[su_note note_color=”#defcdc”] Update Fri 10 PM: After reviewing this evening’s NAM, NBM and Hi Resolution model data, the gist of the forecast below still stands. The rain tapers and ends mid morning Saturday but it will remain unsettled. Cloudiness lingers, and breaks in the clouds, bright patches, even some sun is possible. Widely scattered sprinkles/showers remain possible as well and increase in likelihood again for a short period around 4 PM, as the main front moves through. Temperatures reach a high of about 58º around noon and slowly fall during the afternoon and especially after the frontal passage.
Sunday’s forecast below also remains intact. High 46º. There’s a chance of an upper air impulse bringing sprinkles or flurries after midnight.[/su_note]
…. from Fri morning:
Low pressure will gradually move over us on Saturday and depart on Sunday.
Despite the location of the center of the low being directly over us, the rain will be will not be continuous. It will become spotty.
For Saturday, most models have rain moving through in the morning and to the north of us by afternoon. Any rain lingering will be light and widely scattered. The NBM 1 hour mean rain depiction for 1 PM Saturday gives a better picture. There may even be some “dry slotting” in the afternoon, allowing brighter skies. High 58º!
A weak cold front moves through during the mid to late afternoon Saturday. There may be another burst of showers and it will become windy.
For Sunday, mostly sunny skies, windy and colder with above average seasonable highs of 46º. Winds diminish somewhat during the evening.
[su_note note_color=”#defcdc”] Next Week: Of interest is a dip in the jet stream on Tuesday into Wednesday with the spawning of a coastal low. There appears to be a shortage of cold air needed to make this a snow storm here, but this needs to be watched.
I got up this morning and saw the clouds and even some very light snow flurries. Last night’s models did not show these clouds or flurries. Nor did the higher resolution just-run hourly models like the HRRR or RAP show it.
Did the models capture anything that could explain the weather this morning?
They did, but apparently the model’s algorithms didn’t translate a strong upper air disturbance and moisture into the clouds and flurries that we have.
Last night’s GFS shows a strong area of vorticity (“an upper air disturbance”) passing through along with increased moisture in the upper and mid levels.
Add the humidity, vorticity and upper air cyclonic flow together and you get the weather we have this morning.
If the models were somewhat better, this combination of parameters should have triggered its cloud algorithm to show the clouds we have. But it didn’t.
Based on the same thinking and parameters, the GFS shows humidity and vorticity move off between 12 – 2:30 PM. We should see more sun by that time.
[su_note note_color=”#defcdc”]Update Wed 10 AM: This morning’s high resolution NAMNEST finally shows the light precip and clouds we already know about.
The warm front clearly did NOT make it north of the Philadelphia area, as had been previously predicted by the GFS. It’s in the 40s. The short range models got this one correct yesterday. Not a surprise; the new GFS has been biased to the warm side.
Today’s GFS has caught up with short range models. Here’s the latest GFS below. We don’t get out of the 40s north of the city.
Speaking of temperatures, here’s the latest NBM (National Blend of Models) forecast for New Years Eve- Midnight. Clear skies. Winds 7-9 mph.
[su_note note_color=”#defcdc”]Update Sun 12 PM: So, the current radar shows precipitation moving in sooner than any models had predicted. Rain may start as early as 1 PM.[/su_note]
Most of last night’s models continue with the idea that rain begins in the general Philadelphia area around 3-4 PM today, Sunday. A remaining exception is the high resolution Canadian (HRDPS) which waits until 7 PM for the rain to start.
Rain may be heavy overnight and into Monday morning, then more light and scattered.
A warm front will try to make its way into Philadelphia and areas on a line to its south on Monday morning. If it does, temps will rise into the low 60s.
However, only the GFS has it making its way as far north as Willow Grove. The NAM, NAMNEST and the Canadian HRDPS, GDPS keep it further south. Areas that remain north of the warm front will remain in the low 50s. (Warm fronts are not as distinct as cold fronts.) So it may not get as warm as previously though on Monday.
Most models have a cold front moving through about 2-3 PM, dropping temperatures.