Update Fri @ 10:47 AM — This morning’s GFS (12z) just available shows even less snow by the changeover time of about 10 PM— less than 1 inch. Even less will be left by Monday morning after a night of rain.
Update Fri @ 8:47 AM — Last night’s 00z and 06z model runs show little change in the forecast for Sunday into Monday. Snow starts between 5 and 7 PM Sunday and quickly mixes with and changes to rain about 10 PM, with a mix of sleet and rain, then snow far north.
NAM 06z showing critical temperature (snow/rain) contours (violet-blue-magenta) moving north of Philadelphia at 10 PM with a transition from snow to rain.
If you were to go out and measure the snow at midnight, you’d find maybe 1-2 inches of snow in the immediate PHL area, but with most of the precipitation expected to fall as rain after midnight, there will be about 1/2-1 inch of heavy, slushy mess in the morning.
While some of the ensemble models and NBM continue with a chance of showers or snow flurries during the day Monday, most deterministic models have the precip ending around daybreak.
All keep the temperatures above freezing Monday, so a freeze up is not expected until Monday night.
I’ll update later today if things change dramatically.
Of course, I’ll be updating the storm forecast with my regular weekly feature “Weekend Weather Forecast” Friday evening.
Update Thu @ 10:20 PM — Tonight’s NAM, just available, continues with the same forecast for Sunday evening into Monday. A transition to rain around Philadelphia about 10 PM.
Like the GFS, it has most of the precipitation (rain) ending before daybreak Monday. Likely snowfall is down to a soggy inch or so.
Tonight’s NAM rain/freezing rain/sleet/snow areas at 1AM Monday
Update Thu @ 5:10 PM — (some additional changes highlighted below.) The storm forecast for Sunday evening into Monday has just come into range of this afternoon’s NAM model which forecasts up to 84 hours ahead. While its forecast doesn’t extend into Monday, it shows what we can expect for the start of the storm.
The NAM supports a similar development and track that the GFS has been forecasting since Tuesday. Snow starts between 4 PM and 7 PM, but critical levels of the atmosphere warm up by 10 PM with transition to sleet and just rain by midnight.
NAM 18z forecast for Sunday evening 10 PM. The critical temperature contours will move north of the immediate PHL area starting 10 PM with a change to rain. Most of the storm’s precipitation will fall as rain.
While we might get snow (1-3 inches possible) before midnight, most of the precip with this storm is expected to occur occur between midnight and daybreak Monday with it too warm for anything but rain.
With heavy rain during the night, much of the snow will be compressed down to slush by morning.
The latest GFS has the storm exiting quickly, by early Monday morning with just snow showers.
It’s not clear whether there will be much snow on the back side of the system, but the trend is that there won’t be much.
It should be noted that the GEFS (ensemble) still maintains more of a coastal storm. So there’s a glaring lack of agreement with this storm. Things could change.
The storm expected late Sunday into Monday is expected to bring a mix of wintry weather to us. What most people want to know— “How much snow are we going to get?”
As is often the case with winter storms in Philadelphia, nailing down the snow totals is beyond challenging. This coming storm will likely be an extreme example of challenging.
It’s never a just matter of seeing where the 32º line surface temperatures is. The atmosphere is three dimensional and temperatures at several different layers of the atmosphere are associated with snowfall and other precipitation types.
There are several main techniques that snow / sleet / freezing rain / rain are forecast; basically they all try to figure out the average temperature of layers of the atmosphere: (hover on the underline for the pop-up glossary)
Average predicted 32º temperature at certain levels of the atmosphere, specifically 800, 900 and 975 mb ( NAM FOUS data)
Position of critical thicknesses, which in turn, are associated with average 32º temperatures . The 540 contour of 500-1000 mb thickness is the most basic, although there are other “critical” thickness levels associated with snow and other PTYPES.
Three dimensional temperatures energy referred to as Bourgouin Positive and Bourgouin Negative Energy areas, developed by the Canadian Meteorological Center. (Review the research paper at this link.) The model blend (NBM) includes the Bourgouin parameters.
Position of the 32º contour “average Wet Bulb Temperature above 610 meters above ground level”
Below is the latest GEFS (mode) model with 32º contour temperatures at 800 900 and 1000 mb at 1 AM—
GEFS 06z forecast for Monday at 1 AM. 32º contours at different critical layers of the atmosphere suggest much of Philadelphia and half of Montgomery/Bucks county will be a mix of sleet and rain. Areas north of the blue 32º will be wet snow. Below this will be a mix and rain. “Guaranteed all snow” is near and north of the 540 thickness line“. Shading is six hour precipitation. (Click on image for a larger view.)
Here’s this morning’s NBM showing the Bourgoiun Positive Energy at 5. J/kg contour and the average Wet Bulb Temperature
Today’s NBM 12z for Monday at 4 AM is predicting a much ‘warmer’ storm with the yellow criticalBourgoiun contourfar north of the city. The wet bulb 32º contour is also far north of the city. (Click on image for a larger view.)
What makes this so difficult is that the predicted position of these precipitation type cut offs changes throughout the storm and and will change drastically with this storm. Calculating a snow total, including compaction of snow fallen by a change to rain is a moving target.
Currently I’m still leaning towards the warmer GFS model with less snow, as discussed yesterday. The GFS has us south of the 540 thickness line for much of the night time storm hour meaning rain or a mix.
Update Wed @ 9:03 PM — I’m going to need to change the title of this posting tomorrow to “Storm Late Sunday into Monday”.
This afternoon’s (18z) GFS and GEFS models have become available.
The GFS and ICON continue with a western track and a significant influx of warm air a few hours into the event. Any light snow at the start changes to sleet and rain and with a change back to snow possible but most of the moisture has move northeastward. So little accumulation in the immediate PHL area, according to the GFS.
Unfortunately, this is going to be a complex storm and the above simple forecast isn’t supported by the ensemble versions- the GEFS.
Both the ‘mean’ and ‘mode’ versions of the GEFS are now more similar, providing higher confidence in their forecast. They show a more coastal track with warm air moving in for a shorter period of time after 10 PM Sunday through 3 AM Monday. The thermal profiles are much more supportive of more snow accumulation—
GEFS “mode” version with “Bias-Corrected Precipitation” at 1 AM Monday. This is the warmest point in the storm for us. By 3 AM, the critical thickness lines all move eastward with a change back to a mix of sleet and snow. (Click on image for a larger view.)
In terms of timing, the latest model blend (NBM) shows precipitation starting from south to north about 6-7 PM Sunday and ending late morning or early afternoon Monday.
The NBM has a significant portion of the immediate PHL area in either rain or mixed precipitation. Way too early for snow totals, but tonight’s NBM (00z) snow totals for Blue Bell have a statistical range from 2.0″ -4.6″ by Monday afternoon.
The storm comes into the forecast range of the NAM and Canadian RGEM models tomorrow. Stay tuned
Update Wed @ 4:29 PM — While there’s still 110 hours before a storm affects us later Sunday, the models have trended towards a forecast first made by Monday night’s GFS.
The storm will form from a disturbance that is currently in the eastern Pacific ocean right now—
GOES Pacific Satellite image today showing the disturbance expected to develop into our coastal storm. (Click on image for a larger view.)
The GFS has a westward storm forecast: A deep low pressure system will develop over Virginia Sunday evening and will move northeastward over us later Sunday night into Monday. The storm will have an influx of warm air soon after the precipitation starts and the GFS’s inland track would assure we have mostly heavy rain and high gusty winds.
GFS 12z forecast for Sunday at 10 PM. “Critical thickness” lines (red/lavender/purple) and surface temperature (white) all suggest a thermal profile that will be mostly rain for us. (Click on image for a larger view.)
My bet is on the GFS right now, since it was the first to predict this system and its been very consistent. Today’s European (ECMWF) model is similar to the GFS position. But its forecast of the storm exiting early Monday morning may be too fast.
Unfortunately, the forecast isn’t clear cut, as the GEFS and ICON models have the storm moving over NJ or coastal NJ. (The ICON has measurable snow just northwest of the city.) These models also are much slower with the storm exiting the area Monday.
GEFS 12z forecast for Monday at 7 AM. This is a colder storm, more eastern track with snow. (Click on image for a larger view.)
Just to make things interesting, I’ll throw in another model, the Navy NAVGEM with its slower and eastern track which misses us. (I like the NAVGEM for tropical systems, not so much for winter storms) —
US Navy NAVGEM 12z forecast for Monday 7 AM. (Click on image for a larger view.)
Of course, there’s plenty of time for the forecast to change. The impulse that will create this storm is 3000 miles away at this time. Stay tuned.
Update Wed @ 8:37 AM —The current trend with the forecast is for Sunday’s low pressure system to develop directly over our area, with any snow changing to a mix, then rain, and then back to snow before ending.
The Canadian and European models are part of that general consensus, although the ensemble versions of the respective models continue to place it somewhat more eastward, as more of a coastal storm with the potential of more snow.
According to the model blend NBM, snow will start about 4 PM Sunday and mixing and changing to rain about midnight and then transitioning back to light snow sometime late Monday morning or early afternoon before ending.
The comparison of the GEFS ensemble mean vs. ensemble mode models show some differences in position and huge differences in the thermal profile, with the mean still showing snow here, but the mode too warm for much snow.
Differences in the mean and mode models suggest a non-normal distribution of the ensemble members and higher uncertainty in the forecast.
GEFS 06z ensemble mean showing critical temperatures south of the city. A colder profile and more snow possible. GEFS (ensemble mode with bias-corrected precip) 06z forecast for Monday at 7 AM. Shows low pressure off of Maryland coast, but considerable warm air intruding around its core, causing the critical thickness lines (red, lavender, purple) to be west of our area. This means principal precipitation type will be rain or possibly freezing rain north of the 32º line.
Since the storm is more than 89 hours in the future, we’re still dealing with the extended range and ensemble models and none of the higher resolution short range models. Stay tuned.
Update Tue @ 7:52 PM — Today’s extended forecast models (GFS, Canadian GDPS, ECMWF and ICON) have all followed the lead of the GFS in predicting a near coastal storm Sunday evening into Monday. However, large differences/uncertainties in timing and placement of the center of the storm still need to be clarified in coming days.
Currently, these deterministic models have a mixed bag of precipitation (snow/rain/freezing rain/sleet) for us due to a western track while the statistical ensemble models have more snow with an eastern track. Current timing is Sunday evening through Monday morning.
The latest GEFS ensemble ‘mean’ (as in statistical average) model and the latest GEFS ensemble ‘mode’ model are similar, but are very different from the main GFS, confirming a low confidence forecast.
The storm is forecast to affect us 132 hours from now. There’s plenty of time for things to change. Stay tuned. I’ll update tomorrow morning.
Previously Posted Tue 1:26 PM —
Following my long diatribe about extended range forecasting using ensemble models, another GFS model has come out this morning which supports the trend of a developing coastal storm from last night’s GFS—
GFS 12z forecast for Sunday at 10 PM. Red line is the 540 thickness line. The position of the 540 line suggests that mixed precip- rain sleet, will occur south and east of this line. (Click on image for a larger view.)
The latest Canadian GDPS, ICON and ICON-EPS have a similar forecasts, with some being too warm for snow here.
As a followup to the ensemble forecast diatribe, the latest GEFS (12Z) has come out, which shows more of a snow storm with high uncertainty in the northwest quadrant—
GEFS (ensemble) 12z forecast for Monday 4 AM low pressure position.
This is a developing situation. There’s much uncertainty, especially considering just 24 hours ago, this wasn’t a possibility. Stay tuned.
Originally posted on Tue at 9:15 AM—
The latest GFS has a new wrinkle, with yet another storm developing late Sunday into Monday. The GFS predicts this storm as a significant nor’easter with a mix of precip for our area.
The latest GFS (below) is a statistical outlier, and NOT supported by the other models or its own ensemble. But it’s something to watch. Stay tuned.
This latest GFS (06z) forecast is NOT supported by any other model or even its own ensemble. It is likely an outlier. (Click on image for a larger view.)
