March 18th-19th Possible IVT Snow Event

CPcantmeasuresnow wrote:

amugs wrote:SREFS!!!!

Oh boy, this may be a sleepless night or two worthy. I need some sleep, exhausted form the last storm that I refuse to call by name. One of the many dumb ideas brought to you by TWC.

agreed naming of winter storms is dumb. Theres,way to many to remember. It should only b for hurricanes.

Grselig wrote:Can we name it "Rocky"

No its go b jman da man! Lol can u tell I been have some bubbly lol I hurt my foot and my knee shoveling ice blocks of snow and tripped on uneven snow alcohol and tylenol and hot bath didn't even work ugg.

WOW! 4 km NAM for the weekend shows a Godzilla for eastern LI. Not sure if it gets that much but like I said earlier, I think eastern LI gets the goods Saturday!

Don't count on this snow map to be the bible. It is very tricky this far out to see exactly how the upper systems cut off. I think we all know how they can change within 24 hours of the event

WeatherBob wrote:Don't count on this snow map to be the bible.  It is very tricky this far out to see exactly how the upper systems cut off.  I think we all know how they can change within 24 hours of the event

Yes, like I said, this snow map is very unlikely to verify. However, I have seen in the past with this type of setup eastern LI doing well. Recent example: 3/28/15

Six hours. Not more than six hours before an event can I allow myself to believe anything. I'm too jarred from the lack of model consistency and countless busts/booms. I have no trust! It is cool to see most of our snow "chances" landing in March of all months, tho! Happy late-season tracking. :-)

Math, do you have the records for the upper level maps for this storm? It would be cool to see what they looked like.

Here is my First Call for this event:

Start: 1-4am Saturday far-west, 6-10pm far-east
End: 12-4am Sunday far-west, 7-11pm far-east



Unfortunately, I do not have the energy to do a video worth watching right now, so I am just going to write my thoughts out quickly. This setup looks very similar to the November 19th system we had, where areas north and west of the coastal plain ended up receiving nearly 8" of snow. That event featured a very deep negatively tilted trough that continued to amplify as it headed east-southeastward through the Midwest and into the Northeast. Unlike this event, however, that featured conditions suitable for slantwise convection and CSI (conditional symmetric instability), which allowed snowfall rates to reach upwards of 2"/hr in some areas. Although our current system will also feature a negatively tilted trough diving east-southeastward from the Midwest into the Northeast, this one is not really amplifying; rather, it is maintaining its strength as it traverses the region. Both systems have roughly equivalent PWAT values associated with them, but this one will feature the redevelopment of a secondary coastal low off of the Delmarva or Virginia coastline. This is what is leading to the likely development of an inverted trough (IVT), as rising air being forced by the lower-level coastal circulation begins to encounter the stronger forcing for ascent created by the mid-level (H7 and H5) lows trailing behind. Frank has already done a nice writeup on this, so there is no need to restate everything.

The reasoning behind this map is multi-faceted. First of all, upon my analysis of the Northern Hemispheric and tropical forcing mechanisms, I noted the following: +AO, +NAO, -WPO, +EPO, weak 50/50 trough, and no coherent MJO forcing. Using the teleconnections for March, the -WPO/+AO conditions largely offset each other, with no significant signal attributed to the phases of the +NAO and +EPO in March. However, with the presence of the weak 50/50 trough (really a lagging front) factored in with all of this, I can see a little room for the digging trough to trend a little further south in subsequent runs. This is for two reasons. The first is because of the confluence created by the 50/50 trough and amplifying mini-ridge ahead of the digging Midwestern trough, which should allow this trough to dig a little further and not be inhibited by other unfavorable teleconnections. The other is because I took into this season's tendency for troughs entering the Northeast to deepen as they do so, with a deep trough also centered just offshore of the Pacific Northwest/British Columbia. As you can see, my map reflects a further south and colder scenario than most of today's guidance (save the RGEM and NAM suites, as well as the CMC and UK, as those make up a fair portion of my forecast blend). Regarding totals, I do not feel confident in anything that is being advertised to be over 6" at this time. Even though we have a negatively tilted trough coming into the Northeast and PWAT values of between .4-.8" region-wide, we are lacking ADEQUATE forcing. Isentropic lift is not very strong, even Saturday night into Sunday morning. We are also lacking any jet dynamics to aid in general vertical motion. This means that we will be relying almost exclusively on the ability of the evolution of the low-level circulations to transport enough moisture and create enough low-level vertical motion such that they can then realize the forcing created by the negatively tilted and closed H7 low digging to our southwest and H5 vorticity advection as that low also digs to our southwest. Now, because they are both digging to our southwest and closed off, that should put us in a more favorable zone for ascent through the mid-levels, as that will aid in enhancing mid-level frontogenetical forcing some, but the lack of any jet and decent isentropic lift will limit how much of the PWAT values can be precipitated out. I went with a general 50%, since forcing for ascent will only be occurring through about that much of the atmospheric column (remember, PWAT values are the amount of liquid that would fall if all of the water vapor from the entire column was precipitated out), and assumed approximately 12:1 snow ratios given forecast thermal profiles. This supports my max totals well. Also, I am anticipating some loss to melting along the coastal plain, where precipitation is seemingly going to be enhanced. I feel a general swath of 3-6" is a good place to be given the extreme negative tilt of the troughs early on to the northwest, which should help to squeeze out a little more of that .4" PWAT value given the stronger mid-level frontogenesis it creates and higher snowfall ratios, while further southeast the trough takes on a neutral tilt which will limit the mid-level frontogenesis, have lower snowfall ratios, but have more moisture to work with. I am hoping to be able to release a final call either tomorrow morning or even as late as Saturday morning because of my work schedule, but these are my preliminary thoughts for now.

rb, that is a great explanation of the whole set up. However, if the system cuts off a bit farther south and a little west, snow could shift farther south. I would have to wait till tmrw nights runs to find any trends from the new models coming out now.

WeatherBob wrote:Math, do you have the records for the upper level maps for this storm?  It would be cool to see what they looked like.    

I did not take into account upper levels maps, but I provided a recent example of Suffolk County getting the goods.

But here is the 500 mb map valid 8 PM 3/28/15:

http://www.spc.noaa.gov/cgi-bin-spc/getuadata.pl?MyDate1=150329&Time1=00&MyDate2=&Time2=12&align=V&Levels=500

Also, here were the snow totals from the event:

http://mesonet.agron.iastate.edu/wx/afos/p.php?pil=PNSOKX&e=201503291406

Thanks Math. I do think this upcoming 500 mb upper low does cut off to our south, the storm you are showing has the 500 mb low starting to cut off to our north. So, just based on that, it does look like there is a better than likely chance of getting more than that storm. Have to see where the trends go now. The upper level disturbance is currently over the montana/canada border, you can hardly see it on the upper level maps.

UPTON! (I cannot lol)...

"Interesting Norlun Trough event as closed upper low dives out
of the Great Lakes to the Mid-Atlantic Coast on Saturday - some
21Z SREF Members in excess of 6" (8 or 26 for eastern Long
Island and 5 of 26 in the City). 00Z NAM is rather impressive -
perhaps this is Long Island`s turn? Winds in SREF have potential
for 40 KT with heavy snow...hummm."

WeatherBob wrote:rb, that is a great explanation of the whole set up.  However, if the system cuts off a bit farther south and a little west, snow could shift farther south.  I would have to wait till tmrw nights runs to find any trends from the new models coming out now.

Thanks, Bob!! I am planning on that happening, which is why my snow doesn't extend as far north as most guidance has. I think the edge of the band ends further southwest, but the band of snow itself also narrower because of warm air advection. Even though the mid-level lows will be southwest of us, that still leaves us in the "warm sector". So, it will be the battle between the warm advection and the low-level cold air drainage. I think south of I-78 (ish) the warm air wins, but north of there the cold drainage will hold out and eventually win out, as it typically does in more typical southwest flow events east of the Delaware River. I blended the storm types of a typical southwest flow event and an elevation snow to come up with that map, in addition to the other factors mentioned. Does that maybe clarify it a bit better?

Yes, rb. But the evolution and ejection of the current upper level system can be off by say, 20 miles, maybe 30 miles from the present model runs since the upper level system is very small and weak right now. This difference can cause a considerable shift in position downstream off the coast. If the upper level was more developed, I would have a higher confidence in where the center will move to.

14.1, 86%, 29.87R

NWS went from 1 1/2 inches for me with a mix Saturday, to all snow 3 to 7.Accuweather has me 2 to 4.

nws snow map one last hurrah for winter!

Nws has me for 2 to 4 inches while AccuWeather has me for 1 to 3 I want more lol

Could be a nice little event, 3-6 would be a nice way to replenish the snow pack

rb924119 wrote:Here is my First Call for this event:

Start: 1-4am Saturday far-west, 6-10pm far-east
End: 12-4am Sunday far-west, 7-11pm far-east

Are these start/end times correct?  24 hr duration?

Before leaving for the airport I decided to make a quick map. No time to provide analysis, but norlun events are tricky to forecast so the dark shade of blue could easily shift south into NYC, all of LI, and central NJ tomorrow. Gotta keep a close eye on it.

gigs68 wrote:

rb924119 wrote:Here is my First Call for this event:

Start: 1-4am Saturday far-west, 6-10pm far-east
End: 12-4am Sunday far-west, 7-11pm far-east

Are these start/end times correct?  24 hr duration?

Give or take, yes. But it will largely be light to occasionally moderate precipitation.

WeatherBob wrote:Yes, rb.  But the evolution and ejection of the current upper level system can be off by say, 20 miles, maybe 30 miles from the present model runs since the upper level system is very small and weak right now.  This difference can cause a considerable shift in position downstream off the coast.  If the upper level was more developed, I would have a higher confidence in where the center will move to.

Oh absolutely it can, and I'm banking on the models shifting a little further south than they were, which is why my map looks like it does. But if they don't, or the system shifts too far, then everything changes again ahaha

Frank_Wx wrote:Before leaving for the airport I decided to make a quick map. No time to provide analysis, but norlun events are tricky to forecast so the dark shade of blue could easily shift south into NYC, all of LI, and central NJ tomorrow. Gotta keep a close eye on it.

Nice map, Frank! I was also considering "bubbling" my 3-6" zone around the city, but figured I could edit details like that in a final update haha All in all, we are pretty similar, with yours just a little further northeast than mine.

so far what Mt Holly thinks. they've come some ways because they had about 2" earlier yesterday lol

All of the Nam models show three to six inches of snow for the tri-state area with the 3K and am being the most bullish of six to 10. The heaviest precip comes late Saturday evening into early Sunday morning
Edit the 3K NAM actually stalls the system east of Long Island and it snows till Midday Sunday 8 - 14 inches especially for Long Island

algae888 wrote:All of the Nam models show three to six inches of snow for the tri-state area with the 3K and am being the most bullish of six to 10. The heaviest precip comes late Saturday evening into early Sunday morning
Edit the 3K NAM actually stalls the system east of Long Island and it snows till Midday Sunday 8 - 14 inches especially for Long Island

What!? No way

Rgem is 3 to 5 inches of snow for the area surprisingly Long Island is not in that I think that's wrong