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. So use this at your own risk!!

3 Review of My DJF Forecast Actual DJF Temps Overall, the temperature forecast worked out well in general. I was off over the Plains and underestimated the warmth in the Southwest. But got the general cold east, warm west, idea right. I was right for wrong reasons though as I thought AO/NAO would be negative, but instead were very positive. Main cold driver was EPO/-WPO/+PNA.

4 Winter A strong El Nino will continue through the winter of and this combined with a strong +PDO and other factors appear to point to a rather mild/warm winter for the central/western US with cooler than normal temperatures confined to the Southeast.

5 ENSO State A strong El Nino is ongoing with the strongest warm anomalies from near the dateline to the coast of South America, a canonical look..

6 MEI Index ENSO continued YEAR Jan Feb Mar Apr May June July August Sep Oct Nov Dec The MEI (Multi-Variate ENSO Index) which is probably the most robust and allencompassing ENSO index has been in El Nino territory since March and has strengthened into the strong range in recent months. The Sep value is the 2 nd highest on record. Weekly SST Anomalies ENSO Region Week Aug Aug Aug Aug Sep Sep Sep Sep Sep Oct Oct Oct Currently the El Nino has a rather classic look to it, with the warmest anomalies centered mostly east of the date line (regions 3 and 1.2), not a Modoki (central/western based anomaly pattern regions 3.4 and 4) style. Region 1.2 has cooled in recent weeks, but I am not sure I am ready to say that is going to continue.

7 10/24/15 10/24/97 10/24/82 10/24/72 10/24/91 10/24/87 10/24/86 10/24/02 10/24/09 Comparison of SST Anomalies vs. other years with moderate to strong El Nino s. A comparison of how the SST anomalies looked in some other El Nino episodes at this time is helpful. You can see that no two events look the same, so every El Nino episode will have its own flair. Just looking at things generally 1997, 1982, and 1972 seem to be the closest matches in the ENSO regions. There is much more in the way of warm water in the NE Pacific from the dateline east in 2015 than any of the other years.

8 ENSO continued Available ENSO SST plume forecasts show region 3.4 values peaking in Nov/Dec then weakening through the winter. This is true of the modeling for all ENSO regions. Will this weakening occur and if so does it matter? Region 3.4 will likely begin the winter around C and then possibly weaken to 1.5-2C by Feb. Only 82-83, were stronger since 1950.

9 Strong ENSO Composites Top 4 highest MEI values for December-February period since , , , The commonalities are a strong Gulf of Alaska low, +AO/+NAO signatures, and warm ridges over SE Canada and Europe. Warm winters for many in US. The strong GOA low promotes a mild Pacific flow for North America during this type of winter. This combined with a lack of blocking produces the warm temperature composite on the right.

10 December January February March Dec-Feb

11 I am not a big fan of using data before 1950 due to the lack of observational platforms, but using pre 1950 ENSO region 3.4 anomalies, these 6 winters could also be classified as strong El Nino s. Somewhat similar temperature anomaly patterns to other 4 years post 1950.

12 ENSO Conclusions The assumption is that the current strong El Nino will peak in November/December and then weaken in magnitude as the winter progresses but not rapidly. This rate of weakening could be important. Some events that have seen a moderate to strong El Nino magnitude in the Fall and early winter have seen a cooler turn to the weather pattern for mid to late winter that corresponded with a weakening magnitude of El Nino. Coincidence or Causation? This El Nino will likely not be a Modoki style event during the winter but will be a full basin event with regions 3 and 1.2 remaining on the same level anomaly wise with regions 3.4 and 4 through much of the winter. Despite some similarities with the 4 other strong El Nino winters shown, significant differences remain not only in El Nino spatial SST patterns, but also in other areas of the Pacific, Atlantic, and Indian Oceans. Which means other factors could lead to a different result.

13 PDO State Classic +PDO look to North Pacific. +PDO SST Anomalies -PDO SST Anomalies The PDO has been strongly positive for a while and there is no reason as of now to think phase changes is coming soon.

14 PDO continued Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year Sep DJF ? The PDO value has been > 1 since September of To the left are the top 20 September PDO values and their subsequent winter DJF PDO phase. Every year saw a DJF period feature a +PDO on average with the average DJF value of The years in that list that were also El Nino winters were, (weak/moderate), (strong), (moderate), (moderate/strong), (strong), (weak), (weak), (weak), (strong), (weak), (weak), (weak), (moderate). So statistically we are very likely to see a +PDO this winter. My guess is that our average DJF PDO value finishes in the 1.25 to 1.75 range. The best years from this list of analogs for winter match from ENSO/PDO could be , , , , ,

15 PDO continued Due to the current NPAC SST anomaly configuration, the fact we are currently in an strong El Nino state in the Pacific (which often times coincide with a +PDO), and statistical evidence, I predict we will see a strongly positive PDO this winter.

16 PDO continued Notice the commonalities in these 9 DJF period. Positive anomalies in NW US /western Canada. AO/-NAO signature. Strong sub- Aleutian low and below normal heights over eastern half of US into Europe. All December-February periods averaged 500mb anomalies for DJF PDO >= 1. So a strongly positive PDO would argue for more ridging over western North America and into the polar regions.

17 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. We have moved into a positive (westerly) phase. These phases usually last on the order of months meaning that we should see the positive phase last through the winter. There has been numerous papers and research 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. This correlation did not work out last year. But taken in a vacuum we could assume the QBO phase this winter may favor less high latitude blocking. Again I have low confidence in this indicator as recent winter ( ) and ( ) bucked this correlation significantly.

18 This correlation did not work last year and when looking at the top 10 +QBO DJF, I did not see an opposite pattern. So at best there appears to be a weak correlation with QBO winters and little with a +QBO winter. -QBO 500mb DJF Anomalies This composite is for DJF periods where the QBO average -10 or less (17 winters since 1950). Notice the above normal heights near the North Pole and subsequent below normal heights in the midlatitudes including the central and eastern US.

19 QBO continued The QBO years/winters that I think are the best fit to this year are , , and They produce the 500mb anomaly pattern to the left. This small sample sized composite looked to favor a +AO/NAO, a stronger than normal GOA low, and some troughing over the SE US. Some commonalities with the strong ENSO composites.

20 AMO State There is a large pool of cold water in the north Atlantic, but plenty of warmer than normal water across most of the rest of the north Atlantic leading to a current +AMO state.

21 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 be slightly warmer than normal. 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)

22 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 the stronger the Siberian Fall surface high is in October, which can lead to a disrupted stratospheric polar vortex which can then transfer to the troposphere during DJF in the form of AO. Cohen.

23 Snow Cover continued Above are two graphics showing snow cover extent anomalies compared to normal and to the last 10 years (left from Environment Canada and on the right from NESDIS). Currently we appear to be a little above normal as far as coverage, but not as significantly as we were in It is the rate of change to watch as well.

24 Snow Cover continued Top 5 Oct Eurasian Snow Cover Years DJF composites Bottom 5 Oct Eurasian Snow Cover Years DJF composites Using data from the university of Rutgers You can see the top 5 years show lots of blocking in the Polar Regions and a AO configuration, while the bottom 5 years show below normal heights at the Pole and a +AO configuration.

25 Snow Cover Conclusion So far the amount of October snow cover in Eurasia and it s apparent advance this year would seem to favor the AO influence proposed in Cohen s theory. However, after setting records last winter with Eurasian snow cover, the AO was positive for the winter, so use with caution.

26 Seasonal Model Forecast for DJF JAMSTEC CFS IRI NMME

27 Putting It All Together I think our strong El Nino will influence the pattern this winter and lead to some very typical responses to the atmospheric weather pattern. An more active then normal subtropical jet stream, strong Gulf of Alaska low, and warmth over Canada and the northern US. The strong positive PDO though will favor more ridging over western North America then is typical with stronger El Nino events, and thus I think there will be some bouts of arctic air this winter. When this occurs, combined with the active southern branch, we will see potential for major winter storms, particularly across the southern and eastern tier of the US. In general though this will be a very mild winter for the northern Rockies, Midwest, northern Plains and Great Lakes. I think we will see less arctic air outbreaks this winter then in the previous 2 winters.

28 What Could Go Wrong? While I do expect some commonality with previous strong El Nino winters, blocking is always a wild card. Blocking is very difficult to predict and while there is some correlation with solar activity, Eurasian snow cover, QBO phase, it is largely very unpredictable. If the summer blocking tendency carries over to the winter, we could see a very cold/snowy winter. I expect the El Nino to remain strong through early winter and only slowly weaken. If there is a more rapid weakening, once could argue the forcing associated with the El Nino could weaken and we could see a different global weather pattern develop as the winter progresses. What that would be is not clear. The warm waters in the Northeast Pacific promote more ridging and +PNA/-EPO configurations which would favor more cold than forecast. There is more warm water in this region currently then seen in the other strong El Nino winters. Exactly how will the atmosphere respond to this if at all?

29 Analogs The analogs I like most this winter are and with , as my best secondary analogs. I also gave some considerate to , , and as lesser analogs. All have some issues. But I like different parts of each.

30 Maps December 2015 Temp Anomalies January 2016 Temp Anomalies December will likely be the warmest month of the 3. Many El Nino composites show there is generally more warmth in early winter compared to late winter. I think January could be a transition month to a cooler patter over the Southeast and Eastern US in general.

31 Maps February 2016 Temp Anomalies Dec-Feb Temp Anomalies February has the signal for the coldest month and it wouldn t shock me if it ends up colder then my current map shows. Overall I am forecasting a warmer than normal winter for the north and cooler than normal for the south, but for no where to see an extremely cold winter. I am similar to strong El Nino composites, but a little cooler as I do think the expansive warmer than normal waters in the NE Pacific could help promote period of a EPO/+PNA which will lead to cold interludes particularly in late January through March.

32 Maps Cool Season Snow Anomalies I expect a rather storm winter along the southern and eastern tier with opportunities for snow when cold air is present. Keep in mind though, snow forecasts are not going to very accurate usually as there are so many variables present.