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December 10-12th Clippers


Chicago Storm

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DVN...

 

I HAVE MADE A FEW ADJUSTMENTS TO THE FORECAST GRIDS...BUT HAVE NOT
CHANGED THE OVER ALL THEME OF THIS SNOW EVENT. IT IS CLEAR TO ME
THAT THERE WILL BE A SHARP CUT OFF TO A NIL EVENT ON THE SOUTH
FLANK...WHICH I HAVE ATTEMPTED TO OUTLINE IN THE LAST WSW UPDATE.
LOCATIONS JUST SOUTH OF IOWA CITY TO MUSCATINE TO SOUTHERN HENRY
COUNTY MIGHT WONDER WHAT ALL THE SNOW TALK WAS ALL ABOUT IN THE
END. JUST NORTH OF THE NIL CUT OFF...WE ARE STILL LOOKING FOR A
PERIOD OF MODERATE SNOW...POSSIBLY BRIEFLY HEAVY. AMOUNTS OF 1 TO
3 INCHES SEEM ON TARGET...MAYBE A FEW HIGHER TOTALS IN A NARROW
AXIS MOST LIKELY FROM CID...TO CWI...TO STERLING ILLINOIS. WINDS
SHOULD NOT INCREASE AT ALL UNTIL AFTER THE SNOW COMPLETELY ENDS.
IN THE END...WE MAY NEED TO CANCEL THE SOUTH FLANK OF THE WSW
EARLY TOMORROW SHOULD SNOW NOT FALL SIGNIFICANT ENOUGH TO CREATE A
LASTING BLOWING/DRIFTING THREAT WHEN THE WINDS INCREASE. THE MOST
SIGNIFICANT PIECE OF THIS FORECAST THAT I IMAGINE IS NOT BEING
PASSED ON...IS THAT WEDNESDAY WILL SEE RAPIDLY FALLING
TEMPERATURES AGAIN...WITH DANGEROUS WIND CHILLS IN THE WESTERN CWA
ALL DAY.  

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00z 4km NAM gives Hawkeye about 0.15".  I'm expecting at least 20:1 ratios with this thing, but wouldn't be surprised to see even higher ratios than that.  So CR should be good for 3-4".  We are literally right on the 0.1" line on the 4km.  Should fluff up to a nice 2" snow, but man that cutoff to the south is very sharp.  Could be a big difference in accumulations from the IL QC to the IA QC.

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LOT...

 

920 PM CST

THE CURRENT WINTER WEATHER ADVISORY FOR OVERNIGHT AND WEDNESDAY
MORNING CONTINUES AS IS WITH NO MAJOR CHANGES TO GOING FORECAST
SNOWFALL TIMING OR TOTAL AMOUNTS. HAVE ADDED PATCHY BLOWING SNOW
IN FOR THE MORNING IN THE FAR WEST AND SPREAD IT EASTWARD DURING
THE AFTERNOON AS THE COLD...OR SHOULD WE SAY COLDER...AIR SURGES
BACK IN.

THE POSITIVES TOWARD SNOWFALL WITH THIS SYSTEM REMAIN IN
OBSERVATIONAL AND SHORT TERM GUIDANCE...WITH HIGHER SNOW-TO-
LIQUID RATIOS AND TEMPORARY MODEST RATES...WHILE THE SUBTLE
CAVEATS OF A SOMEWHAT WEAK WAVE AND LOW COLUMN MOISTURE ARE LIKELY
TO BE OFFSET BY THESE ENOUGH TO PRODUCE 2-3.5" SNOWFALL AMOUNTS
AND LIKELY SCATTERED HIGHER AMOUNTS WITHIN THE ADVISORY AREA.

RADAR ECHOES ARE ON THE UPWARD TREND ACROSS CENTRAL IA WITHIN THE
WARM AIR ADVECTION WING OF THE SHORT WAVE SEEN ON WATER VAPOR
PROGRESSING EAST SOUTHEAST. THIS WAVE WILL TURN MORE EAST ALONG A
WELL-DEFINED 850-600MB BAROCLINIC ZONE SEEN ON THIS EVES UPPER
AIR DATA. HAVE ALREADY NOTED 3/4SM VISIBILITY IN IA...LIKELY
ASSOCIATED WITH ONE HALF TO ONE INCH PER HOUR TYPE RATES. WITH
CLIPPER SYSTEMS IT SEEMS THAT THE LOWEST VISIBILITY CAN OFTEN BE
EXTRAPOLATED OUT ALONG THE PATH GIVEN THE WAVE MAINTAINS ITS
INTENSITY. THE RAP CONTINUES TO SHOW A FAIRLY QUICK PROGRESSION
OF THE LEADING EDGE OF SNOW OVER MUCH OF THE FORECAST AREA BETWEEN
12AM-3AM...SO DID NOT GET FANCY WITH TRYING TO ADJUST THE ADVISORY
START TIME. TEMPORARY WEST-TO-EAST BANDED SNOW OVERNIGHT WOULD
SEEM PROBABLE IN NORTH CENTRAL IL...MAYBE INTO THE CHICAGO
METRO...ALONG THE BAROCLINIC ZONE AND FAIRLY WELL-DEFINED
MOISTURE CONVERGENCE. CONVECTIVE ALLOWING/HIGH RES GUIDANCE
INDICATES HIGHER REFLECTIVITY AS WELL IN THE 3-8 AM PERIOD. SNOW
LOOKS TO LINGER OVER CHICAGO THROUGH THE END OF THE ADVISORY TIME.

FORECAST SOUNDINGS CONTINUE TO INDICATE A VERY DEEP DENDRITIC
GROWTH AREA WITH NOT OVERLY HIGH WINDS BELOW THE CLOUD BASE FOR
FRAGMENTING. EVEN THE 00Z DVN SOUNDING HAD THE WET BULB
TEMPERATURE BASICALLY WITHIN THE DENDRITIC GROWTH AREA ALL THE WAY
TO ~10000 FT! THIS SHOULD ALLOW FOR GOOD EFFICIENCY (20:1) OF
MEAGER PWAT VALUES UNDER 0.2 INCH AND MIXING RATIOS IN THE CLOUD
LAYER OF 1-1.5 G/KG. SO WE CONTINUE A LITTLE HIGHER THAT MOST
GUIDANCE FOR SNOWFALL AMOUNTS.

THE MOST PROBABLE TIMING FOR HEAVIEST SNOWFALL RATES IN ROCKFORD
IS 2-6 AM AND CHICAGO IS 4-8 AM. TEMPERATURES SHOULD CONTINUE TO
INCH UP PRIOR TO THE SNOWFALL...AND THEN HOLD STEADY IN THE MID-
UPPER TEENS DURING THE SNOW.

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