Update Thu @ 12:44 PM — On Tuesday, I mentioned the possibility of a coastal low forming after the cold front moves through Sunday evening. The ‘possibility’ didn’t show on yesterday’s models, but last night’s ensemble models began showing something possible.

Today’s GFS shows a full coastal system developing later Sunday night.

Right now the models have the new storm just brushing us as it moves northeastward, but the current forecast for the weekend has begun to change today. I’ll update this evening.

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Update Wed @ 7:04 PM — Continued mild through Sunday afternoon. Different low pressure systems move over the next two days, mostly during the nighttime hours, with gradual partial clearing during the daytime hours.

More specifically, Thursday looks to be mostly cloudy after morning showers move out. High in the mid 50s

Friday will be the nicest day, with sun expected to break through a thin cloud layer and mild and dry. High mid 50s.

The models are in fairly good agreement for this weekend.

New Year’s Eve will be dry and mild but rain will move in several hours after midnight.

New Years Day will be rainy, but mild.

Sunday starts rainy, but the rain becomes more sporadic in the afternoon. The specifics for Sunday’s forecast are not quite final, as a strong cold front moves through on Sunday about 6 PM with showers and gusty winds.

Temperatures drop rapidly. Residual showers may turn to snow flurries.

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Update Tue @ 5:50 PM — A pattern change is shaping up for the country and our area, as cold air has been building up in northern Canada and appears to be poised to make a plunge into the continental US next week, pushing the jet stream southward. This change will occur for our area towards the end of the upcoming weekend. (I recently made mention of my approach to conceptualization of jet stream positions in a recent post about our mild winter.)

A cloudy, unsettled pattern will be with us through Saturday, New Years Day as we’ll be on the warm, moist side of an approaching upper trough. Low pressure systems will be moving up along this slowly approaching boundary, bringing us periods of rain. Timing these systems will be an important part of the forecast.

Of interest is the possibility that low pressure may develop as the strong cold front moves through sometime Sunday. Light snow may be possible late Sunday or Sunday night. Stay tuned.

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Following a beautiful day today, Sunday, the weather model guidance shows a trend towards colder weather and a somewhat unsettled weather pattern.

But first, Monday—

I should begin with the acknowledgement that Monday’s possible snow showers do not merit the amount of attention I’m giving them. But, I just don’t want to get too rusty predicting snow this season…

Update Mon @ 8:47 AM— The latest SREF model just available has changed its forecast to rain only here in the immediate PHL area, with light snow north of Quakertown. Over past years, the SREF precipitation type has done well. I guess we’ll see.

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Updated @ Mon 7:31 AM— Latest GFS in a similar range as the posted graphic last night. NBM percentile predictions continues with 0.00” to 0.22”, a coating at most

All changes to light rain after 2 PM.

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Update Sun @ 11:04 PM — Tonight’s models, as usual, have a range of forecasts for tomorrow’s light snow. The range of accumulations is from about 0.5 – 0.75“ (GFS) to 0.0 (ICON).

Falls between 10 AM and 2 PM.

The NBM which is designed to sort through these discrepancies has a median of 0.00 “ to a possible coating. I think we’ll go with the NBM.

Turns over to light rain after 2 PM.

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Update Sun @ 7:45 PM — The GFS has a statistical ensemble version (GEFS) which just became available. Its snow forecast seems more reasonable for us than the GFS posted earlier—

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Update Sun @ 5:25 PM — This afternoon’s GFS has come in as an outlier with today’s higher resolution models. It forecasts measurable snow for us Monday, while the other models have nothing accumulating. Hard to believe, but here’s the latest GFS—

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The front that moved through Saturday night is to our south and will return as a warm front. Moisture moving up ahead of the front on Monday will encounter a column of cold air. Several models have the light precipitation initially falling as light snow. There is agreement that surface temperatures will be above freezing and little if any accumulation is expected. (The GFS forecasts a coating.)

The NBM also shows snow for early Monday, but extremely light amounts. The NBM median accumulation is 0.00”.

Whatever falls, there will be a changeover to rain.

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For the rest of the week the jet stream configuration over the continental US will transition from the current ridge, with warm air pushing northward, to a flat configuration.

Wednesday, cold air building in Canada is expected to flatten the entire pattern and push a just bit further south, keeping us consistently colder.

The same flat pattern will prevent any major storms from developing near us.

Update Fri @ 10:37 AM — Some of us woke up to some snow pellets (graupel) on the ground from an upper air disturbance that went through about 2 AM. ( FYI – was predicted by the models, but not mentioned here.)

Today, Friday, mostly sunny although a few areas will have some brief periods of cloudiness.

Clouds move in about 4-6 PM this evening.

I’ve updated this forecast from the update earlier this morning. The NAM-NEST kept the showers to our north as shown below. The HIRESW models have more showers in the Philadelphia area on Saturday.

Saturday- cloudy early, then some breaks of sun, then cloudy again. The latest NAM-NEST has the showers Saturday remaining north of our area for the morning and early afternoon—

But the HIRESW models have showers as early as mid morning Saturday. So uncertainty remains about Saturday’s cloud cover and showers. All models keep the showers very light (< 0.10″) Further complicating things is that the HREF ensemble has clearing skies mid afternoon.

High temp 54º ± 3º Becoming WINDY in the afternoon.

Showers move in later Saturday afternoon with the “cold” front, according to the new NBM—

The updated NBM (13z) is closer to the HIRESW and HREF showing showers in the morning, then partial clearing and windy in the afternoon.

Sunday will be sunny. 50-52º Still above average temps.

I’ll try to nail down the details with tonight’s “Weekend Weather Forecast”.

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Update Thu @ 7:19 PM — The main scenario with a warm front followed by a cold front moving Saturday remains intact, although the low pressure system and fronts will be moving through about six hours earlier than previously forecast. Clouds move in Friday afternoon, as warm air aloft moves towards over us.

There’s a lack of clarity for Saturday’s forecast details. The general trend is for milder weather (50-53º) (53º ± 4º. The unusually high spread captures the uncertainty in the forecast.). Some showers in the early morning mostly north of the city, but the model blend (NBM) has some very light (0.01″) sprinkles moving in here early afternoon. Clouds in the morning, possible breaks in the clouds late morning, but overall cloudy. Becoming VERY WINDY late afternoon and early evening.

So very little rain Saturday, but enough clouds and wind to reduce the full enjoyment of the mild temperatures.

Skies clear for Sunday with temperatures somewhat chillier, but still above average seasonal norms. It will remain windy.

I’ll try to nail down the details with tomorrow’s “Weekend Weather Forecast”.

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Update Wed @ 9:47 PM — Christmas eve will be dry and relatively mild with temps in the 40s.

We’ll find ourselves Saturday with an approaching warm front with showers developing by afternoon, especially north of the city. While it will be mild on Saturday, things really warm up Saturday night after the warm front moves north of us. VERY WINDY!

A cold front moves through after midnight Saturday with windy conditions and somewhat cooler temperatures for Sunday.

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Update Wed @ 6:45 AM — Saturday is looking wetter than previously forecast. Updates this evening.

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Previously Posted Tue 6:06 PM —

Our uneventful weather continues this week, as the cold air we have on Tuesday will be gradually displaced by milder air by Christmas and the weekend.

The upper air jet stream flow that remains generally flat to ridge-like will continue into next week, preventing any large storm development.

The extended range ensemble models continue to forecast this flat ridge flow. There will be weak disturbances that move through this flat flow, giving us varying amounts of sunshine or clouds, depending upon the day. The weekend will be relatively mild with highs on Saturday near or above 50º (±3º) and Sunday in the upper 40s!

(Seasonal average highs are 42º – Blue Bell and 44º – Philadelphia.)

There may be some showers on Saturday, although the current GFS is down-playing those. There will be cloudiness at times from these weak disturbances, but there may be some sunshine for us over the weekend as well. Timing these weak disturbances will be main forecast challenge.

No snow is forecast at this time through the end of year! Of course, the pattern may change.

I still don’t detect any buildup of very cold air in Canada. (I mean minus 40º F, not the ‘mild’ minus 25º currently in place. ) Also absent is the usual extremely cold Greenland. It’s just very cold, not extremely cold.