December 19-20 Talking Points - Part 3

[earthlight]

Over the past few days we have seen several jumps on forecast guidance(trends as some would call them, although the only real trend I think we've seen has been west with the Polar Vortex) which have significantly impacted the direction of the forecast this weekend. This post is actually going to draw on the features which were originally on the drawing board five days ago, which actually is a testament to the fact that aside from the PV, the models have been pretty good with the general idea of this pattern. That being said, the fragility of the setup has led to vastly different solutions every few runs.

The image below is today's 12z ECMWF's 48 hour forecast. You can see the major players here. First, the shortwave over the Central US. This shortwave is broad but it's actually pretty deep as far as heights go. Second, you can see the all important shortwave diving southeast through the Plains. This feature is actually imperative in amplifying the trough itself and is the energy that drives the surface low development. It should be noted that this shortwave was sampled at 12z, but not very well. The data region was pretty sparse. At the time of initialization, the shortwave over the southwest US was just barely offshore--but probably got sampled decently well. The setup as you can see has trended extremely favorably for the development of a strong cyclone off the east coast. The Polar Vortex has moved very far west on most of the guidance, and the shortwave over the Plains is forecast to drop southeast very rapidly into the base of the trough which is already neutral tilted near the MS River.

Although we are still seeing varying solutions, the main reason for them (other than the shortwave itself) is the handling of the PV. For example, take a look at today's 18z NAM compared to it's 12z run over the Great Lakes. There is an area of increased confluence this run which de-amplifies the height field and causes the solution to be a bit further east. These are features we will have to watch very carefully over the next 24 hours or so.

The question has to be asked at this point--how confident should we be in this thing coming back west? I say we have certainly gained some significant confidence over the past 12 hours. The 18z NAM is really the only model at the moment showing a no-go solution. The GFS brings 0.25" liquid back to Northeast New Jersey--and that was after a very large correction to the west.

The Euro has now had the storm just right, and just east of the benchmark on the last two runs respectively. With it's ensembles falling into a similar track, I think it's time we begin recognizing this as "increasing probability of a major snowstorm". The Euro's superior resolution and 4DVAR initialization scheme only add to this idea. We still have a ton of time to watch, discuss and analyze.

[pazzo83]

Seems like Upton isn't biting just yet:

Long term /Saturday through Wednesday/... Sat through Monday... changes to this portion of the forecast weren't significant due to model discrepancy in handling a coastal storm that threatens to impact the area. The European model (ecmwf)...which had been much more consistent vs. The GFS...had shown a less amplified and more progressive pattern...taking the storm well out to sea after emerging from the southeast states. The 00z/16 European model (ecmwf) now closes an 500 mb low near the 40/70 benchmark Monday morning...drawing the storm over Cape Cod by Monday evening. This solution is a western outlier...and is probably over strengthening southern stream shortwave energy...although its origins are not yet adequately sampled just yet. The GFS meanwhile has trended towards the previous European model (ecmwf) runs with a flatter flow aloft and a surface low track far enough away such that we get no precipitation over the County Warning Area. The gefs mean has also shifted much farther east and depicts a similar storm track to that of the operational 00z/16 GFS run. Although overall confidence is still on the low side...there remains a threat for a significant storm to impact the tri state area. Saturday should be dry with colder than normal high temperatures. For now...kept probability of precipitation no higher than chance category from late Saturday night through Monday. In spite of a lack of confluence and high pressure to the north locking in cold air...thermal profiles and forecast track of the storm (east of 40/70) favors an all-snow event. Too early and too much uncertainty to get too detailed with the amount and placement of the heaviest precipitation...or determining where mixed precipitation/rainfall could occur.

[SnowGoose69]

Thats still the older Upton discussion though, with the 12Z Euro doing the same I'd like to see what they say, I would imagine not much different since still this is very on the fence as far as a hit.

[Kaner587]

Seems like Upton isn't biting just yet:

that is before the 12z euro so it was probably prudent for them not to be a little more bullish...if 18z GFS comes even further west expect their next update to be more bullish

[SACRUS]

nice discussion earthlite,

Just to recap

12z (12/16)

NYC QPF

GFS: 0.25 - 0.50

GGEM : 0.50 - 0.75

ECM : 1.5 - 1.75"

UK : looked OTS

GFS ENSEMBLES : Mean was west of OP 0.75 (+)

GGEM ENSEMBLES : Mean was East of Op

ECM ENSEMBLES : mean was in line with OP

[Kaner587]

that is old

that is their 12pm discussion

check their 4:03 discussion

lol their new one is actually QUITE bullish

[pazzo83]

Yea just saw that, my bad.

[supermeh]

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...

-- Changed Discussion --MODELS HAVE COME INTO MUCH BETTER AGREEMENT ON THE MEDIUM TO LONG

RANGE. FOR NOW HAVE GONE WITH A BLEND OF THE 00Z ECMWF/12Z GFS/12Z

CMC GLOBAL THROUGH MONDAY...THEN REPLACED THE 00Z ECMWF WITH THE

12Z ECMWF FROM MONDAY NIGHT ON. THE 00Z ECMWF DEVELOPS A WARM CORE

AT 850 HPA WITH ITS LOW BY SUNDAY NIGHT...APPARENTLY DUE TO

CONVECTIVE FEEDBACK PROBLEMS...SO ITS LOW IS PROBABLY A TAD ON THE

STRONG SIDE...AND A TAD TOO FAR WEST...IN ADDITION TO ITS

QUESTIONABLE THERMAL FIELDS...SO PREFERRED NOT TO USE IT DURING

THE CORE OF THE POTENTIAL STORM FOR EARLY NEXT WEEK.

THE CONSENSUS SUGGESTS A POTENTIALLY SIGNIFICANT SNOW STORM FROM

LATE SATURDAY NIGHT INTO MONDAY...ESPECIALLY FROM NYC EAST. DO HAVE

SOME CONCERN THAT THE MODELS MIGHT HAVE TRENDED TOO FAR WEST...SO

DO NOT WANT TO PUT THE WESTERN 1/3 OF THE CWA IN THE MAIN THREAT

AREA QUITE YET...THOUGH IF CURRENT TRENDS HOLD...THAT IS STILL A

POSSIBILITY.

IT IS QUITE LIKELY THAT LOW PRESSURE WILL BE DEVELOPING OF THE

CAROLINA COAST BY SUNDAY MORNING IN RESPONSE TO A 500 HPA TROUGH

DIGGING INTO THE SOUTHERN MID-ATLANTIC STATES...THE ENERGY DRIVING

THIS SHOULD REACH THE PACIFIC NORTHWEST TONIGHT. THIS LOW THEN

TRACKS NEAR THE 40/70 BENCHMARK LATE SUNDAY NIGHT...AND TO AROUND

CAPE COD BY LATE MONDAY.

THE MODELS NOW SUGGEST THAT A WEAK HIGH WILL BE TO THE

NORTH...HELPING TO TAP INTO RE-ENFORCING COLD AIR...WITH A MAINLY

N TO NW WIND FLOW WITH THIS STORM...AS A RESULT...ONCE AGAIN

DISCARDING THE 12Z ECMWF...AM EXPECTING ALL SNOW AT THIS TIME. IF

MIXED PRECIPITATION WERE TO OCCUR THOUGH IT WOULD BE OVER THE TWIN

FORKS AND POSSIBLY SE CT.

IN ADDITION TO THE SNOW...STRONG WINDS ARE LIKELY...FOR NOW

HOLDING TO NEAR OR BELOW ADVISORY STRENGTH...BUT THERE EXACT

STRENGTH IS STILL TO BE DETERMINED.

BASED ON THE ABOVE...WOULD EXPECT SNOW TO DEVELOP FROM S TO N LATE

SATURDAY NIGHT...AND BECOME HEAVY AT TIMES...ESPECIALLY FROM NYC

EAST...SUNDAY AFTERNOON/EVENING...THEN GRADUALLY TAPER OFF MONDAY

FROM SW TO NE. THE HWO WILL BE UPDATED TO REFLECT THIS THINKING.

DEPENDING ON THE EXACT TRACK/STRENGTH OF THE STORM...THERE IS SOME

SUGGESTION THAT THE DEFORMATION AXIS ON THE BACKSIDE OF THE STORM

COULD LINGER OVER THE AREA...WITH LIGHT SNOWS CONTINUING POSSIBLY

INTO TUESDAY. HAVE SLIGHT CHANCE POPS MONDAY NIGHT/TUESDAY TO

REFLECT THIS.

[pazzo83]

Still only calling for 40% chance of snow here in the city.

[bluehens]

Boy do I hope the 18z GFS at least holds serve......any sort of sizable move east on it may crash the server.

[LVblizzard]

Based on that discussion, Upton is throwing out the 12z Euro but it's much better than the older discussion that was posted.

[Kaner587]

Based on that discussion, Upton is throwing out the 12z Euro but it's much better than the older discussion that was posted.

they aren't throwing it out they are just correcting for the biases they believe it has. If they threw it out I doubt they'd be as bullish

[Joe4alb]

Still only calling for 40% chance of snow here in the city.

remember there are certain guidelines they have to follow when using percentages when talking about events that are still several days away. A situation could be 100% and you will still see a 40 - 50% on an official forecast, don't get discouraged.

[KEITH L.I]

Boy do I hope the 18z GFS at least holds serve......any sort of sizable move east on it may crash the server.

18z model runs in the past have been very unreliable..I have seen them lose storms and have it come back at 00z...I have also seen them make bombs and lose it 6 hours later..really the next big cycle is the 0z runs tonight

[Stormlover74]

That's why I was so surprised to see Mt Holly going with 80-90% 3 days out

remember there are certain guidelines they have to follow when using percentages when talking about events that are still several days away. A situation could be 100% and you will still see a 40 - 50% on an official forecast, don't get discouraged.

[SACRUS]

Boy do I hope the 18z GFS at least holds serve......any sort of sizable move east on it may crash the server.

It wuold be nice to have some conistency on all of the guidance. What a challenging storm this one is turningo out to be. 2010 ending on an exciting note.

[Joe4alb]

That's why I was so surprised to see Mt Holly going with 80-90% 3 days out

i believe that is against protocol if I remember correctly. When I was forecasting in SUNY Albany 40% 3 days out was considered a max.

[yhbrooklyn]

they aren't throwing it out they are just correcting for the biases they believe it has. If they threw it out I doubt they'd be as bullish

It's not really bullish for the NYC area.

THE CONSENSUS SUGGESTS A POTENTIALLY SIGNIFICANT SNOW STORM FROM

LATE SATURDAY NIGHT INTO MONDAY...ESPECIALLY FROM NYC EAST. DO HAVE

SOME CONCERN THAT THE MODELS MIGHT HAVE TRENDED TOO FAR WEST...SO

DO NOT WANT TO PUT THE WESTERN 1/3 OF THE CWA IN THE MAIN THREAT

AREA QUITE YET.

[bluehens]

I agree wholeheartedly but that still doesn't mean that many people won't be lining up on the edge of the building.

18z model runs in the past have been very unreliable..I have seen them lose storms and have it come back at 00z...I have also seen them make bombs and lose it 6 hours later..really the next big cycle is the 0z runs tonight

[KEITH L.I]

I agree wholeheartedly but that still doesn't mean that many people won't be lining up on the edge of the building.

It's also very good to have Euro on your side within 72 hours..that's a givin!

[Kaner587]

It's not really bullish for the NYC area.

THE CONSENSUS SUGGESTS A POTENTIALLY SIGNIFICANT SNOW STORM FROM

LATE SATURDAY NIGHT INTO MONDAY...ESPECIALLY FROM NYC EAST. DO HAVE

SOME CONCERN THAT THE MODELS MIGHT HAVE TRENDED TOO FAR WEST...SO

DO NOT WANT TO PUT THE WESTERN 1/3 OF THE CWA IN THE MAIN THREAT

AREA QUITE YET.

bullish in terms of what they were thinking before

[A-L-E-X]

It's also very good to have Euro on your side within 72 hours..that's a givin!

Lets put it this way-- we're looking a lot better than we did this time yesterday!

The trend is your friend

[bluehens]

I agree that I just want to see some continuity......the GFS doesn't have to come in as strong as the Euro right now but a completely OTS solution would be a bit disconcerting. My fear here with any sort of phasing systems is a major fail ala 2001 or a later developer like 2000.

Mt. Holly has Wilmington DE at 70% POPs for Sunday so they're thinking we'll see something.

It wuold be nice to have some conistency on all of the guidance. What a challenging storm this one is turningo out to be. 2010 ending on an exciting note.

[gkrangers]

It's not really bullish for the NYC area.

THE CONSENSUS SUGGESTS A POTENTIALLY SIGNIFICANT SNOW STORM FROM

LATE SATURDAY NIGHT INTO MONDAY...ESPECIALLY FROM NYC EAST. DO HAVE

SOME CONCERN THAT THE MODELS MIGHT HAVE TRENDED TOO FAR WEST...SO

DO NOT WANT TO PUT THE WESTERN 1/3 OF THE CWA IN THE MAIN THREAT

AREA QUITE YET.

No reason to go nuts yet either. If we get through the 00z suite tonight with a good hit further west, then that would be a good time to get a little more bullish.

[A-L-E-X]

No reason to go nuts yet either.  If we get through the 00z suite tonight with a good hit further west, then that would be a good time to get a little more bullish.

Theyre not really bullish because this thing is still a few days away lol.  But you really have to like the trends.

[yhbrooklyn]

No reason to go nuts yet either. If we get through the 00z suite tonight with a good hit further west, then that would be a good time to get a little more bullish.

Absolutely.

I'm excited, but I have to keep it tempered.

It's only one run. I know Euro is king, but it can have bad runs too.

[A-L-E-X]

bullish in terms of what they were thinking before

Significant from NYC east means 6 inches plus and WSW criteria-- thats as much as you can say 3 days out lol.

[cleetussnow]

No reason to go nuts yet either. If we get through the 00z suite tonight with a good hit further west, then that would be a good time to get a little more bullish.

Then it will be a 48 hr or less forecast. They played it right in their grids. They kept 30% pops on the grids even when the models were bailing yesterday.

[Alpha5]

Anyone see the HPC maps

http://www.hpc.ncep.noaa.gov/qpf/fill_99qwbg.gif

Look a lot different than what Upton seems to be thinking for the area

[A-L-E-X]

Anyone see the HPC maps

http://www.hpc.ncep....fill_99qwbg.gif

Look a lot different than what Upton seems to be thinking for the area

That does show the "significant" snowstorm they were talking about from the city on east.  I would say that 0.5 qpf is the border of what I would call "significant" with 12:1 ratios, that's 6 inches.  Peak of 0.86, which would be around 10 inches.... so there's your forecast of 6-10" right there (if that's what theyre thinking.)