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sebastiaan1973

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  1. We speculated about this back in October: another QBO disruption. EOF analysis of NCEP/NCAR 10-100hPa equatorial zonal winds shows no QBO progress since January. It will be fascinating to see where it goes from here.
  2. We have to be honest. Although I highly respect Stewart (Glacierpoint) his ideas of February don't realise. The first part of his forecast was excellent.
  3. You shouldn't pay attention to Hellmann. It's a defintion by Gustav Hellmann, especially for Continental Europe. It is the sum of all negative daily temperaturs over 24 hours. So the first of January has an average temperature of -2c, the next day -3c. That combines to 5. And so on. For England it doesn't make sense, because such days are even more rare than in the Netherhands. There is a reason UKMO doesn't use Hellmann https://en.wikipedia.org/wiki/Gustav_Hellmann
  4. Quite remarkable charts. With lowest pressure around 990 hPa. Which is in this time of the year not often seen.
  5. With some intresting research The North Atlantic Oscillation (NAO) and eddy-driven jet contain a forced component arising from sea surface temperature (SST) variations. Due to large amounts of internal variability, it is not trivial to determine where and to what extent SSTs force the NAO and jet. A linear statistical–dynamic method is employed with a large climate ensemble to compute the sensitivities of the winter and summer NAO and jet speed and latitude to the SSTs. Key regions of sensitivity are identified in the Indian and Pacific basins, and the North Atlantic tripole. Using the sensitivity maps and a long observational SST dataset, skillful reconstructions of the NAO and jet time series are made. The ability to skillfully forecast both the winter and summer NAO using only SST anomalies is also demonstrated. The linear approach used here allows precise attribution of model forecast signals to SSTs in particular regions. Skill comes from the Atlantic and Pacific basins on short lead times, while the Indian Ocean SSTs may contribute to the longer-term NAO trend. However, despite the region of high sensitivity in the Indian Ocean, SSTs here do not provide significant skill on interannual time scales, which highlights the limitations of the imposed SST approach. Given the impact of the NAO and jet on Northern Hemisphere weather and climate, these results provide useful information that could be used for improved attribution and forecasting. -> JOURNALS.AMETSOC.ORG
  6. At 10 hPa, I see the lowest m/s this season. At 150 hPa, I see some support for an Atlantic Ridge. Do we get after the toppler another period of colder air from the North?
  7. Hi Malcom, sorry, I wanted to thank you. 

    Sebastiaan

    1. Blessed Weather

      Blessed Weather

      No probs Sebastiaan. I did wonder. I'll pop over there and edit the name.
      Cheers.

  8. < 15 m/s at day 10. Lowest value I've seen from EC.
  9. Well, to me GFS6-oper is alone. E.g. EC at 240h, 10 hPa, 17 m/s.
  10. On November 18, the World Climate Service issued its seasonal forecast and discussion for winter (December-February) 2019-2020 in the U.S. and Europe. WCS seasonal outlooks include an overview of expected climate anomalies, key drivers, and risk factors for the upcoming three-month season, and our forecast document contains detailed discussion of available predictors, including dynamical model forecasts and statistical and analog guidance. Analog analysis has long been a staple of the WCS methodology for seasonal forecasting, and we invariably rely on diverse sets of analogs derived from numerous aspects of current global climate patterns. Only on rare occasions do we focus on individual analog years, because the climate phase space is rarely a very good match to any previous year in the modern history in more than a few respects; there are nearly always significant differences from any candidate analog year. Moreover, the degrees of freedom in the climate system are too numerous to expect a close correspondence with any past year to continue into the future, and so any search for “the perfect analog” is a fool’s errand (even if the climate were assumed to be unchanging). Nevertheless, the latest WCS seasonal forecast report discusses a very notable confluence of similarities between the present climate and that of winter 2002-2003. In keeping with usual practice, the WCS winter forecast is not unduly influenced by the 2002-03 analog, but the degree of similarity is so striking that it is worth considering the 2002-03 outcome as a plausible outcome and risk scenario for winter 2019-20. The following list of similarities was presented in the forecast document and is reproduced here: 2002 is the top QBO analog year, based on the 12-month evolution of the 30mb QBO index The Indian Ocean Dipole was strongly positive in September and October 2002, and tropical convective patterns were similar; 2002 is the third best analog year for 200mb equatorial velocity potential A region of highly anomalous warmth developed in the northeastern Pacific in late summer 2002 and persisted through autumn, similar to 2019 Arctic sea ice was low in summer 2002 and set a record at the time for low September ice extent The October Northern Hemisphere circulation was similar in 2002, with Arctic blocking, a trough over northwestern Europe, and a very similar ridge-trough pattern over western North America. The November MSLP pattern was also similar with respect to high pressure north of Scandinavia, low pressure near the British Isles, and low pressure south of the Aleutians. October 2002 had the highest snow cover on record (1967-present) over North America; October 2019 was the third highest The 10mb polar vortex was much stronger than usual in the first half of November 2002, but the Arctic Oscillation was negative for November, similar to this year. No other year provides such a close match to this unusual combination, although 2018 is also a good analog. Remarkably, 2002 is nearly a perfect match to 2019 (better than any other year since 1900) for Central England Temperature in August, September, and October. Perhaps coincidentally, October 2002 England & Wales precipitation was also nearly identical to 2019, and November was also extremely wet (October-December 2002 was the third wettest such period on record). 2002 and 2019 are the only years on record with a strong Southern Hemisphere SSW (Sudden Stratospheric Warming) event; both of these occurred in late austral winter and led to Southern Hemisphere blocking in both October and November. The most significant difference between 2002 and 2019 is that 2002 had more warmth in the eastern equatorial Pacific and much less in the West Pacific; El Niño was more classical (East Pacific) rather than Modoki-like. There was also much less anomalous warmth in the subtropical and northern North Pacific in 2002. In view of the extensive and remarkable similarities betweeen 2002 and 2019, it is tempting to conclude that winter 2019-2020 will be closely analogous to 2002-2003, with a strongly blocked pattern, unusual cold in the eastern two-thirds of Europe and the eastern United States, and reduced precipitation and wind in central and northern Europe (see below). However, as noted above, the WCS approach is to treat individual analog years with caution, regardless of how impressive the similarities appear to be. The combined consensus of a large array of dynamical and statistical predictors is a more reliable guide to likely seasonal patterns, and the WCS forecast is constructed accordingly, but the winter of 2002-2003 should be regarded as a plausible alternative scenario. Climate Pattern Similarities Between 2002 and 2019 | World Climate Service WP.WORLDCLIMATESERVICE.COM Discussion of similarities between late autumn conditions in 2002 and 2019
  11. According to Cohen we -here in NW-Europe shouldn't expect much of the possible SSW. Impacts In my opinion it is crunch time for the Northern Hemisphere (NH) winter. A minor sudden stratospheric warming (SSW where a warming of at least 25°C occurs in the polar stratosphere) is likely and a major mid-winter warming (MMW where the zonal mean zonal wind at 10hPa and 60N reverses from positive to negative) is possible in mid-December. I include in Figure i the temperature animation of the stratosphere and impressive warming is being predicted by the GFS, enough to at least qualify for a minor warming. Based on the GFS forecast, some regions of the polar stratosphere could see a 70°C (126°F) jump in temperature in a matter of days! I saw that some members of the GFS ensemble showed an MMW as early as early December, but I think this is likely too soon. I believe regardless of the timing and magnitude of the event it will have impacts on the NH weather. I would argue that some of the predicted features in the tropospheric circulation are related to the anticipated PV disruption. The models are predicting a mid-troposphere low pressure over Northern Siberia starting next week. This is very close to the predicted location of the stratospheric PV starting this week. The other predicted main feature in the polar stratosphere is ridging/high pressure centered near Alaska. This will likely be associated with a tropospheric feature/reflection as well. Something similar occurred last December with ridging in the interior of North America and very mild temperatures across the continent. A repeat is possible but my sense of the trends this fall is that the ridge will likely setup further west, forcing a colder solution but admittedly it’s a tough call. In last week’s blog, I argued that the increase in the vertical energy transfer and the PV disruption is looking more like an “absorptive” event and less like a “reflective” event and that seems to be even more true this week. Leading up to an “absorptive” event while the stratospheric AO trends negative the tropospheric AO trends positive with milder temperatures across the mid-latitudes and colder temperatures in the Arctic. Though many of the trends are not particularly strong, based on today’s forecast plots included in today’s blog all those trends are apparent. The forecast for Europe is consistent with these expectations with an increasing westerly flow and milder temperatures. Milder trends are also predicted for eastern North but those trends might run into more resistance due to record low sea ice in the North Pacific sector of the Arctic and the well above normal sea surface temperatures (SSTs) in the eastern North Pacific especially in the Gulf of Alaska. Those features could help to promote ridging near Alaska/Gulf of Alaska with downstream troughing in North America with colder temperatures bucking the trends from the vertical energy transfer. Regardless of the amplitude, I expect some cold weather from the SSW most likely in eastern North America and Northern Asia. However, if the predicted SSW is relatively minor with a quick recovery in the stratospheric AO, even possibly becoming strongly positive, would favor a positive tropospheric AO. Then an extended mild to very mild period across the NH mid-latitudes could ensue from late December through much of January. I would expect at some point another PV disruption that would reverse the weather to colder but by then an overall mild winter would be almost a certainty. Last December (before the AO blog was archived), I did analyze some MMW that were of the displacement variety. First, event to event variability is large but there are two PV displacements that occurred relatively recently in mid-December – December 15, 1998 and December 16, 2000. These are calendar dates close to when our polar vortex model predicts as the time most likely for an MMW this upcoming December. If the SSW occurs mid-December then it takes about two weeks for the related circulation to propagate down from the stratosphere to the troposphere with a colder weather regime to follow, starting around the holidays. The winter of 1998/99 and 2000/01 are two very different winters with winter 1998/99 being relatively mild and winter 2000/01 being relatively cold in eastern North America and Northern Asia. There was also an MMW in late December 2001/early January 2002 followed by a mild winter. However for now, I am leaning towards a colder solution more similar to 2000/01 more so than 1998/99 given the low sea ice in the North Pacific sector of the Arctic (though based on the weather forecasts, I expect a lot of ice to form over the next two weeks) and the very warm SSTs in the eastern North Pacific. Also, in general the Arctic is warm. I wrote many blogs last winter on the surprisingly cold Central Arctic that may have interfered with the downward propagation of a negative AO. So far this fall, the central Arctic has not been cold. Also the QBO (quasi biennial oscillation) is easterly or at least trending east. An easterly QBO favors a more negative AO than a westerly QBO (see Labe et al. 2019). The following is some of the relevant text and figures copied from last December: “I have included in Figure ii the stratospheric PV a week or longer prior to when an MMW was observed and then in Figure iii the observed temperature anomalies across the NH the month when an MMW was observed and the following one or two months. Winters included are 1998/99, 2000/01, 2001/02, 2003/04, 2005/06 (in the Cohen and Jones paper this is listed as PV split - the cold Europe and blockbuster February snowstorm along the mid-Atlantic are consistent with this - but at least the beginning resembles a PV displacement), 2006/07 and 2007/08. Prior to 1998, the most recent stratospheric PV displacement was in 1987. My personal preference is not to use analogs prior to the era of amplified Arctic warming (pre-1990) for current winters. In all of the stratospheric PV displacements since 1998 the stratospheric PV is displaced towards northern Eurasia with the exception of February 2007 when the stratospheric PV was displaced towards Greenland. Also, the flow around the stratospheric PV in all winters was directed from Siberia towards eastern North America (5 winters) or to Europe (2 winters). The predicted stratospheric PV displacement (see Figure 13 below) is consistent with all the previous stratospheric PV displacements of the past two decades. Looking at the surface temperature anomalies during and following all seven stratospheric PV displacements shows more variability. Temperatures across eastern North America are below normal four of seven two or three winter months during and following the stratospheric PV displacement. Temperatures across much of Europe are below normal really only once for the two or three winter months during and following the stratospheric PV displacement. Temperatures across Asia are below normal three of seven two or three winter months during and following the stratospheric PV displacement.” https://www.aer.com/science-research/climate-weather/arctic-oscillation/
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