For tonight’s light snow, the latest NAM QPF is 0.15 inches water, the GFS 0.10 and NBM 0.07. This translates into 1, possibly 2 inches of snow ending about 7 AM Friday.
Tonight’s NAM and GFS have light snow starting late afternoon Saturday, quickly mixing with sleet and some freezing rain, then all rain during the early evening hours. There may be an inch or so of accumulation before the changeover, with higher amounts far northwest.
Rain continues heavy through the night. Temperatures are now forecast to only reach the 40s overnight.
With daybreak Sunday, rain changes back to mix of rain, sleet and snow as temperatures plummet to 30 by noon. Temperatures drop further as the day progresses. Precipitation also ends by noon. Accumulations as shown in the NAM graphic below still hold. Will have a better handle on specifics tomorrow.
I did want to mention that many of the NWS research websites and experimental models are unavailable due to the government shutdown.
Winter Weather Update Thurs 1 PM
For tonight’s light snow, this morning’s NAM is clocking in with a QPF of 0.15 inches water. The GFS 0.13 inches water. 1-2 inches of snow starting 10-midnight and ending about 7 AM Friday.
For the weekend storm, IGNORE the graphic from the ECMWF I posted earlier. As I said, I think it’s overestimating the snow forecast big time.
This morning’s NAM and GFS have light snow starting late afternoon Saturday, quickly mixing with sleet and some freezing rain, then all rain during the early evening hours. Rain continues heavy through the night.
With daybreak Sunday, rain changes back to mix of rain sleet and snow. Minimal accumulations expected, even north and west of the city.
Here’s the likely snow totals by the end of the storm:
Winter Weather Update Thurs 8 AM
For tonight’s light snow, the NAM is still clocking in with a QPF of 0.17 inches water. 1-2 inches starting 9-11 PM and ending about 7 AM Friday.
The models are all on board for an earlier and faster storm Saturday into Sunday. (Previous model runs had the changeover back to snow about noon, Sunday. Now the changeover is predicted during the morning Sunday and precipitation ends early afternoon.)
There are considerable differences in the predicted thermal structure during the changeover. The latest European (ECMWF) has significant snow for Philadelphia. (See map below). I don’t currently think this is correct.
Based on the average NAM/GFS models, 2-4 in Philadelphia by Sunday noon. I think this may be too high as well. Stay tuned.
Winter Forecast Update Wed 11:00 PM
Tonight’s NAM has the snow starting about 9pm Thursday night. QPF values 0.16 inches water. 1-2 inches of snow. Ending about 7 am Friday morning.
Additionally, the Saturday night storm is depicted by the NAM to be even faster, exiting our area early morning Sunday, almost 8 hours faster than previously predicted by the GFS. (The NAM forecast range has just come into Sunday morning.) That would mean even less snow on Sunday morning. The uncertainty with this weekend storm seems less about track and more about phasing and speed. The trend is less phasing and more speed, faster exit..
Things continue to evolve with the storm for Sunday. But first, let me talk about the light snow for late Thursday night into Friday morning.
A warm front type system will bring light snow, starting around midnight Thursday and ending about 7 AM Friday morning. QPF values range from 0.16 NAM to 0.14 GFS to 0.07 NBM. Most likely accumulation is 1-2 inches.
Now for the storm on Saturday night into Sunday. Here are the current trends –
The latest GFS and FV3-GFS have the storm moving faster. Light snow starts about 4-5 PM Saturday afternoon, but quickly mixes with and changes to rain.
Later model runs will allow a better guess of how much snow will accumulate before the changeover to rain.
Heavy rain Saturday night into Sunday morning. The rain will be accompanied by increasingly high winds. Late Sunday morning, a strong cold front will pass through; the rain will change to snow as temperatures drop from the 50s to the 30s by mid afternoon and into the 20s by 7 PM.
While the total QPF is about 2.00 inches water, it now appears the the changeover to snow on Sunday will be brief and only about an inch or two of snow will accumulate in and around PHL as temperatures drop. It appears that the influx of cold air will cut off much of the moisture, reducing snow totals.
Much of the precipitation will become spotty by late afternoon and should be over early evening. The big impact will be winds and rapidly sinking temperatures. Unclear how much icing will be a problem, but it’s likely since wet areas will freeze over.
Reviewing today’s models, there isn’t much change from this morning’s update. The small disturbance that will accompany a warm front will bring some light snow Thursday night. Current QPF is 0.12 inches water, or 1-2 inches from midnight Thursday to daybreak Friday.
Most models have the large storm for late Saturday into Sunday tracking far enough inland that the precipitation will be mostly rain for PHL and immediate suburbs; the changeover to snow looks to be late enough to limit snow accumulations significantly. Stay tuned.
Winter Storm Outlook Update Tues 8 AM
The small disturbance that will accompany a warm front will bring some light snow Thursday night.
The large storm for late Saturday into Sunday is now forecast to track far enough inland that the precipitation will be mostly rain for PHL and immediate suburbs; the changeover to snow on Sunday may be late enough to limit snow accumulations significantly. This scenario has been one that has alternated with a more coastal track.
One more thing...
While focusing on the weekend, I neglected to mention a warm front type snow that is forecast to occur this Thursday night into early Friday morning. There’s some question whether it will be snow or a mix, but tonight’s NAM has it as snow. An active week coming up. Stay tuned.
The first truly impressive storm of this winter season is forecast to affect our area in the late Saturday through Sunday timeframe.
This storm has been predicted by the long range models for well over two weeks. All models are currently on-board for some sort of winter precipitation, very windy conditions, followed by deep cold.
As always, there are differences among the models regarding the speed of the initial impulse, degree of intensification and the precipitation type. The GFS, the FV3-GFS and the ECMWF (European) are the most impressive.
As of today, it appears that precipitation will start as rain Saturday night, then change to a mix of sleet and rain and then a final changeover to all snow from west to east during the day Sunday.
As we know, the models didn’t accurately depict the the past weekend precipitation, so it’s a shot in the dark to predict snow totals 6 days in advance.
That said, a 6-10 inch snowfall (based on the latest ECMWF) with high winds is looking possible at this time, north and west of Philadelphia. Philadelphia may stay a mix for a longer period. This storm looks to be impressive, with high winds. Stay tuned.
Tonight’s NAM has moved down the QPF to 0.37 inches water for Philadelphia. The graphic with the tight gradient below is still a realistic forecast for snowfall — areas in far Northeast Philadelphia will have less snow than areas near the airport. Areas central to Philadelphia can expect 3-4 inches.
The accumulation will be extremely location dependent, south to north.
Here are some changes— light snow continues into Sunday evening in areas south. Far South Jersey May have over 1 foot of snow.
An unusual forecast is unfolding, with a tight gradient of precipitation and larger possible snow totals. With these tight gradients, I usually go with the NAM model.
This morning’s NAM had a QPF of 0.92 inches water at the airport, rapidly diminishing to very low amounts at Trenton. This seemed like an error. This afternoon’s NAM run is still running much higher than other models, with a QPF of 0.42 inches water at the airport and a tight gradient of decline. I don’t usually use graphics for snow totals but the sharp gradient in amounts can only be depicted graphically:
This morning’s models also depicted the sharp gradient, but the QPF values were about half of the NAM prediction.
Based on the NAM, 2-5 inches will fall, depending on location north to south.
Snow will start between 10 and midnight tonight and will continue to about 1 PM on Sunday. It’s possible that tonight’s model runs will back off on the high QPF values, but you’re reading this blog to find out what the possibilities are.
This mornings NAM data just available shows a QPF of 0.15 inches water. This morning’s GFS has a QPF of 0.11 inches water.
Based on the average, we’re back to 1, possibly 2 inches of snow, starting about 9-11 PM and ending about daybreak on Sunday. Considerably less the further north one is from Philadelphia.
There’s been tremendous consistency and continuity with the models and the storm track. I’ll update tonight with my usual Weekend Weather Forecast about 10:45 PM.
Forecast Update Fri 10 AM
This mornings NAM data just available shows a QPF of 0.15 inches water. The GFS data is not yet available this morning, but last night’s 06 UTC (1AM EST) run showed a bit higher- 0.22 inches water for Philadelphia.
The forecast remains on track for 2 inches, possibly 3, in Philadelphia with lesser amounts to the north. Light snow starts early evening Saturday and ends during the morning hours on Sunday.
There’s been tremendous consistency and continuity with the models and the storm track.
I’ll update tonight with my usual Weekend Weather Forecast about 10:45 PM.
Forecast Update Thurs 12 PM
This morning’s models (NAM, GFS, FV3-GFS) are in remarkable agreement with a forecast of light snow starting early Saturday evening and ending sometime during the day on Sunday. Most of Sunday should be snow-free with the exception of light flurries.
The total QPF is about 0.18 inches water, which translates into about 2-3 inches of snow at these temperatures.
Today’s GFS is down-playing intensification with throw-back snow later on Sunday.
There is increasing agreement among the models that the low pressure system will track to our south. We’ll be on the northern fringe of the precipitation shield.
Over-running moisture ahead of the low cause light snow to fall late Saturday afternoon or Saturday evening. About 1 inch or so is expected in the general Philadelphia area by Sunday morning. (Total QPF water equivalent is 0.10 inches) The NAM and GFS are in agreement, as is the FV3-GFS.
Sunday will be cold but little if any snow is expected during the daytime hours. Any snow will fall to our south in Delaware.
The GFS has some intensification and development of the low as it reaches the coast Sunday evening. It shows some throwback moisture, allowing some additional snow Sunday night. An additional 1 inch of snow is possible Sunday night.
As everyone who reads my blog knows, we really can’t make an accurate call on QPF and snowfall amounts until 12-18 hours before the event. Stay tuned.