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November Winter Forecast Update

Ja

Quick and to the point for people who are interested in just the short version: I am expecting this winter to be 1 below normal temperatures (Appalachian Mountains and west of the Appalachian Mountains), with the remainder of the Southeast being slightly below normal temperatures 2)slightly  above precipitation for the Southeast, with below normal precipitation west of the Appalachian Mountains 3)Above normal snowfall for the Appalachian Mountains, the Mid-Atlantic and the Northeast, near normal snowfall for the Southeast.

***NEW GEOPOTENTIAL HEIGHT ANOMALY PROJECTION***

January 20th - February 15th (projection)

My most similar winters used were : 1951-1952, 1966-1967, 1969-1970, 1977-1978, 1978-1979 and 2008-2009. Note that I treat 1951-1952 and 2008-2009 as nullifying winters; they are included here to drive down the intensity of the other 4 winters. These two are weighted less than the others.

This time, I have added in other primers, such as Siberian snow cover, Arctic ice coverage, strength of the Polar Vortex, and estimations of what I believe the blocking pattern regime will be based on the other 3.

L'Heureux's Weather calculated temperature anomalies from December - February.

The JAMSTEC model (thanks Brad for the link) projection for this winter (temperature anomalies) for December - February. Color scale is from -1.5*C to 1.5*C.

Note that I have subbed out 1952-1953 from the previous forecast, and have added 1969-1970 and 1977-1978. I added 1969-1970 as a warming Pacific scenario, and inserted 1977-1978 to show a westerly oriented weak El Nino strengthening.

L'Heureux's Weather daily precipitation anomalies. Blue = wetter, red = drier.

The JAMSTEC model (again, thank you Brad for the link) projection for this winter (precipitation anomalies) for December - February. Color scale is from -1.5mm/day to 1.5mm/day. Blue = wetter, red = drier.

So in one of these La Nada winters, both the subtropical jet and the polar jet stream have their periods of dominance. The similarities between the Japan model and my output is encouraging that this may be useful for the future.



The Arctic Oscillation is a measure of the strength of the polar vortex; it does this by measuring pressure anomalies in 1)the mid-latitudes 2)the polar regions. For simplicity purposes, I will break down the polar vortex to something we can all relate to: a coffee cup.

Stir a cup of coffee. For this test to work, we need some coffee grinds in our coffee cup. These coffee grinds represent our cold air reservoir. Now, spin your cup in a counter clockwise direction. Notice how when the circulation grows stronger, the grinds/cold air becomes more concentrated under the center. Imagine the coffee itself is a representation of "pressure height anomalies." The center is lower and the edge is higher. This would be an example of a positive phase of the Arctic Oscillation (my red box above). The Arctic has anomalously low surface pressure and the mid-latitudes have anomalously high pressure. Now, put a spoon in your coffee and block the circulation. The circulation will distort and the coffee grinds will spill out from the center. The circulation may even break in half. Now the center is not as low and the edge is not as high. The distorted flow would be something like a negative Arctic Oscillation.

In real life, when the polar vortex is very circular, colder air is bottled up in the Arctic. But when the circulation is less circular, cooler air from the Arctic is pushed into the United States and Europe instead. February 2009, February 2010, and December 2010 were notorious for the flow breaking down entirely and reversing in the Arctic stratosphere, because the stratosphere over the Arctic was warmer than the stratosphere in the mid-latitudes.

The changes of the storm track and pressure patterns between the positive phase of the NAO and the negative phase of the NAO.

The North Atlantic Oscillation is a branch of the Arctic Oscillation but is important enough on its own to mention. A positive phase has a stronger than normal low pressure near Iceland and a stronger than normal high pressure over the Azores. A negative phase has weaker pressures over both.

The PNA is a little trickier but think of it as an equivalent to what the NAO does for the east coast, over the west coast instead. In a positive phase, there is a ridge over the northwest, a trough south of Alaska, and a trough over the Southeast. This is another helping hand in distorting the Polar Vortex. There is a theory that the PNA is a bridge between ENSO and the Arctic. The literature is still a little muddled and I will not include much here, except that in an El Nino winter the PNA tends to lean positive.

Impacts of the NAO and PNA on the storm track

We have two primary cold spells: the first is from December 24th - January 4th and the second is from February 10th - February 21st. We have three secondary cool spells from December 10th - December 18th, January 20th - January 25th, and January 27th - February 10th.

We have a primary warm spell March 15th onward, and two secondary warm spots: January 6th - January 15th and March 6th - March 15th.

Anything not in red or blue is "neutral".

So again, quick and to the point, I am expecting this winter to be 1)slightly below normal temperatures (Appalachian Mountains and west of the Appalachian Mountains), with the remainder of the Southeast being at near normal temperatures 2)slightly below normal precipitation for the Southeast, with above normal precipitation west of the Appalachian Mountains 3)Above normal snowfall for the Ohio River Valley and the Northeast, near normal snowfall for the Southeast. Northwest Flow will play a larger role for the Appalachian Mountains this winter while the rest of the Southeast may have its snow opportunity in 1)Late December 2)February. The battle zone certainly seems to be in the Mid-Atlantic and the Northeast this year.

The Pacific Northwest stays chilly but near neutral for the winter. The northern plains I expect to have drier conditions for the winter, while the Rockies get a time share deal from a boosted subtropical jet and a polar jet, both of which are competing for the #1 slot. I would wait for those + AO periods for precipitation for the northern Rockies and a -AO period for precipitation in the southern Rockies.

500mb Height Anomalies December - February

The Deep South will be robbed of moisture until those -AO periods due to the battle zone being farther north (Midwest, Ohio River Valley). My concern for these areas and with the Southeast comes from increased ice storm chances late December (primary risk Midwest, secondary Mid-Atlantic) and February (primary risk Mid-Atlantic, secondary risk Midwest).

 And for more clarity how snow cover over Siberia plays a role....
Image 1 = November jet stream,  brown = October 15th snow cover
Image 2 = November jet stream,  November Geopotential Height Anomalies

Note that the years with the greater onset of Siberian snow cover have stronger warm anomalies pushing into the Arctic (2009, 2010, 2012). The strongest points in the Jet Stream correspond to the greatest north/south pressure gradient.