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2/13 Significant/Major Winter Storm Discussion & Observations


Northof78
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1 hour ago, NEG NAO said:

So now at 0Z so far its the Ukie/Nam/Canadian VS. GFS/ICON/EURO  didn't look at Ensembles

I am still going with slushy couple of inches near the coast more inland - I would expect winter weather advisories having a chance to be issued late Monday for Tuesday in NYC metro

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4 minutes ago, LibertyBell said:

a coating to 2 inches seems like a safe bet.

Honestly I would be happy with 1 inch.

 

Only way this is more is if the storm slows down and passes off the Del Marva and is stronger - like some of the solutions from a couple days ago....along with the current NAM

namconus_ref_frzn_us_44.png

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Well, the last holdout for significant snow for areas south of 78 and along 95 from Philly to NYC, the NAM, has caved to the rest of the guidance, with much less snow south of 78 at 6Z vs. 0Z. The 6" line is now along 78 insted of from Philly to Pt. Pleasant. Oh well. The 0Z CMC still had 6" along and N of 276/195, but it was much warmer than the NAM at the surface, so ratios would be far worse. Would be nice to not have this keep moving warmer and wetter so that those of us along 95 might still get the 1-3" shown by the NBM. I'll be bold and go with 2" for my house in Metuchen vs. 2.5" on the NBM and 1.5" from the NWS.

Image

Image

snowfall_acc-imp.us_state_ne_s.png

 

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No solid insight on when modeling goes into the NBM, except when this runs (19z,01z etc), I dont think it has the latest instantaneous modeling... example 19z cycle.  I doubt that it has the 18z model run, nor the 18z ensemble. 

I was stunned by the sudden upward shift in the 19z NBM yesterday as well as the post of the NWS probs for 4" etc.  It didnt make sense. I can only think it incorporated some of the SREF which was broader snowfall for NJ.  This is where we have to take this stuff with a grain of salt since I don't know exactly the percentages of modeling and statistical temp guidance that drops into each NBM cycle. I'll bet most on here don't know about LAMP stats. 

So... I think something was wrong with the 19-22Z/10 NWS probs for 1,4"  and NBM amount guidance since it briefly radically shifted. 

My confidence in the EPS snowfall forecast on the warm side gradient of the event continues below average... This event or non event will tell us more.  

I definitely would not use 10 to 1 ratios on the warm side of the axis... it doesn't recognize ptype or sweeps it over a 3 -6 hr qpf. 

I see the GEFS +snowdepth change continues meager... if this verifies again, then I think everyone has to show more appreciation for this guidance.  Event has yet to occur so I won't badger on my concerns but I'm a Canadian has to have it (not the 10-1 excess on the warm side gradient) and the GEFS +snow depth change has to be used as strong consideration of what we expect. 

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2 hours ago, RU848789 said:

Maybe if I stay awake and keep posting good things will keep happening, lol. the 6Z GFS just came out snowier than 0Z, especially for areas between 195 and 78, including NYC. Maybe the "trend" north has stopped.

sn10_acc-imp.us_state_ne_s.png

 

sn10_acc-imp.us_state_ne_s.png

why does the GFS have a very strange zig zag rain snow line along the south shore lol?

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NWS ensemble guidance probs for 1,4,8", the NWS overall 5AM deterministic forecast which went into the swath of watches, and the 07z/11 NBM which looks like a reasonable baseline for me and where I started with a forecast elsewhere.

Please be careful about relying on 10 to 1 snowfall on the warm side of the max axis... it's fraught with ptype-qpf-ice pellet conversion issues. 

I'm even concerned about getting 5" of snow up here in Wantage of extreme nw NJ.  

I'll check back late today...

Screen Shot 2024-02-11 at 5.38.23 AM.png

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Screen Shot 2024-02-11 at 5.38.59 AM.png

Screen Shot 2024-02-11 at 5.40.34 AM.png

Screen Shot 2024-02-11 at 5.41.38 AM.png

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9 minutes ago, MJO812 said:

No one is using those maps

Check out the SNE thread. 

 

Those maps don't tell the whole story right now - I am still going with the 2 slushy inches NYC immediate metro and more inland which I have been saying for days now BUT the metro is still sitting right on the fence and  can still fall off on either side - situation is too close for comfort in either direction IMO relying on some guidance that can be unreliable at times IMO...........especially 2 days out

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12 hours ago, donsutherland1 said:

The 0z NBE has increased forecast snowfall to 3.7" in NYC, even as it increased its estimated low temperature to 37. Only a single case exists with such a snowfall and a low temperature of 35 or above. Either the NBE will reduce the estimated low temperature or future cycles will reduce snowfall amounts. I suspect it will be the latter, as occurred with the GFS MOS's moving from an estimated 4"-6" to trace to 2".

The 2/11 12z NBE has slashed its forecast snowfall amount to 1.7". It maintains a low temperature of 37.

image.png.34348d53b1ad3107d006b30e9695a876.png

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10 minutes ago, NEG NAO said:

Those maps don't tell the whole story right now - I am still going with the 2 slushy inches NYC immediate metro and more inland which I have been saying for days now BUT the metro is still sitting right on the fence and  can still fall off on either side - situation is too close for comfort in either direction IMO relying on some guidance that can be unreliable at times IMO...........especially 2 days out

Agree

I like a dusting to inch on colder surfaces here but hopefully we get a surprise .

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On 2/10/2024 at 8:07 AM, qg_omega said:

Looks like rain to white rain for the coast, maybe an inch or two at the end on the grass.  Would be great storm if we had a normal February airmass

Mostly white rain, temps in the mid to upper 30s lol

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Now that we have a moment: Pivotal weather users may have noticed the HRRR now has 

Model snowfall!!   Total Snowfall: Model Ratio (in).  That is a big deal. 

Please read how snowfall is produced.  Model Ratio, Kuchera and Positive snowdepth change  are the recommended with 10 to 1 last. 

It's a long but worthy read while we wait for models. I just took a snapshot but Kuchera is described as well, though not appended here. 

-----

On the other hand, the snow depth variable does attempt to account for melting, compacting, and sublimation on a representative ground surface, and is even able to take advantage of minute-to-minute changes in the soil model state while doing so. So, in that regard, it can be more useful for estimating the ground accumulation at the end of a snowstorm than our 10:1 and Kuchera snowfall products. Still, this benefit is offset by the substantial pitfalls of using very imprecise SLRs and typically treating sleet as snow.

Conceptually, users should realize the snow depth variable is just a byproduct of internal model considerations around surface fluxes; this is a domain of physics where the precise snow depth may not be quite as crucial as the total mass of frozen precipitation covering the ground. As such, using model snow depth to forecast snowfall is subject to caveats and errors that are of similar magnitude to 10:1 or Kuchera, and it may perform even worse in some situations!

Explicit model snowfall

The HRRRv4 and RAPv5 (implemented at NCEP in December 2020) began providing explicit forecasts of snowfall; to our knowledge, this is a first in mainstream operational NWP. These forecasts can be found on Pivotal Weather as the “Total Accumulated Snowfall” parameter. Although the HRRR/RAP use a very simplistic SLR that is a strict function of 2-m AGL temperature, they are able to assess SLR and melting at every model time step, which affords far greater temporal precision than our post-processed products (e.g., Kuchera).

In addition, the NWS National Blend of Models applies relatively sophisticated SLRs to each input model’s QPF. Although NWS NBM precipitation forecasts are essentially post-processed ensemble means, and therefore may tend toward smoothing out maxima in forecasts beyond the first 12-24 hours, the NBM’s SLR approach is more advanced than any individual NWP model on Pivotal Weather.

Summary and practical recommendations

  1. Our snowfall products generally attempt to forecast the snow that falls to the surface; not necessarily the snow pile you see on the grass, interstate, your rooftop, or anywhere else after a long storm. There are some caveats with Kuchera (penalizes warm temperatures in part to account for on-ground melting) and accumulated positive depth change (explicitly accounts for melting, albeit with model data file frequency as a confounding factor) — but none of these products will consistently provide an accurate forecast of final ruler-measured snow depth, even if the model’s QPF and vertical profile are spot on!

  2. We recommend using the Kuchera snowfall products in most situations. Kuchera snowfall is certainly imperfect, but from our perspective, it is the least flawed practical option on the table right now for most models.

  3. Model snow depth can also be quite useful if your main forecast problem is the final ruler measurement on a natural surface, especially for events where melting is a major concern. This product sometimes includes sleet, however, and the SLRs it uses implicitly are probably less accurate than Kuchera much of the time.

  4. The 10:1 ratio snowfall products exist primarily as a very conservative estimate for legacy purposes, and because they are painless to compute. For certain datasets, we may not be able to compute Kuchera, leaving 10:1 as the only practical option. In the future, we may consider removing some or all 10:1 products, but they still may have value to some users as a baseline approach that is easy to compare between all datasets.

  5. In our view, the best path forward toward more accurate and less confusing NWP snow forecasts is for modeling centers to track snowfall internally during integration, rather than just liquid equivalent frozen hydrometeors. The current situation leaves it to end users like us to apply SLRs (and, in some cases, infer precipitation type) based on limited and temporally sparse data. Even a simplistic in-model SLR algorithm estimating the expected crystal type and riming (with the benefit of full-grid data and microphysics parameters), applied much more frequently during integration than publicly available data files, may yield drastically better snow forecasts than today’s. ADDENDUM: the RAP and HRRR have started providing explicit snowfall forecasts as of December 2020, so hopefully more models will follow suit in the near future!

 

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