Quantitative Precipitation Forecast
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000 FOUS11 KWBC 230837 QPFHSD PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 437 AM EDT THU MAR 23 2017 VALID 12Z THU MAR 23 2017 - 12Z SUN MAR 26 2017 DAYS 1 THROUGH 3... ...WEST COAST... SHORT WAVE ENERGY CROSSING THE PACIFIC NORTHWEST DURING DAY 1 WILL TAP DEEP PACIFIC MOISTURE...RESULTING IN HIGH ELEVATION SNOWS ACROSS THE OLYMPICS AND CASCADES OF WA. A LONG WAVE TROUGH COMES ASHORE ACROSS THE WEST COAST DURING DAY 2...BRINGING WITH IT LOWERING SNOW LEVELS AND THE POTENTIAL FOR HEAVY SNOWFALL. THERE WAS GENERALLY GOOD MODEL AGREEMENT WITH RESPECT TO THE OVERALL SYNOPTIC SETUP...SO THE THERMAL PORTION OF THE FORECAST WAS BASED ON A MULTI MODEL BLEND. THE QPF PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF. DAY 1... THE BEST LIFT WITH SHORT WAVE ENERGY CROSSING THE PACIFIC NORTHWEST OCCURS DURING THE LAST PART OF DAY 1. AHEAD OF THE SHORT WAVE...SNOW LEVELS RISE ABOVE 6000 FEET IN THE WARM AIR ADVECTION PATTERN. THIS RELEGATES SNOWFALL ACCUMULATIONS OF THE HIGHER TERRAIN OF OLYMPIC RANGE AND NORTHERN WA CASCADES...WITH LOCAL 6 INCH AMOUNTS EXPECTED OVER THE OLYMPICS. FURTHER SOUTH...A MOISTURE PLUME AHEAD OF A LONG WAVE TROUGH IMPACTS NORTHERN CA. SNOW LEVELS RISE AHEAD OF THE LONG WAVE TROUGH...RESULTING IN 2 TO 4 INCHES OF SNOWFALL ACROSS THE HIGHER TERRAIN OF THE TRINITY AND SISKIYOU MOUNTAINS IN NORTHERN CA. DAY 2... THE MAIN THRUST OF THE MOISTURE PLUME IMPACTS NORTHERN AND CENTRAL CA DURING DAY 2. THE COMBINATION OF UPSLOPE FLOW AND SYNOPTIC SCALE ASCENT SUPPORT HEAVY QPF ACROSS THESE AREAS...AS SNOW LEVELS FALL TO 4500 TO 5500 FEET DURING THE DAY. THERE IS ENOUGH QPF TO SUPPORT AN AXIS OF 8 TO 12 INCHES OF SNOWFALL ACROSS THE HIGHER TERRAIN OF THE SIERRA NEVADA RANGE IN CA....WITH LESSER AMOUNTS (GENERALLY BETWEEN 4 AND INCHES) EXPECTED ACROSS THE SHASTA AND TRINITY MOUNTAINS. THE MOISTURE PLUME IS NOT AS ROBUST ACROSS THE PACIFIC NORTHWEST DURING DAY 2...BUT SNOW LEVELS HERE ALSO FALL TO 4500 TO 5500 FEET. THIS SHOULD ALLOW STRIPS OF 3 TO 6 INCHES OF SNOW OVER THE HIGHER TERRAIN OF THE OR CASCADES...WITH 4 TO 8 INCH AMOUNTS OVER THE WA CASCADES. DAY 3... AS THE LONG WAVE TROUGH COMES ASHORE ACROSS THE WEST COAST...MUCH OF THE MOISTURE IS SCOURED AHEAD OF THE BEST LIFT. SUBSEQUENTLY...MAINLY LIGHT SNOWFALL AMOUNTS ARE EXPECTED ACROSS THE NORTHERN WA CASCADES...WHERE LOCAL 2 TO 4 INCH SNOWFALL AMOUNTS WERE PLACED. ...GREAT BASIN/ROCKIES... A LONG WAVE TROUGH CROSSES THE WASATCH RANGE IN UT EARLY ON DAY 1...FOCUSING MOISTURE FOR HEAVY SNOWFALL. A CLOSED MID LEVEL LOW FORMS AT THE BASE OF THE LONG WAVE TROUGH OVER THE SOUTHERN ROCKIES LATER ON DAY 1...WHICH SPINS UP SURFACE LOW PRESSURE. THE COMBINATION OF MID LEVEL FRONTOGENETIC FORCING AND UPSLOPE WILL RESULT IN HEAVY SNOWFALL ACROSS THE CO ROCKIES...AS WELL AS THE BIGHORN RANGE IN WY. THE EXITING MID LEVEL SYSTEM WILL TAKE THE MID LEVEL FORCING WITH IT DURING THE FIRST HALF OF DAY 2...BUT HEAVY SNOWFALL IS STILL EXPECTED OVER PORTIONS OF EASTERN CO BEFORE IT DEPARTS. FINALLY...ANOTHER LONG WAVE TROUGH CROSSES THE ROCKIES DURING DAY 3. MOISTURE IS NOT AS ABUNDANT FOR THIS TROUGH...SO MAINLY LIGHT SNOWFALL AMOUNTS ARE EXPECTED. THERE WAS GENERALLY GOOD MODEL AGREEMENT WITH RESPECT TO THE OVERALL SYNOPTIC SETUP...SO THE THERMAL PORTION OF THE FORECAST WAS BASED ON A MULTI MODEL BLEND. THE QPF PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF. DAY 1... A LONG WAVE TROUGH CROSSES THE GREAT BASIN DURING THE FIRST HALF OF DAY 1. UPSLOPE FLOW AHEAD OF SURFACE LOW PRESSURE DEVELOPING OVER UT/CO WILL FOCUS THE MOISTURE IN THE COLUMN ON THE WASATCH AND UINTA RANGES IN UT. SNOW LEVELS DROP FROM 9000 TO 6000 FEET ACROSS THESE RANGES...AND THERE IS A MULTI MODEL SIGNAL FOR 4 TO 8 INCHES OF SNOWFALL...WITH THE HIGHEST AMOUNTS OVER THE NORTHERN WASATCH RANGE AND THE UINTAS. FURTHER NORTH...THE COMBINATION OF MOISTURE AND LIFT WILL BECOMES FOCUSED BY UPSLOPE FLOW OVER THE GRAND TETONS AND THE BIG HORN RANGE IN WY...MAINLY DURING THE MIDDLE OF DAY 1. AGAIN...THERE IS A MULTI MODEL SIGNAL FOR 4 TO 8 INCHES OF SNOWFALL...WITH THE HIGHEST AMOUNTS OVER THE BIG HORNS. AS THE LONG WAVE TROUGH CROSSES THE CENTRAL AND SOUTHERN ROCKIES DURING THE SECOND HALF OF DAY 1...A CLOSED MID LEVEL LOW FORMS OVER SOUTHEAST CO. MOST OF THE 00Z MODEL SOLUTIONS SHOWED THE DEVELOPMENT OF THE MID LEVEL FRONTOGENETIC FORCING ON THE NORTH SIDE OF THE CLOSING MID LEVEL LOW...THAT ROTATES TO THE NORTHWEST SIDE AFTER 24/00Z. THE BEST LIFT IN THE MID LEVEL FORCING OCCURS DURING THE 24/06Z AND 24/12Z TIME FRAME. THAT AREA OF LIFT IS AUGMENTED BY INCREASING UPSLOPE FLOW ACROSS THE PALMER DIVIDE OVER THE CO FRONT RANGE. INITIALLY...BOUNDARY LAYER CONDITIONS ARE WARM ENOUGH TO RESULT IN A PRECIPITATION PHASE ISSUE ACROSS THE FRONT RANGE. HOWEVER...THE PRECIPITATION CHANGES TO SNOW AS THE COLUMNS MOISTENS...AND BECOMES ISOTHERMAL JUST BELOW FREEZING. THERE SHOULD BE ENOUGH QPF TO SUPPORT AN AREA OF 12+ INCHES OF SNOWFALL OVER THE HIGHER TERRAIN...TAPERING TO LESS THAN 6 INCHES WHERE THE CHANGEOVER OCCURS LAST (AIDED BY DYNAMIC COOLING ASSOCIATED WITH THE CLOSING MID LEVEL LOW. FURTHER SOUTH...LOCAL 4 TO 8 INCH SNOWFALL AMOUNTS WERE PLACED OVER THE SAN JUAN RANGE IN CO...MAINLY ABOVE 7000 FEET...AS UPSLOPE FLOW FOCUSES MOISTURE ON THAT RANGE. DAY 2... THE MAIN BAND OF FRONTOGENETIC FORCING WILL PIVOT ACROSS THE FRONT RANGE OF THE CO ROCKIES DURING THE FIRST PART OF DAY 2...AS THE CLOSED MID LEVEL LOW EXITS ACROSS THE SOUTHERN PLAINS. THERE COULD BE SOME PRECIPITATION TYPE PROBLEMS EARLY...UNTIL DYNAMIC COOLING AND MOISTENING OF THE COLUMN RESULT IN A PROFILE THAT SUPPORTS SNOW. THERE WAS A MULTI MODEL SIGNAL FOR 6 TO 10 INCHES OF SNOWFALL NEAR KLIC...EXTENDING NORTH TO SOUTH OF KAKO. FURTHER NORTHEAST...MODEL SOUNDINGS WERE NOT COLD ENOUGH TOP SUPPORT AN ALL SNOW SCENARIO...SO AMOUNTS WERE RAINFALL OFF TO LESS THAN 2 INCHES STRETCHING INTO SOUTHWEST NE DURING DAY 2. DAY 3... THE NEXT LONG WAVE TROUGH CROSSES THE GREAT BASIN AND ROCKIES DURING DAY 3. MOISTURE BECOMES THE LIMITING FACTOR FOR SNOWFALL...BUT DYNAMIC LIFT AND UPSLOPE FLOW SHOULD MAKE THE MOST OF THE MOISTURE IN THE COLUMN. LOCAL 2 TO 4 INCH SNOWFALL AMOUNTS WERE PLACED OVER THE HIGHER TERRAIN OF EASTERN NV...UT INTO THE ABSAROKA RANGE IN NORTHWEST WY AND SOUTH CENTRAL MT. ...UPPER GREAT LAKES INTO THE NORTHEAST... MOISTURE STREAMING ACROSS A FRONTAL BOUNDARY EXTENDING FROM THE UPPER GREAT LAKES TO THE NORTHEAST DURING DAYS 1 THROUGH 3 WILL RISE UP AND OVER COLDER AIR AT THE SURFACE...RESULTING IN THE POTENTIAL FOR FREEZING RAIN EACH DAY. THE 00Z ECMWF MAY BE TOO FAST DROPPING THE FRONT SOUTHWARD...ESPECIALLY ON DAY 3. OVERALL...THERE WAS GOOD MODEL AGREEMENT ON THE LOW LEVEL PLACEMENT OF THE COLD AIR...SO THE THERMAL FIELDS WERE BASED ON A MULTI MODEL BLEND. THE QPF PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF. DAY 1... MOISTURE RIDING UP AND OVER A LOW LEVEL FRONTAL BOUNDARY EXTENDING FROM SOUTHERN MN INTO WESTERN NY STATE WILL PROVIDE SUFFICIENT LIFT FOR QPF ALONG AND NORTH OF THE FRONT ACROSS THE UPPER GREAT LAKES DURING DAY 1. THE BROAD WARM AIR ADVECTION PATTERN PRODUCES A WARM NOSE ACROSS MN INTO NORTHERN WI AND THE UP OF MI DURING THE DAY. AS THE VERTICAL MOTION INCREASES...THE WARM NOSE BETWEEN 750 MB AND 800 MB WILL ROB THE COLUMN OF ITS ABILITY TO SUPPORT SNOW ACROSS THIS AREA. HOWEVER...LOW LEVEL COLD AIR REMAINS LOCKED IN PLACE BY A LOW LEVEL EASTERLY FLOW. CONDITIONS AT THE SURFACE WILL INITIALLY BE CRITICAL FOR FREEZING RAIN...BUT COLDER AIR DRAINING SOUTHWARD AHEAD OF HIGH PRESSURE OVER ONTARIO SHOULD REINFORCE THE LOW LEVEL COLD AIR IN THE ABOVEMENTIONED LOCATIONS. THIS SHOULD BE ENOUGH QPF TO SUPPORT AN AREA OF 0.01 TO 0.10 INCHES OF ICE STRETCHING FROM THE ARROWHEAD OF MN ACROSS FAR NORTHERN WI AND THE UP OF MI. FURTHER EAST...THE PRECIPITATION IN THE BROAD WARM AIR ADVECTION PATTERN MOISTENS THE COLUMN ACROSS WESTERN AND CENTRAL PA INTO WESTERN AND CENTRAL NY STATE TOWARD THE END OF DAY 1. THE LOW LEVELS REMAIN COLD DURING THIS PROCESS...BUT MODEL SOUNDINGS SHOWED A WARM NOSE BETWEEN 750 MB AND 800 MB...WHICH SUPPORTS A FREEZING RAIN AND SLEET MIX. ICE AMOUNTS ARE EXPECTED TO REMAIN BELOW 0.05 INCHES. DAY 2... THE LOW LEVELS SHOULD WARM ENOUGH TO LESSEN THE THREAT FOR ICING ACROSS THE UPPER GREAT LAKES DURING DAY 2. PATCHY AREAS OF 0.01 INCHES OF QPF WERE PLACED OVER NORTHERN WI INTO THE UP OF MI. HOWEVER...AS COLD AIR BLEEDS SOUTH FROM HIGH PRESSURE OVER ONTARIO...THE COVERAGE OF ICING COULD INCREASE LATE DURING THE PERIOD. FURTHER EAST...THE WARM AIR ADVECTION PATTERN MOISTENS THE COLUMN ACROSS UPSTATE NY INTO NORTHERN NEW ENGLAND DURING THE FIRST PART OF DAY 2. MODEL SOUNDINGS SHOWED A SATURATED COLUMN JUST BELOW 0 DEGREES C...WHICH SUPPORTS A PERIOD OF SNOW ACROSS THIS AREA. THE COLUMN EVENTUALLY LOSES THE ABILITY TO SUPPORT SNOWFALL...BUT NOT BEFORE A GENERAL 1 TO 3 INCHES SNOWFALL STRETCHING ACROSS THE ABOVEMENTIONED AREAS. COLD AIR AT THE SURFACE BECOMES TRAPPED BY THE SNOWFALL...SO THAT A PERIOD OF ICING IS EXPECTED THE SNOWFALL ENDS. ICING AMOUNTS ARE GENERALLY EXPECTED TO REMAIN BELOW 0.10 INCHES. COLD AIR IN PLACE WILL BECOME TRAPPED IN VALLEYS OF THE TERRAIN STRETCHING ACROSS THE SOUTHERN TIER OF NY STATE INTO PORTIONS OF THE LOWER HUDSON VALLEY OF NY INTO NORTHEAST PA AND NEARBY NORTHWEST NJ. THE COLD AIR SHOULD BE FAIRLY SHALLOW...SO ICING OCCURS BEFORE 24/18Z...WITH ICE AMOUNTS REMAINING BELOW 0.10 INCHES. DAY 3... COLD AIR CONTINUES TO BLEED SOUTHWARD ACROSS THE UPPER GREAT LAKES INTO UPSTATE AND NORTHERN NEW ENGLAND DURING DAY 3....AS HIGH PRESSURE MOVES FROM ONTARIO INTO QUEBEC. THE COLD AIR WILL BE DIFFICULT TO DISLODGE...AS THE NEXT SHOT OF PRECIPITATION CROSSES THE AREA. MODEL SOUNDINGS SHOWED A WARM NOSE NEAR 3 DEGREES CELSIUS BETWEEN 800 AND 850 MB...AND THE MODEL SOUNDINGS SHOWED A FREEZING RAIN PROFILE HERE. LOCAL 0.10 INCH ICE AMOUNTS ARE POSSIBLE ACROSS PORTIONS OF THE NORTHERN LP OF MI INTO PARTS OF FAR UPSTATE NY. HAYES $$

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