March 20-April 2, 2005 Pattern Discussion

[donsutherland1]

Last week’s discussion raised a number of issues of what one might expect for the March 6-19, 2005 period.

• Overall, the East should remain generally cooler than normal, though moderation is likely after mid-month…

A storm moving off the Mid-Atlantic coast and heading northeastward from there could bring at least some accumulations of snow from Philadelphia to Boston during the 3/17-19 period. Parts of the northern Mid-Atlantic and Northeast could see a significant snowfall.

During the March 13-17 timeframe, temperatures generally averaged 2°-4° below normal with the coldest anomalies being centered in the Mid-Atlantic region. New England was a little milder. On March 17, a storm moved off the East Coast but too far to the south to bring snowfall to the Mid-Atlantic and New England.

• Temperatures will likely average below normal in the Ohio Valley, Midwest, and Great Lakes region. The March 16-18 period could see Chicago, Detroit, and Cleveland receive accumulating snow.

During the March 13-17 period, temperatures averaged much below normal in this region. Chicago, Detroit, and Cleveland saw temperatures average more than 6° below normal with Cleveland averaging more than 9° below normal.

Light snow moved into this region on March 17 and continued on the 18th at some locations. On March 17, Chicago picked up 0.6” and Detroit a trace. Detroit saw additional light accumulations on March 18.

• The Central Plains will likely see a mild day on Saturday (March 12) before a new shot of colder air arrives. March 13-17 could see readings run below normal with some light snow perhaps around 3/16-17. After some moderation beginning on the 18th, cooler air could return during or after the following weekend.

On March 12, the temperature soared to 65° in Kansas City. Overall, during the March 13-16 period, temperatures averaged below normal. By March 17, temperatures were somewhat above normal. No accumulations of snow resulted.

• For the most part, warmer than normal, dry weather should persist in Seattle. The March 17-21 period could briefly experience readings falling to near or even somewhat below normal.

Through March 15, temperatures averaged above normal in Seattle. Somewhat cooler air arrived on March 16. Also, Seattle received 0.14” rainfall on March 16.

• The warmth should ease in Los Angeles after March 13. However, through the duration of the period, readings are likely to remain somewhat above normal. Rainfall should remain below normal.

Moderation in the much above normal warmth occurred starting on March 12. Nevertheless, the temperature spiked to 75° in Los Angeles on March 16 after an unseasonably cool start to the day. Through March 16, no rainfall was recorded.

The March 20-April 2 Ideas:

The following information noted in a post on March 17 should provide a reasonable broad framework for the continuing evolution of the pattern through the rest of March into the first part of April:

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.

• Although one will likely see the cold ease in the Mid-Atlantic and Northeast starting late next week, readings through the rest of the month and into the first two days of April will probably average near or somewhat below normal for the period as a whole. Parts of the northern Mid-Atlantic and southern New England, including Boston, will likely see some snow changing to rain Saturday night into perhaps early Monday. A modest accumulation is possible in Boston. Interior sections could see an appreciable snowfall.

During the March 24-25 period, an additional storm could move off the East Coast bringing rain to Washington, DC to Philadelphia and possibly rain or snow changing to rain to New York City and Boston. Behind the storm, a colder air mass should return for the weekend of March 26-27.

In the longer range, moderation will likely occur in the closing days of March.

• After a cool end to the weekend, temperatures should moderate across the Ohio Valley, Midwest, and Great Lakes region. Rain could affect this area during the March 23-24 period. The weekend should be somewhat colder before milder air returns.

• The week of March 20-26 should see generally warmer than normal conditions across the Central Plains states. However, the March 19-20 period could see the risk of severe weather in parts of Texas and the Deep South. The March 22-23 timeframe could see a storm bring rain to such cities as Kansas City and St. Louis. The weekend looks to be somewhat cooler but readings could remain near or somewhat above normal.

• The closing weeks of March could see the emergence of a wetter pattern in the Pacific Northwest that should see April come out with normal to above normal precipitation. A detailed discussion of this development is provided in a separate post.

At present, the March 19-21 period could see at least a moderate rainfall in the Seattle area. In the wake of this storm, temperatures should remain close to normal.

• Los Angeles will likely see additional rainfall next week during the March 22-24 timeframe. Readings should generally remain close to normal, though March 24 has the potential to be unseasonably cool. March should end on a dry and mild note.

April Snowfall: More for Boston, but Most of the Mid-Atlantic Likely Finished:

The latest ENSO analogs suggest that while snowflakes might be seen across the Mid-Atlantic, from largely south of the greater New York City area, no accumulations are likely during April. Boston still appears poised to receive additional accumulations in April.

• Top ENSO Matches:
Boston: Average Snowfall: 1.9”; 6/24 (25%) years with 1” or more; 10/24 (42%) years with more than a trace
New York City: Average Snowfall: 0.7”; 6/24 (25%) years with more than a trace; 16/24 (67%) of years with a trace or more
Washington, DC: Average Snowfall: 0.0”; 3/24 (13%) of years with more than a trace; 10/24 (42%) of years with a trace or more

• Top ENSO Matches and NAO:
Boston: Average Snowfall: 1.1”; 5/20 (25%) years with 1” or more; 8/20 (40%) of years with more than a trace
New York City: Average Snowfall: 0.8”; 4/20 (20%) years with more than a trace; 11/20 (55%) of years with a trace or more
Washington, DC: Average Snowfall: 0.0”; 2/20 (10%) years with more than a trace; 6/20 (30%) of years with a trace or more of snow

• Top ENSO Matches and QBO:
Boston: Average Snowfall: 3.9”; 3/8 (38%) years with 1” or more; 5/8 (63%) with more than a trace
New York City: Average Snowfall: 0.5”; 1/8 (13%) years with more than a trace; 6/8 (75%) years with a trace or more
Washington, DC: Average Snowfall: 0.0”; 0/8 (0%) years with more than a trace; 1/8 (13%) years with a trace or more

• Top ENSO Matches and PDO:
Boston: Average Snowfall: 2.9”; 4/14 (29%) years with 1” or more; 5/14 (36%) years with more than a trace
New York City: Average Snowfall: 1.0”; 2/14 (14%) years with more than a trace; 8/14 (57%) years with a trace or more
Washington, DC: Average Snowfall: 0.0”; 0/14 (0%) years with more than a trace; 5/14 (36%) with a trace or more

Given the above situation and the idea that the negative NAO regime will likely give way to a positive one within the first 10 days of April, I believe it is reasonable to expect that Boston will likely see 1” or more snowfall in April, NYC should see at least a trace of snow, but Washington, DC will likely see no accumulations of snow.

A Look Back: The March 24, 1875 Mid-Atlantic Snowstorm:

The following were select accounts of this late-season Mid-Atlantic storm:

Baltimore: Snow commenced falling about 7 o’clock in this morning and continued steadily until 11:30, reaching a depth of several inches when it ceased.

Harrisburg: A snowstorm set in at 5 o’clock this morning. It is still snowing fast.

Newcastle, DE: The storm continues, with a heavy fall of snow. Thermometer, 34.

Philadelphia: Snow commenced falling here about 8 o’clock [in the morning] and continues unabated. The weather is moderating, and the snow melts as it falls.

Washington, DC: A heavy snowstorm commenced early this morning and continues at noon.

Source: “The Snowstorm,” The New York Times, March 25, 1875.

Occasional Website Link: “The Storm King: Some Historical Weather Events in the Pacific Northwest”

http://oregonstate.edu/~readw/

[BritBob]

Fantastic post Don, as always. One of which I agree with, you`ll be pleased to know

[donsutherland1]

Thanks, Bob. I hope all is going well for you. It might take a little longer for the cold anomalies to end in eastern Canada but the emerging ENSO analogs are still pointing toward what might be a relatively warm summer.

[donsutherland1]

Several additional points concerning the April snowfall prospects noted in the opening thread:

Historic Climatology:
• Boston: Average Snowfall: 1.4”; 39/113 (35%) years with 1” or more; 55/113 (49%) years with more than a trace; Most: 22.4", 1997
• New York City: Average Snowfall: 0.9”; 41/135 (30%) years with more than a trace; 89/135 (66%) years with a trace or more; Most: 13.5", 1875
• Washington, DC: Average Snowfall: 0.2”; 17/116 (15%) years with more than a trace; 63/116 (54%) with a trace or more; Most: 9.3", 1924

ENSO Trends Are Important for Boston/Southern New England:
The seasons that are snowiest in April are those that see a strengthening El Niño through the April-June period. Of those 9 seasons since 1950, 8 (89%) saw 0.1" snowfall or more in April. 6 of those 9 seasons (67%) saw 1" or more snow in April. Average monthly snowfall came to: 5.1".

In contrast, all other seasons saw an average of just 0.8". 13/46 (28%) received 1" or more snow in April and 19/46 (41%) received 0.1" or more snow. A majority of those 1" or above seasons occurred when SSTAs were either holding generally steady or warming during the April-June period.

An April 1982-type event is unlikely for April 2005:
Although, the SSTAs in the ENSO regions are not too dissimilar from those through February 1982, there is a critical difference: 1982 experienced a rising El Niño event; 2005 is in the midst of a fading one and I fully expect that the 2004-05 El Niño will be declared finished by the end of April. April 1997 also saw a rising El Niño event. Those differences between 1982 and 1997 will likely prove decisive.

[donsutherland1]

Overnight, the footsteps of the possible pattern change grew louder...

850 mb temperature forecast from the 3/21 0z ECMWF:

• Boston
3/26 0z: +0.8°C
3/27 0z: -1.2°C
3/28 0z: +6.2°C

• New York City
3/24 0z: +0.0°C
3/25 0z: -0.6°C
3/26 0z: +2.6°C
3/27 0z: +3.2°C
3/28 0z: +6.7°C

• Washington, DC
3/23 0z: +1.8°C
3/24 0z: +4.1°C
3/25 0z: +1.3°C
3/26 0z: +6.1°C
3/27 0z: +7.2°C
3/28 0z: +8.0°C

It appears that shortly after the 7-day period, a brief shot of somewhat cooler air could move into the Northeast and Mid-Atlantic region.

AO, NAO, PNA:

Longer-term, there is even stronger support for the Arctic Oscillation going positive just prior to April 1. The GFS ensembles remain greatly divided as to whether the NAO will go more strongly negative around April 1, but 77% of the cases in April saw the NAO average positive when the AO averaged positive. Thus, I'd expect that given historic experience with regard to extreme blocks and the AO/NAO relationship, sometime during the first 10 days of April, a positive NAO regime will lock into place.

The PNA is forecast to go positive in the closing days of March.

Things to keep in mind with regard to the NAO/PNA in April for the East:

• Positive NAO/Moderately Positive PNA: generally widespread warmth with the highest probability of warm anomalies in the Northeast and Southeast.

• Positive NAO/Moderately Negative PNA: generally cool across the Northeast; Warm in the Mid-Atlantic and Southeast.

Overall, I expect the above two combinations to be more common than those with a negative NAO during April (expected flip in the NAO per extreme blocking situations, historic experience that 60% of the time when March sees the NAO average negative, April has a positive one; VAwxman's research that shows an overwhelming share of the time, a March NAO of -2.000 or below is followed by a positive/neutral one in April). Thus, warmth, especially across the Mid-Atlantic and Southeast looks good. This would tie in well with the tendency for warmth in the East following weak El Niño winters.