The Coming Negative NAO: Implications

[sphinx]

FWIW,

The Extended Eta (06 GFS initialization), the 00 UT GFS, and the 06 UT GFS are showing a moderate wintry event for the Mid-Atlantic in the March 2-3 time frame.

[donsutherland1]

Sphinx,

Such a moderate event would probably seem reasonable. With a strong block, the storm would not really be able to cut much farther north than its 168-hour position on the ECMWF. Instead, it would probably have to "bowl" eastward or jump to the coast in Miller B fashion.

The following are the maps from the 2/23 12z Euro (168 hours) vs. those 1 day before the onset of KU storms in the 2/20-3/20 period (which assumes the storm on the 3/2 12z ECMWF is the candidate storm).

Notice that the ECMWF depiction and the February 1969 event are those that had the most pronounced higher heights in the East just prior to the event.

ECMWF: 168 hours:


March 17, 1956:


March 1, 1960:


February 21, 1969:


February 21, 1987:


March 11, 1993:

[MiamiensisWx]

How do you think this will effect the upcoming hurricane season, donsutherland1?

May you answer my question above?

[Steve H.]

I believe the potential exists for something rather large in the timeframe you speak of, namely the first few days of March. Large high pressure in NW Canada lifting and the UL low in eastern canada adjusting westward in tandem could buckle the jet and allow bombgenesis along the mid atlantic region and possibly up to SNE. A very tricky pattern here with potential volatility. Winter is by no means over! Should be an interesting coupla weeks. DT is also seeing this, but keeps the threat to the middle Atlantic states. Cheers!!

[donsutherland1]

CapeVerdeWave,

Right now, I'm thinking that the 2006 season will be an active to perhaps very active one. I'm not yet sure about areas at greatest risk of landfall. The current blocking episode will have no bearing on the season, as it should be long gone before the first tropical cyclone develops.

[donsutherland1]

Correction:

I inadvertently posted the wrong information for Boston concerning days with high temperatures < 40° for the 2/20-29 period:

Percentage of Days with Highs < 40°:

Boston:
NAO > 0: 38%
NAO -0.99 to 0: 56%
NAO -1.99 to -1.00: 57%

[donsutherland1]

FWIW, if one compares the 2/24 18z GFS with the 2/25 0z run, one finds the following:

18z 138 hours: Low centered just west of Toronto
0z 132 hours: Low centered in western New York State

Whether or not this is the start of the southward trend remains to be seen. In my view, with the growing block, a solution that would take the storm to the Great Lakes or into southern Ontario is not very likely. Moreover, I believe this is the first indication of modeling trends to come.

Overall, I'm still thinking that the storm translates eastward to a position just off the Delmarva and then tracks east-northeast/northeast from there. Hence, areas from PHL/NYC to Boston could be in line for a moderate and possibly greater snowfall around 3/2. DCA/IAD to BWI might also get into the snowfall if the storm tracks a little farther to the south.

At this time, I don't believe the block will be so strong so as to squash the storm to the Southeast.

[donsutherland1]

Morning thoughts...

1) The block continues to build. NAO figures:

2/22: -0.115
2/23: -0.493
2/24: -0.771
2/25: -1.114

2) The GFS continues to increasingly "sense" the growing block. As a result, it continues to adjust the track of the 3/2 system to the south:

Forecast Position of Low for 3/2 12z:
2/24 18z run: Just west of Toronto
2/25 0z run: Western New York State
2/25 6z run: Western Pennsylvania

3) I continue to expect that the system could move offshore/redevelop off the Delmarva. At its farthest south, given the forecast strength and position of the block, I don't see it moving offshore farther south than the Virginia Capes.

[donsutherland1]

Several quick points on the 3/2 event:

1) The block is real (possibly near -2.00 by March 1 then fading as it retrogresses after that).
2) This block is not comparable to last year's superblock that peaked at around -6.00 and overwhelmed the East with cold.
3) A look at the ensemble means suggests that the block isn't the problem. Instead, the trough in the east is too small and the ridge to its south is too large.

As a result of this situation--it's something that I should have noticed earlier--a more northward track to the 3/2 storm is probably reasonable. I had expected a southward adjustment but should have paid greater attention to the small size of the eastern trough.

[Stephanie]

Our local forecasts are not showing any real cold outbreaks after this weekend and tomorrow. We have rain in the forecast for this week as well.

I've said it for MANY years now - once March is here, bring on Spring!

[donsutherland1]

The Moderate Block: Final Statistics

The 3/6 data showed that the NAO had broken above zero with a reading of +0.068 today. Hence, the block ended in the 3/5-12 timeframe as suggested by past history with such moderate blocks.

The following were the statistics from the just-ended moderate block.

Mean Temperatures:
Expected Mean Temperatures:
Boston: 32.8°
New York City: 36.0°
Philadelphia: 36.6°
Washington, DC (DCA): 40.6°

Actual Mean Temperatures:
Boston: 27.2° (5.6° below the historic mean)
New York City: 32.8° (3.2° below the historic mean)
Philadelphia: 34.0° (2.6° below the historic mean)
Washington, DC (DCA): 38.4° (2.2° below the historic mean)

Percentage of Days with Highs < 40°:
Expected Percentage of Days:
Boston: 57%
New York City: 39%
Philadelphia: 34%
Washington, DC (DCA): 19%

Actual Percentage of Days:
Boston: 67%
New York City: 50%
Philadelphia: 50%
Washington, DC: 17%

Total Snowfall in the 2/22-3/5 Period:
This is where the block tended to underperform.

Boston: 2.5"
New York City: 1.3"
Philadelphia: Trace
Providence: 6.6"
Washington, DC (DCA): Trace