The latest NAM data has become available. QPF totals have risen to over 0.50 inches water. Upper atmosphere temperatures marginally support snow or snow mixed with sleet and ice pellets early morning in the immediate PHL area. Inner western suburbs willl likely be all snow.
The upper atmosphere cools causing precipitation to be all snow by late morning or early afternoon.
Surface temperatures about 30-32 degrees for much of the day. Winds will be light. It will feel like less of a “storm” and more of a light gentle snowfall that accumulates over the duration of the day.
Snow starts before daybreak with a coating to an inch expected by daybreak. The snow or snow/sleet continues at a light rate becoming all snow by afternoon. Slightly heavier snow during the mid to late afternoon before ending about 7-8 pm.
Depending upon the amount of morning sleet, snowfall totals 3-5 inches, possibly 6 if it’s all snow.
The GFS data becomes available in about an hour . I’ll update if things change.
10:45 pm update GFS model similar enough that I’m sticking with the above Forecast.
The afternoon run of the NAM just became available. QPF values for PHL are now over 0.5 inches water. The critical temperatures of the upper atmosphere are now not as warm as the earlier run from PHL and westward. Still slightly warm for dry snow—some sleet or ice pellets will mix in reducing snow totals. Approximately 3-5 inches possible for PHL and immediate west is the current call.
The afternoon model runs are not the best, since they don’t incorporate new radiosonde upper air data. (That’s those weather balloons.) Will have a better handle on snow totals with the evening runs. Will update by about 9:30 PM tonight.
The latest GFS and NAM data has become available and there are some new wrinkles in predicting the snow totals. The QPF values are still in the general 0.30-0.40 inch range, which if cold enough, could translate into 3-5 inches of snow for Philadelphia.
BUT, the latest track of the storm is close enough that some warm air in the upper levels of the atmosphere will mix in during the mid-day hours, especially in PHL and eastward. This means that we will wake to some accumulation of snow at daybreak Saturday, but there’s now a strong possibility that it will mix with freezing rain and some sleet during the late morning and early afternoon, greatly reducing snow totals. Areas in NJ will be even warmer aloft, with more sleet, rain and freeezing rain. Areas to the west of PHL will have less QPF (“Quantity of Precipitation Falling”), but will likely remain mostly snow.
More info later this afternoon and tonight. Stay tuned.
The chance of snow was first picked up by the statistical models as early as last weekend. There had been much downplay by the deterministic models (GFS, NAM) for much of the week, until yesterday, when the models jumped on board with a light snowfall 1-3 inches.
Each successive model run has increased the QPF for Philadelphia and NJ, with last evening’s run showing 3-5 inches of snow based on QPF of 0.30 to 0.40 inches water.
The 1 AM run of the models (they are run four times daily) continues with this trend with the NAM having a QPF of 0.40 inches water and the GFS now up to 0.50 inches water. Last night’s statistical models show an almost certain snowfall for Philadelphia and more significant accumulations for central and parts of southern NJ. Current predictions for Philadelphia are 4-6 inches, more in NJ.
We won’t have a best guess for snowfall until tonight’s models are out. The early NAM data becomes available after 9 PM, the GFS data becomes available after 10 PM. I’ll follow the trends during the day, (time permitting) and I’ll update between 9 and 10:30 PM.