We have decreased our November HDD projection from 576 to 500. This compares to the 5-year normal value of 576, the 10-year normal of 551, and the 30-year normal of 564. Last week, there was substantial uncertainty regarding the projected temperature distribution during the entire month due to mixed variables. This week, confidence is now high for a very warm start of November. In fact, we feel the first half of November will be distinctly warmer than normal (below normal HDDs). There is still a large amount of uncertainty regarding the second half of the month and as a result, our second half of November HDD projection is near the 10-year normal. Still, this sums up to well below normal HDDs for the month as a whole. Why has certainty risen so much for the start of November as compared to the end of the month? For one thing, the beginning of November is now at the tail end of the two-week forecast. Model performance within the two week lead window is generally much better than in the three or four week lead window. As we mention in the forecast discussion below, the 11-15 day forecast is one that features broad warmth, with a strong zonal flow across much of the Nation.
Second, the projected location and configuration of the stratospheric Polar Vortex (PV) during the 14-15 day forecast is one that favors warmth over the United States. October is the first month when the PV begins to spin up as the sun angle decreases and daylight decreases. This increases the temperature gradient between the North Pole and the equator resulting in an increase in westerly winds that starts in the stratosphere which then over time extend down into the troposphere, helping to force circulation patterns across the Arctic and Mid-latitudes. When the PV “buckles” over a region or is perturbed, cold air is able to funnel down across lower latitudes. The figure above shows the current and projected (Nov 5) location and configuration of the 50mb PV. Currently, the PV is weak but perturbed/elongated. Over time, it is projected to increase in strength, become more circular in shape and be positioned over western Eurasia. Historically, this projected setup of the PV in early november is a very mild one for much of the U.S. Moreover, this projected setup is a stable one for the PV. Hence, if the November 5 PV position verifies as forecast, it would likely stay that way for quite some time - at least another 7 days. This is why our days 16-25 leaning is quite warm.
Finally, the most current Euro weeklies are also projecting warmer than normal conditions during the start of November. This, combined with the discussed variables above, is providing high confidence for a warm first half of November.
The overall theme to the forecast this morning has remained consistent since earlier this week. A zonal flow (west to east) will dominate the pattern into the beginning of November, which will keep temperatures above normal across the majority of the Nation. In the details, the forecast trended slightly warmer today, and thus heating demand trended lower.
Over the next 5 days, the most anomalous warmth will be located across the Midwest and southern States. Thereafter, temperatures will moderate across the Nation. Weak ridging will build in across the West Coast towards the end of the forecast period, while the Eastern third of the Nation remains the most variable in terms of temperatures. A few brief surges of cool air are projected to descend over the Eastern third of the Nation over the next two weeks, which will dampen temperatures to near normal levels. Overall, the pattern continues to favor warmth across most of the Nation through the beginning of November; thus, heating demand will be below normal.
Late last week, we also talked about snow cover and how the placement and advancement of early snow cover across the Northern Hemisphere is one of the key predictors of the winter ahead. The figure above shows the most current Northern Hemisphere snow cover (top) in comparison to the snow cover extent at the same time last year (bottom). Since last week, snow cover has continued to build across eastern Canada and has trekked further south across most of Eurasia, with the exception of western Russia.
As mentioned in Friday’s report, the ultimate key to predicting the wintertime (Dec - Feb) temperatures across the Midwest East region is the Eurasian snow extent. In general, the more advanced the snow cover extent in late October, the colder the wintertime temperatures across the Midwest East region.
The figure on the top right shows the two-month progression of Eurasian snow cover extent for the past ten years from September through October. The black line represents the extent of snow cover this year, while the red line represents the ten-year average. So far, Eurasian snow cover extent is above the 10-year normal, but not above the level of the past two years (maroon and green lines), which were years of record cold across the Midwest East region.
The figure on the bottom right shows the snow cover extent over North America. Snow cover extent across North America is also above the 10-year normal level. In fact, snow cover extent has now greatly exceeded the extent in 2013, but is not at the same level as 2014.
There are only ten more days until the end of October, and thus these last ten days will be extremely important in terms of our thoughts on the overall wintertime (Dec – Feb) pattern.