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.
“If you have time to indulge, would you mind explaining the difference between the ICON and NBM models (the later I believe is a blend of models?), with an eye towards which rain forecast for tomorrow (modest or healthy) is the one you lean towards?”
Good question and I’ll do my best to [try] to keep it simple and straightforward.
First, I should say the latest NBM model has moved towards the ICON model with over 1.25 “ rain (ICON model- 1.6”) for Friday afternoon through Saturday morning. mid-afternoon, then cloudy.Sunday cloudy with light rain again towards evening. Clearing Monday early morning.
Complete forecast on Friday.
NBM
The NBM is the “National Blend of Models”, a blend of many different models that is run hourly by NOAA. It’s an increasingly accurate composite model that statistically evaluates numerous other model’s inputs, determines their biases and errors, and comes up with a better forecast. For temperature forecasts alone, it’s pretty complex—
Statistical step evaluation for temperature (MAE= Mean Absolute Error) Source: NOAA
Numerous models are part of the blend. While it is called the “National Blend of Models”, the NBM now incorporates the European Models, Canadian Models and the Australian models. It includes all of the NOAA models- GFS, GEFS, NAM, NAM-NESTHRRR, HREF, SREF, HIRESW, RAP, GFS-MOS, NAM-MOS, EDKMOS, etc.
On an hourly basis, the NBM starts with the hourly HRRR model , adds each of these other listed models above on a regular schedule, and compares their forecasts to something called URMA, which is an error-corrected measurement of actual observed conditions six hours previously. It basically sees which model is doing better from actual measurements six hours earlier and weights that towards a better composite forecast.
The NBM model is available as a standard model in “grib” format and also available as a text file.
The ICON model is a German global (and regional) weather numerical prediction model from Deutscher Wetterdienst. It is run 4x daily, every 6 hours. What’s interesting about the ICON is its unique attempt to reduce computational errors by slicing/dicing up the atmosphere into icosahedrons (pyramids) instead of spherical cubes, which is what the new GFS-FV3 uses. FV3= is Finite Volume Cube).
All numeric weather prediction models divide up the atmosphere three- dimensionally. The geometry of each model, and the number of vertical levels and the horizontal size (resolution) determines its computational load and the amount of computational error created. (Many of the equations for weather prediction become infinite series calculations that require cut off. Successful geometric calculation can reduce the cut off error at the top of the troposphere. )
I’ve already gotten too technical, but the ICON is what is called a “non-hydrostatic model”, meaning it’s good at small scale vertical motions associated with thunderstorms and precipitation.
Why do I use the ICON model? It seems to do very well with temperatures, cloud cover and high precipitation events. I’ve been surprised how good it is. When the NBM shows high statistical spread (uncertainty), I lean towards the ICON forecast if it’s within the NBM spread.
It’s been a challenge for me to learn how to download the ICON and convert it. Since it’s icosahedral in geometry, it takes one of my old iMacs, used as a dedicated server, over an hour to re-interpolate the model into the standard latitude and longitude format and reduce its file size to something manageable. And that’s the time needed to download and convert only a few parameters!
I don’t know why the ICON is not included in the NBM and I’m not aware of any research papers comparing the ICON to other models. But I’m sure they’re out there somewhere on Google.
