February 20-March 5, 2005 Pattern Discussion

[donsutherland1]

Last week’s discussion raised a number of issues of what one might expect for the February 13-28, 2005 period.

Briefly, before I begin assessing the earlier ideas and where things could be going, I will note that a cold front moved off the East Coast of the United States on February 16. Per the case made earlier for a dramatic shift in the pattern around or just after mid-month, I believe this cold front marked the beginning of that pattern change.

To be sure, some remain skeptical of the pattern change and its evolution. Briefly, the following two points should be considered:

• The anti-pattern change case had been premised on the idea that the NAO would not go negative. It has done so and the debate now focuses on just how strongly negative it will go.
• The anti-pattern change case also rested on the idea that the SSTAs were unfavorable and had not changed appreciably. In the Pacific, that assumption was shattered when the January PDO came in positive. For October through December, it had been negative. This is a large-scale change with major teleconnective ramifications, specifically the greater tendency for the PNA to average positive.

Anyhow, that’s food for thought. At this point in time, my confidence remains as high as it was when I wrote the lengthy discussion in favor of the pattern change. The fact that the change is no longer well out into the future but is now occurring should lend confidence to the idea that the second half of February should prove both colder than normal in the East and snowier than normal there (from DCA to BOS), as well.

Now, let’s examine some key points from last week’s pattern discussion:

• This pattern change will likely dominate the sensible weather for the remainder of the month and possibly into early March once it unfolds. Overall, a cold and sometimes stormy scenario appears likely to develop. Some of the storms will likely be suppressed and a significant arctic outbreak is possible.

Right now, I have no major changes to this thinking. The change is now unfolding, the NAO and AO are forecast to crash and the PNA to rise. The southern jet is likely to remain active through the rest of the month.

• The February 13-16 period should see temperatures run near to somewhat above normal in the Mid-Atlantic and Northeast. Sunday (13th) could be cooler than normal. Some rain, possibly beginning as some frozen precipitation, could occur late on Monday into Tuesday but amounts won’t be significant.

Temperature anomalies were as follows (2/13-16):
Boston: +6.0°; 2/13: Normal; 2/14 –1°
New York City: +9.0°; 2/13 +3°
Washington, DC: +9.3°; 2/13 +4°

I am increasingly aware that although I had the right idea about a milder period, I have been understating the warmth. That point is duly noted and shows up in the anomalies posted above.

February 14 saw some rain, which briefly began as some snow and ice outside the major cities from Philadelphia north and eastward. In some areas well outside the cities, a small accumulation of snow was reported.

• A clipper could deposit a minor accumulation of snow, particularly from Philadelphia to Boston. Afterward, a cold air mass could send minimum temperatures to the upper teens and lower 20s from Philadelphia northward. The most likely timing for these readings would be the February 18-19 period.

This morning, light snow was falling across eastern Massachusetts, including the Boston metropolitan area.

• After the 20th, the potential will exists for a possible Arctic outbreak. With the NAO projected to tank—and strong support for such a situation—the closing 7-days of February could average much below normal in the East.

Right now, there still appears to be some possibility of this and it is showing up on the February 18 0z run of the ECMWF. For now, it is a matter that will bear watching.

• In addition, there should be accumulations of snow from Richmond to Boston. Some part of the East Coast could see a major snowstorm during that week [week of January 20]. With an active southern jet, abundant cold air available, and increasing blocking, a cold and stormy pattern should set up.

A moderate, overrunning event appears likely to bring some accumulations of snow from Philadelphia to Boston. Accumulations are even possible in Washington, DC and Baltimore. The timing for this event appears to be the January 20-22 timeframe. Richmond appears unlikely to receive accumulating snow from this storm. But the pattern should offer an additional 1-2 threats before the month ends.

• The Southeastern U.S. will likely see colder weather move into the region on the 17th. The cold air mass won’t be all that impressive.

The temperature reached 69° at Atlanta on February 16. Cooler air arrived on the 17th.

• The period of February 21-28 will likely see temperatures average below normal across the region including such cities as Atlanta and Raleigh. The potential exists for at least some snow in Raleigh during this period. Ice even as far south as Atlanta is a good possibility.

It appears that readings during this timeframe will likely average near normal to somewhat above normal for the period as a whole, as the below normal cold likely won’t arrive until late in this period. The odds of snow and ice through at least the first half of this period are looking much less than they did earlier.

• The Central Plains should see a important rainstorm affect the region this weekend (12th and 13th). A cold air mass should bring colder weather to such cities as Omaha and Kansas City by midweek (February 16). Milder air will attempt to return. However, a storm could move across the central or southern Plains by next weekend and bring behind it a significantly colder air mass.

The February 12-13 period saw the following rainfall amounts:
Kansas City: 1.18”
Omaha: 1.15”

The February 13-14 period saw temperatures run 15° and more above normal at Omaha before a colder air mass arrived on the 15th. At Kansas City, the February 13-15 period saw temperatures average 12° or more above normal on each day. Colder air arrived on February 16. The computer guidance continued to point to the return of some milder air for the weekend before colder air would return afterward.

• Rain from the Central Plains should move into the Ohio Valley and Midwest on Monday (February 14). Colder weather should arrive on the 16th and some flurries or light snow could precede it. The February 21-28 period could be colder to much colder than normal.

The February 13-15 period saw the following precipitation amounts:
Chicago: 1.02”
Detroit: 1.12”

A strong cold front moved through the region during the latter part of the 15th (at Chicago) and Detroit on the 16th. Chicago saw a trace of snow. Detroit picked up 2.4” of snow on February 16.

• The Pacific Northwest will see moderate temperatures turn colder for a brief period. However, after the February 13-16 timeframe, temperatures will grow gradually milder. The remainder of the month could see readings average warmer than normal. Before the warmup, the temperature could fall below freezing for at least one night in Seattle.

During the February 13-16 period, Seattle saw temperatures average 4.8° below normal. The temperature fell below freezing in Seattle during the mornings of February 14-16. The February 15 low was 26°.

• California will likely see wetter than normal conditions continue through at least February 20 and perhaps longer as the southern jet remains energetic. Wetter conditions should also extend into the Southwest.

The February 11-12 period saw 2.29” of rain fall at Downtown Los Angeles. This wet pattern appeared likely to continue as a storm headed for California late this week. Flagstaff received 1.81” of precipitation (2/11-12) and an additional 0.21” on February 15.

The February 20-March 5 Ideas:

The mid-month pattern change will continue to evolve over the course of the upcoming week. The NAO and AO are likely to accelerate their descent and the PNA is likely to commence an increase. The blocking is especially pronounced on a number of 2/18 0z model runs, which are now increasingly sensing the synoptic evolution of the pattern. Meanwhile, the southern branch of the jet stream is likely to remain active through the remainder of the month into at least the first part of March. Meanwhile, the Canadian ensembles have continued to shift toward a colder look, especially across southern and central Canada. Warmth is increasingly evident in the Canadian West and far north. If above normal height anomalies can link up, this could be an indication that more serious cold could be in the picture toward the latter part of the period specified above. Right now, that’s not ensured but it is something to watch for, especially as the dual blocking regime becomes more pronounced.

• For now, in the East, the February 20-27 period should be colder than the February 13-19 timeframe (Mid-Atlantic and New England). A storm will likely bring some accumulations of snow from Philadelphia to Boston later Sunday into Monday. The storm could end as some rain especially the farther south one goes. This will not be a big storm and it should be a quick-mover given that the blocking regime is just now beginning to evolve. It likely will not become the proverbial “50/50” low.

The midweek period should see more moderate temperatures, perhaps somewhat above normal readings, especially the farther south one goes. However, another system that will likely move off the lower Mid-Atlantic Coast Thursday into Friday will probably bring some light snow or flurries to parts of the northern Mid-Atlantic and even perhaps southern New England. In its wake should be another cold air mass. This cold air mass will likely be similar to the one now moving into the East for the weekend but has some potential to be several degrees colder.

The February 28-March 5 period should see generally colder than normal readings persist in this region. The threat of a more significant storm could be on the rise.
• The Southeast will probably not see noticeable cold during the February 20-27 period. However, in the wake of a storm that should bring a cool rain to such cities as Atlanta and Raleigh, colder air should move in for the end of the week.

The February 28-March 5 period could average colder than normal.

• Midwest: The first part of the coming week should see moderation in Chicago and Detroit. However, a colder air mass should push into the region by Wednesday or Thursday (February 23-24) and it could bring some accumulations of snow to such cities as Detroit and Cleveland.

• The weekend looks to be balmy in the Central Plains. However, a system moving across the region could bring some rain, ice, or even snow to parts of the area by midweek next week. Behind the storm, readings should be trending downward. Another shot of even colder air is likely the following week.

• Seattle’s prospects are for gradually building warmth after a briefly shot of cooler air brings seasonal to somewhat below normal readings to the Pacific Northwest for Sunday and Monday. Readings should be rising to above normal levels late in the week and should average above normal the following week.

• In California, the rains should continue to soak the region from time to time. An important storm could bring rain to Los Angeles Sunday or Monday. Another storm could arrive late in the week.

The Precyclonic Environment Prior to Big East Coast Snowstorms:

In their outstanding two volume work, Paul Kocin and Louis Uccellini describe the precyclonic environment that has preceded many of the major storms documented by them. With the major pattern change now getting underway, a few of the characteristics they pointed to should be noted:

• An upper-level trough over eastern Canada
• An upper-level ridge over the western United States/Canada (PNA+)
• Sometimes unimpressive short-waves: Prior to the development of heavy snowfall, many cyclone-producing troughs are poorly defined only 1-2 days before the snow events and have not yet consolidated into a significant short-wave trough in the eastern United States.
• Major pattern changes: In some instances, major snowstorms occur as a part of major changes to the larger-scale circulation pattern… In particular, the snowstorm of 5-7 February 1978 occurred as part of a dramatic change in the circulation regime across North America…that saw a blocking pattern emerge with an upper cutoff anticyclone over north-central Canada and a cutoff low over the northeastern United States and southeastern Canada… The January “Blizzard of ‘66” also occurred in a period of major upper-level circulation change… With the President’s Day Storm of February 1979, a major circulation change also occurred but not in the same manner as during either February 1978 or January 1966. This storm occurred during a transition from a persistent large-scale circulation pattern that brought extremely cold temperatures to the northeastern United States to one that became more zonal as the storm exited the East Coast, signaling the end of the cold regime.

Source: Paul J. Kocin and Louis W. Uccellini, [I]Northeast Snowstorms: Volume I: Overview, Boston: American Meteorological Society, 2004, pp.91, 97.

Therefore, with a major pattern change having begun to unfold, persistent signals of a possible major precipitation event(s) on the GFS ensembles, and an active southern jet, there is at least some possibility of a significant East Coast snowstorm. Moreover, one should not necessarily be lulled into complacency by seemingly innocuous-looking short-waves, as such has often been the case 1-2 days before the event.

The highest risk for a major event, which is not guaranteed, would probably be during the first 7-10 days of March once the NAO begins to rise from its forecast very negative levels.

Washington, DC: “Build it [negative nao] and will they [snow] come?”

Some things to consider for the major blocking that is forecast to develop:

• “50/50” lows/Grand Banks Vortices don’t magically appear ahead of blocks. Why? Because there is little in the flow to cut them off except in rare circumstances until the block is strong enough to slow systems down. Usually when a block grows strong enough, at some point, system will cut off and form a “50/50” low. Thus, the fact that there is no “50/50” low right now is meaningless when it comes to looking at the medium and longer-term in the evolving cold pattern. So, no need to despair early on.

• Many of the very strong blocking regimes have seen the development of such a low for part of their life cycle.

• Of the 5 strongest blocking regimes during February, 4 (80%) saw Washington, DC receive 8” or more snow during the February-March period.

• Of the 10 strongest blocking regimes during February, 7 (70%) saw Washington, DC receive 6” or more snow during the February-March period and 5 (50%) saw Washington, DC receive 8” or more snow.

• Of the 10 strongest blocking regimes during February, none saw Washington, DC receive less than 3” snow during the February-March period.

In conclusion, don’t be overly concerned about the absence of a “50/50” low at this point. If history is any guide, such a low will likely develop as the blocking regime strengthens. Odds also strongly favor Washington, DC seeing additional accumulating snow this winter.

[Jrodd312]

Great discussion as always Don. I believe that a monster is out there ready to set up during the February 28- March 5 period.

[donsutherland1]

Thanks, John.

I believe things are still evolving nicely. As I write this here in the NYC suburbs, there's a burst of light snow. I'll be in DC this weekend and do not believe that DC/BWI is out of the game as far as some accumulations are concerned.

Long-range, the pattern still has a lot of potential. The NAO is now showing signs of tanking (< -1). Last night's 0z EC was encouraging and the 6z/12z runs of the NAM are now behaving pretty consistently.

[WXextreme]

Thanks, John.

I believe things are still evolving nicely. As I write this here in the NYC suburbs, there's a burst of light snow. I'll be in DC this weekend and do not believe that DC/BWI is out of the game as far as some accumulations are concerned.

Long-range, the pattern still has a lot of potential. The NAO is now showing signs of tanking (< -1). Last night's 0z EC was encouraging and the 6z/12z runs of the NAM are now behaving pretty consistently.

Don great post!
Others are raising concerns that the Arctic air will retreat back over the pole in this time frame. So, I guess I'm wondering what are your thoughts on this? Will there be a lack of arctic air when the NAO and AO finally tank?

[donsutherland1]

WXextreme,

Thanks for the kind words.

I'm not sure that is the most likely scenario or even a likely one. Model guidance performs at its worst during large-scale pattern changes such as the one that is now unfolding. I believe the 2/18 0z run of the ECMWF hinted against the idea that the Polar Vortex would move over the Pole to Siberia.

[Brett Adair]

Don, Great Post first of all. I must say, that I have to agree with the ECMWF idea of a 50/50 senario shaping up with the PV forming later next week. A split flow pattern like this has been a GREAT setup in the past for a southeast snowstorm. This rather fast and active southern stream will pose as a pump for moisture and energy impulses to invade, now all we need is for a descent PV to setup and allow for the arctic air to invade from the north.

[donsutherland1]

From the February 13-28 discussion from February 11:

...a cold air mass could send minimum temperatures to the upper teens and lower 20s from Philadelphia northward. The most likely timing for these readings would be the February 18-19 period.

Code: Select all

City   18th   19th
BOS    20     13
DCA    23     20
NYC    18     16
PHL    24     17


[donsutherland1]

Thanks, Brett.

I very much like how the ECMWF is evolving. Hopefully, before the pattern ends, its potential for wintry weather will be realized both in the Southeast and Mid-Atlantic. Much of the MA just missed out on the 2/20-21 event (trace of snow at both DCA and BWI), coatings reported beginning in northern MD, and 2" or above (many 6" reports) from PHL to Boston. Of course, that 2/20-21 event was not the only event likely. It's one of several possible opportunities.

[KWT]

the intresting thing is that we have strong northen blocking and a strong southern arm of the jet stream,and i can't see it ending in realible timeframe at the very least,and with the northen jet almost dead(for at least 144hrs)I can't see at least the pattern for northen blocking with a Greenland high ending for another 2-3 weeks yet possibly.

[jimvb]

There seems to be a disconnect between the weather model runners and the weather websites. The Weather Channel calls for moderate weather in the first week of March, with temperatures in the 40s and 50s in the Mid-Atlantic states, and for rain on Feb 28 and March 4 (two dates with important events for me which could be wiped out by snow). Accuweather also calls for temperatures in the 50s and rain on those two dates. But that's not what the GFS seems to be saying. It seems to be saying that a big area of cold is going to come in from Canada with the -10 line going into VA and even the Carolinas at times, and with the 0 line south of the Mid-Atlantic on Feb 28 and Mar 4 and that indicates snow instead of rain.

Is the GFS really going to be right? And why do the weather websites call for warmer weather than the GFS and other models indicate?

[donsutherland1]

Jimvb,

Those websites generally offer a variation of the MOS (usually several hours behind the actual model runs e.g., this morning's figures are probably from the 0z data). In the extended range, the MOS trends toward climatology given that the reliability of modeling becomes less over larger timeframes.

This is where an examination of synoptic patterns can be very valuable. Unfortunately, such an examination requires human effort and it cannot be automated. Thus, at this point in time, it is not possible to use the synoptic information to adjust the website numbers.

Finally, the GFS is not all alone in pointing to a colder pattern than what might be implied on some of the sites. The NAO is now an incredible -5.10 (off the charts) and there is the growing prospect that a major storm early next week could bring unseasonably cold air into the eastern United States. The outcome of that storm is also not yet certain.

[Jrodd312]

Jimvb,

Those websites generally offer a variation of the MOS (usually several hours behind the actual model runs e.g., this morning's figures are probably from the 0z data). In the extended range, the MOS trends toward climatology given that the reliability of modeling becomes less over larger timeframes.

This is where an examination of synoptic patterns can be very valuable. Unfortunately, such an examination requires human effort and it cannot be automated. Thus, at this point in time, it is not possible to use the synoptic information to adjust the website numbers.

Finally, the GFS is not all alone in pointing to a colder pattern than what might be implied on some of the sites. The NAO is now an incredible -5.10 (off the charts) and there is the growing prospect that a major storm early next week could bring unseasonably cold air into the eastern United States. The outcome of that storm is also not yet certain.

That is too negative. lol. If it gets too negative doesnt all the cold air just go to the south? That is what local mets here are saying about next week's possible noreaster.

[Ed Spacer]

I belive the NAMS are actually on settling the low moving closer a bit north in its slingshot into the atlantic.However,if it DOES intensify quicker(which all the severe weatyher in the south seem to indicate) then my 6"-8" estimate may shift to a 12" estimate.The speed will be too fast for a super noreaster.

[donsutherland1]

John,

If the PNA is strongly positive or positive and rising fast, the 2/28-3/1 system should have a chance at coming farther up the coast as the trough would be sharper. If the PNA is flat, it would likely be squashed well to the south and out to sea. The March 1962 system is an example of one storm that went out to sea well to the south due to a generally strongly negative PNA-extreme negative NAO.

[donsutherland1]

Some very brief notes:

The February 27-March 12 pattern discussion should be up some time this evening. The situation concerning next week's storm is very complex and, given the extreme nature of the block in place, is something that the modeling might have difficulty handling.

Having said this, several points are in order:

• Per the earlier discussion on extreme blocks, one should remain wary of the modeling that is pointing to the westernmost tracks for the possible storm.

• The idea that the 500 mb low dropping into the Great Lakes region will "capture" the surface low and pull it inland is not by any stretch of the imagination a done deal. The January 1966 and February 1967 snowstorms saw strong 500 mb lows fail to absorb the surface low.

• The 2/28-3/1 event won't be the last possible winter storm to affect parts of the East.

• The proverbial die has been cast for a colder than normal March in the East while dry weather persists in the Pacific Northwest.

[Fodie77]

Some very brief notes:

The February 27-March 12 pattern discussion should be up some time this evening. The situation concerning next week's storm is very complex and, given the extreme nature of the block in place, is something that the modeling might have difficulty handling.

Having said this, several points are in order:

• Per the earlier discussion on extreme blocks, one should remain wary of the modeling that is pointing to the westernmost tracks for the possible storm.

• The idea that the 500 mb low dropping into the Great Lakes region will "capture" the surface low and pull it inland is not by any stretch of the imagination a done deal. The January 1966 and February 1967 snowstorms saw strong 500 mb lows fail to absorb the surface low.

• The 2/28-3/1 event won't be the last possible winter storm to affect parts of the East.

• The proverbial die has been cast for a colder than normal March in the East while dry weather persists in the Pacific Northwest.

I'll be looking foward to your discussion.