Little change in the forecast since my “outlook” was posted yesterday.
Fast moving high pressure will bring cold temperatures for Saturday. Saturday starts with some cloudiness, but skies clear by early afternoon. It will be cold. High 38º (based on the NBM model) or 43º ( based on the EKDMOS). I think the colder model will be correct. It will be breezy.
Early Sunday morning, a weak warm front moves across with some clouds early. Skies clear for sunshine by mid morning. Milder but still below average temps. (High 43º NBM or 45º EKDMOS)
Rain for Monday and Monday evening. A strong cold front moves through late Tuesday.
Rain, turning to snow showers early Wednesday morning. Very cold Wednesday and Thursday. I’ll keep an eye on the system for late Tuesday into Wednesday.
For you weather model fans, the NBM (National Blend of Models) gets an upgrade to version 3.2 next week Dec 17th. The NBM combines several short and medium range model forecasts, including the European and Canadian models and statistically weights them according to their performance over their first six hour forecast accuracy. It’s run every hour. It’s been very good at predicting rain vs snow and total precipitation.
Sat 5PM Update: Current Trends: Precipitation starts later, about 7:00 AM Sunday. Several hours of light sleet and freezing rain possible before changing to rain late Sunday morning. Far north and west will have extended sleet/freezing rain.
There have been ongoing changes with the onset of the precipitation, suggesting the models are having trouble with this complex surface and upper air system.
For Monday, still a coating of snow for PHL and immediate counties north and west on Monday. Areas north of Trenton and Doylestown will have more— potentially a few inches of snow on Monday. I’ll update later this evening.
…from Sat morning:
Sat AM Update: After reviewing last night’s model runs, here are the current trends— For Sunday morning, all models show a period of sleet and freezing rain until about 10 AM Sunday morning, longer duration far northwest. Below, the SREF (Short Range Ensemble Forecast Model) shows sleet/freezing rain in magenta, snow in purple, rain in green.
SREF model 7 AM Sunday Categorical PTYPE (precipitation type). White line is freezing line at surface (actually measured 6 feet above ground).
The rest of Sunday, we’ll have rain, heavy at times.
Still uncertainty about Monday, especially Monday evening. The SREF, NAM and ECMWF (European) have light snow with a coating of accumulation possible, more north of Allentown. The GFS and the CMC (Canadian) have snow showers, very light and scattered, no accumulation in our area. I’ll continue to update again this weekend.
…from Fri evening—
The latest models have come in. Based on the latest NAM, NBM and WRF, there are some changes in the forecast. The background for the weather this weekend is discussed in previous posts.
Saturday will be dry. Some sun early with high, thin cloudiness increasing throughout the day. Lower level clouds move in later in the afternoon than previously forecast. Very light winds. High 44.
About 3-5 AM Sunday morning, precipitation starts, likely as sleet, freezing rain and wet snow mixed in. After daybreak Sunday, sleet continues and mixes and changes to rain during the morning. Some freezing rain possible north and west early. Driving may be affected early. Tonight’s WRF models have freezing rain/sleet until mid morning.
Rain, heavy at times Sunday. High 45.
A secondary low may linger and intensify off the coast on Monday, with cold air changing precipitation to wet snow on Monday and Monday evening. Still unclear if there will be any accumulation, but a coating to 1 inch possible by Tuesday morning.
Fri Late Afternoon Update: The forecast for the weekend remains on track. I’m updating because the Monday forecast period now falls into the range of the NAM model.
For Monday: The latest NAM model supports the scenario discussed as a possibility over recent days— a secondary coastal surface low is held back or regenerated along the coast due to the upper atmosphere low pressure system to the west playing catch-up with the surface system.
Once they come together and become “vertically stacked”, the coastal system will further intensify. This setup allows the collision of cold air and wrap-around moisture to create the chance of snow Monday afternoon and evening over our area. About an inch is a possibility by daybreak Tuesday morning, although warm surface temperatures may interfere with accumulation. I think there’s still uncertainty about the amount of snow on Monday and I expect better clarification over the weekend. Stay tuned.
NAM forecast for 1 AM Tuesday showing Simulated Radar and Critical temperatures supporting snow.
…from Friday morning—
Fri AM Update: There’s still uncertainty with the forecast for Monday. As mentioned over past days, the strong upper atmosphere low remains west of the surface low on Monday. Precipitation lingers while the “critical thickness” levels indicating lower level/upper level temperatures supporting snow moves to our south and east, allowing a change from rain to snow.
So the possibility of light snow or snow showers on Monday still a possibility. A range of warmer surface temperatures may keep us from getting much, if any, accumulation. Below is the SREF (Short Range Ensemble) “statistical mean” forecast. Stay tuned.
SREF Forecast for Monday at 6:45 AM showing critical thickness and upper/ surface low pressure systems. Critical thckness for snow shown as red and magenta lines. Green blue shading is precipitation.
….from Thursday night:
Happy Thanksgiving!
A wet and windy Sunday….
The trends mentioned in the previous posts continue. A deep low pressure system will spawn a secondary coastal low late Sunday as the upper atmosphere low pressure system hangs back.
Saturday will be dry. Sunny in the morning with increasing cloudiness expected by early afternoon. High 48. Precipitation, originally forecast to start Saturday afternoon is further delayed until the hours before daybreak Sunday.
For Sunday, precipitation will be rain, heavy at times. QPF values around 1 inch of rain. There is a possibility the the precip starts as wet snow before daybreak but warm temps will change everything to rain. High 43. It will be windy.
The deep low pressure will intensify off the coast. Colder air will be brought in before daybreak on Monday. There is the chance of change back to wet snow at that time. No accumulation is forecast by the models.
Monday will be cloudy, windy and colder. High 40. The upper atmosphere low and the coastal low will combine to our east. The models are forecasting snow showers during the day Monday, but surface temperatures will be above freezing. Little or no accumulation is forecast by the models.
I think there’s some uncertainty in the model forecast for Monday. Some of the snow showers could be more than forecast, but I don’t like to second guess the models. Monday is just beyond the forecast window for the shorter range, higher resolution models. Stay tuned.
Sun AM Update: Rain and showers linger and end 1-2 PM. Clouds hang in until clearing about 3-4:30 PM Sunday, earlier south and west.
Sat AM Update: Rain starts Saturday evening about 7PM.
This morning’s models have the showers lasting until 12-1 PM Sunday. Some sun possible 2-4 PM Sunday.
Fast moving systems will affect our weather over the coming weekend.
High pressure builds in for early Saturday, with mostly sunny skies in the morning that will be followed by increasing cloudiness late morning and early afternoon as low pressure approaches from the southwest. High 48. The daytime hours will be dry.
Tonight’s models are in reasonably good agreement that rain moves in between 5 and 7 PM Saturday evening.
Moderate rain expected Saturday night into Sunday morning.
Light showers may linger through the morning hours Sunday as an upper air disturbance behind the main surface low passes through.
It’s expected that skies clear during Sunday afternoon, but there are differences in the models with the timing of the clearing. High 49. It will be windy.
