March 6-19, 2005 Pattern Discussion

[donsutherland1]

Last week’s discussion raised a number of issues of what one might expect for the February 27-March 12, 2005 period.

The cold pattern in place since just after mid-February has proved to be a prolific snow producer, especially from the northern Mid-Atlantic to New England. Given the blocky pattern that is likely to continue, cities from Washington, DC to Boston and back through the Ohio Valley and later into the Plains should all see additional accumulations of snow before the month is finished.

Those who enjoy winter weather should savor this special stretch of weather that remains ongoing. There are hints that April could see the evolution of a very different pattern (emerging ENSO analogs, climatology of extreme blocks, and SOI analogs).

Key points from last week’s pattern discussion:

• Per the synoptic experience with extreme blocks and the ECMWF/ECMWF ensembles, I expect a powerful storm to pass just the New Jersey coast toward Cape Cod. There should be stronger high pressure over Quebec than is currently shown on some of the modeling. Right now, details remain sketchy but I believe odds of seeing the following snowfall before changeover issues are as follows:

Boston: 4” or more: High; 6” or more: Moderate
New York City: 4” or more: High; 6” or more: Moderate
Philadelphia: 4” or more: High; 6” or more: Moderate
Washington, DC: 4” or more: Moderate; 6” or more: Low

The potential does exist for more snowfall in some or all of these cities…

Accumulations were as follows for the February 28-March 1 snowstorm:
Boston: 8.1”
New York City: 7.7”
Philadelphia: 5.6”
Washington, DC: 2.2”

The detailed forecast put out prior to the storm was bad and greatly overstated amounts as the storm wound up tracking farther to the east than assumed in that forecast.

• In the wake of the February 28-March 2 event, look for colder air to surge into this region. Low temperatures could well fall into the upper teens and lower 20s before moderation takes place after midweek.

Colder air moved into the eastern United States on March 2. Low temperatures on March 3 included:

Boston: 20°
New York City: 23°
Philadelphia: 23°
Washington, DC: 25°

• An energetic clipper could bring additional light snow or flurries to parts of this region, especially from Philadelphia northward during the March 3-4 period. After March 7, per the GFS ensembles, there is a signal for possibly another major storm.

The clipper threat had diminished per the latest computer guidance with little snow expected. After March 7, there appeared to be the prospect for a significant storm in the March 9-11 period.

• Throughout the period, look for temperatures to average below to much below normal.

Through March 4, readings averaged well below normal.

• The February 27-March 7 period should prove quite snowy across the Midwest and Ohio Valley. However, the best snow from the major February 28-March 2 storm may fall just to the east of this region. Look for the entire period covered by this pattern discussion to see below normal temperatures.

Temperatures averaged below normal in this region through March 4. Snowfall amounts during the February 27-March 3 period included:

Chicago: 1.2”
Cleveland: 8.9”
Detroit: 4.3”

• February 27 could see some rain in the Central Plains states. Through March 5, the Central Plains states should see readings average generally above normal with the greatest warmth likely during the March 2-4 period. The March 6-12 period should see somewhat cooler readings and there could be a shot of cold air that brings readings below normal.

Small amounts of rain fell across the Central Plains on February 27. The February 27-March 3 period saw readings average somewhat below normal to near normal but much warmer air took hold beginning on March 2 with readings in the 50s and later 60s.

March 3 highs:
Kansas City: 65°
Omaha: 51°

March 4 highs:
Kansas City: 68°
Omaha: 62°

• California will likely see a new storm arrive with some rain during the February 28-March 1 timeframe. Rainfall should not be as copious as recent rains. The overall period covered by the pattern discussion should remain wetter than normal. March 5 could see temperatures become noticeably warmer in California. The following week, likely after March 7, renewed rains could arrive.

Los Angeles received some showers but no serious rainfall during the February 28-March 3 period. Temperatures were forecast by the modeling to reach the mid 60s by March 5 and the 70° by around March 8.

• Dry and relatively warm weather should continue in Seattle through the period covered by this pattern discussion.

Temperatures ran much above normal during the February 28-March 3 period in Seattle. The city picked up 0.16” rain.

The March 6-19 Ideas:

In the cold will come a parade of clippers. Some will fizzle. One or two might sizzle. The southern jet might even join the parade at some point. That pretty much sums up the pattern that is likely to continue through the duration of the period covered in this discussion. There could be a brief period where milder air pushes as far north as New England on Tuesday. However, colder air will quickly rush back into the region Tuesday night. Behind that front, things could get really interesting.

The GFS ensembles (both with respect to the PNA and NAO) and historic experience concerning extreme blocking situations during negative SOI regimes/neutral or warm ENSO winters suggest higher than average potential for something big in the near- to medium-term. Following this outlook is a more detailed discussion on that matter.

Per the GFS ensembles, the NAO is likely to remain negative and could actually begin to fall anew later next week. The progged AO also is consistent with such an outcome. Consequently, in my view, from midweek next week through at least midmonth, storms are more likely to take a coastal or offshore track than an inland one.

• The Mid-Atlantic and Northeastern states are likely to see temperatures moderate this weekend. Temperatures could reach the 40s into New England, especially on Tuesday. The first clipper could bring some rain and snow showers to parts of the Mid-Atlantic tomorrow. Another system will likely bring rain and gusty winds to the East on Monday night and Tuesday (March 7-8). In the wake of this system colder air will likely pour into the East.

The March 9-10 and March 12-13 periods could see snow affect parts of the Mid-Atlantic and Northeast. The first system will likely bring at least some accumulations to Philadelphia, New York City, and Boston. Even Baltimore and Washington, DC have a chance at a light accumulation. Further south, Raleigh could experience perhaps some snow and ice before a possible change to rain/showers on March 9-10. The second system could bring accumulations to Washington, DC, Baltimore, Philadelphia, and possibly all the way to New England.

Throughout the period temperatures will likely average below normal in all the above-mentioned cities.

• Colder than normal weather should continue through the duration of the period in the Ohio Valley and Midwest. After some rain in such cities as Chicago and Detroit around March 7, colder air should return. It is not out of the question that these cities see some accumulations of snow before the rain ends. March 8 could mark the start of a prolonged spell of below to much below normal temperatures. March 9-10 could see additional snow in this region. Over the next two weeks, I expect that Chicago will reach and exceed 40” snow for the season.

• In the Central Plains, weekend warmth will likely be replaced by colder temperatures on March 8. On March 6, Omaha could actually see the mercury soar into the 60s. During the March 7-8 timeframe temperatures will likely trend downward.

Cities such as Omaha, Kansas City, St. Louis, and Bismarck will likely all receive some accumulations of snow during the period. The March 9-11 period offers one such opportunity for accumulating snow at least some of these cities.

• The Pacific Northwest looks to remain warmer and drier than normal through the duration of the period. Temperatures at Seattle should be trending higher by March 7. The March 10-12 period offers the potential for temperatures to soar into perhaps the middle 60s or above at Seattle. Afterward, somewhat cooler air should begin to filter back into the region and some showers could occur.

• The drying process should continue in California for a time. Temperatures will likely trend upward during the weekend in California. Much above normal readings are likely in the March 8-12 timeframe for Los Angeles as the PNA crests then begins to fade. It is not out of the question that Los Angeles sees the temperature soar above 80° on one or more days during the height of the warmth. After the following weekend (March 12-13), the threat of new rainfall could increase.

Looking for a Major Snowstorm?

During times when the SOI was negative and there were either neutral or El Niño conditions, extreme blocking situations have sometimes been followed by major snowstorms.

If there is to be a such a snowstorm before the current pattern changes, it will likely occur before the NAO goes positive. Once the NAO has gone positive, given how late in the month that could be, the threat of such a storm will likely be greatly reduced.

Briefly, aside from the experience concerning extreme blocking patterns, two other factors are at least likely to be consistent with past occasions that saw late-season major snowstorms (snowstorms that started Feb. 15 or later) documented in Paul Kocin’s and Louis Uccellini’s Northeast Snowstorms (American Meteorological Society, 2004):

• March ENSO Region 3.4 Anomaly: Likely > -0.20°C (10/11 91% of cases)
• March QBO: Highly likely above -6.00 (9/11 82% cases)

Possible Problem:

• March SOI: No late-season snowstorm occurred in a month where the SOI averaged below -12.6. However, two somewhat earlier storms occurred with a far more negative SOI (Feb. 1978 -24.4 and Feb. 1983 -33.3).

Going back to the matter of timing, the following sample concerns storms that occurred when a negative NAO environment preceded their start. By that, I mean that the NAO was negative 5 days prior to the start of the storm.

Late-Season with Negative NAO Environment that Preceded Them:
• Number of storms: 6 (55% of late-season storms)
• Negative NAO at start of storm: 5 (83%)
• Negative NAO 5 days after start of storm: 4 (67%)

All KU Storms with Negative NAO Environment that Preceded Them:
• Number of storms: 18 (55% of all storms)
• Negative NAO at start of storm: 14 (78%)
• Negative NAO 5 days after start of storm: 10 (55%)

Note: The sample includes the February 2003, December 2003, and January 2005 storms.

Consequently, late-season storms that follow a negative NAO environment, are more likely to occur at a time when the negative NAO is likely to be sustained for at least a time after the event. They are also overwhelmingly likely to commence when the NAO is still negative.

Therefore, once there is sufficient support for the development of a positive NAO, the arrival of the positive NAO could mark the end of the major snowstorm threat for the current negative NAO regime. The highest NAO recorded at the start of a late-season storm that occurred following a negative NAO environment was +0.208 (March 1993 superstorm).

The opportunity for a major snowstorm would likely be greatest when the NAO is making a sharp change. In the aforementioned sample of 11 storms, 8 (73%) occurred when the NAO had a range of 2.000 or higher during the period beginning 5 days before the storm and ending 5 days after. 7 (64%) of occasions had a range of 2.500 or above.

Another factor supporting the possibility of an “enhanced precipitation” event in the East is the sharp rise in the PNA that is now underway. Research by Heather Archambault has found that major increases in the PNA have their highest correlation with major East Coast precipitation events in March, though weaker correlations exist for other months. The last such major swing in the PNA—January 7 (-6.138) – January 24 (+2.804)—culminated in the January 22-23, 2005 blizzard that buried parts of the northern Mid-Atlantic and New England under 1-2 feet of snow with locally higher amounts. The December 5-16 period in which an earlier large-scale increase in the PNA took place also saw the Mid-Atlantic and Northeast receive substantial precipitation during the December 6-11 timeframe.

Given the NAO and PNA signals, odds favor any such storm occurring prior to March 20 and possibly prior to midmonth. The fact that the March 3 12z ECMWF and March 4 0z GGEM blew up separate systems into sub-970 mb monsters suggests that the models might be sensing the potential that could lie just ahead. Of course, such a storm is not a certainty. However, with NAO and PNA signals, historic experience concerning extreme blocks, and the hints on the modeling, things are looking interesting.

If another major snowstorm does occur, Winter 2004-05 would become only the 7th winter since 1950-51 to have two or more such storms for the East Coast. The seasons that saw two or more such storms are:

1960-61 3
1986-87 3
1957-58 2
1966-67 2
1968-69 2
1977-78 2

Looking farther ahead, a logical question that arises concerns whether the current negative NAO regime is likely to break anytime soon. The latest GFS ensembles argue that it is likely to continue through midmonth. Additional factors suggest that a positive NAO regime is not likely to set in until the first week in April.

• Past experience with extreme blocks. Considering the extreme blocks that peaked in the February 15-March 15 timeframe (the 2005 block peaked on February 25), a sustained positive NAO regime typically didn’t arrive until April. 5/6 (83%) cases saw it arrive after April 1 (typically around April 5). 1/6 (17%) case saw it commence on March 24. The latest case saw its arrival delayed until April 18.

• NAO climatology: April has often been a “graveyard” for blocking regimes. Of the 25 occasions where March saw a negative NAO that followed a negative NAO in February, just 10 saw April average negative. 60% saw a flip to positive in April. Of the 31 years in which the NAO averaged negative in March irrespective of the February NAO, only 13 saw the NAO remain negative in April. 58% of all years with a negative NAO in March saw the NAO go positive in April.

March Snowfall Update:

Last week, I noted, “Given the pattern likely to predominate the first half of March, I believe it is reasonable to expect that Boston will likely see 8” or more in March, NYC should see 6” or more, and Washington, DC 3” or more.”

Snowfall climatology also argues for such amounts in Boston and New York City:

In Boston in seasons that saw 30” or more in January and 15” or more in February, 4/5 (80%) received 10” or more in March. Average March snowfall came to 13.7”.

In New York City, in seasons that saw January and February each receive 15” or more snow, 4/5 saw NYC receive 6” or more snowfall in March. The average March snowfall was 7.1”.

In seasons that saw Boston receive 50” or more snow (current figure: 78.2”), NYC 35.0” or more (current figure: 37.0”), and Philadelphia 29.5” or more (current figure: 29.9”), the least snow received in Washington, DC was 17.4”. Given that Washington, DC has seen just 11.7” so far, this historic information also implies that Washington, DC will likely see additional accumulations of snow in March.

Totals through March 4 came to:

Boston: 6.1”
New York City: 2.9”
Washington, DC: Trace

A Look Back: March 10, 1856: A Day of Extreme Cold:

March 10, 1856 proved to be one of the coldest days ever recorded in the eastern United States during March. In several cities, subzero cold was recorded and some of those temperatures would demolish existing all-time monthly low temperatures.

Reports for March 10 included:

• Albany: “The thermometer in this city ranged from zero to 7° below zero, this morning.”
• Boston: “At sunrise this morning, the thermometer in this city stood at zero.”
• Buffalo: “The thermometer this morning stood at 15° below zero…”
• Cleveland: A low of –10° was recorded.
• Louisville: “The weather here is very cold.”
• Montreal: “The mercury stood at 12° below zero in this city this morning.”
• New York City: 4° at 7 am.
• Philadelphia: “The mercury, at daylight this morning, had fallen to zero.”
• White River Junction, Vermont: “[T]he mercury was 26° below zero.”

Source: “Great Severity of the Weather,” The New York Daily Times, March 11, 1856 and “The Weather,” The New York Daily Times, March 12, 1856.