Expert Forecast Opinion

CRITIQUE ON THE FORECAST - TONIGHT AND SUNDAY
 
RIDGE OF HIGH PRESSURE THAT HAS BROUGHT SUNNY WEATHER AND ABOVE
NORMAL TEMPERATURES THE PAST FEW DAYS WILL SHIFT EAST OF THE STATE
TONIGHT. WILL FINALLY EXPERIENCE AN INCREASE IN CLOUDS THIS EVENING
AHEAD OF A SHEARING UPPER LEVEL WAVE SEEN ON WATER VAPOR PUSHING
INTO THE WESTERN GREAT LAKES. THERE IS ALSO A RIBBON OF HIGHER
LOW-LEVEL THETA-E SITUATED FROM LAKE MICHIGAN TO EAST-CENTRAL
INDIANA...PERCHED ALONG THE FORWARD FLANK OF A THERMAL RIDGE
ASSOCIATED WITH A DECAYING SURFACE WAVE OVER THE PLAINS. GFS/NAM
HAVE A FAIRLY GOOD HANDLE ON THE ADVANCING LOW-LEVEL MOISTURE BELOW
850MB...AND SHOW VERY LITTLE SEEPAGE INTO SOUTHEAST MICHIGAN
TONIGHT. GIVEN LACK OF LOW-LEVEL MOISTURE ADVECTION OVERNIGHT
COUPLED WITH SIGNIFICANT DRY AIR SHOWN BELOW 700MB ON 12Z DTX
SOUNDING...WILL REMOVE MENTION OF PRECIPITATION FROM THE FORECAST
OVERNIGHT OUTSIDE OF A CHANCE TO SEE A FEW FLURRIES OVER LENAWEE
COUNTY. HAVE UNDERCUT GUIDANCE VALUES FOR OVERNIGHT MINS FROM FLINT
NORTHWARD GIVEN LESS MOISTURE AT THE MID-LEVELS THAN MODELS ARE
FORECASTING. EXPECT TEMPERATURES TO FALL QUICKLY AFTER SUNSET NORTH
OF FLINT WHERE MID-LEVEL MOISTURE WILL BE SLOWER TO ARRIVE.

CLOUD COVER WILL CONTINUE TO INCREASE ON SUNDAY AS THE DEVELOPING
STORM SYSTEM OVER THE PLAINS BEGINS TO LIFT NORTHEAST. ALL MODELS
NOW HOLD GOOD MOISTURE ADVECTION AND ISENTROPIC ASCENT OFF UNTIL
AFTER 00Z (7PM EST)...AND WILL KEEP THE FORECAST DRY. WILL EXPECT
ANOTHER WARM DAY NONETHELESS AS WARM AIR ADVECTION AND SOME SUNSHINE
NORTH OF I-69 HELP TEMPERATURES AGAIN SURGE FROM THE MID 30S TO NEAR
40.

REMAINDER OF THE PERIOD

ATTENTION IN THE LONG TERM REMAINS ON THE PENDING STORM SYSTEM
THAT WILL BRING ACCUMULATING SNOW FOR THE SUNDAY NIGHT THROUGH
MONDAY EVENING PERIOD. LATEST WATER VAPOR/UPPER AIR ANALYSIS
SHOWING THE MAIN SHORTWAVE OF INTEREST MOVING INTO SOUTHERN
CALIFORNIA THIS AFTERNOON. ELONGATED POSITIVELY TILTED TROUGH
REMAINS POSITIONED TO THE NORTH STRETCHED FROM THE NORTHERN PLAINS
INTO THE NORTHERN/ CENTRAL ROCKIES. BETTER SAMPLING OF THESE
FEATURES OVER THE LAST 12 HOURS IS STARTING TO PROVIDE BETTER
OVERALL MODEL CONSENSUS IN CAPTURING THE SYNOPTIC
EVOLUTION...ALTHOUGH SOME SMALLER SCALE DETAILS STILL REMAIN
ELUSIVE AT THIS TIME SCALE.

SURFACE CYCLOGENESIS WILL COMMENCE THROUGH SUNDAY AS THE SOUTHERN
STREAM WAVE EMERGES INTO THE SOUTHERN PLAINS. THE ASSOCIATED
SURFACE LOW WILL THEN LIFT NORTHEAST...MOVING FROM NEAR STL AT
06Z MONDAY TO NORTH-CENTRAL OHIO BY 00Z TUESDAY. THE TRACK OF THE
LOW LEVEL CYCLONE CERTAINLY PLACES SOUTHEAST LOWER MICHIGAN UNDER A
FAVORABLE TRACK FOR ACCUMULATING SNOWFALL. LEAD PUSH OF THETA-E
ADVECTION/ISENTROPIC ASCENT WILL INITIATE THE ONSET OF SNOW AS THE
COLUMN SATURATES FROM SOUTHWEST TO NORTHEAST DURING THE EARLY
MORNING HOURS MONDAY. WHILE NOT OVERLY IMPRESSIVE...THE ASCENT
LOOKS SUFFICIENT ENOUGH TO DETER ANY RESIDUAL DRY AIR FROM BECOMING
A LARGER PLAYER IN THE SATURATION PROCESS.

WIDESPREAD SNOWFALL WILL CONTINUE THROUGHOUT MONDAY AS THE MID LEVEL
DEFORMATION DYNAMICS MIGRATE ACROSS THE AREA. LATEST MODEL
SOUNDINGS AND CROSS SECTIONS DEPICT A SUSTAINED REGION OF LIFT ON
MONDAY THAT AT LEAST BRIEFLY EXTENDS INTO THE BETTER DENDRITIC
GROWTH ZONE...ESPECIALLY FROM THE I-69 CORRIDOR SOUTHWARD. ALSO
NOTING A MODEST FGEN RESPONSE DURING THIS TIME ACCOMPANIED BY A
PERIOD OF LOWER STATIC STABILITY /EPV BELOW -1/. SPECIFIC HUMIDITY
WILL CLIMB TO AT LEAST THE 3 G/KG RANGE...WITH THE MOIST NAM
TOUCHING 4 G/KG IN THE SOUTH. TAKING ALL THIS INTO ACCOUNT...
SUPPORT IS CERTAINLY THERE FOR SOME HIGHER END SNOW AMOUNTS
/OVER 6 INCHES/.

PROPER PLACEMENT OF THIS AREA REMAINS THE BIGGEST CHALLENGE...
ESPECIALLY WITH REGARD TO THE WATCH HEADLINE DECISION. REMAINING
QUESTIONS STILL CENTER ON WHERE ANY MESOSCALE ENHANCEMENTS WILL
SETUP AND WHETHER MORE MODEST SNOW TO LIQUID RATIOS /10-12:1/ CAN BE
OVERCOME BY AN OVERACHIEVING QPF FIELD AS SUGGESTED BY THE 12Z NAM
/QPF UPWARD OF .75" ACROSS THE SOUTH HALF/. STILL A BIT EARLY TO
PINPOINT THIS...ALTHOUGH THE CHARACTER OF THE DYNAMICS WOULD SUGGEST
SOMEWHERE WITHIN THE ARB-DTW TO I-69 CORRIDOR WILL SEE THE HIGHEST
TOTALS. CONCERN EXISTS FOR POINTS SOUTH TO THE OHIO BORDER FOR MORE
SUPPRESSED SNOW RATIOS /10:1 OR LESS/ AS THIS AREA FLIRTS WITH A
MORE MARGINAL THERMAL PROFILE/MIX POTENTIAL...AS WELL AS THE
THREAT FOR A PERIOD OF MID LEVEL DRY SLOTTING. TOWARD THE
NORTH...PORTIONS OF THE SAGINAW VALLEY/THUMB MAY FIND ITSELF
DISPLACED SLIGHTLY FROM THE BETTER ASCENT AND HIGHER QPF.

WITH ALL THIS SAID...FAVOR A GENERAL 5-7 INCH SNOWFALL ACROSS MOST
OF THE REGION THROUGH MONDAY EVENING WITH SOME BANDING POTENTIAL
PUSHING A FEW LOCALES INTO THE 8 INCH RANGE. DESPITE THE LINGERING
UNCERTAINTIES...CONFIDENCE IS JUST HIGH ENOUGH TO WARRANT A WATCH
ISSUANCE. COUNTIES ON BOTH THE SOUTHERN AND NORTHERN FRINGES OF
THE WATCH LOOK THE MOST MARGINAL...BUT WILL ALLOW FOR SLIGHT
ADJUSTMENTS WITHIN THE MODEL THERMAL/MOISTURE PROFILES AS THE
AFOREMENTIONED SHORTWAVE ATTAINS EVEN BETTER SAMPLING OVER THE
NEXT 24 HOURS.


BEYOND MONDAY...A PIECE OF THE POSITIVELY TILTED NORTHERN STREAM
TROUGH WILL SETTLE ACROSS THE GREAT LAKES IN THE WAKE OF THIS SYSTEM
FOR TUESDAY. A SUFFICIENTLY MOIST LINGERING AIR MASS WILL SUPPORT
ADDITIONAL SNOW SHOWER DEVELOPMENT UNDER THE ENHANCED FORCING
THROUGH THE DAY...WITH ADDITIONAL ACCUMULATIONS UP TO AN INCH
POSSIBLE. ADDITIONAL NORTHERN STREAM ENERGY WILL THEN DROP
SOUTHEAST AND CLOSE OFF AS IT MIGRATES ACROSS THE REGION ON
WEDNESDAY AND THURSDAY. THIS WILL MAINTAIN A MOIST CYCLONIC FLOW
WHILE PROVIDING A PERIOD OF MODEST COLD AIR ADVECTION. THIS WILL
CONTINUE TO WARRANT A MENTION FOR SNOW SHOWERS THROUGHOUT THIS
PERIOD. TEMPERATURES WILL SETTLE A FEW DEGREES NORMAL DURING THIS

. . posted at 633 PM EST Saturday February 20 2010

 



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