A sudden turn of events at the 500mb level of our atmosphere on recent guidance in regards to the Saturday night storm is quickly grabbing our attention in the weather community. Originally, models were showing a potent mid-level trough coming over the area but not producing much in the way of precipitation due to the progressive flow keeping the trough positively/neutrally tilted. The big story of the weekend was going to be the extremely cold temps and wind chills approaching -20*F for the NYC area.
That has changed.
We are still going to see those type of temps. on Sunday into Monday, but now we have to also consider significant snow accumulation for the area Saturday night into Sunday.
EURO Model:
If we take a look at the set-up at H5 we notice a couple of important features. Firstly, the western ridge is collapsing but not before it merges with the cut-off / thumb ridge in northern Canada. This allows positive heights to extend through western/central Canada which helps plunge the next piece of upper air energy into the eastern CONUS.
By Saturday morning the "new" ridge is formed from the merger and the very potent northern stream vort is about to enter the northeast. The +PNA actually plateaus at one point on Saturday instead of continuing on its negative trend, so this is a good indication that the -AO ridge is very real and not some fantasy.The question is...how far south does the H5 vort get? This is where models conflict. Ultimately, the exact position of this upper low will have a huge influence on how this storm transpires at the surface.
The EURO and CMC are the furthest south with the trajectory of the upper level low. In response, the transfer of energy from the surface low that's located over the Great Lakes / Upstate NY is off the southern coast of NJ. The secondary low develops further south and areas around NYC see a sizable snow event. The GFS transfers the energy much further north because it's still keeping the overall flow progressive which inhibits the upper level low from digging further south.
The secondary low that forms off the coast is just one piece to the puzzle. There is a strong signal on guidance of a norlun developing over the area as well. The incredible difference in air temps (warm over the western Atlantic and cold filtering in behind the trough), very potent upper level vorticity, and a mid-level trough turning negative results in instability which leads to a norlun / inverted trough. The convergence of the arctic and tropical air masses brings about 700mb frontogenesis and a narrow band of snow covering an isolated area is bound to develop. These are very tricky to forecast and are often times difficult to pinpoint until within 12-24 hours of the event.
With the GFS being the furthest north with the upper level low's, it does not bring the best H7 VV (vertical velocity - lift) int our area. It's mainly N&E of NYC.
The CMC is entirely different. With the H7 low located south of LI, it's able to bring those VV's crashing into the immediate coast and snowfall rates are coming down at an impressive clip.
The CMC model is also very bullish on bringing the mid-level trough negative, another reason why we're seeing the norlun develop further south. This is beginning to get support from the RGEM and its ensembles, as well as, the EURO, SREFS, and some of the GEFS members. This is why this storm is suddenly turning heads, because this trough was never expected to go negative. Anytime you have a bowling ball H5 vort over the area with a negatively tilted trough that raises eyebrows very quickly. Very quickly...
The GFS is on its way to going negative, but it's not quite there for the NYC Metro area. However, the last few runs have shown it trending in the right direction and it will be extremely interesting tonight to see if it caves to foreign models.
It is difficult for me to say at this time what snow accumulations will be for the NYC Metro area. There will definitely be snow, that I am pretty positive, but how much is the million dollar question. Determining where the secondary surface low develops is step one. Step two will then be to figure out where the norlun sets up. Since both these things are still unknown, it's very tough for anyone to really say otherwise on snow amounts. The POTENTIAL snow amounts could be as high as 12-15" with high ratios expected. The minimum I think will be 2-3". So the scale is anywhere from 2 to as high as 15" of snow. It's just premature for me to actually forecast amounts right now. Gun to my head, I would say 4-8" for NYC metro beginning Saturday night and persisting into Sunday. With the high winds and the falling snow, blizzard conditions can be expected across the area. Blizzard warnings will not be issues unless there is a significant snowfall expected, but that does not mean it will not be "blizzard-like" outside if snow is falling. Whiteout conditions are likely.
If a couple of things are certain that's the high winds and bitter / dangerous cold. Wind gusts on Sunday could reach Tropical Storm strength, and Hurricane force strength along the east-end of LI and the Cape's of New England. I would expect there to be power outages in some locations which would be awful since that could mean no heat for some people. With temps. expected to get down below 0 Sunday night around NYC metro with wind chills between -10*F and -20*F...this calls for a dangerous situation that could possibly develop. I urge you to prepare for the worst even if you do not see much in the way of snow accumulation.
We'll see what happens tonight with the 00z runs! Stay tuned to both forums for all the updates.
Have a great night,
Frank
www.njstrongweatherforum.com
www.usa-wx.com
