Will the Extreme 2005 Block Sizzle or Fizzle?

[donsutherland1]

For those who are interested in how the current extreme blocking regime fares against the strongest on record:

Current Reading: -5.104 Rank: 67th Lowest on record

Lowest NAO Readings:
1. -6.967 March 3, 1962
2. -6.962 February 13, 1978
3. -6.718 March 10, 1951
4. -6.641 March 2, 1962
5. -6.523 February 14, 1978

Blocking Regimes where the NAO fell to -5.500 or Below:

• March 1951: Low: -6.718 (3/10); 3 days at -5.500 or below
• February 1955: Low: -5.608 (2/15); 2 days at -5.500 or below
• January 1958: Low: -5.692 (1/22); 2 days at -5.500 or below
• March 1958: Low: -6.014 (3/9); 3 days at -5.500 or below
• February 1960: Low: -5.991 (2/11); 3 days at -5.500 or below
• February-March 1962: Low: -6.967 (3/3); 6 days at -5.500 or below
• December 1963: Low: -5.653 (12/18); 1 day at -5.500 or below
• February-March 1965: Low: -6.289 (2/22); 4 days at -5.500 or below
• February 1978: Low: -6.962 (2/13); 5 days at -5.500 or below
• January 1979: Low: -5.560 (1/25); 1 day at -5.500 or below
• January 1985: Low: -5.638 (1/19); 1 day at -5.500 or below
• March 2001: Low: -5.744 (3/2); 1 day at -5.500 or below

For those who are interested, the following extreme blocking regimes occurred during a month in which the SOI averaged -10.0 or below:

January 1958, December 1963 and February 1978. All saw a "Kocin-Uccellini" snowstorm occur either following the extreme block (January 1958 and December 1963) or leading up to it (February 1978).

Of the six episodes that saw a more modest negative SOI (above -10.0), 3 saw such a snowstorm (March 1958, February 1960, and January 1979). All such snowstorms followed the extreme block. Two saw no such storms (March 1951 and January 1985). The two that failed to see such a snowstorm occurred during La Niña winters.

Considering that February 2005 is all but certain to see the SOI average -10.00 or below and Winter 2004-05 was a weak El Niño winter (not a La Niña one), should the NAO drop to -5.500 or below in coming days, the probability of a Kocin-Uccellini snowstorm at some point as the extreme blocking episode begins to weaken could be high.

Given the storms associated with these blocking regimes:

The following percentage of cases saw 4" or more snowfall at select cities:

Boston: 83%
New York City: 100%
Philadelphia: 100%
Washington, DC: 83%

The following percentage of cases saw 6" or more snowfall at select cities:

Boston: 83%
New York City: 100%
Philadelphia: 100%
Washington, DC: 67%

The following percentage of cases saw 10" or more snowfall at select cities:

Boston: 50%
New York City: 100%
Philadelphia: 67%
Washington, DC: 33%

Given these statistics, should the NAO reach -5.500 or below, while a major Appalachians storm might be a possibility--a "miss" per the Kocin-Uccellini definitions--it is far less likely than a full-fledged Kocin-Uccellini storm that would likely substantially impact at least 3 of the 4 cities from the Washington, DC, Philadelphia, New York City, and Boston pool.

[donsutherland1]

Just a quick update...

The 2/24 NAO is now -5.648. This makes the 2005 blocking episode the 10th strongest on record.

What might this mean for the February 28-March 1 event? At this point in time, I believe the critical question concerns where the system will track. Will it track out to sea or come northward closer to the coast? Given the magnitude of the block and the fact that the PNA is not overwhelmingly positive, I don't like the argument that the system will ultimately come up west of the Appalachians.

Resting on the GFS Ensembles, I'm assuming that the PNA will be at or above +1.00 perhaps closing in on +2.00 and rising at the time the event comes around. This means that the trough should be somewhat sharper than modeled by the 2/24 0z out-to-sea solutions.

So, my very early thoughts:

1) The storm won't be crushed and sent harmlessly out to sea. Will it come close enough to bring a crippling snow or just some accumulations to the MA and NE? I don't know yet. I would not be surprised if the ECMWF begins to trend toward the northwest in future runs. In fact, per a post by Wes (former HPC meteorologist), he learned that the 0z ECMWF ensembles were closer to the coast than the operational model.

2) The storm probably won't come up west of the Appalachians. Nevertheless, given how good Joe Bastardi can be at pattern recognition, I'm wary of saying more than "probably won't come west of the Appalachians."

3) At some point before the pattern breaks, a Kocin-Uccellini-type snowstorm is possible (see first post in this thread for that discussion)--not guaranteed, of course, but a much greater possibility than in most other years.

Whether or not it is this early-week event or a later one that can become such a storm remains to be seen. Past situations with extreme blocks have had a greater tendency for such events to occur when the block was weakening and not near the peak of its extremes. The GFS ensembles might suggest a later event but I'd feel uncomfortable trying to pin down any such event as a better candidate and will stick with the idea that such an event is very possible in the near- to medium-term.

For now, a good section of the Mid-Atlantic is receiving an appreciable to significant snowfall. Southern New England should do well later in the day and at night.

[JETSTREAM BOB]

-------------- Maybe,but the trend is my friend...........and maybe only New England,but don't think much south of N.Y. / Pa. border will see anything significant for the rest of the 2005 winter season.....Joe Bastardi has been saying since Mid-Jan it's coming"GET READY",but we have failed to see much snow except for New England this 2005 winter season...............and not going to see any until probably 2007 winter.....JSB has spoken..............!!!!!!!!!!!!

[krysof]

will you be happy if you get 6 inches of snow?

[SnowGod]

The models really have not pegged a "storm" per say down. They have continuely had a storm around the coast(on or off) and another low over the lakes with the arctic air. I am not sure how it will play out. I can't see it going West or Up the Apps with the NAO so negative, but oddly enough one of your "record" departures in 1965 had a whopper of a snowstorm west of the apps that season around February 22-28 period, however it was a moderate La Nina(surprise, surprise).

My view now? The southern low goes up the coast, the lakes low doesn't merge to Northern New England.

[donsutherland1]

Just a quick note:

The final figure for 2/25 was -6.026. This makes the current blocking regime the 5th strongest on record.

[ChiTownMC]

Just a quick note:

The final figure for 2/25 was -6.026. This makes the current blocking regime the 5th strongest on record.

How long have they been keeping records?