STORM2K

Prior to this morning, the NAO had spent 9 of its last 10 days in positive territory. On April 19, the NAO fell to -0.848. The GFS ensembles (4/19 0z run) suggest that the NAO could fall to perhaps -1.500 or below. Afterward, they show a mainly negative NAO into at least the first part of May.

Since 1950, there have been 29 years that saw the NAO fall to -1.500 or below in the April 20-May 5 timeframe and 18 years that saw it fall to -2.000 or below in that same period of time. The lowest recorded value was -4.516 set on April 25, 1995.

Unlike the strong blocks that form during the winter, mid-to-late spring blocks last for far shorter durations. Since 1950, the 29 aforementioned blocks lasted an average of 12 days. Those that bottomed out at -2.000 or below lasted 13 days on average. The most prolonged such block was that of April 17-May 11, 1954, which lasted 25 days.

Looking more closely at the historical experience, one finds:

• 9/29 (31%) of such blocks lasted less than 10 days and 21/29 (72%) lasted 15 days or less.
• From the time the blocks peaked (GFS ensembles point to the April 27-29 timeframe), 26/29 (89%) ended in 10 days or less.

Consequently, if the historic experience holds true, a reasonable timeframe during which the new block would end would be the May 1-10 period.

If one examines the MJO, it has now moved into Phase 7. By the May 4-13 period, it should move into Phase 2, which is correlated with a positive NAO. Consequently, the MJO appears to lend further support to the NAO's flipping to positive in the aforementioned timeframe.

Thus, given the historic experience and MJO data, I believe that the NAO will likely return to positive territory in the May 1-10 timeframe. Before then, the East could experience several days where the minimum temperature could fall to unseasonably chilly levels (probably 45° or below in NYC, 43° or below in Boston, and 47° or below in Washington, DC) on one or more days, especially during the April 25-May 5 period. It is possible that Boston might even see the mercury fall below 40°. Moderation will likely commence around May 5 +/- several days. Near or just after midmonth, ridging could again be pronounced in the East with above normal temperatures.

FWIW, perhaps as a hint, out of the 12 situations that saw the NAO bottom out at -1.500 or below during April 27-May 5 period, 8 (67%) saw NYC reach 90° or above in May.

Does that imply a return to wet weather during May in Seattle?

TT-SEA,

This block is nowhere near as strong as the superblock earlier this year. I don't believe it will have much bearing beyond perhaps the Great Lakes/Midwest region.

The April 20 0z ensemble forecast for the NAO is now showing a look that I believe is reasonably consistent with the opening post in this thread. That's a big change from the April 19 0z run, which showed mainly a negative NAO through the forecast period:

NAO Update...

The NAO bottomed out at -2.225 on April 27; the GFS ensembles did an excellent job in pinpointing the timeframe in which the NAO would bottom out. The GFS ensembles are pointing to a near-term return to positive for the NAO.

Also, the MJO is now in Phase 1. On that front, things are also progressing rather nicely. So, overall I have no big changes in my general thinking. As for May, I'm thinking the Northeast will wind up normal to somewhat above normal, the Mid-Atlantic should finish somewhat above normal, and the Southeast somewhat below normal to near normal.

Don...

Do you have a list of years that had a strong block in February and early March and again in April like this year AND had similar ENSO conditions??

TT-SEA,

Since 1950, when NAO statistics are available on a daily basis, 5 years saw a superblock (NAO: -5.500 or below) in February or March and another strong block (NAO: -1.500 or below) in late April-early May: 1951, 1955, 1960, 1962, 1978.

Four of those years saw neutral ENSO conditions for the April-June timeframe: 1951, 1960, 1962, 1978.

One of those years saw a weak El Niño during the winter: 1978

1978 appears to be the closest match with regard to the NAO and ENSO.

Thanks Don.

How close was the spring of 1993 to meeting these conditions?

TT-SEA,

The negative NAO has a much greater impact on the East/Ohio Valley than the Pacific Northwest. 1993 did not resemble 2005 with regard to extreme blocks.

Instead, March 1993 had one of the strongest positive NAO regimes on record--all of March 2005 was negative. On March 16, the NAO reached +4.417, the 4th highest value on record for March. Overall, March 1993 saw the NAO average +1.044 (9th highest on record for March). March 2005 had an average NAO of -2.757 (3rd lowest on record for March).

Also, late April-early May saw just a positive/strongly positive NAO. Late April 2005 experienced a strongly negative NAO, which is now fading.

From April 19:

• ...21/29 (72%) lasted 15 days or less.
• From the time the blocks peaked (GFS ensembles point to the April 27-29 timeframe), 26/29 (89%) ended in 10 days or less.

Consequently, if the historic experience holds true, a reasonable timeframe during which the new block would end would be the May 1-10 period.

Today, the NAO rose to +0.693. Thus, if one measures the former block against the historic experience, it lasted a total of 13 days and ended 5 days after it had peaked at -2.225 on April 27.