Sun Dec 13 Meteorology for Chicago
Brett Crapser
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1217 PM CST SUN DEC 13 2009
DISCUSSION...
335 AM CST
MAIN FORECAST CHALLENGES THIS MORNING ARE WITH LINGERING FZDZ
THIS MORNING...PRECIP CHANCES MONDAY...AND TEMPERATURES THROUGH
THE PERIOD.
FAIRLY IMPRESSIVE SHORTWAVE CONTINUES TO MOVE EASTWARD ACROSS THE
MID-MISSISSIPPI VALLEY REGION THIS MORNING WITH THE WIDESPREAD RAIN
AND EVEN ISOLD THUNDERSTORMS CONFINED TO OUR SE MOST COUNTIES.
STRATUS DECK HAS BEEN BUILDING DOWNWARD ACROSS THE REMAINDER OF THE
AREA WITH STRONG SHEAR WITH THE CLOUD LAYER MAKING THIS CLOUD DECK
QUITE EFFICIENT AT DRIZZLE PRODUCTION. AMBIENT TEMPS ARE GENERALLY
AT OR ABOVE FREEZING ACROSS THE CWA...HOWEVER TEMPS OF MANY SURFACES
ARE JUST BELOW FREEZING RESULTING IN A LIGHT ACCRETION OF ICE TO
FORM. CALLS TO LOCAL LAW ENFORCEMENT SUGGEST THAT AN ARGUMENT COULD
PROBABLY BE MADE TO EXPAND THE ADVISORY INTO DUPAGE AND WILL
COUNTIES WITH REPORTS OF ICING ON LESSER TRAVELED/UNTREATED
ROADS...BUT GIVEN THAT DEWPOINTS/TEMPS CONTINUE TO RISE THINK THAT
THE ICING THREAT WILL BE RAMPING DOWN THE NEXT COUPLE HOURS.
ELSEWHERE...DEEP SNOWPACK WILL KEEP PAVEMENT TEMPS AROUND FREEZING
AND THUS RESULT IN A CONTINUATION OF THE FZDZ THIS MORNING.
AS WINDS SHIFT TO THE WEST LATER THIS MORNING SLIGHTLY DRIER
BOUNDARY LAYER AIR WILL ADVECT INTO THE AREA AND SHOULD PUT THE
KABOSH ON DZ/FZDZ. WHILE DZ WILL END AM NOT EXPECTING THE STRATUS
DECK TO GO ANYWHERE TODAY AND HAVE UNDERCUT MOS HIGHS A BIT GIVEN
THE EXPECTED THICK STRATUS AND NEUTRAL/WEAK CAA.
NEXT SYSTEM WILL BE FAST ON THE HEELS OF THIS ONE WITH PRESSURE
FALLS ALREADY COMMENCING OVER THE CENTRAL HIGH PLAINS WHERE LEE
TROUGH IS PROGGED TO CONSOLIDATE INTO A SFC LOW TODAY. THIS LOW IS
PROGGED TO MOVE QUICKLY EASTWARD AND ACROSS NORTHERN ILLINOIS MONDAY
MORNING. WARM AIR ADVECTION WILL BE RAMPING UP TONIGHT...THOUGH MUCH
OF THE INITIAL LIFT WILL GO TOWARD SATURATING THE COLUMN AND BE
FOCUSED ALONG/NORTH OF THE IL/WI LINE. COULD SEE SOME LIGHT WINTRY
PRECIP DEVELOP LATE TONIGHT OVER FAR NORTHWEST COUNTIES...BUT BETTER
THREAT APPEARS TO BE NORTH OF THE IL/WI BORDER AND PLAN TO KEEP POPS
LOW FOR NOW. ALONG/RIGHT OF THE SFC LOW TRACK LIMITED MOISTURE
SHOULDN`T SUPPORT MUCH IN THE WAY OF PRECIP MONDAY...BUT DID HANG
ONTO SOME LOW POPS FOR SHRA.
MODELS CONTINUE TO SUGGEST THAT THE AIRMASS MOVING INTO THE AREA
BEHIND THIS SYSTEM...WHILE COLDER...WILL NOT BE AS BRUTAL AS THE
PREVIOUS COLD SNAP. NUDGED TEMPS UP ANOTHER CATEGORY OR SO TUESDAY.
DIDN`T MAKE ANY ADJUSTMENT TO TEMPS BEYOND TUESDAY BUT WITH H8 TEMPS
PROGGED TO WARM TO ABOVE AVERAGE BY THURSDAY SUSPECT OUR GOING
TEMP GRIDS MAY NEED SOME UPWARD TWEAKING TUES NIGHT THROUGH
THURSDAY.
IZZI
&&
AVIATION...
1217 PM CST
1800 UTC TAFS...PRIMARY AVIATION FORECAST CONCERNS WILL BE
CONTINUED POTENTIAL OF AT LEAST IFR CIGS THROUGH MUCH OF THE
PERIOD.
COLD FRONTAL BOUNDARY HAS LOST MUCH OF ITS STEAM THIS MORNING
WITH THE BULK OF THE UPPER FORCING MOVING ACROSS SOUTHERN CANADA
AND MID/UPPER LEVEL FLOW NEARLY ZONAL ACROSS WESTERN GREAT LAKES
REGION. SFC HIGH PRESSURE HAS BEEN TRYING TO BUILD SOUTHEASTWARD
FROM THE NORTHERN PLAINS BUT ALREADY BEGINNING TO SEE SFC PRESSURE
FALLS DEVELOP ACROSS THE SOUTHERN/CENTRAL PLAINS IN RESPONSE TO
ONSET OF CYCLOGENESIS DOWNSTREAM OF NEXT BROAD PACIFIC UPPER
TROUGH APPROACHING THE ROCKIES THIS MORNING. THIS LOW WILL
CONTINUE TO DEVELOP THIS AFTERNOON AND IS EXPECTED TO TRACK ACROSS
NORTHERN ILLINOIS SUNDAY MORNING. ANOTHER FAIRLY STRONG LOW LEVEL
JET AND BRIEF SHOT OF ENHANCED WARM/MOIST ADVECTION WILL PRECEDE
THIS SYSTEM. ALREADY A VERY STRONG INVERSION NOTED IN ACARS FLIGHT
DATA AROUND 3K FT WHICH SHOULD BE LOWERING ESPECIALLY LATER
TONIGHT WITH STRONGER LOW LEVEL WARM ADVECTION. SFC OBS FROM
ACROSS THE AREA INDICATE WIDE RANGE IN REPORTED CIGS FROM LIFR TO
HIGH END IFR/LOW END MVFR. COULD BE A BRIEF WINDOW OF IMPROVEMENT
TO LOWER END MVFR LATE THIS AFTERNOON/EARLY THIS EVENING BUT
WINDOW FOR IFR FREE CONDITIONS APPEARS TO BE NARROW AT THIS TIME
AND HAVE REINTRODUCED IFR CIGS THIS EVENING. CANNOT RULE OUT LIFR
CONDITIONS ESPECIALLY AT RFD IN CLOSER PROXIMITY TO STRONGER
MOISTURE ADVECTION TONIGHT...BUT SEE AT LEAST POTENTIAL OF LIFR
CIGS ALL TERMINALS. HAVE INCLUDED LIFR CIGS ONLY AT RFD FOR NOW BUT
WILL HAVE TO MONITOR FOR POSSIBLY GOING A LITTLE LOWER IN FUTURE
UPDATES/FORECASTS. MODEL TIME/HEIGHT SECTIONS DO INDICATE SOME
INCREASED UVM IN ASSOCIATION WITH STRONGER WARM ADVECTION SURGE
LATE TONIGHT AND DO SEE A DISTINCT POSSIBILITY OF MORE DRIZZLY
TYPE PRECIPITATION DEVELOPING AGAIN LATE TONIGHT/MONDAY MORNING.
HAVE KEPT MENTION -DZ AT ALL TERMINALS EXCEPT RFD WHERE LINGERING
SNOW COVER MAY KEEP NEAR SFC COLD ENOUGH TO SUPPORT SOME -FZDZ.
WEST WINDS SHOULD DIMINISH THROUGH REMAINDER OF THIS AFTERNOON AS
SFC PRESSURE GRADIENT WEAKENS BETWEEN PRESSURE SYSTEMS AND THEN
SOUTH SOUTHEAST WINDS WILL DEVELOP THIS EVENING IN RESPONSE TO
DEVELOPING LOW PRESSURE SYSTEM ACROSS THE PLAINS. WINDS WILL SHIFT
TO THE NORTHWEST MONDAY AFTERNOON AS LOW MOVES INTO LOWER MICHIGAN
TAKING A COLD FRONT ACROSS NORTHERN ILLINOIS. CONDITIONS SHOULD
IMPROVE TO MVFR MONDAY AFTERNOON BEHIND THE FRONT AS DRIER LOW
LEVEL AIR ADVECTS INTO THE REGION.
MARSILI
&&
MARINE...
330 AM CST
GUSTY SOUTHWEST WINDS WILL DIMINISH AND VEER WEST AND THEN NORTH
BEHIND A COLD FRONT CROSSING THE LAKE THIS MORNING. THE NEXT LOW
PRESSURE SYSTEM APPROACHES ON MONDAY AND PASSES ACROSS THE
SOUTHERN PART OF THE LAKE. AS A LARGE HIGH PRESSURE CENTER BUILDS
ACROSS THE UPPER MIDWEST ON TUESDAY...NORTHWEST WINDS WILL AGAIN
INCREASE IN THE PRESSURE GRADIENT AHEAD OF THIS HIGH AND BEHIND
THE DEPARTING LOW. GALES WOULD AGAIN BE A POSSIBILITY FOR TUESDAY
AFTERNOON AND EVENING...THOUGH AT THIS TIME IT APPEARS THIS WOULD
BE PRIMARILY IN THE GUSTS.
LENNING
&&
LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&
$$