The NAM has joined the rest of the models with a continued chance of on and off light showers and drizzle through the evening.
I don’t recall any of the models showing the fog that has developed. With the fog and heavy low cloud deck, it doesn’t look like the mid 60s that was heavily advertised will become a reality. High temperatures now looking to be about 59.
Skies clear on Monday. A period of cold weather for much of the upcoming week and weekend with rain rather than snow looking possible for end of next weekend.
From last night:
Tonight’s NBM (National Blend of Models) has rain tapering and ending about 2-3 pm on Sunday. The NAM has it ending a bit earlier, the GFS has the chance of continued light showers into the evening. Either way, it will cloudy and mild. High 63.
Outlook- A winter-like broad dip in the jet stream will bring cold temperatures for the next week or so. Stay tuned.
Thurs Noon update: Wow, more snow than I had forecast! The low level atmosphere temperatures are at or below freezing. The NAM upper atmosphere critical thickness level seems to have fooled me this time around. It works in the winter, apparently not in November. 😱 I’ll make a note of it!
So, let’s try this again. Based on the latest NBM (which I’ve been ignoring, except for the precipitation start time), the changeover to sleet and then rain will occur about 1 -2 PM in PHL. This is a tough business.
Based on the latest NAM and NBM model data, the forecast from last night is on track.
Precipitation is moving in, as shown on radar, at 10:30 AM. On this radar image, it’s easy to see how the precipitation is hitting a wall of cold air, eroded at the northern edges—
This is a sleet to rain event, with some snowflakes possible at the start. The latest NAM supports the possibility of a brief change to snow before daybreak Friday. (Little or no accumulation.)
Last night’s models have changed little from previous runs. (Please see my previous posts.)
Light precipitation starts about 8AM Thursday in PHL (earlier to the south). Low level temperatures remain at or below freezing for an extended period of time, while the temperature profile in the upper atmosphere is too warm to support snow.
While the precipitation may start as some light snow, it will quickly change over to all sleet in PHL and the immediate surrounding counties. Some freezing rain is also possible early.
QPF values in the morning are about 0.20 inches water and during the afternoon, 0.70 inches water. Much of this will fall as rain.
The sleet will mix with and eventually change to rain. Cold air damming scenarios have sometimes lasted longer than predicted by the models, so it’s difficult to tell when this changeover will occur. An extended period of sleet can occur in far northern and western suburbs. (Allentown) East of the city, in NJ, any sleet will change to rain even earlier.
Surface temperatures at or below freezing about 7 AM Thursday but rise slowly into the mid 30s.
The expected secondary storm formed by the merger of the upper low Thursday night now appears to be too warm for any snow. Rain is expected late Thursday night into Friday morning.
Just a quick update,based on tonight’s NAM model data that just became available.
Much of what I wrote about earlier today still holds. While the lower levels of the atmosphere remain below freezing for an extended period on Thursday due to “cold air damming”, the upper levels of the atmosphere are forecast to be too warm to support snow Thursday.
I believe this will be a sleet event, eventually changing to sleet and rain; rain possibly heavy in the afternoon. While some early snowflakes are possible, it should quickly change to sleet.
I’m sure there are forecasts out there for snow, based on preset precipitation type algorithms. But thermal profiles have worked well for my forecasts in the past and I’m going to stick with it this time. This forecast is for Philadelphia and immediate suburbs. Areas far north and west may have more freezing rain and sleet accumulating.
The scenario with the upper low enhancing a secondary surface low Thursday night into Friday morning is also supported by tonight’s NAM. However, it’s looking less likely that enough cold air will allow a changeover to snow .