Mid-Month Pattern Change Appears Highly Likely

[donsutherland1]

Linda,

The cold pattern is, at least in my view, nearing its end. But that process will likely require 2-3 weeks before a warmer pattern moves into place.

First, a note from the earlier verification: The final NAO figures are even more impressive than the preliminary ones. On February 25, the NAO fell to -6.026, the 14th lowest value of all-time. This makes the 2005 block, the 5th strongest block on record.

Also, the 2005 block saw 9 days (using some very recent preliminary data) where the NAO was at or below -5.000. That ties the all-time record set in both the 1958 and 1962 super blocks.

Here's the way I see the pattern evolving, pieces of which were noted in some of the recent weekly pattern discussions, and some newer information:

1) Per the historic experience with past late-season extreme blocks and NAO/ENSO climatology, I believe the worst of the cold anomalies in the East will be finished 3/24 +/- a few days.

2) Even afterward, blockiness will probably be recurrent for a time, so this less cold period may still have some quite cold days interspersed with a growing number of milder ones (far more milder days relative to colder ones than the first 20 or so days of March).

The latest GFS ensembles (3/17 0z) show the NAO going negative after briefly reaching positive territory. This reinforces the idea of blockiness being recurrent until the positive NAO regime sets in.

3) Most of the extreme blocking situations saw the negative NAO replaced by a sustained positive NAO during the first 10 days of April (earliest: March 24; latest: April 18).

4) When the NAO undergoes a regime change, look for more widespread, persistent, and meaningful warmth. I believe April will average normal to above normal in the Mid-Atlantic/Southeast with New England being a tougher call (analogs are split between the cooler and milder option: the curse of backdoor cold fronts?). I would not be surprised if either the Mid-Atlantic or Southeast averaged several degrees above normal e.g., DCA saw a mean temperature of 58° or above for April but those are just initial thoughts.

5) Once the NAO pops positive, I believe that the threat of any KU-type storm will have effectively ended. Of course, at this point, I don't see such a threat on the horizon.

6) Once April arrives, at least per the analogs that are looking increasingly strong to me, the threat of meaningful snow in the Mid-Atlantic will be all but finished. I don't believe DCA will see more than a trace of snow in April and perhaps won't see anything. Even NYC probably won't see much, but perhaps a little more than a trace. I'll offer greater details in tomorrow's weekly pattern discussion.

[Lindaloo]

Sorry don, that ahem was in no way meant for you.

Your analysis is excellent as usual.

[donsutherland1]

Linda,

I just thought I'd add where I believe things are going as perhaps you or others might have wanted an additional follow-up.

Best wishes.

[TT-SEA]

Do you have any ideas on the Pacific Northwest during this pattern change?

We seem to have switched to a wetter regime. We had widespread rain around Seattle yesterday and good mountain snow for the first time in a long time. We had 1.50 inches of rain at our house even though the official total at the airport was much less.

This weekend and early next week also looks wet. It has been 2 months since we have seen a pattern like this set up.

Do you think this will continue? What are the odds of a wet April around here?

[donsutherland1]

TT-SEA,

First, the pattern change discussion actually concerned the mid-February pattern change.

Second, emerging analogs suggest that April could see normal to somewhat above normal precipitation in the Pacific Northwest. The recent showers (rains in some places) might well be an opening shot toward the change to a wetter pattern.

[Lindaloo]

Linda,

I just thought I'd add where I believe things are going as perhaps you or others might have wanted an additional follow-up.

Best wishes.

Oh! Thank you very much!

I am in the deep south and I do not know what to think about this continuing damp, cold scenario. This is unusual for this region in March. Although we did have that blizzard in March of 93, which as you probably know is more than likely never going to happen again, not in March anyway.

[donsutherland1]

Linda,

Cooler than normal weather in March in the Deep South is actually the norm following weak El Niño winters. April typically sees a moderation in temperatures:

[Lindaloo]

I did not know that. I guess I am having a bad case of Spring Fever. Thanks.

[donsutherland1]

You're welcome, Linda.

[Stormsfury]

April 1991 in the Southeast was quite warm as a whole, with many afternoons w/scattered thunderstorms around on a local level. (based on memory).

Aprils 1993 and 1994 were somewhat variable but as a whole slightly ABV normal WRT to long term climo. I used to have all the numbers specifically from day to day from 1991-1997 as I used to keep daily logs of highs/lows, precip, winds, etc ... and still have those somewhere around here (on hardcopy) ...

SF