOUTHEAST HALF
OF THE CWA APPEARS TO BE THIS EVENING WITHIN THE WARM CONVEYOR
BELT...WHILE OVER NORTHWEST CWA HAVING A CHANCE NOT ONLY WITH WARM
CONVEYOR BELT BUT ALSO WITH TROWAL PROGGED TO DEVELOP LATE
TONIGHT. GIVEN THE EXPECTED PRECIP TYPE PROBABLY STAYING ALL SNOW
ALONG THE NORTHWESTERN FRINGES OF OUR CWA...ANY THUNDER COULD
REALLY BOOST SNOWFALL TOTALS AND SNOWFALL RATES.
FINALLY...WITH PRECIP TYPE LOOKING INCREASINGLY LIKELY TO BE MOSTLY
RAIN OVER A MAJORITY OF THE CWA...CONCERN IS GROWING REGARDING HEAVY
RAINFALL TOTALS. A QUICK DROP BELOW FREEZING TOMORROW WILL LIKELY
DECREASE THE AMOUNT OF RUN OFF...HOWEVER EVEN ONE INCH OR MORE
RAINFALL TOTALS WILL LIKELY RESULT IN CONSIDERABLE STANDING
WATER...WHICH WILL FLASH FREEZE WEDNESDAY AS TEMPERATURES BEGIN TO
PLUMMET WITH BLACK ICE AND SLICK SPOTS A DISTINCT POSSIBILITY.
A FULL AFTERNOON DISCUSSION WILL BE ISSUED BY AROUND 3 PM.
IZZI
&&
PREV DISCUSSION...
949 AM CST
INITIAL PERUSAL OF MORNING DATA LEADS TO SOME CONCERNS REGARDING THE
GOING FORECAST. MODELS REMAIN CONSISTENT WITH A SURFACE LOW
TRACKING FROM MISSOURI ACROSS THE CHICAGO METROPOLITAN AREA
TONIGHT...ALL THE WHILE EXPLOSIVELY DEEPENING. THE 12Z WRF-NAM HAS
TRENDED A BIT FARTHER SOUTH WITH THE TRACK PUTTING IT IN LINE WITH
THE LAST COUPLE DAYS OF RUNS OF THE GFS WHICH HAVE BEEN VERY
CONSISTENT. THE TRACK OF THE SURFACE LOW WOULD FAVOR A SWITCH OVER
TO SLEET THEN RAIN OVER MUCH OF THE CWA THIS AFTERNOON INTO EARLY
THIS EVENING...HOWEVER THE GFS AND WRF-NAM HAVE BEEN CONSISTENTLY
ADVERTISING THERMAL PROFILES REPRESENTATIVE OF ALL SNOW THROUGH
THE EVENT...EVEN EAST OF THE LOW TRACK. FINALLY THIS MORNING THE
12Z WRF-NAM AND 12Z RUC HAVE TRENDED TOWARD MUCH WARMER THERMAL
PROFILES WHICH ARE SUPPORTIVE OF A CHANGE OVER. MORNING UPPER AIR
ANALYSIS REVEALED A NEARLY 20C TEMP GRADIENT AT H8 FROM KDVN TO
KLZK...WITH KSGF SOUNDING SHOWING A VERY IMPRESSIVE WARM LAYER
WITH H8 TEMPS OF 5C (A COUPLE DEGREES WARMER THAN THE WRF-NAM). AS
CYCLONE BEGINS TO RAPIDLY INTENSIFY VERY STRONG LOW LEVEL JET WILL
DEVELOP AND BEGIN TO TRANSPORT THIS MILDER AIR RAPIDLY NORTHWARD
THIS EVENING HELPING FACILITATE THE CHANGE OVER TO SLEET THEN
RAIN.
WILL HOLD OFF ON MAKING ANY CHANGES TO GOING FORECASTS FOR
P-TYPE...TOTALS...OR HEADLINES UNTIL 12Z GFS COMES IN.
HOWEVER...IT DOES APPEAR AS THOUGH A BACKING OFF ON SNOW TOTALS
WITH MORE OF A TRANSITION TO MORE RAIN TONIGHT WOULD BE IN ORDER
WITH MIDDAY FORECAST UPDATE. STRONG COLD AIR ADVECTION IN THE WAKE
OF THE STORM WOULD ALLOW FOR A SWITCH OVER TO SNOW LATE TONIGHT
INTO WEDNESDAY MORNING WITH THE POTENTIAL FOR A QUICK-HITTING
SHOT OF SNOW ON THE BACKSIDE OF THE SYSTEM.
ONE OF THE BIGGEST STORIES WITH THE STORM MAY WELL BE THE
WINDS...BOTH AHEAD OF AND BEHIND THE STORM. THE 12Z WRF-NAM HAS
SURFACE LOW DEEPENING 12MB IN 6 HOURS THIS EVENING AS THE LOW
APPROACHES THE AREA. SUCH EXPLOSIVE DEEPENING OF THE SURFACE LOW
WILL LIKELY RESULT IN VERY IMPRESSIVE ISALLOBARIC RESPONSE AND
WOULDNT BE SURPRISED TO SEE EAST WINDS GUST TO 40 MPH OR POSSIBLY A
BIT HIGHER FOR A TIME THIS EVENING BEFORE THE SURFACE LOW MOVES
OVERHEAD.
VERY STRONG WINDS ARE ALSO EXPECTED ON THE BACKSIDE OF THE LOW IN
THE COLD AIR ADVECTION REGIME WEDNESDAY. DEPENDING ON SNOWFALL
AMOUNTS AND CONSISTENCY OF THE SNOW ON THE GROUND...THERE COULD BE
NEAR GROUND BLIZZARD CONDITIONS IN OPEN AREAS
WEDNESDAY...PARTICULARLY ACROSS AREAS THAT SEE SIGNIFICANT SNOW
ACCUMULATION TODAY AND TONIGHT.
IZZI
&&
AVIATION...
1200 UTC TAFS...
THE SNOW WILL REMAIN IN NORTH CENTRAL ILLINOIS...NORTHEAST ILLINOIS
AND NORTHWEST INDIANA TODAY AND TONIGHT. THERE IS AN UPPER AIR
FRONT THAT LIES FROM NORTHERN IOWA AND NORTHERN ILLINOIS. WARM AIR
IS BEING LIFTED UP THIS FRONT AND CONDENSING PRODUCING SNOW. WE
WILL FORECAST IFR CEILINGS OF 800 TO 1000 FEET. THERE MAY BE SOME
PERIODS WHERE THE CEILING WILL BE 1500 FEET. WE ARE GOING TO
FORECAST THESE CONDITIONS FOR OHARE, MIDWAY, ROCKFORD, DUPAGE AND
GARY AIRPORTS. THE WIND WILL INCREASE TO EAST AT 20 KNOTS WITH
GUSTS 30 KNOTS LATER THIS AFTERNOON. A STRONG LOW WILL BECOME MORE
ORGANIZED IN COLORADO THIS MORNING AND IT WILL MOVE INTO MISSOURI
THIS AFTERNOON. THEN BY WEDNESDAY MORNING THE LOW WILL OVER LAKE
MICHIGAN. THE WIND WILL BECOME SOUTHWEST AFTER 10 UTC AND THE
SPEED WILL BE 15 KNOTS GUSTING TO 25 KNOTS. THE CEILING WILL BE
IFR IN THE MORNING.
&&
MARINE...
308 AM CST
A LARGE HIGH WILL MOVE ACROSS LAKE MICHIGAN TODAY. THE HIGH
EXTENDS FROM SASKATCHEWAN TO VIRGINIA THIS MORNING. BY THIS
EVENING THE HIGH WILL BE OVER NEW ENGLAND AND QUEBEC. THE WIND
WILL BE EAST AT 10 TO 20 KNOTS THIS MORNING. AN INTENSE LOW WILL
MOVE INTO OKLAHOMA. THIS LOW WILL GIVE A STRONGER EAST WIND THIS
AFTERNOON WITH A SPEED OF 25 TO 35 KNOTS. SMALL CRAFT ADVISORIES
AND GALE WARNING HAVE BEEN ISSUED FOR THIS AFTERNOON AND TONIGHT.
SNOW WILL FALL OVER THE LAKE LATE AFTERNOON. A MIXTURE OF SNOW
SLEET AND RAIN WILL OCCUR TONIGHT OVER SOUTHERN LAKE MICHIGAN AND
SNOW WILL FALL IN THE NORTH HALF OF THE LAKE. THE LOW WILL BE IN
CENTRAL ILLINOIS MIDNIGHT TONIGHT AND OVER SOUTHEAST LAKE MICHIGAN
BY WEDNESDAY MORNING. THERE WILL BE SNOW AND GALE FORCE WIND
WEDNESDAY AND WEDNESDAY NIGHT. BY WEDNESDAY NIGHT THE LOW WILL BE
OVER LAKE HURON. THEN THURSDAY...THE LOW WILL BE OVER QUEBEC. THE
WIND WILL DECREASE AND THERE WILL BE SNOW SHOWERS OVER LAKE
MICHIGAN.
&&
LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER STORM WARNING...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011 UNTIL 6 PM WEDNESDAY.
WINTER WEATHER ADVISORY...ILZ012-ILZ013-ILZ014-ILZ019-ILZ020
UNTIL 6 PM WEDNESDAY.
IN...NONE.
LM...GALE WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-LMZ742-
LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ868-LMZ870-LMZ872-
LMZ874-LMZ876-LMZ878...7 PM TUESDAY TO 4 AM FRIDAY.
SMALL CRAFT ADVISORY...NEARSHORE WATERS
UNTIL 7 PM TUESDAY.
&&
$$