MegaMike

Go to this link, https://vlab.noaa.gov/web/mdl/nbm-weather-elements then click the parameter you're curious about. If the field has a 'CONUS' option, click it. You'll get a table that specifies model weights. Despite the description, I still can't make sense of the NBM snowfall maps that correspond to their model weights. Something else seems missing... The map seems too high and I don't see much that supports those values b/n the SREFs, EPS, GEFS, and GFS.

It's frustrating, for sure. I think to be 100% satisfied with their documentation, they need to provide an example for one case study. ie... Provide a table of snowfall accumulations (and exactly what field/how they processed snowfall) for all ensemble members + diagnostic models, take the weighted sum of all ensembles/models, then show the final result superimposed onto their NBM map. When I ran the calculations as you did, my value was far off from what the map showed too. I do trust that their documentation is correct (consisting models and weights wrt time), but something else does seem missing. I may ask the developers when I have time, but I think Don already did this.

From hour 61-84 the NBM snowfall (it's different per variable) product composes of the GEFS (30 members; weighted 24.75%), EPS (50 members; 41.25%), GFS (weighted 4%), and SREF (10 members; weighted 30%). Likely, either the SREFs, and or EPS, beefed up snowfall a little bit... The GEFS/GFS still look poor.

Excellent! Now try running these modeling systems (similar to the AIGFS and EC-AIFS). They'll perform better and you'll get a good challenge out of it https://github.com/google-deepmind/graphcast

Based on the link I sent, let's see if this checks out... Both the EPS and GEFS have the same weights. 1.2% for each individual member (1.2%*[50+30]=96%)... The GFS has a weight of 4%, so the ensemble snowfall contribution would be, 9 = 8 GEFS + 1 EPS SF_ens = 9 * 0.012 * 6" + 1 * 0.012 * 16" SF_ens ~ 1" NBM snowfall is SF_ens + 0.04*SF_GFS. I don't know what the GFS had for the city at 12z, but it could be calculated; NBM = 5.9" based on the map 5.9" = 1" + 0.04 * SF_GFS SF_GFS = 4.9"/0.04 = a very unrealistic answer... So something is off. Do you have the 6z totals? Mathematically, that's how the NBM (imo) calculates its mean.

Oh, the irony! I just commented about this in a different thread. At hour 84+, for snowfall specifically, the NBM uses the EPS (50 members), GEFS (30 members), and the GFS (https://vlab.noaa.gov/documents/6609493/32850490/CONUS_SNOICEACCUM.pdf)... So there must be some GEFS/EPS members that still support a (significant) snow event. You'd have to look at the individual members themselves to determine how (are there a few members skewing the mean, are there two different 'camps', etc..) that map you posted above is produced. I thought it was odd too. For skewness, you could also look at quartile ranges provided at https://sites.gsl.noaa.gov/desi/?x4dLocations=["City"]&chart=x4d&lat=40&lon=-105&theme=dark&timeZone=local&hourFormat=12&x4dviewState={"latitude"%3A40.5%2C"longitude"%3A-100%2C"bearing"%3A0%2C"pitch"%3A0%2C"zoom"%3A4}&dset=HREF-CONUS&clusHghlgt=true&x4dMapStyle=3D&x4dMaps={"basemap"%3A{"value"%3A"Mapbox"}%2C"mapboxBasemap"%3A{"value"%3A"Satellite"%2C"parentValue"%3A"mapboxBasemap"}%2C"Airports"%3A{}%2C"ARTCC"%3A{}%2C"Cities"%3A{"checked"%3Atrue}%2C"Coastlines"%3A{"checked"%3Atrue}%2C"County+lines"%3A{}%2C"Countries"%3A{}%2C"Country+lines"%3A{}%2C"Countries+NonUS"%3A{}%2C"Country+NonUS+lines"%3A{}%2C"CWAs"%3A{}%2C"Graticules"%3A{}%2C"HUC+6+(CONUS+%26+OCONUS)"%3A{}%2C"HUC+8+(CONUS)"%3A{}%2C"PSAs"%3A{}%2C"RFCs"%3A{}%2C"Roads"%3A{}%2C"State+lines"%3A{}%2C"Tribal+Lands"%3A{}%2C"Vulnerability"%3A{"value"%3A"Social+Vulnerability+Index"%2C"parentKey"%3A"Vulnerability"%2C"parentValue"%3A"Vulnerability"}%2C"Watches%2FWarnings%2FAdvisories"%3A{}%2C"Zones+(public)"%3A{}%2C"Zones+(fire+wx)"%3A{}%2C"Zones+(coastal+marine)"%3A{}%2C"Zones+(offshore+marine)"%3A{}}&preferredFontSize=14&x4dDset={"renderOptions"%3A"t2"%2C"plotargs"%3A[{"fields"%3A["t2"]%2C"fieldOption"%3A"statisticalMeasures"%2C"trackingID"%3A"tracking_643d2e8c-941e-4f4a-99f4-e0f3eeab9bd4"%2C"layerorder"%3A1769554913471}]%2C"name"%3A"statistics"%2C"default"%3Atrue}&x1dGroup=Default&x1dSection=overview&x1dSingleField=t2&x1dGraphStyle=pdf&x2dGraphStyle=boxwhisker

The 13z NBM has a 12-18" mean contour for SE MA (looks similar to the map DIT posted). According to dat' link (https://vlab.noaa.gov/documents/6609493/32850490/CONUS_SNOICEACCUM.pdf), snowfall values at 84hr+ are calculated from 30 GEFS members, 50 ECMWF members, and the GFS. Needless to say, some individual members still support this event. 12z GEFS fcst hr 138: 12z EPS fcst hr 138 (last available hour at the moment):

Snow up to thy' neck... The trooper is 24" tall and snow is ~2/3rds to the top of the figurine. I'd call it ~13-16" (removing some inches that drifted from the roof). Just shoveled the driveway and measured just under 11" on my car. Getting a fine snow that steadily accumulates. Spent about 45mins shoveling and picked up a half inch. Good day overall and it's still snowing!

Yea, that band coming through Boston now produced large dendrites in Wrentham. Enjoy it!

Ignore it for the coastline. Snow depth is a diagnostic product from each modeling system. It's masked for bodies of water... If you interpolate snow depth from a body of water (always 0") to a place over land (>>>0", in this case), it'll always have a tight gradient for coastal locations.

I think @EastonSN+ is asking for someone to post it. Generally, a little less QPF for all locations... I wouldn't overanalyze it though. It's about time to focus on regional/meso-scale models.

It uses several different algorithms + a bunch of different modeling systems < https://vlab.noaa.gov/documents/6609493/6665561/NBM_v4.2_Eval_SlideDeck.pdf >. Definitely an improvement over the traditional, 10:1 methodology.

It's a great website and there's a lot provided! It's free and the HREF (and others) is available too. For the weenies: https://sites.gsl.noaa.gov/desi/?x4dLocations=%5B%22City%22%5D&chart=x4d&lat=40&lon=-1[…]1dSingleField=t2&x1dGraphStyle=pdf&x2dGraphStyle=boxwhisker For now, I'd stick with BUFKIT although the NBM does incorporate the Cobb snowfall algorithm. For S&Gs, the 13z NBM has ratios ~12-20:1 throughout the event. It's a bit early for this (and snowfall maps), but man... I miss following potential snow events.

13z NBM will appease most of the forum... For a mean at D4.5, I think this is impressive.

12z diagnostic mslp, prate, type, etc... and 12z ensemble spread at forecast hour 132: