PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 413 PM EST FRI NOV 22 2013 VALID 00Z SAT NOV 23 2013 - 00Z TUE NOV 26 2013 ...DAYS 1-3... GREAT BASIN/SOUTHWEST/CENTRAL AND SOUTHERN PLAINS... A LARGE CLOSED UPPER LEVEL LOW SPINNING JUST OFF OF THE SOUTHWEST COAST IS EXPECTED TO SLOWLY TRACK EAST OVER THE NEXT COUPLE OF DAYS...REACHING THE SOUTHERN PLAINS BY MID-DAY MON. GIVEN THE CURRENT SURGE OF ARCTIC AIR ALREADY IN THE SOUTHERN PLAINS...IT APPEARS A SIGNIFICANT ICE STORM IS ON TAP FOR THE SOUTH...WITH A FOCUS FROM CENTRAL TEXAS INTO THE ARKLATEX REGION. THERE IS DECENT MODEL AGREEMENT OVERALL BUT THE FORECAST WAS WEIGHTED MOST TOWARD THE 12Z NAM/GFS AND 00Z ECMWF WITH THE TRACK OF THE UPPER LOW AND LOW LEVEL COLD AIR. THE 09Z SREF MEAN WAS USED OVER THE COMPLEX TERRAIN OVER THE SOUTHWEST IN ADDITION TO THE NAM...BUT THE SREF WAS WEIGHTED LESS FROM SUN-MON GIVEN THE BROAD NATURE OF ITS LIGHT TO MODERATE QPF. FROM TONIGHT INTO SAT EVENING...THE MAIN FOCUS WILL BE WITH THE CLOSED LOW MOVING INTO THE SOUTHWEST WITH STRONGLY DIFLUENT FLOW IMPACTING THE FOUR CORNERS REGION. SNOWFALL HERE WILL BE ELEVATION DEPENDENT WITH SNOW LEVELS ONLY DOWN TO ABOUT 6000 FT - 7000 FT FOR THE MOGOLLON RIM IN ARIZONA...BUT AN INITIALLY ROBUST STREAM OF MOISTURE WILL BE SURGING AHEAD OF THE UPPER LOW. THIS MOISTURE STREAM WEAKENS AS THE LOW MOVES INLAND DURING THE DAY ON SAT BUT STRONG MID-UPPER LEVEL DYNAMICS TIED TO THE UPPER LOW AND LOW LEVEL UPSLOPE SHOULD FAVOR LOCALLY HEAVY AMOUNTS INTO THE WASATCH OF UTAH AND SAN JUANS OF COLORADO. A COMBINATION OF UPSLOPE FLOW INTO THE SOUTHERN HIGH PLAINS BUT MID-LEVEL DRY AIR SHOULD LIMIT A MIXTURE OF SNOW/ICE TO MAINLY LIGHT AMOUNTS ACROSS TEXAS AND NEW MEXICO. FROM SAT EVENING INTO SUN EVENING...MODERATE TO HEAVY SNOW WILL MOVE INTO NEW MEXICO AHEAD OF A 700 MB LOW WHICH IS EXPECTED TO ELONGATE AND PERHAPS OPEN UP BY 00Z/25. UPSLOPE FLOW ACROSS THE SOUTHERN HIGH PLAINS IS EXPECTED TO TRANSITION TO MORE OF A SOUTHERLY DIRECTION WITH THE APPROACH OF THE UPPER TROUGH...BUT CONTINUED WARM ADVECTION ATOP THE COLD AIR MASS WILL CONTINUE WITH MODERATE TO HEAVY SLEET AND 0.25 INCH ICE ACCUMULATION LIKELY WITH A PROMINENT WARM NOSE CENTERED NEAR 750 MB AND COLD LAYER FIRMLY IN PLACE VIA NORTH TO NORTHEASTERLY WINDS. NAM/GFS SOUNDINGS FROM 48 HOURS AGO APPEAR TO BE POORLY RESOLVED WITH THE MAGNITUDE OF COLD AIR CURRENTLY IN PLACE ACROSS WEST TEXAS...BUT IT IS NOT CLEAR IF THIS TREND WILL CONTINUE WELL INTO THE SHORT RANGE WITH TODAYS NAM/GFS. IT WAS GENERALLY NOTED THAT THE NAM WAS COLDER IN THE BOUNDARY LAYER ACROSS THE SOUTH THAN THE GFS AND ECMWF...BUT THIS DIFFERENCE WAS SPLIT IN THE FORECAST THINKING IN AN ATTEMPT TO MITIGATE THE KNOWN COLD BIAS IN THE NAM. FROM SUN EVENING TO MON EVENING...REMNANT SNOW SHOWERS WILL CONTINUE BENEATH THE COLD MID-LEVEL TEMPERATURES OF THE UPPER LOW BUT THE MOISTURE SOURCE WILL BE SHUNTED EASTWARD ACROSS TEXAS. LOW LEVEL TEMPERATURES ARE THE BIG QUESTION MARK FOR THIS TIME PERIOD WITH THE NAM/SREF DEPICTING 24 HR ICE ACCUMULATIONS OF .50 TO 1.0 INCH FROM THE TEXAS HILL COUNTRY TOWARD THE SOUTHERN SUBURBS OF DALLAS/FORT WORTH. THIS APPEARS OVERDONE BUT THE MODELS HAVE GOOD AGREEMENT WITH A .50 TO 1.5 INCH STRIPE OF LIQUID EQUIVALENT IN THE 24 HRS ENDING 00Z/26 ACROSS TEXAS INTO LOUISIANA...WITH THE NORTHERN EDGE CO-LOCATED WITH A DEFINED WARM NOSE ATOP A DEFINED SUB-FREEZING LAYER JUST ABOVE THE SURFACE. QUESTIONS OF THE DEPTH OF THE COLD DOME AND SURFACE TEMPERATURE STAYING BELOW FREEZING TO SUPPORT ICE EXIST. PROBABILITIES REFLECT A LOW-MED CHANCE OF 0.25 INCHES OF ICE...WITH SLEET AND MAYBE SOME LIGHT SNOW FARTHER NORTH TOWARD THE RED RIVER AND WEST-CENTRAL ARKANSAS FOR DAY 3.
Posted on: Fri, 22 Nov 2013 21:36:44 +0000
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