Quantitative Precipitation Forecast
Issued by NWS

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000 FOUS11 KWBC 230851 QPFHSD PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 450 AM EDT MON OCT 23 2017 VALID 12Z MON OCT 23 2017 - 12Z THU OCT 26 2017 DAYS 1 THROUGH 3... THE PROBABILITY OF HEAVY SNOW IS LESS THAN 10 PERCENT DAY 1. DAY 2... ...UPPER GREAT LAKES... COLD AIR ADVECTION WEST OF THE TRACK OF SURFACE LOW PRESSURE CROSSING THE UPPER GREAT LAKES ON DAY 2 COULD ALLOW FOR A CHANGE OVER FROM RAIN TO SNOW OVER PORTIONS OF THE WESTERN UP OF MI. THERE WAS GENERALLY GOOD MODEL AGREEMENT WITH OVERALL SYNOPTIC SETUP...SO THE THERMAL PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE ECMWF AND GFS...AND THE QPF PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF. THE GFS/NAM SHOWED ENOUGH COLD AIR AND MOISTURE IN THE COLUMN TO SUPPORT A PHASE CHANGE FROM RAIN TO SNOW OVER THE WESTERN UP OF MI BEFORE THE EVENT ENDS. THE NAM FORECASTS A SLIGHTLY FASTER TRANSITION PLUS PRECIP LASTS LONGER...SLOWING POTENTIAL FOR MORE ACCUMULATION. THE ECMWF CONTINUES TO SHOW SLOWER ARRIVAL OF COLDER AIR...LEADING TO NO ACCUMULATION AS 925 MB TEMPERATURES STAY ABOVE FREEZING. DAY 3... ...MT ROCKIES... A FRONT PROGRESSES SOUTH ACROSS MT WITH AN INCREASE IN RELATIVE HUMIDITY WITH THE FRONT AND VEERING OF THE LOW LEVEL WINDS TO THE NORTH PRESENTING THE LIKELIHOOD OF SNOW IN TERRAIN FACING THE FLOW WEST OF BILLINGS AND SOUTH OF GREAT FALLS. THE MODEL MAJORITY SHOWS A COUPLE OF INCHES IN THESE MNTNS WITH UPSLOPE FLOW AND THE OPERATIONAL ECMWF INDICATING 4 INCH MAXIMA LEADING TO A LOW RISK OF FOUR INCHES IN THE AREA. THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT. PETERSEN $$

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