Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 07:50 PM GMT del 18 novembre 2009
Good afternoon!!! Since the weather pattern has been relatively benign the last few weeks and that is going to continue, I thought it would be interesting to examine warmer than normal Novembers and see if there is any direct correlations to patterns in the winter. Meteorology forecasting is based on several things one of them including climatology and past weather patterns. The better knowledge a forecasting has of past weather conditions, the better forecast a meteorologist will have especially medium and long term range forecasting. While one could say seasonal forecasting is just a guess, much of it is based on solid meteorological logic on past weather patterns and current conditions against previous years. But what makes it so difficult is that every season is unique and completely different from the next, so these analog years only can be a base. Much of the past weather temperature departure data below is courtesy of the Millersville University Climatology data for Lancaster, Pennsylvania Link. I thought this would be a mean location for the Northern Middle Atlantic including the states of Pennsylvania, Maryland, and Delaware. Currently the culprit or catalyst for the warmer weather east of the Mississippi is the Alaska Vortex carving a trough across central Alaska with well below normal temperatures and widespread snowfall.
Anchorage, Alaska Tower Cam...
Fig. 1- Many parts of Alaska are coated in several inches to feet of snow. Also lows of -25F are common across the northern tier near Barrow, Alaska these past few nights. Courtesy of Tazmanian and his tower cams on his blog.
This Alaskan Vortex was the main culprit for the last few winters and problems with inland storm tracks across the continental United States. But there are signs of this vortex retrograding and I will talk about the more towards the end of the blog. Also another issue is a displaced polar vortex. Currently the eastern Hemisphere is experiencing well below normal temperatures especially towards Asia. In fact Beijing, China experienced there heaviest snowfall on record this past week. This polar vortex is displaced and allowing a milder flow over the western hemisphere including much of North America. And will still have ongoing building cold air over the northern Arctic where temperatures have been well below normal. Also with above normal heights near the Hudson Bay of near 534dm, this is preventing any conformed blocking from the Hudson Bay to Greenland. But what is interesting and I think many people are beginning to forget is this is relatively typical for November. In fact take a look at Denver, Colorado. They are currently under another winter storm watch for 6-12inches of snow. November is one of their snowiest months of the year, so this pattern of western troughs with below normal temperatures in Alaska and displaced polar vortices is very common this time of year. But the question is how long this pattern will continue as it varies from year to year. Take 2006-2007, that winter featured an extremely mild November, December, and first half of January courtesy of all of the above conditions remaining stationary. But then a pattern change and shift in the global jet allowed for one of the coldest Februarys on record with several large snowstorms during the rest of the winter over the eastern half of the United States. So the first objective for the blog will be to analyze pure temperature trends from November to winter since 1914. Reminder these temperature records are out of Lancaster, Pennsylvania... My winter mean temperature consists of December, January, and February months.
Normal Departure Novembers... (+.5SD to -.5SD)
10/37 winters had a normal temperature mean
12/37 winters had a above normal temperature mean
15/37 winters had a below normal temperature mean
*Note: Generally equal chances follow temperature patterns for normal mean temperature Novembers. Little to no correlation
Above Normal Departure Novembers... (x>.5SD)
12/32 winters had normal temperature mean
7/32 winters had above normal temperature mean
13/32 winters had below normal temperature mean
*Note: Again small correlation but looking through data warmer than normal Novembers are followed by near normal or below normal temperature winters.
Below Normal Departure Novembers... (x<.5SD)
9/25 winters had normal temperature mean
8/25 winters had above normal temperature mean
8/25 winters had below normal temperature mean
*Note: Zero correlation for below normal temperature Novembers correlating to temperature patterns for winter.
As you can see there are very few correlations between November and ensuing winter average temperatures. But the highest correlation is between above normal temperature Novembers followed by normal or below normal winter mean temperatures. Here are a few interesting winters with severally altering weather patterns from November to winter (December, January February)
Out of those above five winters, one of them stands out as far as temperature patterns quite similar to November 2009. I think most areas will likely top out around +1.8 to +2.5 for the Northern Middle Atlantic for mean temperatures this November 2009. Yes I know that completely throws off my November forecast, but O well. Anyways 1961-962 and 1977-1978 have very similar temperature patterns for this November correlating to colder winters. I thought it would be appropriate to note the global patterns during those Novembers and correlate to the ongoing ENSO and global jet patterns of November 2009 to see if we can make any connections. As noted above the final mean temperature of November does not appear to have much effect on the ensuing winter mean temperature with generally equal chances. But I thought it was interesting to note the lowest mean proportion was above normal Novembers to above normal winters with only a 22% chance.
A glance at 1977-1978...
Fig 2- Courtesy of the NCDC. In general much of the nation featured above normal temperatures with a very similar temperature spread as November 2009.
Upon first glance at the ENSO records from November 1977 an ONI reading of .6 was read for an October, November, and December reading. This was part of a warm phase. Similar readings in 2009 with a reading of .9 found from August, September, and October. 1977-1978 featured a weak El Nino, unlike this year where forecasts will likely top out around moderate El Nino status at the height of the ENSO event somewhere around December. From past history archives weak El Nino events are less volatile as far as weather patterns go, therefore if a cold pattern establishes early in December, then it is likely to last through a majority of the winter. Where weak El Nino events tend to have more transient weather patterns without dominate temperature patterns establishing for long periods of time. In no way or another do I believe this winter will feature the arctic blasts that occurred in 1977-1978 making it one of the coldest on record. I did do some jet stream analysis in the 500mb range during the entire month of November 1977 finding some interesting similarities... Link. First off there were a series of cutoff lows across the nation particularly the south along with a series of winter storms across the Rockies. An Atlantic high offshore helped to funnel in the warmer air across the east along with a southeast ridge towards the end of the month. The last interesting note is the state of the PDO during 1977. It was near neutral or slightly positive, which is nearly identical to this year. The ENSO and PDO is a pretty close match to 2009. Again I really do not believe we will see the record cold of that winter as already evident, patterns seem very progressive and transient. But I think the connection can be made correlating a similar November temperature anomaly to a following below normal winter and/or only December.
A glance at 1985-1986...
Fig 3- Courtesy of the NCDC. A much larger temperature contrast across the nation with well below normal temperatures in the west with well above normal temperatures in the east.
Immediately looking at 1985-1985 it was featured as a weakening La Nina year with generally neutral Nino 3.4 region sea surface temperature anomalies. Also important to note a developing El Nino occurred towards the latter of 1986. Pretty much this can be thrown out of the analog bin for that reason alone. But my interesting note when I looked at the 500mb analysis for the month (see link above), I found a very similar jet stream especially towards the east coast with a strong southeast ridge during the middle and end of the month with a deep trough and coastal storm during the first week of the month. This is pretty similar to the actual pattern switch timing for November 2009. Other than this interesting jet stream analysis, 1985-1986 is not an analog for this year as the ENSO conditions remain severely different.
So in conclusion November temperature trends have very small correlations but the dominate correlation is a warmer than normal November allowing for a near normal or colder than normal winter. Temperature anomalies so far for November 2009 range from (+1)-(+1.3) over much of the Northern Middle Atlantic. With little relief from the warmer than normal weather pattern temperature anomalies for the month will like top off around +2F or so, but is there a pattern change coming. The second half of this blog will try to take a look at that scenario. First of all yes I believe a pattern change will begin to unfold in the ensuing weeks particularly after Thanksgiving. Guidance is suggesting a return to transient troughs around the fourth week of November towards Thanksgiving. But during pattern changes the computer models can be a bit too anxious with pattern changes resulting a week or so later. At this current time I see a pattern change towards the first week of December with hints at the pattern change after Thanksgiving with weak troughing especially over New England.
Here is the current mean pattern for most of November...
Fig. 4- Courtesy of Penn State Meteo. A few things of interesting note already some of which mentioned earlier in this blog. The two areas of well below normal temperatures globally are across Asia and Alaska where the polar vortex remains displaced. Current PNA conditions remain negative along with a positive AO. This is allowing for abnormal weather 1000-500mb temperatures of near 540dm even towards the Hudson Bay. There has also been some slow ice development over the Arctic towards Greenland and that is courtesy of the southwest flow. No blocking is evident over the northern Atlantic, unlike October which featured strong rex blocking over Canada. But signs of a balance of equilibrium is occurring as the global jet and balance of the equatorial Pacific may allow for some colder air to enter the continental United States. First let’s look at the Madden-Julian Oscillation...
Fig 5- GFS ensembles are forecasting a general progression to phase 3 with weak forcing then possibly progressing back to phase 2.
Fig 6- Courtesy of Raleighwx. Phase 2 composites for December generally suggest widespread troughing over the east coast.
The MJO remains highly variable so forecasts remain very uncertain. It also does not appear MJO forcing will be too strong towards the 10-15 day range, which may limit its impact on the global 500mb pattern. Interesting to note also is that the HIRES GFS even suggests a higher chance of retrograding back to phase 2 for the MJO towards December. Looking at teleconnnections we are beginning to see a slight turn to a more favorable negative west-based NAO. Link. GFS and GFS ensembles are beginning to start to compromise towards the pattern for the start of December. ECMWF weeklies also suggest a similar pattern with towards the end of the run on the 11/14/09 0z run favoring some positive PNA ridging and a negative EPO. In general it appears the Alaska vortex with retrograde back as the favorable Aleutian Low, which we saw during much of October. My concerns though remain that some of the long range guidance suggests a continued active north Pacific storm track, which favors troughing over the west. But at least now guidance is beginning to suggest an end to this benign weather pattern. What I do not see is any extreme arctic blast, just a general relaxation in the ridging pattern over the east coast. My early thoughts suggest a volatile pattern in December with near normal temperatures. I think I can generally make the statement that chances of a snowstorm over the northern Middle Atlantic this November are very slim and the only chance would be during the last week of the month or so only if the pattern could find the needle in the haystack; that is we would need luck. But come December changes are coming, while small at first, they may be big in the long run. Anyways have a fantastic day!!!
"Here northeast of Harrisburg 2009-2010 winter statistics"
Current Snow Cover- 0inches
Monthly Total- 0inches
Seasonal Total- 0inches
Winter Weather Advisories- 0
Winter Storm Warnings- 0
Ice Storm Warnings- 0
Blizzard Warnings- 0
Freezing Rain Advisories- 0
Winter Storm Watches- 0
Lowest High Temperature- 41degrees
Lowest Low Temperature- 23.5degrees
Wind Chill Advisories- 0
Wind Chill Warnings- 0
(Snow Storms Stats)
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