wdrag

My final post on this thread and for this evening. The amounts you see where 4+...its going to be 2-3"/hour. May have some TS+ 1/8 Mi. This is the one in 10 chance from Mt Holly, the overall Winter Storm Severity Index - both collaborated at 3P this afternoon, the 00z/3 3K NAM snowfall as a start, and the 00z/3 HRRRX. You want big snow, you go south and get there by 5AM. Should be a 6 hour window of some embedded white out down in s NJ somewhere. I checked 18z/3 NAM soundings. It all looks quite good for all snow after 8AM.

I'm not sure who is saying 2" max there? As a planner, you can know the deterministic and be alert that the forecast may go awry on the heavy side. The SREF and all ensembles on this system have not been wild, only trending northward.

New NAM at least 1" NYC now..starts NYC around 10A-Noon.

Hi... before the 00z/3 NAM says I'm wrong... the SREF has value, especially the SPC SREF. While it can be wrong, especially gradients, better not ignore it when it has these amounts. I'm pretty sure we'll see a stripe of a foot or so in the DelMarva, far s NJ. Sharp lift and strong UVM. So whether it has too much snow on LI/NYC, (and there has to be doubts), I cannot cast aside the SREF (not within 24 hours). Added the 21z SREF for LGA which is now up to 7". This in part driven by several members up near 20". Even so, of the 22 members, only 1 is ZERO. All other members more than 1" there. In my opinion, It's going to snow in NYC---maybe not much but I think we need to be prepared for 4 in NYC and count on one inch, especially if the 00z NAM follows the 00z/3 HRRR north. I could be wrong and I don't like eating humble pie, but tomorrow morning should be fast changing on radar.

Checked 18z GGEM, EC, 15z SREF... not much recession if any... this suggests some modeling is going to be major error in the northern side gradient change between NYC-LI and Toms River. HRRR/HRRRX look a little more like the NAM...s of NYC. Going to be some large errors, especially 12z/18z EC if this ends up like the much further south NAM. May post once more before going to rest, then up early to recheck.

Not threading Jan 7 til i get a handle, either 9P tonight or 6A or 6P Monday.

I'll be offline 3P-830P - someone is welcome to add the WATCHES and WARNINGS map when it finally completes at 5PM. I'll add the winter storm severity index at 9P and we all can update all the modeling-data-observations from that time forward. I suspect our first mPing wintry precip will show up in southern edge around 3A-4A.

A moderate to major 6-12 hour snowstorm will occur Monday, mainly from somewhere within the I95 corridor southward. Snow, sleet, rain or freezing rain should begin between 3 and 6AM in NJ and NYC-LI-s CT around 6A-8A. Any mixed precip will change to all snow soon after beginning. Temperatures may be still be just above freezing at the start, but drop into the mid 20s by 10AM with wind chill down into teens and north-northeast winds gusting 20 to possibly 30 MPH. Drifting snow should develop wherever amounts exceed 4". 12z/2 NAM 900-500MB and 800-600MB banding suggests 1-2"/hour snowfall for a couple of hours during the height of the storm across central NJ and southern-eastern LI. Details of how much and where the sharp cutoff on the northern fringe (NYC), NNJ/s CT/se NYS still to be determined but too many models have slowly shifted northward the past day of cycles to prompt concerns for a significant impact storm. 03z and 09z/2 SPC SREF modeling has broad range of solutions but suggest close to 5" LGA with the blizzard conditions remaining just offshore. Let's keep the modeling-storm discussion in the January thread til after the 9PM (00z/3 NAM cycle has posted through Monday). MPing may be helpful. The 12z/2 ECMWF Kuchera snowfall map has been added per our thanks and appreciation to the ECMWF-Weather.US. This is a single operational member of all the modeling but represents potential... This could shift a bit either direction with time and amounts might end up significantly less in some areas but overall, it seems to have the axis of high impact travel concern fairly well outlined.

Courtesy the EC and Weather.US... I appreciate their data. I have to think the GFS/SREF has been better than most other modeling, in advance...all catching up to the GFS. Northern fringe of the GFS can be argued drier but there is no doubt (in my mind) about a high impact storm very close to NYC. I'll fire up the OBS-nowcast thread... lets leave it alone til about 10PM then have at it...keep all the discussion of the storm here til we get to within 6 hours of starting. Thanks... Good job on getting this thread started and enjoy. Here is one EC op model Kuchera solution from the 12z/2 cycle. It's not what the final solution will be, but I'll believe a big 6-12" snowstorm is coming to s NJ and very close to NYC. If its mixed with sleet, amounts less, but removal still difficult.

I hope our forum members read. I'll rereview and attempt to clarify.

That increase by the way was added between 0830-10z... but 08Z is what helped bias the conservative ensemble approach. It's quite a process with few staff at 330AM to make big changes to the products...

On the above: I'd rewrite 20% chance of less than 1" to 80% chance of greater than 1" Then go from there... Ate least I know to expect snow. The probs we derive are generated from collaborated 3 hourly or sometimes 1 hour snowfall information blended with the National Center. These tend to miss the high end of storms, and I think tend to be conservative but the latter is my opinion only. They are only as good as the modeling we chose to use... Note: I add the 08z NDFD graphic for snowfall. No wonder we are a little behind. Nada Philly area and LI. now the 17z... note big increases including 2"LI and 5" s NJ. I think we had some info at 08z that ddid not get into the NWS NDFD forecast. Hence the thread.

Added the criteria for 12 hour WATCH snowfall. It's verified as an average in the zone-county. THIS I NOT a NWS snowfall forecast map, only answering some of the base criteria for issuing a WATCH. Pleas make sure this is not mis interpreted as a snowfall forecast map. Color coded here you are. Also a link to the NWS snow probabilities and Winter Storm Severity Index is below, as of 10AM. https://www.weather.gov/phi/winter

Also, probably gone unnoticed GFS nothing for the 7th... GGEM snowstorm I95 nwwd. Mondays storm is first up. If the GFS and SREF fail too high or the NWS ensemble blend and NAM are too low. There modelers will argue sensitivity to the interactions...they are correct.

Criteria. My guess still 6 in 12 and 8 in 24. and/OR combined PUBLIC IMPACT. If I was in the drivers seat with more internal info and collaborating across the NWS borders, I'd argue for a watch = brief high impact potential in the 8A-4P time frame. It's a watch. Rush hour impact or residuals of and clearly there is big bust potential. Can't be afraid to be wrong. Preparation is critical for the infrastructure, already impacted by Covid. They can use the probs to help prep folks. If it misses south, the guidance is there to support the decision making. I add the possibly biased high SREF chc of 1"/hr. Use the legend for prob. I checked blizzard conditions and those are just 20 mi offshore of LI and the NJ coast at 18z Monday. By the way this graphic data is probably not used in NWS... i still it valuable for me from the old school days which for me still work well, with the exception of not having the improved details of modeled soundings and snow growth. Graphic added 1209P

I sort of do too... and NAM idea of dry layers near 850 MB looks good NYC, but this system is showing better 500MB separation and tending to go negative as it whips out to the se of Cape Cod. I do not like the SREF being so abundant this far north (especially SPC SREF plumes). It won't take much for big snow growth and I expect we will see 1-2"/hr snowfall rates for a couple of hours mid late morning somewhere very close to Toms River... may still be south of there but I think now is not the time to hold onto a snowfree forecast. Folks need to be aware of potential and if it misses south of NYC... excellent. I just don't think our modeling is that good to be so determinant that nothing will happen, in the face of the ensembles, GFS/ SPC SREF plumes etc. Mt Holly made the right move, 6 hours late but enough to prep their folks. Going to be quite a storm for 6-12 hours Baltimore area to ACY and probably a bit further nw.

Soooooo much variability. I think a watch will be needed for a portion of NYC subforum...up to and including Monmouth County to eastern LI. A watch is not a warning but it signifies potency and be alert. NWS can describe the options in the details. Watches should verify about 50% of the time, probably higher verification in shorter lead time. These graphics from the SPC SREF are surprising to me but not so for the GFS. Wow! Both the 03z and 09z have mean some where near 5" LGA. This is up from 0.2" 15z cycle yesterday and 2" in the 21z cycle yesterday. Big banding signal for for S NJ. NYC best time for a burst of snow seems 10A-1P.

SPC HREF from the 12z cycle... further north and heavier. Close call NYC.

GFS positive snow depth change: sure involves NYC and 1 foot amounts in the axis. Going to be an amazing GFS coup if this works out. NAM, ensembles, SPC HREF will be unbelievably defeated. Soon we will know. I still think lots of mixed precip DCA-Cape May or Dover DE. Thicknesses are still warm at the start and sounding still yet to be subfreezing. It will change rapidly shortly after precip begins. SPC HREF will be far more conservative than the GFS on the northern side of the storm.

Mt Holly finally issued a WSA for the southern part of their area. It will show up soon.

All this modeling needs to be published and allow reality to verify. I may not be posting 3P-8P. Will review at 930PM and possibly start an OBS thread at that time if we are looking at more than a trace LI central NJ. If these ensembles BUST now (thanks for hotdog), then NWS will need to review the mix. Seeing less than a 30% chance of 1" at Toms River is telling in the 14z/2 NWS ensembles. EPS amounts can be biased high because 10 to 1 ratios, even mixed wintry. Where I think ensembles tend to fail is along the gradient edge, (go either way) and in the core, not high enough probs. However, modelers will tell you it's a matter of dispersion of solutions. Warrants further conversation. In the meantime, I hope we have a better system than this for the rest of the winter.

So, 12z NAM, HRRR, HRRRX matches more conservative forecast as only flurries or no snow NYC N and W, but e LI and central NJ Toms River south near the sharp cutoff. Also, some modeling is showing heavy snow near DC-Baltimore. Caution---sleet mixed in may but back amounts but still result in treacherous travel there. I think the further s you go in the axis, the greater the chance of mixed qpf. Added a few graphics: 1) 09z/2 SREF too abundant too far north and not enough mix south to limit amounts. 2) 12z HRRRX amount through 2PM. 3) NWS 14z /2 ensemble chance of 4" Note how conservative - that is because I think it models mixed types better. 4) NWS 14z/2 prob for 1" VERY difficult for NYC to see an inch, maybe all of LI too. I do not know but I lean drier and lower amounts in our area. That does not mean I'm correct but NWS conservative posture, barring 12z GGEM/EC probably works for our subforum with advisory conditions possible e LI snd central NJ. Further it's a mess where being on the road in the morning is not where I'd want to be.

Will add the following: Potential 6-12 hour snowstorm even close to NYC 4A-4P Monday... here is some guidance that is conflicting but warrants concern for a potential 6+" event for parts of e LI, central NJ titrating down to flurries I84. Exactly what modeling is best... your call. We won't know for sure til Monday night which modeling has been most accurate in the longer and shorter range. NWS ensemble at 09z for >1" is still quite conservative NYC as is the SPC SREF. The HRRRX is also conservative but the normal SREF, last image is bullish. Click for more detail including legend for amounts. There may be a little melting at the start, otherwise, where it snows, it should become quite slippery on all surfaces. Driving possible e LI and central NJ coast if the heavier amounts occur.

No thread for Monday. The sounding provided by eduggs is helpful to illustrate a potential GFS problem. Nevertheless, keeping an eye upon. 18z EC is a touch north as is GGEM (both shown below). However NDFD (NWS forecast digital database) is basically nil, as is the prob for >1" (shown below). Will rereview at 7AM Sunday but no thread, at least for now. 22z/1 WPC chance of >1" of snow continues receding southward Monday. I know a lot of folks are talking big hit in mid Atlantic but this does not imply that at all. It defies GFS, GGEM and EC. So unless it's mixed precip and or melting at 33-34F, it seems that some of these stronger hitting models are going to be too heavy on snow, or the WPC ensemble is far far too conservative.

Okay, The following are multiple cycles of the EC OP Kuchera for our area. Consistent that something will happen in the northeast. Where it rains and snows, definitely uncertain and I've not yet looked at EPS. But regarding what the modeling has shown... whether it's important? To me it is of value and validates expressing some interest. The time of the cycle and it all ends at 12z/8 except I think inadvertently ended one cycle at 00z/8. There is one cycle missing since it as basically a null event down here se of Ithaca. Also again: My thanks to the ECMWF and Weather.US for these graphics.