2010 - Second Half of the Winter
| 05 February 2010
As we head toward the February Vacation period and the enter the second half of the season, I thought it would be a good idea to look further ahead than I normal do in one of these reports. As I put this summary together, the mid Atlantic region is about to endure its second paralyzing snowstorm of this season, with another one in the barrel for 4-5 days from now...that one looks as though it will be able to impact New York and New England, too. Aside from a 7 to 10 day thaw in mid to late January, colder than normal weather has dominated the eastern two thirds of the country since early December. Snow has fallen as far south as Miami. Hundreds, if not thousands of records associated with harsh winter weather have been set, including 13 straight days of sub-freezing lows in Tallahassee. In the West, a split jet stream flow has brought occasional rounds of snow to the Northwest, with more appreciable snow amounts to the northern Rockies where the Pacific moisture interacted with the western fringe of arctic air masses close to the Continental Divide. The southern Sierra and southern Rockies have benefited from the enhancement of the southern branch of the jet stream, courtesy of the El Nino in the Pacific Ocean. That relationship will continue to be a beneficial one for the remainder of this winter, not only for Southwestern resorts, but for ski areas 2,000 miles further east. The same general pattern that has dominated the season thus far will remain intact much of the time as we move through February and March. Why?
Here's a list of the large scale factors that point toward a continued domination of cold and snowy weather in the eastern half of the country for much of the remainder of the season.
EL NINO: The waters of the central equatorial Pacific Ocean are still warmer than normal, and the El Nino is still in place. That means that there will be extra energy and water vapor available to systems that pass through that area. Many of those upper level impulses eventually work their way across the southern United States, and when they interact sufficiently with energy in the northern branch, as is the case with today's major snowstorm, big ticket items are the result. The El Nino is weakening, and that process will continue into the spring. Typically, when El Nino's are losing their strength, later winter/early spring weather in the eastern half of North America is colder and damper than normal. A development in the past week will only serve to enhance the impact from El Nino in the next few weeks...a burst of low pressure in the southwest Pacific, the magnitude of which has not been seen in the 20+ years that have been monitored, has cut loose, and as that impulse moves toward Mexico and eventually the Gulf of Mexico, the strong upward motion will serve to reinforce the southward movement of cold air from the north. As air rises, something has to replace it, and as an area of strong rises moves by just to the south of the U.S., cold air gets drawn southward. Cold air and snow will visit the Deep South again this winter in my opinion, and the movement of a burst of upward motion across the Southwest and into the Gulf suggests that one or two very strong storms will form in that region in the coming weeks...if the northern and southern branches of the jet hook up when they do...look out in the East...again.
STRATOSPHERIC WARMING: This phenomenon is a very effective precursor of cold outbreaks during the wintertime. There is a strong correlation between such events and a blocking pattern over the north Atlantic that leads to cold outbreaks over eastern North America. A late November episode tipped off the cold that developed in December, and a second episode got underway several weeks ago and continues as February unfolds. If anything, the latest stratospheric warming is stronger than the early season event. The persistence of high latitude blocking will serve to continue the delivery of cold air masses from Canada and lock in the cold pattern east of the Continental Divide until further notice.
LOW SOLAR: The sun continues to struggle to produce sunspots, although solar cycle (SC) 24 is now underway. Activity has increased in the past month, but it overall, the number and coverage of sunspots is still very meager. A good number of solar physicists believe that we have entered a 20-40 year period of a relatively quiet sun. The last time we saw a sequence of cycles similar to SC 22, 23, and the start of SC24 was the late 1700s, when the "Dalton Minimum" was getting underway. While this clearly is a potential longer term indicator, low solar activity is factored into the identification of analog years...the 3 most prominent analog years I am using right now, 1958, 1969, and 1978, were all low solar years.
COLD WATER NORTHWEST OF HAWAII: There has been a sizable pool of colder than normal water situated northwest of the Islands for much of this winter. In that position, the cold air can feed into troughs that form east of the Islands. Upper level troughs east of Hawaii are strongly correlated with downstream troughs in the eastern United States. It will take a while for the cold water to dissipate, so the eastern trough will continue to dominate, with more of a tendency toward ridging in the West.
HIGH LATTITUDE BLOCKING: Several times this winter, I have referred to the Arctic Oscillation (AO), which is a clear indicator of high latitude blocking when it is in its negative phase. The AO has been in negative territory much of this winter, and it looks as though it will be staying there for a while. Take a look...
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The red portion of the graph is a forecast for the next 2 weeks...you can plainly see that we have had a negative AO for most of the winter...notice the brief run at positive territory around mid January...just as the thaw got underway. When the AO is negative, high pressure dominates in the polar regions, which tends to help the cold air spread out to the lower latitudes. Right now, blocking extends from Greenland and Iceland westward to much of the eastern third of Canada. Over the next 4-6 weeks, the block will slowly migrate to western Canada, and as it does, it will allow a northwesterly flow to persist over much of central and eastern Canada, which will sustain the delivery of cold air into the central and eastern U.S. The arrival of the block in western Canada will mean that the season will end without a resumption of the snowy period that marked the beginning of the season back in November.
Eventually, as we start to see the transition into spring and the lower latitudes start to warm, the northern branch of the jet will start to lift northward, and the warmer than normal air associated with the block will start to spread out to the south. As a result, I believe that milder than normal weather will first develop along the Canadian border in late March and continue through April...the Great Lakes and Northeast appear to be in line for a mild and benign April, so there promises to be some terrific spring skiing and riding this year before the season comes to a close. The season in the mid Atlantic will likely last longer than normal, due to residual cooler than normal temperatures persisting further south, as well as deeper than normal bases as we head into March. When this winter finally winds down, we will have seen an extraordinarily persistent blocking pattern through much of the season, which is consistent with other winters where high latitude volcanoes erupted during the prior warm weather months. Remember, we had Mt. Redoubt in Alaska last winter, and an eastern Russia volcano let go in early summer. Both eruptions spewed sulfur dioxide into the stratosphere, which, for reasons that are not fully understood, correlates well with high latitude blocking the following winter.
ANALOGS: As you know, the identification of periods when these and other large-scale features were similarly aligned is a big part of my forecasting methodology. This winter, I have been leaning on several analogs that were very strong in terms of global indicators that are consistent with what is in place now. Those years are 1958, 1969, and 1978. 1966, 2003, and 2007 have also provided some guidance, but the first three have been prominent much of the time since autumn. With that in mind, here is a compilation of temperatures for the period February through March for those three years...
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I don't know about you, but I like what I see in terms of favorable weather for the latter stages of the season in the Midwest, Northeast, and mid Atlantic regions. You can see that the deviations are larger south of I-80, with some relaxation along the Canadian border, which is consistent with the idea that the block will bring somewhat milder air relative to normal later in March as is spreads southward from Canada. Now, precipitation amounts during those months were close to average, and I do think that there will actually be a little less than normal precip in the Ohio Valley and especially in the Great Lakes and northern New England, where a drier northwesterly flow will dominate. Short wave troughs embedded in the flow will bring periods of light to moderate snow to these areas, which will serve to keep the surfaces predominantly packed powder, even if the major storms stay further south. The mid Atlantic will be wetter (whiter) than normal, due to my belief that the southern stream will continue to produce storms that will reach that region, even if they are suppressed south of New England. It is worth noting that all three analog years has major snow events...1958's mid February blizzard spread heavy snow from the Florida panhandle to New England and it was a part of a series of storms that brought an amazing 88.5 inches to Stroudsburg (eastern), PA during the two month period. 1969's late February storm dropped 10 feet of snow on Wildcat Mountain, which forced the closure of the resort for several days. The Blizzard of 1978 was a monster in southern New England...my mother, who ran a nursing home in northern Rhode Island at the time, could not return home (25 miles away) for a full week.
SUMMARY: This pattern has the potential to produce tremendous amounts of snow in the East, for sure, but it will require the cooperation of many different facets of the weather to generate the storms to turn the trick. The Southwest has every reason to be optimistic about the second half of this season, as well. The Upper Midwest will remain cold, but natural snow will be somewhat limited, the product of the passage of Alberta Clippers, for the most part. The central Rockies will continue to be between the two branches of the jet, and will have to depend on the periodic interaction between Pacific moisture and a glancing blow from the western edge of arctic air that will be focused further east. As for the Northwest, well, there will be some occasional snow as Pacific energy tries to penetrate the persistent western ridge, but seasonal amounts will be a disappointment, especially after the fast start. Seldom do we see a given pattern dominate to the extent that the western ridge/eastern trough configuration has thus far this winter, and all indications are that the same setup will dominate the remainder of the season, as well.
