Winter Forecast. Allan Huffman RaleighWx

Transcription

1 Winter Forecast Allan Huffman RaleighWx

2 Disclaimer Seasonal forecasting is difficult and you are always learning. I attempt to look at all factors I understand and have seen correlate in the past and put together a forecast. Sometimes it works sometimes it doesn t. So many factors go into a forecast and one that isn t accounted for properly can wreck a forecast. This process is highly nonlinear. So use this at your own risk!!

3 Review of My DJF Forecast Actual DJF Temps Last winters forecast did not work out well. It ended up being cool/cold in the NW/Northern Rockies, and warm over much of the rest of the country. It is possible there was some lag in the warmth accumulated by the super El Nino of that allowed warmth to linger longer than I thought it would. But ultimately it was a neutral ENSO, +PDO, strong +QBO, +AMO winter, and there were not many analogs to that was the best match from that perspective, and it did feature a DJF with above normal temps in the Midwest/NE and cool west, but was overall much cooler than It was a bad forecast, not much left to say except try to learn and move on. I did say that last winters forecast had much lower confidence than the usual low confidence, so I wasn t too surprised it busted.

4 Winter Headlines The climate index assumptions are that ENSO will be in a weak La Nina state, the PDO will average neutral to weakly positive, the QBO will be in a strong easterly state (-), and the Atlantic (AMO) will remain warmer (+) than normal. So LN, +PDO, -QBO, +AMO. My thinking is that we will see a very typical weak La Nina type look to the jet stream pattern over the Pacific but the Atlantic holds the key to how much if any cold patterns the East/Midwest see this winter. The thinking is that we will see bouts of a NAO, but probably but on average more +NAO patterns then NAO patterns. I expect the 1 st ½ to 2/3 of the winter to likely lack sustained arctic cold in the eastern 1/3 of the US, but we could see potentially a cold close to the winter for the East.

5 ENSO State We currently look to be in a borderline weak La Nina, from a SST anomaly perspective, but at the moment we are not seeing rapid cooling in any ENSO region and notice the coolest anomalies are located over eastern equatorial Pacific regions.

6 MEI Index ENSO continued YEAR Jan Feb Mar Apr May June July August Sep Oct Nov Dec The MEI (Multi-Variate ENSO Index) is probably the most robust and all-encompassing ENSO index. After a brief flirtation with El Nino status in the late Spring and early Summer the index has been falling through the neutral category the last few months. Weekly SST Anomalies ENSO Region Week Aug Aug Aug Aug Aug Sep Sep Sep Sep Oct Oct Oct Oct ENSO SST trends have been rather chaotic the last few months. Overall regions have been trending cooler and in general eastern regions (1.2 and 3) have been cooler w.r.t normal than western regions (3.4 and 4) have been.

7 ENSO continued Available ENSO SST plume forecasts are in general good agreement. They show a definitive turn to a weak La Nina by November with the event peaking around -1C in region 3.4 in Dec/Jan and then warming back to neutral by March. In general, I think we can see most model data, real time SST temps, and subsurface data supports a weak La Nina forecast this winter season.

8 DJF periods where MEI averaged -0.5 to -1 Weak La Nina Composites DJF periods where region 3.4 averaged -0.5 to -1 I plotted up composites of weak La Nina ranges for the MEI and region 3.4 to see if there were any signals. Use these with caution, I just wanted to see any potential signals. We definitely see a tendency for positive height anomalies in the Gulf of Alaska, Bering Sea, and into Siberia as well as over Greenland. With a negative anomaly signal over western/central Canada, northern Rockies/Plains of the US. Cool favored in NW and N Rockies/Plains, but interesting the MEI/3.4 groups differ over the SW and SE. In general cool is favored in the north, and warmth in the south.

9 Other ENSO thoughts. Another factor I looked into, was the distribution of SST anomalies across ENSO regions. The theory being if warmer waters (relative to departure from normal) are centered say over the Date Line as compared to in the eastern equatorial pacific then this could promote higher than normal convection over the DL area, and perhaps encourage more MJO phase 8/1/2 behavior. Instead of using the TNI (trans Nino index (region 1.2 minus region 4), I just averaged regions 3.4/4 and subtracted the average of regions 1.2/3 from this to give me an idea on where the warmer/cooler anomalies set up. Positive values indicated positive SST anomalies were situated more over the central/western eq. Pacific and negative values indicated the opposite.

10 Weak La Nina Years where regions 3.4/4 anomalies averaged greater than regions 1.2/3. Weak La Nina Years where regions 3.4/4 anomalies averaged less than regions 1.2/3. Notice that in both composites strong negative height anomalies are favored in western Canada and positive anomalies in the Gulf of Alaska. The main difference is that the years with warmer anomalies over the central/western Pacific favored more Alaskan ridging and cross polar flow as well as blocking over Greenland and a widespread colder pattern for the US from the Rockies east, while the opposite pattern favored a more expansive SE ridge, a less classic NAO look and seemed to favor bottling the coldest air in Canada.

11 The current SST anomaly map certainly shows a look where regions 3.4/4 anomalies would be warmer than regions 1.2/3 anomalies. This evolution of this is somewhat hard to predict going forward, but most models do show the colder anomalies being over regions 1.2/3 through the winter as of now.

12 Region 3.4/4 warmer than Region 1.2/3 DJFs Year Reg 4 Reg 3.4 Reg 3 Reg 1.2 (3.4/4-1.2/3) Region 3.4/4 cooler than Region 1.2/3 DJFs Year Reg 4 Reg 3.4 Reg 3 Reg 1.2 (3.4/4-1.2/3) Above I have listed the weak La Nina years where regions 3.4/4 were warmer anomaly-wise than regions 1.2/3 (top) and the opposite on the bottom. I also included my predictions of where I think each region will average this DJF. As you can see if I am right, then would be in the top group of weak La Nina DJF seasons. That would produce an ENSO analog list of: , , , , , , , and

13 December ENSO Analogs Of the 8 weak LN years, I chose as analogs, the composite message is warmth over the eastern half of the country for December with the cool weather over the 4 Corners/SW. Obviously there is variability as always.

14 January ENSO Analogs Of the 8 weak LN years, I chose as analogs, the composite message is cooler than normal weather over much of the country, except Florida. The 500mb signals are for a NAO, and a strong +TNH. Of course 1967 and 1975 were quite warm.

15 February ENSO Analogs Of the 8 weak LN years, I chose as analogs, the composite message is cooler than normal over the Midwest/Lakes/NE/MA and warmer in the SW and Gulf Coast. The 500mb signals are a NAO and continued +TNH.

16 ENSO Conclusions The assumption is that we will have a weak La Nina ENSO state this winter with an MEI and ENSO region 3.4 averaging in the -0.5 to -1C range. It seems likely that eastern regions of the equatorial Pacific will be cooler (anomaly wise) then central/western regions. This theory may or may not have some legitimacy. I used it to separate out different weak La Nina years. ENSO state is just one factor that can have an influence on the seasonal global circulation pattern. Thus it is best to use this in consideration of other factors.

17 PDO State +PDO SST Anomalies -PDO SST Anomalies The north Pacific SST anomalies are a bit chaotic at the moment. It is probably best to characterize PDO as neutral to weakly positive at the moment. As the current SST anomaly images more closely resembles the +PDO composite.

18 Subsurface Cool subsurface water in central North Pacific. Warm subsurface water near western North America The 150 m depth averaged SST anomalies show a neutral to modest weak +PDO signature with warm subsurface anomalies right near the west coast of North America and colder subsurface anomalies in the central North Pacific. This could argue that we may see a neutral to weakly positive PDO continue into the winter. However, the signal is certainly not robust thus predictability is low. It would seem a raging PDO is not in the offing anytime soon though. Thus the best bet is neutral to weakly positive for the PDO phase this winter.

19 PDO continued Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Using the JISAO PDO method, the monthly PDO value has been positive every month since January However the value has been rather weak the last 3 months, with the average for JAS only 0.17, so effectively neutral The current depth averaged SST anomalies combined with persistence, and the fact that I don t think we see the La Nina become too strong leads me to forecast a neutral to weak positive PDO this winter with the JISAO PDO calculation averaging between 0 and 0.75 for the Winter. However, this is low confidence. Only 3 years from my weak La Nina analog group featured a positive PDO (1.01), (0.5) and (0.47). Those years tended to favor cold in the N. Rockies, Plains, Midwest, and TN/MS River Valleys. With weaker cool signals in the East/NW and warmth in the SW.

20 PDO Summary As of now I favor a winter PDO that will average in the 0 to range. I make this argument based off the current weak +PDOish look to the depth averaged SST anomalies in the Pacific, persistence, and my forecast lack of a moderate/strong La Nina which would serve to turn the PDO to a more sustained negative look. However, confidence is low and if the forecast were to bust, I think it would bust towards a weak negative PDO. Those years tended to be warmer, particularly over the central US.

21 QBO State Year J F M A M J J A S O N D The QBO is a measure of equatorial stratospheric wind direction oscillations. The QBO switched from a positive (westerly) phase to a negative (easterly) phase this summer. Right now it appears the easterly phase will likely continue and be at strong levels through the winter of There have been papers and research performed that has linked the QBO to high latitude blocking, particularly in the winter. The correlation has been that negative phases (easterly) of the QBO in the winter can be linked to increased blocking and negative phases of the Arctic Oscillation (AO) and the opposite can be true with a positive(westerly) phase. A more negative AO means cold air is displaced from the polar regions into the mid-latitudes. My own research shows that the strongest correlation with a -AO for a DJF period is when the DJF QBO averages in the -6 to 1.5 range. 16 of 18 DJF periods with that QBO average value had a DJF AO. I expect the QBO this winter to average at -15 or lower. There have been 9 winters when the QBO has averaged that negative, , , , , , , , , and of those 9 winters saw a AO, although only 4 of those 9 winters saw a NAO. Of those 9 winters, 4 could be classified weak La Ninas, , , , and Unfortunately there was no clear trend with those 4 winters. 2 of the 4 had a NAO. So while a negative QBO could certainly tend to favor higher latitude blocking this winter, there is not enough evidence of a linkage of a strong QBO to a weak La Nina to significantly factor it in to my forecast.

22 QBO Summary Year J F M A M J J A S O N D My prediction is that the QBO will remain easterly this winter and average at -15 or less for the DJF period. However, no clear link was seen in weak La Nina years and strong QBOs to AO phase. My conclusion would be that perhaps the negative QBO will slightly favor increased blocking but that is certainly not said with high confidence. If one tried to drill down a little bit more and link this with the AMO, only one of those 4 weak La Nina winters with a strong QBO had a +AMO and that was which had a very strong AO for the winter. But with only 4 years to look at, there is certainly not enough there to base much on.

23 AMO State Currently warmer than normal SSTs abound in most of the Atlantic, indicating a solid +AMO at the moment.

24 AMO Year J F M A M J J A S O N D The AMO has been mostly in a warm (positive) phase since I expect the Winter AMO to continue warmer than normal based off of persistence of the current warm phase and the current look to the sea surface and subsurface temperature anomalies. The main correlation I use, is to favor winter analogs that also fall in warm long term AMO phases. (1995-present and 1940s-early 60s)

25 Eurasian Fall Snow Cover/Advance Research in the last decade by Judah Cohen and others, have linked October Eurasian snow cover and the rate of change to the phase of the winter AO and the subsequent winter temperature pattern pdf Not only the amount of snow cover but the rate of change in October are considered important The summary is that the more snow covered Siberia is in October and the quicker the snow cover advances particularly south of 60N, the stronger the Siberian Fall surface high is in October, which can lead to a disrupted stratospheric polar vortex which could then transfer to the troposphere. during DJF and result in a AO confguration. Cohen.

26 Snow Cover continued The graphic on the left shows snow cover extent anomalies for Eurasia compared to normal (from Environment Canada) and then graphically you can see it for the Northern Hemisphere. Currently we appear to be above normal as far as coverage goes. However this correlation did not work well the last couple of seasons, so for now I give it only slight consideration.

27 Seasonal Model Forecast for DJF JAMSTEC CFS IRI NMME

28 Putting It All Together Confidence is increasing that we will see a La Nina this winter, most likely in the weak range. Composites of weak La Ninas favor a cold western/central Canada, interior Pac NW, N Rockies, and N Plains. However, there is variability in other regions. My forecast is that the combination of an equatorial SST anomaly distribution where the coldest anomalies are further east (which could favor above normal convection near the Date Line), strong QBO (which could favor increased chances for high latitude blocking), neutral PDO, and +AMO will favor a winter that is colder than normal over the N Rockies, Interior NW, N Plains, and changeable across the Midwest/eastern US. I feel pretty good that the weak La Nina composite Pacific pattern will likely verify, but the Atlantic pattern is questionable. The trend in recent winters and even this Fall is for a positive NAO. Which would likely lead to a warm winter for the East.

29 Analogs Year MEI PDO AMO QBO Reg 4 Reg 3.4 Reg 3 Reg 1.2 Solar NAO AO PNA EPO WPO (3.4/4-1.2/3) ????? I see the above winters as the best matches for the upcoming winter with my projections for some of the climate index values for this upcoming winter. As is usual there are no perfect matches, but in looking at the projected ENSO region anomalies and orientation, weakly positive PDO, positive AMO, negative QBO, and low solar <1000. From the above list is one of the best matches. If the PDO were to dip weakly negative, emerges as a pretty good analog. Both of these winters were rather cold and featured a NAO and AO. The reality though is that it has been since the winter of since we last saw both the AO and NAO average negative, and the winter NAO hasn t been negative since that winter as well. If the NAO/AO are both positive, then the warmer winters of and are more viable, although both those years had some climate index differences with what I expect, as both had negative AMOs and negative PDOs.

30 Maps December 2017 Temp Anomalies January 2018 Temp Anomalies I believe we will have a hard time sustaining any AO/-NAO through the first half of the winter. Thus I expect a rather mild/warm December, particularly over the eastern US. Some arctic air masses will make inroads into the NW, N Rockies, and N Plains but I expect little significant progress into the East. As we head into January, I do think the arctic air mass strength will increase w.r.t normal, and further inroads to the south and east will occur. However, I do expect the southeast ridge to fight back and thus the East will be rather variable with some impressive cold shots but also impressive warm-ups as well. The January pattern if I am right, could produce some ice storm threats across the Ohio Valley, Mid-Atlantic, and Northeast.

31 February 2018 Temp Anomalies Maps Dec-Feb Temp Anomalies I do think as we head into late January and February, that we could see a period of AO/- NAO. My analog package favored late winter cold as opposed to early winter cold. Thus, I think the major winter impacts will likely come this month for the Midwest/Lakes/East, with a mild close to the winter on the West Coast. Overall, I think the winter will feature some periods of a negative AO/NAO, but overall likely average positive. If this occurs at stronger levels then I think, then overall winter will be warmer then I show, especially in the East. However, if I am wrong about the AO and NAO, my forecast will bust too warm. Right now, it is hard to predict a AO/-NAO due to the persistent winter trend this decade, and current short term trends.

32 Maps Cool Season Snow Anomalies I expect snowfall to be above normal for the cool season in the interior NW, N Rockies, N Plains and western Lakes, with below normal snowfall over the central/southern tier of the US.

33 Final Ramblings Seasonal forecasting is difficult, therefore I fully expect to miss aspects of this forecast, hopefully more right than wrong. Right now, I am banking on the AO/NAO remaining mostly positive the first 1/2 to 2/3 of winter. This is due mostly to persistence, so that assumption could be dramatically wrong which would throw at least my Dec/Jan forecasts into the toilet. I do think the analog package favors a colder back ½ of winter as opposed to a first ½ of winter. I feel rather good that we will see a typical weak La Nina type jet stream pattern in the Pacific/North America but the Atlantic may be key. If there is no blocking, the eastern US/Lakes could see a very warm winter. My thinking is that we will see more +NAO regimes then NAO regimes, but that we will not be completely shut out on that end.