Latest NAM and GFS data from the 1PM (18z) afternoon model run. Both models now colder; little chance of changeover to sleet in PHL. Both have the snow starting late evening on Friday in PHL and earlier south and west of the city.
The NAM cranks out 1.55 inches water (QPF) and the GFS cranks out 1.98 inches of water (QPF). The snow algorithms suggest 20-30 inches of snow possible by late Saturday night.
The next model run is at 7PM (00z) and the data starts rolling out in raw form about 9:30 PM for the NAM and 10:45 PM GFS.
As we get closer to the actual ‘event’, confidence in the predicted QPF values increases. But with both models having such high QPF values and similar thermal profiles, the chance of a major snow storm is looking like a sure bet.
10 pm update: tonight’s NAM has a QPF of over 1.80 inches water. What’s new- starts between 7-10pm. Heaviest snow and highest winds Saturday morning. A lull late Saturday afternoon for the “dry slot” , then several more inches wrap-around Saturday night. Snow totals 20-30 inches for PHL by Sunday morning.
11pm update. Tonight’s GFS shows a 1.61 inches water. No dry slot and ending Saturday night. Taking average of NAM and GFS 20-24 inches snow total.