Chris's Corner - Could This Be the First 1"+ Snow of Season?

WE ARE STILL LOOKING AT OUR FIRST WIDESPREAD SNOWFALL FOR THE AREA
TONIGHT INTO SATURDAY MORNING. THE MAIN CHANGES/CONCERNS ARE WE WILL
SEE MORE RAIN AT THE ONSET BEFORE A TRANSITION TO SNOW, AND THIS
TRANSITION WILL NOT BE UNIFORM ACROSS THE CWA.

THE OVERALL SET-UP HASN'T CHANGED DRASTICALLY. AN UPPER TROF/LOW
CURRENTLY CENTERED IN THE NORTH CENTRAL PLAINS WILL DIG SOUTHWARD
THROUGH THE PLAINS TONIGHT. INCREASING LARGE SCALE FORCING
ASSOCIATED WITH THE UPPER TROF (WHICH IS A BIT GREATER NOW), GOOD
MID LEVEL FRONTOGENETICAL FORCING, ISENTROPIC LIFT, AND DIVERGENCE
ALOFT ASSOCIATED WITH THE ULJ AND DECREASING WAVELENGTH WILL RESULT
IN AN INCREASING AND LARGE AREA OF PRECIPITATION/SNOW. THIS AREA
OF SNOW WILL BE ANCHORED BY A ROBUST AND WIDE WEST/NORTHWEST-
EAST/SOUTHEAST BAND ASSOCIATED WITH THE STRONG FRONTOGENESIS AND
MID LEVEL LAPSE RATES OF 6-6.5 DEGC/KM. THIS BAND SHOULD START
POKING INTO NORTHEAST MO LATER THIS AFTERNOON WITH THE PRECIPITATION
REALLY EXPANDING THIS EVENING, AND THEN GRADUALLY TRANSITIONING
SOUTHWARD OVERNIGHT AS THE UPPER TROF AND FORCING SHIFT
SOUTHWARD.

TEMPERATURES HAVE GOTTEN A BIT WARMER THAN EXPECTED TODAY AND
THIS WILL RESULT IN A SLOWER CHANGEOVER FROM RAIN TO SNOW. THE
CHANGEOVER WON'T BE UNIFORM OWING TO PRECIPITATION INTENSITY AND
ABILITY TO COOL THE LOW LEVELS. IT WILL TAKE LONGER IN CENTRAL AND
SOUTHEAST MO, PREDOMINATELY OVERNIGHT. THE CHANGEOVER SHOULD
OCCUR SOONER ACROSS NORTHEAST AND EAST-CENTRAL MO INTO WESTERN IL
WHERE THE MORE ROBUST BAND WITH HIGHER PRECIPITATION RATES IS
EXPECTED. SEVERAL RECENT AMDAR SOUNDINGS WITH ASCENTS OUT OF
LAMBERT INTERNATIONAL ARE SHOWING THE WARM LAYER ALOFT EXTENDS TO
A HEIGHT OF ABOUT 850 MB, HOWEVER THE OVERALL MAGNITUDE IS NOT
VERY STRONG WITH TEMPS BELOW 1 DEGC, AND THIS SHOULD BE ERODED
ONCE THE PRECIPITATION AND INTENSITY RAMPS UP. CURRENT THINKING IN
THE ST. LOUIS REGION IS FROM AROUND 02-04Z.

THIS WILL BE A WET SNOW AND WILL ACCUMULATE AND COMPACT. AT TIMES
SNOWFALL RATES WILL BE FROM 0.5-1.0 PER/HR CONTRIBUTING TO
ACCUMULATION ON ROADWAYS AND DETERIORATING DRIVING CONDITIONS. WE
ARE STILL THINKING THAT NORTHEAST MO AND WEST CENTRAL IL WILL SEE
THE HIGHEST TOTALS OWING TO LONGEVITY AND INTENSITY WITH 6+ INCHES
POSSIBLE, WHILE MUCH OF THE REMAINDER OF THE REGION WILL BE IN
THE 3- 5 INCH RANGE. SOUTH CENTRAL IL WILL SEE THE LEAST SNOWFALL.

CURRENT THINKING IS THE SNOW WILL WIND DOWN PRETTY QUICKLY BETWEEN
15-18Z ON SATURDAY AND MAY COMPLETELY END BY EARLY AFTERNOON ACROSS
THE FAR SOUTHERN CWA AS THE FORCING SHIFTS SOUTHWARD ATTENDANT WITH
THE DIGGING UPPER TROF/LOW.

THERE HAS BEEN LITTLE CHANGE TO THE HEADLINES OTHER THAN TO START
AND END SOME OF THE ADVISORIES SOONER.