FXUS63 KLOT 090003
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
603 PM CST TUE DEC 8 2009
.DISCUSSION...
315 PM CST
TONIGHT THROUGH WEDNESDAY NIGHT...
VERY LITTLE CHANGE TO THINKING FROM PREVIOUS UPDATED AFDS ALREADY
ISSUED REGARDING THE STORM. LARGE AREA OF MODERATE TO LOCALLY
HEAVY PRECIPITATION CURRENTLY ENCROACHING ON THE CWA. PRECIP
LIKELY ALL RAIN IN FAR SOUTHERN CWA WITH NORTHERN EDGE OF PRECIP
LIKELY BECOMING INCREASINGLY MIXED WITH SLEET/SNOW AS SHIELD
ADVANCES NORTHWARD INTO COLDER AIR MASS FARTHER NORTH OVER THE
CWA. WHILE TEMPS ACROSS CENTRAL AND NORTHERN PORTIONS OF CWA HAVE
RISEN ABOVE FREEZING SUSPECT THAT THEY WILL LOSE A COUPLE DEGREES
THROUGH WET BULB PROCESSES AS HEAVIER PRECIP MOVES IN. PRIOR TO
PRECIP SWITCHING OVER TO SLEET/RAIN STILL THINK THERE IS A WINDOW
OF OPPORTUNITY FOR MUCH OF THE CURRENT HEADLINE AREA TO PICK UP
BURSTS OF HEAVY SNOW...WHICH COULD ACCUMULATE RATHER QUICKLY.
GIVEN THE ARRIVAL OF THIS SNOW DURING RUSH HOUR DON'T SEE ANY
ADVANTAGE IN LOWERING HEADLINES AT THIS POINT...THOUGH AS
MENTIONED EARLIER...EVENING SHIFT WILL LIKELY BE ABLE TO START
TRIMMING COUNTIES OFF THE SOUTHERN PORTIONS OF THE ADVISORY AREA.
NO SENSE IN RE-HASHING TONIGHT'S REASONING...STILL ANTICIPATE A
CHANGE OVER BACK TO SNOW FROM WEST TO EAST VERY LATE TONIGHT INTO
WEDNESDAY MORNING AS STORM SYSTEM PULLS OUT. HARD TO SAY HOW MUCH
SNOW WILL ACCUMULATE DURING THE MORNING WEDNESDAY AND ALSO TOUGH
TO SAY HOW "BLOWABLE" ANY SNOW THAT ACCUMULATES WILL BE. GIVEN
LIKELY ADVISORY CRITERIA WINDS WEDNESDAY...IF THE SNOW IS
SUFFICIENTLY BLOWABLE THEN WE COULD BE LOOKING AT NEAR GROUND
BLIZZARD CONDITIONS IN OPEN AND RURAL AREAS. GIVEN UNCERTAINTY
WITH SNOW TEXTURE AND AMOUNTS WILL NOT BE DOING ANY BLIZZARD
HEADLINES AT THIS TIME. ALSO GOING TO HOLD OFF ON A SECOND PERIOD
WIND ADVISORY AT THIS TIME SINCE ITS NOT CLEAR WHERE THERE MAY BE
WINTER WEATHER ADVISORIES OUT THUS NEGATING THE NEED FOR WIND
ADVISORY.
THE BIG STORY WILL QUICKLY TRANSITION TO TEMPERATURES AND WIND
CHILLS WEDNESDAY NIGHT. WHERE WE END UP WITH A FEW INCHES OR MORE
OF SNOW ON THE GROUND TEMPERATURES WILL LIKELY REALLY BOTTOM OUT
WEDNESDAY NIGHT INTO THURSDAY MORNING. HAVE SIGNIFICANTLY UNDERCUT
GUIDANCE WEDNESDAY NIGHT OVER NORTH CENTRAL ILLINOIS WITH TEMPS
LIKELY TO DROP TO AROUND 10 BELOW OR POSSIBLY COLDER. WITH WINDS
CONTINUED AT 10 TO 20 WE SHOULD SEE WIND CHILLS SOLIDLY IN
ADVISORY CRITERIA RANGE AND POSSIBLY EVEN FLIRTING WITH WARNING
CRITERIA IN SPOTS.
IZZI
THURSDAY THROUGH TUESDAY...
COLDEST DAY OF THE WINTER THUS FAR ON TAP FOR THURS AS ARCTIC AIR
SURGES SOUTH INTO THE AREA BEHIND DEPARTING STORM SYSTEM AND IN
ASSOCIATION WITH SPRAWLING SURFACE HIGH PRESSURE ACROSS THE
CENTRAL PLAINS INTO THE OHIO AND TENNESSEE VALLEYS. WILL UNDERCUT
MOS GIVEN EXPECTED H850 TEMPS OF -18 TO -20 DEG C PROGGED TO BE
OVERHEAD MAX TEMPS THURS MAY NOT GET OUT OF THE SINGLE DIGITS
ACROSS NORTH CENTRAL IL WITH TEENS THROUGHOUT CHICAGOLAND AND
SOUTH OF I-80.
BROAD UPPER LEVEL TROUGH ACROSS EASTERN NORTH AMERICA GRADUALLY
EASES WITH TIME WITH RISING HEIGHTS ACROSS THE FORECAST AREA LATE
WEEK INTO THE WEEKEND. SURFACE HIGH PRESSURE BUILDING NORTHWARD
INTO THE SOUTHERN GREAT LAKES INTO SATURDAY SETS UP A PERIOD OF
RATHER TRANQUIL WINTER WEATHER WITH TEMPERATURES MODERATING EACH DAY
REACHING BACK NEAR SEASONAL AVERAGES BY SATURDAY.
WEAK UPPER LEVEL SHORTWAVE CAUGHT UP IN FAST ZONAL FLOW RIPPLES EAST
QUICKLY OVERHEAD SAT NIGHT WITH A WEAK SURFACE REFLECTION IN THE FORM
OF A TROF SLIDING EAST ACROSS THE FORECAST AREA. AIR MASS REMAINS
MOISTURE STARVED HOWEVER AND AT THIS POINT QUESTION THE PROSPECTS
EVEN FOR FLURRIES SO HAVE OPTED TO MAINTAIN A DRY FCST SATURDAY
NIGHT AND EARLY SUNDAY.
SOMEWHAT MORE PRONOUNCED INVERTED TROF DEVELOPS LATER SUNDAY
INTO MONDAY WITH SOMEWHAT BETTER MOISTURE AND WARM AIR ADVECTION
SUPPORT. HAVE THUS INTRODUCED CHANCE POPS FOR SUNDAY NIGHT INTO
MONDAY. SECONDARY SHOT OF POLAR AIR SURGES SOUTH BEHIND THIS SYSTEM
LATER MONDAY INTO TUESDAY BRINGING A RETURN TO LAKE EFFECT SNOW
DOWNWIND OF LAKE MICHIGAN HOWEVER WIND TRAJECTORIES SHOULD KEEP SNOW
BANDS TO THE NORTH OF LAKE AND PORTER COUNTIES. MEANWHILE SURFACE
HIGH PRESSURE AGAIN ESTABLISHES ITSELF BEHIND THE TROUGH.
ED F
&&
.AVIATION...
700 PM CST
0000 UTC TAFS...THE DEVELOPING WINTER STORM WILL BRING A PLETHORA
OF PROBLEMS FOR AVIATION WEATHER CONCERNS.
CATEGORICALLY...IFR CONDITIONS WILL PREVAIL FOR THE REMAINDER OF
THE EVENING THROUGH THE MORNING HOURS. AS THE CENTER OF THE SYSTEM
APPROACHES THE TERMINALS AND CORE OF COLDEST AIR MOVES
OVERHEAD...EXPECT THAT CIGS WILL HAVE THE POTENTIAL TO DROP TO
LIFR. LATEST RADAR IMAGERY SHOWS A STRONGER BAND OF PCPN MOVING
INTO NRN IL/IN AND VISIBILITY HAS DROPPED BLW 1SM. ALSO...PCPN
TYPE STILL REMAINS SNOW OR A SNOW MIX...WHICH IS ALSO CONTRIBUTING
TO LOWER VIS. AS WARMER AIR ADVECTS FROM THE SOUTH...THE
SNOW/MIXED PCPN WILL CHANGE OVER TO RAIN...ALLOWING VIS TO IMPROVE
A BIT...BUT SHOULD STILL REMAIN IN IFR RANGES. EXPECT THAT ORD/MDW/DPA/GYY
WILL THEN REMAIN ALL RAIN UNTIL THE SFC LOW MOVES TO THE NORTH AND
EAST OF THE AREA AND DEEP LAYER COLD ADVECTION BEGINS. RFD IS THE
MAIN QUESTION MARK AS FAR AS PCPN TYPE IS CONCERNED. LATEST MODEL
SOUNDING SHOW A BRIEF PERIOD OF SOME WARM AIR INTRUSION...ALLOWING
FOR THE POSSIBILITY OF MIXED PCPN...THOUGH EXPECT THAT RFD SHOULD
PREDOMINANTLY REMAIN FROZEN.
WINDS WILL BE A CONCERN FOR THE DURATION OF THE EVENT AS WELL.
WIND SPEED WILL BE ELY AND INCREASE OVER THE FIRST FEW HOURS OF
THE PERIOD AS THE SFC LOW APPROACHES FROM THE SOUTHWEST. THE
CENTER OF THE LOW IS EXPECTED TO TRACK BETWEEN CHICAGO AND
ROCKFORD...SO WINDS WILL TEMPORARILY WEAKEN AND VEER THROUGH SELY
TO SWLY OVERNIGHT AND THEN STRENGTHEN DRAMATICALLY BY DAYBREAK.
RFD WILL GO THROUGH A SIMILAR SPEED TREND BUT DIRECTION SHOULD
BACK NORTHERLY BEFORE SETTLING INTO WLY IN THE MORNING. AS THE LOW
PULLS OFF TO THE NORTHEAST...COLD ADVECTION WILL INCREASE AND
WINDS WILL INCREASE TO 25-30KT WITH FREQUENT GUSTS TO 40KT. AS THE
WINDS INCREASE AND THE PCPN TURNS BACK OVER TO SNOW...BLOWING SNOW
WILL BECOME A CONCERN. HAVE MAINTAINED MVFR VIS AS THE SNOW
DECREASES ON THE TAIL END OF THE SYSTEM...BUT IF THE SNOW BECOMES
DRY ENOUGH AND WINDS REMAIN STRONG ENOUGH...MAY HAVE TO ADJUST VIS
LOWER WITH THE BLOWING SNOW.
KREIN
&&
.MARINE...
143 PM CST
A RIDGE OF HIGH PRESSURE OVER THE EASTERN GREAT LAKES WILL MOVE
EAST TO NEW ENGLAND TONIGHT. LOW PRESSURE OVER THE TEXAS PANHANDLE
WILL INTENSIFY RAPIDLY AS IT MOVES NORTHEAST TO CHICAGO BY
WEDNESDAY MORNING. THE LOW WILL MOVE TO LAKE HURON BY WEDNESDAY
EVENING...AND INTO QUEBEC THURSDAY. GALE FORCE EASTERLY WINDS WILL
DEVELOP THIS EVENING. AS THE STRENGTHENING LOW APPROACHES SOUTHERN
LAKE MICHIGAN LATE TONIGHT...STORM FORCE WIND GUSTS ARE
LIKELY...MAINLY OVER THE NORTH HALF OF THE LAKE. IN THE WAKE OF
THE DEEPENING LOW...STORM FORCE WIND GUSTS ARE LIKELY MAINLY
OVER SOUTH AND CENTRAL PART OF THE LAKE WEDNESDAY INTO WEDNESDAY
EVENING. HIGH PRESSURE WILL MOVE INTO THE SOUTHERN PLAINS AND
LOWER MISSISSIPPI VALLEY THURSDAY AND FRIDAY. THIS RIDGE OF HIGH
PRESSURE WILL MOVE ACROSS THE WESTERN GREAT LAKES FRIDAY NIGHT AND
SATURDAY...ALLOWING WINDS AND WAVES TO CALM DOWN.
ALLSOPP
&&
.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-LMZ777-LMZ779-LMZ868-
LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL 3 AM WEDNESDAY.
GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-
LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...9 PM WEDNESDAY TO 4 AM
FRIDAY.
STORM WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-LMZ868-
LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...3 AM WEDNESDAY TO 9 PM
WEDNESDAY.
GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
LMZ868...9 PM WEDNESDAY TO NOON FRIDAY.
GALE WARNING...NEARSHORE WATERS UNTIL 4
AM FRIDAY.
SMALL CRAFT ADVISORY...NEARSHORE WATERS
UNTIL 7 PM TUESDAY.
&&
$$
