Update Fri @ 9:52 AM — No significant change in the snow forecast. I’m going to borrow from my previous posts which basically captures the forecast:
From yesterday: “The models have all moved to a weak, unphased system with development occurring further from us. Totals of a coating to an inch are now forecast. “
From Tuesday: “A cold front moves through Saturday night and stalls. Low pressure develops in the southeast coastal area. Track appears offshore, but several models have some moisture over-running the stalled frontal boundary with some snow possible here on Sunday.”
So a coating to an inch is the forecast. The uncertainty is where.
Some models have the maximum precip from the city westward.Some have the maximum precip from the city eastward!
Temperature profiles run from a bit too warm to accumulate to cold enough to accumulate somewhat. Regardless of specifics the models are in agreement that much of it will be a rain or wet snow at the start, further reducing potential accumulations, then changing to snow.
The NAM has the highest accumulations (1.25″) and has the snow lasting into the afternoon. The GFS has the least, in some areas zero. The Canadian RGEM is forecasting about 1.2″ of snow.
Timing: The models have the snow/rain mix starting at 2-3 AM Sunday morning and ending either mid morning (ECMWF) or early afternoon (NAM).
I’ll update with my regular Weekend Weather Forecast later this afternoon/early evening.
Update Thu @ 7:46 AM — Last night’s models have uniformly moved to a weak, unphased system with development occurring further from us. Totals of a coating to an inch are now forecast. With the strong model agreement last night, I think we can lower the focus on this ‘storm’. Unless there are dramatic changes (not expected), my next update will be my regular Weekend Weather Forecast, usually posted late Friday afternoon or evening.
BTW, I should mention that the storm now falls into the range of the NAM, which shows 1-2 inches of snow. While I’m a big fan of the NAM for snowfall, its forecasts for such things are often not credible until about 36 hours ahead of the event.
Update Wed @ 8:28 PM — Another brief update. This afternoon’s ECMWF just became available as well as the ICON model. Both have moved towards a westward track and more snow for us. The ICON has 3″. Forecast is similar to this morning’s GFS which has subsequently moved in the other direction.
Here is the latest ECMWF 18z. (data available only until Sunday 7AM time frame)
Today’s 18z ECMWF forecast for 7 AM Sunday. Any precip (green-blue) north of the white freezing contour is snow. (Click on image for a larger view.)
We’re still in the 90+ hours time frame, requiring the global models. New updates tomorrow morning. The potential snow will then be in the forecast range of the RGEM, NAM and SREF models.
Update Wed @ 6:11 PM — Significant lack of agreement with the model forecasts for Sunday. The GFS and ECMWF have the jet stream phasing late, with the storm to our east. Minimal accumulation (1″ or so).
Today’s ECMWF 12z model forecast for Sunday 10 AM (Click on image for a larger view.)
The German ICON and the Canadian GEPS and GDPS have some phasing, enough to keep the storm near the coast, or in the case of the Canadian, inland!
In all cases, the amount of precipitation currently forecast would lead to 1, maybe 2 inches of snow. The Canadian’s western track would have a mix with rain.
Either way, no model is forecasting a major storm at this time. Things are likely to change. Stay tuned.
Previously Posted Wed 10:02 AM —
Over the past three days, a seemly benign weather pattern with a warm up on Saturday now shows signs of what I like to call “interesting weather” for Sunday.
A broad upper air trough was long expected to form low pressure system in the southeast coast: this had been expected to just brush us.
The GEFS has been suggesting for two days that more precipitation might develop. Here’s the scenario: A cold front moves through Saturday night and stalls. Low pressure develops in the southeast coastal area. Some moisture over-running the stalled frontal boundary results in some snow possible here on Sunday. Track of the coastal storm depends upon degree of upper air phasing.
The latest GFS (06z) shows a northern-southern jet phased upper system with significant upper air energy—
Today’s 06z GFS shows increasingly phased system between the northern stream jet (1) and the southern stream jet (2). The colors represent wind flow within what’s called ‘jet streaks’. White represents winds approaching 200 mph! Phasing causes the upper flow to become more ‘amplified’ resulting in storms moving more northward vs northeastward. (Click on image for a larger view.)
The GFS is close to its ensemble version, the GEFS (suggesting increased confidence), but other models are not all on board with a phased upper flow.
Here’s a comparison of the GFS with the last night’s ECMWF (European). The ECMWF is not phased
Previously Posted Wed 10:02 AM —
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The difference for us between a phased system and unphased is a snowfall of >4 inches….. vs 1″, maybe 2″(unphased).
The German ICON and Canadian GEPS were somewhat closer to the unphased ECMWF. So this is “still up in the air”
Stay tuned. I’ll have a better sense of the trend late this afternoon after the other global models and ensembles have become available.
Update Fri @ 4:54 PM — I sure hope my forecast is correct. The temperature is dropping. The latest MRMS shows the back end of the precipitation near upper Montgomery County is changing to snow. Most areas still rain or what’s called “cold stratiform rain”
MRMS at 4:41 PM The MRMS shows actual precipitation type: rain vs snow. Snow is shown appropriately as white. *Click on image to animate .* (Click on image for a larger view.) Super-imposed are current observed temperatures.
Update Fri @ 3:34 PM —
“All this rain, I hope it doesn’t turn to snow”
It won’t, at least around here. I’m sticking with the NBM and Canadian RGEM models which clearly show the precipitation departs before the cold air comes in—
This afternoon’s NBM model (16z) showing the precipitation into NJ at 7 PM with the surface 32º contour (white) just making it into Philadelphia. With the wet bub freezing temperature (violet contour) and the Bourgouin Positive Energy contour (yellow) just to our south, it would be cold enough for snow, BUT there will be NO precip falling! (Click on image for a larger view.)
I can see why there were forecasts for a change to sleet and snow. Many models had showed it a few days ago. Even today’s ECMWF shows the possibility.—
ECMWF 12z forecast for 7 PM this evening. At first glance it looks like sleet just to the west of the 32º contour, BUT precipitation displayed as a 3 hour accumulation, not radar simulation, so this is really precip that fell between 4-7 PM. Just the way it works. This is a debut graphic for me. It’s first time I’m posting a graphic of ECMWF downloaded raw grib data. (Click on image for a larger view.)
As mentioned in the caption above, after years of not being freely available, the European Center for Medium Range Weather Forecasts (ECMWF) has made their data available in almost (1 hour delayed) real-time. I never thought I would see it but like the US, Canadians and Germans, the ECMWF is participating in the “Open Data Project”.
Prior to this, the ECMWF data was limited to subscription only, on the order of > $10,000 US annually.
Update Thu @ 4:48 PM — The NAM and GFS has joined the other models in forecasting virtually no mixed/frozen precipitation in our immediate Philadelphia area Friday. This is comparable to the forecast first made by the Canadian RGEM model and our own NBM over two days ago.
Basically, the bulk of the moisture and precipitation leaves before the cold air comes in. While the NAM-NEST below shows temperatures dropping below freezing here around 5 PM, the latest RGEM has it an hour or so later.
This afternoons 18z NAM-NEST showing freezing line contour (white) and freezing line contour at 3200 feet (violet) Friday at 5 PM. The green areas is exiting rain and the widely scattered red is possible freezing rain. (Click on image for a larger view.)
Update Thu @ 7:15 AM — Last night’s models continue with the dichotomy in the thermal profiles for Friday with the GFS and NAM having the change to sleet->freezing rain->snow Friday afternoon around Philadelphia and immediate surrounding counties, while the Canadian RGEM and NBM continue with a delayed entry of the cold air until early evening after the precipitation has effectively ended. Some transition is possible in the Allentown area, but not the immediate PHL area.
I continue to lean towards the RGEM and NBM forecast which means little if any transition and little or no accumulation in the evening. Icy conditions possible later evening from pooled water freezing on roadways and sidewalks. I’ll keep an eye on it.
Update Wed @ 5:55 PM — This morning’s and afternoon’s models haven’t changed much regarding the possible transition to sleet/snow on Friday. While there are several models (GFS, NAM, HREF) that have temperatures dropping below freezing as early as 1 -3 PM in Philadelphia with a change to sleet->freezing rain->snow Friday afternoon,the NBM and Canadian RGEM keep temperatures above freezing until early Friday evening with most of the precipitation having ended.
Little or no accumulationwith this scenario in the Philadelphia area/ immediate suburbs, although icy conditions possible from remaining wet surfaces freezing over after 8 PM. I’m leaning towards this Canadian RGEM/NBM forecast. I’ll keep an eye on it.
Update Wed @ 8:57 AM — This morning’s model blend (NBM model) continues to back away from any significant sleet or snow following the cold front passage Friday.
The NBM keeps the surface temperatures above freezing until after 7 PM. The thermal profiles show a dusting/coating of snow northwest suburbs and a brief frozen mix as temperatures drop. Again little, if any, accumulation.
Today’s 12z NBM: Forecast for 7 PM Friday. Surface freezing (white line) wet bulb freezing temp (purple contour), Bourgouin Positive Energy contour (yellow line) (snow north of this line.) (Click on image for a larger view.)
Update Tue @ 4:36 PM —Today’s GFS and GEFS continue with a change to mixed precipitation Friday afternoon, following the cold front passage.
However, this is beginning to look like the event from a few weeks ago (Jan 17th) where rain was supposed to changeover to snow, but we got almost nothing. At the time, the Canadian GEM and NAM were forecasting little to no accumulation and they turned out to be correct.
Despite the GFS/GEFS continued colder forecast, today’s Canadian GEM has a couple of hours of mixed precip (sleet/wet snow) late Friday afternoon in the northwest suburbs with little accumulation.
Canadian RGEM forecast for 5 PM. Violet is mixed precipitation. (Click on image for a larger view.)
I’m leaning towards this. So I think you can forget about any meaningful snow/sleet accumulation Friday.
Previously Posted Mon 8:15 PM —
An interesting scenario is setting up for late Thursday into Friday.
First, we’ll be getting a brief January thaw in February this Thursday with temperatures reaching the low 50s.
Unfortunately, there will be considerable cloudiness with this warmup before heavy rains move in Thursday night. As much as 2 inches of rain are possible from late Thursday into Friday morning.
That’s where things get interesting. A frontal boundary will slowly move through Friday morning and surface temperatures will drop.
GEFS showing red 540 thickness line and approximate position of cold air aloft. Notice the white contour (32º surface temperature) is to our south along with low pressure developing in North Carolina. A wide range of possibilities from mixed precipitation to snow is possible, depending upon the development of this second low. (Click on image for a larger view.)
A second area of low pressure may form along the front and move up bringing a transition to a mix of wet snow or accumulating snow. There’s a wide range of possibilities and uncertainty with this setup.
GEFS shows a mean snowfall of 3 inches in Blue bell but with a standard deviation of of over 7″ That’s a lot of uncertainty. (Click on image for a larger view.)
Update Thu @ 11:13 AM — The new GFS model’s snow forecast just became available—
GFS 12z forecast Snow totals by 8 PM Saturday. (Click on image for a larger view.)
I’ll try to make sense of these giant differences later this afternoon. Right now I’m inclined to go with the GFS, but that could change.
Update Thu @ 10:17 AM — This morning’s early models are becoming available and their higher resolution forecast ranges are taking us to 1 PM or 7PM Saturday timeframe.
The dichotomy between the NAM and GFS still exists. Here’s the very latest NAM snow forecast—
NAM 12z snow forecast through Saturday 7PM (Click on image for a larger view.)
Despite the new NAM forecast, the other new models entering into the picture this morning are tending towards the much lower snow totals of the GFS.(about 1 inch in Philadelphia and less west)
The new GFS comes out just after 11 AM. I’ll take a look and update. Stay tuned.
Update Thu @ 8:43 AM — A quick update with last night’s 00z and 06z model trends. The GFS remains as the eastern outlier with the NAM back with a significant snowfall. European ECMWF still shows 4-6″.
Here’s current snow totals by model (for grid point: Blue Bell PA)
Current model snow forecasts by model for Blue Bell PA. (areas east will have more; this is only one point data)
More updates later… stay tuned.
Update Wed @ 11:11 PM — A quick look at tonight’s GFS just available shows virtually no snow in Philadelphia from this storm . Only eastern NJ. Stay tuned.
Update Wed @ 9:47 PM — This afternoon’s Canadian RGEM and German ICON are very similar to the earlier GFS with low snow accumulations in the immediate PHL area and westward.
However, just when I thought I could put this storm to bed, tonight’s models have started to become available. The NAM has reverted back to somewhat higher snow totals—
NAM snow totals for Saturday 4 PM (Click on image for a larger view.)
Tonight’s model blend (NBM) maintains a wide range of snow totals, reflecting remaining uncertainty.
Tonight’s NBM (00z)— Be certain to look at the wide percentile ranges in smaller font
NBM Median (50 percentile) snow forecast, with 25 percentile and 75 percentile in smaller type (Click on image for a larger view.)
The latest GEFS (18z) shows increased uncertainty of position after 10 AM Saturday in the storm’s western quadrant—
GEFS forecast at 1 PM Saturday shows increased uncertainty (high standard deviation) in northwestern quadrant; likely too late to affect our weather. (Click on image for a larger view.)
Not clear whether the uncertainty translates into higher snow totals.
The NAM-Nest forecast isn’t much use yet since it only forecasts up to 60 hours.
The new GFS becomes available about 11:05. I’ll update if I’m not asleep! 🙂
Update Wed @ 5:13 PM — Wow, this storm at least for Philadelphia, has fizzled out. The latest GFS just available continues with the trend of the storm moving away from us.
GFS forecast for Noon on Saturday. Precipitation barely makes it into Philadelphia. (Click on image for a larger view.)
GFS snow totals—
This afternoon’s 18z GFS just available Snow accumulation by Saturday afternoon (Click on image for a larger view.)
Update Wed @ 3:54 PM — A quick update. Today’s GFS, GEFS and Canadian GEM are all pointing to an eastern storm track, much lower snow accumulations. The SREF just available has backed away from extreme totals of the NAM as well.
Incredibly (and not atypical for the NAM) the 18z NAM just became available and it has the storm virtually missing us! Those wild swings in the NAM are the reason “Never ignore the NAM” does not apply for its longer range forecasts. Seen it before!
Latest GEFS snow totals with standard deviation (sd)—
GEFS forecast, mean with standard deviation. For those rusty about statistics, a standard deviation is a quantification of spread (uncertainty) in a group; in a normal distribution, approx 67% of the models fall within 1 standard deviation of the mean. (Click on image for a larger view.)
I’ll have new graphics a bit later this afternoon, but the GFS forecast just below is looking about right for now.
Update Wed @ 11:14 AM — I wasn’t going to update until later, but I took a peek at the new GFS that just came out and it still has a minimal storm for us! ( A fraction of the snowfall of the NAM forecast).
GFS snowfall (latest)—
GFS Snowfall totals by 7 PM Saturday (Click on image for a larger view.)
I’ll try to make sense of this later this afternoon. Stay tuned.
Update Wed @ 10:04 AM — The new morning models are coming in and despite the shift of the GFS eastward last night, it’s almost close enough to invoke the mantra, “Never ignore the NAM”
The storm is now within forecast range of the NAM model. The NAM continues with its more coastal (westward) storm with severe intensification Saturday morning. The NAM has moved into the range of the European ECMWF
Today’s 12z NAM show intense deepening of this storm (976 millibar central pressure) and large precip shield with a more westward track. (Click on image for a larger view.)
Here are the latest NAM snow totals. Too early to hang one’s hat on this forecast, but I think it’s helpful to know the range currently being forecast—
This morning’s NAM snow forecast by Saturday 7 PM (The white contours are every 2 inches.) (Click on image for a larger view.)
Stay tuned for more updates, likely later this afternoon.
Update Wed @ 8:02 AM — A quick update:
Note that this morning’s GEFS models are tracking a bit more eastward, resulting in a rapid decrease in snow totals from Philadelphia westward.
Here’s the latest NBM percentile snow totals by Saturday afternoon. Note the tremendous range between the 25th percentile and the 75th percentile.
The 50th percentile is the larger number. (Percentiles are tricky concepts….the 50th percentile indicates that half of the models have this snowfall amount or less;half of the models have more. It’s NOT the model average, nor is it necessarily the most likely amount.)
NBM model snowfall percentiles. 25, 50 and 75th percentile. The 50th percentile, also referred to as the median, is the larger number. (Click on image for a larger view.)
Update Tue @ 9:56 PM — This storm will come into the range of more models tomorrow. For tonight, we have a new GEFS and a new SREF which provide some new insights about the uncertainty in the forecast path and speed of the storm. Both model ensembles have the mean position of the low pressure system in similar positions, and similar to other models, although the SREF is a bit more east.
SREF and GEFS—
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Both suggest the most uncertainty (standard deviation) is northeast of the center, suggesting that the storm may move faster than shown and/or the storm will be deeper than shown at 7 AM.
The GEFS also suggests slightly greater uncertainty (?) west of the center, suggesting that the storm may be slightly closer to the coast.
I still don’t yet see much support for the European ECMWF model, which is predicting a major snowstorm for us.
I should mention that the latest ICON model has shown a track consistently eastward with little snow for us.
Also just available is tonight’s NAM which forecasts out to 84 hours. The NAM currently is fairly close to the coast at 7 AM and is forecasting about 2-3 inches by that time. While my regular readers know my mantra, “Never ignore the NAM”, that applies just 24 hours before an event. This far out, the NAM storm tracks are unreliable.
NAM forecast Saturday 7 AM (Click on image for a larger view.)
Update Tue @ 5:25 PM — With this afternoon’s models, there’s been some interesting trends. The GFS has trended a bit closer to the coast compared to the GEFS as well as its position forecast from this morning. That said, it keeps most of the snow in NJ, with a fast taper to an inch or so in Philadelphia.
I did get a chance to look at limited data from the European ECMWF, which currently forecasts a deeper storm than the GFS, closer to the coast. It has a significant snowstorm for New Jersey into the Philadelphia area. I’m not ready to get on the ECMWF band wagon.
This storm will be huge and intense. It seems that despite its eastward track, the precipitation shield may be large enough to still throw back snow over our region. At this point, any accumulation forecast would be just a guess. But it’s fair to say that New Jersey will likely see more than Philadelphia and accumulations will decrease from east to west.
The meso-scale models that extend their forecasts out 84 hours (NAM, Canadian RGEM) are finally coming within forecast range of this upcoming weekend’s storm.
The latest NAM is interesting in that it has a decidedly closer, more westward position of the storm at 84 hours (1 AM Saturday, its current forecast limit ) —
Today’s 18z NAM model forecast for Saturday 1 AM. The position of the main low center is much closer to the coast with snow extending into our area. (Click on image for a larger view.)
(This afternoon’s Canadian RGEM just available, has the storm moving much faster and eastward. Right now, it’s in its own camp, but it leans towards little snow in Philadelphia.)
The tonight’s run of the NAM will become available about 9:45PM. I’ll update after that time. Stay tuned.
Update Tue @ 12:49 PM — For the snow lovers out there not willing to give up on hope, the latest Canadian Global model still shows about 2-3 inches of snow for us on Saturday —
Canadian Global 12z forecast for Saturday 7 PM showing precipitation type and 32º line (Click on image for a larger view.)
So the track of the Canadian model is not necessarily closer, but it shows a wider precipitation shield than the GFS (below).
By the way, I took a look at the GFS snow totals and it does show about 1-2 inches in Philadelphia, despite the eastward track.
Update Tue @ 12:02 PM —This morning’s new GFS ( 12z )just became available. Very consistent with its previous statistical counterpart, the GEFS. The GFS still shows its snow shield barely making it into Philadelphia. (see snow total update above) Things can still change, and I hate to disappoint the snow lovers out there, but there has been too much model consistency to believe at this time that it will come closer to us.
This morning’s latest GFS forecast little change in track, essentially missing us. (Click on image for a larger view.)
This morning’s ICON-EPS (ensemble) model just became available. It no longer shows uncertainty towards us and the east coast. Instead, uncertainty is in speed and direction northeastward—
ICON-EPS (ensemble) 12z forecast for Saturday 7 AM showing pressure and pressure standard deviation (shading) Uncertainty is no longer towards us. (Click on image for a larger view.)
We’re at the 96 hour forecast point, still only in the range of the global models. We have to wait for the forecast point to get to 60-84 hours for other models to make their forecasts. Other global models and ensembles will be available this afternoon.
Update Tue @ 9:01 AM — Last night’s model runs (00z and 06z) of the global models moved the coastal storm decidedly eastward, away from us. That said, there’s still highest uncertainty (standard deviation) in our direction.
Graphics below show consistent eastern track, with snow shield just brushing our area. Uncertainty graphic (darker colors) show similarity with greatest uncertainty westward and northeastward—
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So this storm was never shown to hug the coast and current forecast track is consistent between models and with previous runs. Still, there is uncertainty as shown by high standard deviations to the east and northeast. Some snow still possible, as we get the northwestern fringe with current track. Stay tuned.
Update Mon @ 8:18 PM — Comparing today’s 18z GFS, GEFS, Canadian GEPS, and ICON model’s s forecast at 7AM Saturday—
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The above graphic shows that the latest version (18z) of the GFS is forecast to be further away from us than either the 18z statistical version (GEFS) or the the latest ICON. The Canadian 12z GEPS (not run at 18z) is very different with speed and position. Click on the above graphic to animate.
Update Mon @ 5:24 PM — I was just about to consider a separate web page for this possible upcoming weekend storm. But the very latest GFS 18z just became available and it has the storm taking an eastern track, mostly missing us.
GFS 18z showing eastern track of storm with precipitation missing us. (Click on image for a larger view.)
In this time frame, the statistical ensemble models are generally used, to try to minimize the “model of the hour” or “model of the day” effect. But it is interesting that there’s been this shift eastward again. Stay tuned.
Update Mon @ 11:45 AM —Here’s the latest on the potential Nor’easter for the Philadelphia area late Friday into Saturday.
While previously looking like it would miss us, the latest trend is for the storm to be closer to the coast. The GFS as well as the GEFS mean and GEFS mode have all trended westward. The potential for several inches of snow by Saturday morning is there. The Canadian models have different speeds but somewhat similar westward trends.
Here’s the very latest GFS forecast for 7 AM Saturday—
GFS forecast 7 AM Saturday showing precipitation rates. Anything west and north of the red 540 thickness line would be snow. The other thickness lines along the coast suggest NJ would have sleet and freezing rain. An evolving situation with much time for things to change. (Click on image for a larger view.)
This is an evolving situation with plenty of time for things to change in a multitude of ways before Friday. Stay tuned.
Previously Posted Sun 9:07 PM —
January Thaw? The almostannual warm up usually occurring the third to fourth week in January is nowhere in sight. It looks like we’re going to have to wait (until at least February) to get our annual thaw.
A cold, wide upper air trough will remain with us for this week, with a slight warm up on Tuesday before another cold front moves temperatures at or below seasonal averages.
This is what the models are showing for this week.
A relatively quiet seasonably cold flow will change to a highly amplified flow by Friday. Energy in the northern jet is expected to dive south Friday and phase with the southern jet stream, creating an intense coastal storm. Multiple models are forecasting this storm and multiple models have the storm somewhat missing us.
Previously Posted Sun 9:07 PM —
GEFS statistical mode forecast with bias-corrected precipitation—
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Right now, we would be on the very western fringe of the precipitation (snow), with the main storm missing us. As is always the case with a forecast 5 days ahead, things are likely to change. Stay tuned.
