Winter Forecast. Allan Huffman RaleighWx

Transcription

1 Winter Forecast Allan Huffman RaleighWx

2 Winter Combination of weak/moderate El Nino/+PDO/-QBO and well above average snow cover and snow cover increase this Fall in Siberia point to a winter featuring extensive blocking and colder than normal temperatures for the eastern/central US.

3 ENSO State Most of the equatorial Pacific is warmer than normal.

4 ENSO continued 60 Daily SOI Series As of 10/27, since 8/3 only 11 days have reported a positive SOI value. So one can say the SOI is in a persistent negative phase indicative of El Nino conditions.

5 ENSO continued The trade winds have weakened in recent weeks indicative of a more El Nino like circulation pattern developing.

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 May, although it weakened some in September. ENSO Region Week Aug Aug Sep Sep Sep Sep Oct Oct Oct Oct In recent weeks all ENSO regions have warmed into the EL Nino category.

7 ENSO continued Available ENSO SST plume forecasts show region 3.4 values to be in the 0.5 to 1 range through the winter, which is in the weak El Nino range.

8 ENSO Conclusions The assumption is that El Nino conditions will be present in the equatorial Pacific, but the magnitude will likely be in the weak to at most moderate range most if not all the Winter. Although confidence is low, I assume we will see the SST warm anomalies congregate more in the central/western eq. Pacific as opposed to the eastern Pacific as the winter progresses.

9 ENSO Composites The commonalities in this composite is high latitude blocking (-AO/-NAO), sub Aleutian low, and cooler than normal heights in the eastern US. All December-February periods with an MEI value in the 0.5 to 1.25 range. (Weak to Moderate El Nino)

10 PDO State Classic +PDO look to North Pacific. +PDO SST Anomalies -PDO SST Anomalies The primary PDO index has been positive since January. And the SST anomaly configuration as of late October shows a classic +PDO look.

11 PDO continued Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year Sep DJF ? As you can see above, the PDO has been positive since January. To the left are the years since 1900 when the PDO has been 1 or greater in September. Every year saw a DJF period feature a +PDO on average. So statistically we are very likely to see a +PDO this winter.

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

13 PDO continued Notice the commonalities in these 8 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. Very similar to El Nino composites in Winter. All December-February periods averaged 500mb anomalies for DJF PDO >= 1.

14 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 negative (easterly) phase. These phases usually last on the order of months meaning that we should see the negative 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). A more negative AO means cold air is displaced from the polar regions into the mid-latitudes.

15 QBO Continued 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.

16 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.

17 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)

18 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.

19 Snow Cover continued Above are two graphics showing snow cover extent anomalies compared to normal (left from Environment Canada and on the right from NESDIS). Both show we are running way above normal for Eurasian snow extent. This year is well above all of the last 10 years.

20 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.

21 Snow Cover Conclusion The abnormal amount and advance of October snow cover this year offers an additional argument that we will see lots of high latitude blocking this winter leading to a persistent AO state.

22 Putting It All Together The combination of a weak-moderate El Nino, -QBO, and +PDO is a combination that has produced a predominant DJF AO in the past. There have been 8 winters since 1950 with this combo, all 8 have produced an average negative DJF AO. Those years are , , , , , , , and

23 Putting it All Together continued The climate indicators appear to be pointing to a cold winter, ENSO/PDO/QBO analogs, Eurasian snow cover, etc. The question is when do we flip from this relatively mild mid/late Fall pattern to a colder pattern. Late November? Early December? Christmas? January? Obviously when this occurs will have a big impact on the final winter temperature anomaly composite. I favor the coldest anomalies to be in January/February with the month in most question being December.

24 Analogs The three analogs the match most the indicators I have discussed are , , With the 2 nd best group , , , , These years have large scale matches but things like wrong AMO phase, lower snow cover, -PDO make them less of a match.

25 Maps December likely will be the mildest month nationwide, although I am forecasting cooler than normal in the East and South. January could be cold east of the Rockies with the heart centered over the Southeast/Ohio Valley.

26 Maps February has the signal for the coldest month with the potential for most of the US outside the NW to experience a very cold month with frequent arctic blasts.

27 Maps An El Nino with lots of blocking should favor a southern storm track with Gulf lows and east coast snowstorms/nor easters likely, with relatively less snow over the Plains/Midwest especially compared to last year.