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Last Updated: 7:57 PM GMT on November 16, 2009
— Last Comment: 2:14 AM GMT on November 22, 2009
For 95% of the contiguous United States the first 5 days of October has started below normal, with about half the country decidedly so, running 5-12 degrees subpar. For much of the Western US and Northern Plains this is a marked change from the month of September when temperatures ran 4-8 degrees above normal for the month. Several locations even recorded their warmest or second warmest September on record. The reason for the record warmth was an anomalously strong ‘Rex block’ over Canada that was bookended by a pair of equally persistent areas of troughiness; one over the Gulf of Alaska and another over Greenland which were both responsible for numerous areas of cyclogenesis during the Month of September (see fig.2). But near the end of the month things began to change. The ‘Rex block’ over Canada began to break down as a series of low pressure areas broke away from the circulation over the Gulf of Alaska and made their way into the Western US. Meanwhile, the +NAO pattern (low pressure near Iceland/high pressure near the Azores) across the North Atlantic pulled a complete one-eighty. Over the last week incredible geo-potential height rises have occurred from Hudson Bay to southern Greenland while a deep closed trough formed near the Azores (see fig.3). In fact, the surface reflection of this deep cyclone over the Northeast Atlantic would undergo a slow process from a cold-core to a warm-core storm, eventually becoming Tropical Storm Grace.
 Fig.1 - Temperature anomalies for the US through the first six days of October.  Fig.2 - Chart depicting storm tracks over the northern hemisphere for the period 9/5/09-10/4/09. Note the high frequency of storminess in the Gulf of Alaska and in the vicinity of Iceland and the lack of storminess over the Western Us and south-central Canada.  Fig.3 - 500-hPa height anomalies for the northern hemisphere from September 5th to October 4th. Of note is the strong 'Rex Block' over Canada during the month of September transitioning to a pattern of high-latitude blocking, lowering heights across the mid-latitudes, especially in the western hemisphere from the West Coast of North America to the central Atlantic Ocean. That brings us to where we are today. The pattern changes over the last week are responsible for pushing the jet south into the US, allowing colder air from Canada to become incorporated into the series of Pacific storm systems to march across the country. So cold has this air been, the first major snowstorm across the Intermountain West has already struck, leaving 1-2’ of snow across the Northern Rockies with 3-6” of snow at lower elevations. Snow even raced out into the Northern Plains with up to 4” reported in North Dakota. Unfortunately, those hoping for an Indian Summer following this first major cold blast shouldn’t count on it; this is merely a preview to what’s to come over the next week. One of the coldest air masses on record for this early in the Autumn will be diving straight south from the Arctic, arriving in stages with the first due on Wednesday morning. Low pressure will develop over the Canadian Prairies on Tuesday, helping to drive the first in a series of cold fronts into Montana and the Northern Plains. Vigorous shortwave disturbances diving down the backside of the deep, broad longwave trough carved over North America will not only reinforce the cold air but also but down even colder air from Canada so that by the end of the week a large area of much below normal temperatures will reside over the eastern 2/3rds of the country, save the East Coast where the front may slow before finally pushing through by the beginning of next week. Under the core of the cold airmass (leeside of the Rockies, central/northern Plains/Upper Midwest) expected to invade the US temperatures will average 20-30 degrees below normal and many daily records for both minimum and low-maximum temperature are in jeopardy of falling. This would also leave large portions of the country 10 to 15 degrees below normal or more through the first two weeks of the month. With so much of the country expected so far below normal through the first two week of the month, will the record for the coldest October on record fall? To answer we need to look back at the coldest October on record – 1925. Does this October bear any resemblance to October of that year? Thus far, yes.  Fig.4 - Contiguous United States temperature record for October (1895-2008) Credit:NOAA One of the overriding factors determining the breadth of the cold air across the US in October of 1925 was the succession of three abnormally cold strong surface high pressure areas from northern Canada into the United States. Each one brought progressively colder air with the final arctic high delivering the coldest temperatures on record (to that point) in October with many stations breaking daily and all-time monthly record lows for both minimum and low maximum temperature. During the last cold snap of the month temperatures fell well below zero across a wide area of the Upper Midwest. The coldest reading coming from Montana of -28°F! Temperatures of 15-20 below zero were also common on the coldest night of the cold snap across North Dakota and Iowa, with the zero degree isotherm penetrating into Missouri. Also of note is a very rapid succession of systems across the country with very little time between storms for building heights and a return of warmth. This also contributed to a high amount of snowfall for October 1925, the most on record as well (see fig.5). And not to be outdone, a very rare occurrence for October, not one center of low pressure came ashore the Pacific Northwest during the month of October 1925, as most troughs made their way into the US via Canada after tapping into the cold air that made October 1925 the coldest on record.  Fig.5 - Total snowfall for the month of October 1925. Click for larger image. As the next cold front drops south from Canada this week it will also be followed by an abnormally strong, cold surface high pressure cell (~1040mb). Additionally, there will be a very fast west-southwest to east-northeast jet stream racing across the country. Along the East Coast of North America a ~150kt jet stream will be exiting the coast, reminiscent of what one would expect during a winter season but certainly not October. This highly-amplified jet will bring down numerous disturbances from Canada between the 7th and 13th of the month where they will combine with sub-tropical moisture along the Gulf Coast to produce several heavy rain events, also a concurrent pattern during October 1925.  Fig.6 - GFS forecast 850mb temps, 6-hour precipitation, MSLP for 8am on October 9th, 2009. Note the strong areas of surface high pressure building down from Canada into the Northern Rockies/Plains.  Fig.7 - GFS forecast 850mb temperatures for 2pm October 11th, 2009. Cold air covers much of the northern tier of the country with the 0°C isotherm extending far to the south, from the central Plains to the Northeast Coast. Of course, this is only the first two weeks of the month. Looking ahead to the third week of October there are signs that Pacific energy arriving from north of Hawaii could impact the Pacific Northwest, spilling milder maritime air across the western half of the country, at least. However, there are also signs of a re-amplification of the Gulf of Alaska low/near-West Coast ridge after week three and continuing through the remainder of the month which should, in turn, initiate a re-amplification of the longwave pattern over the US. This is still over 10 days out before signs of a pattern change start to take hold and a lot can change by then. Either way, by mid-month temperature anomalies will be deep in the hole.  Fig.8 - GFS ensemble mean 500-hPa height anomalies centered on 8pm October 21st, 2009. Below normal heights in the Gulf of Alaska indicate a stronger than normal Gulf of Alaska low. Above normal heights over the Labrador Coast also indicates blocking around Greenland may persist through the end of the month, helping to keep cold air funneling into the US from Canada. The year that holds the rank of second coldest October on record was 1976. Once again, this was a month that was dominated by a high-amplitude jet stream across North America with two abnormally cold, strong areas of arctic high pressure crossing the country. The coldest period of October 1976 occurred from the 17-22nd of the month when strong high pressure built down from Canada delivering a record breaking cold airmass across the eastern 2/3rds of the country, ending the growing season for many areas. Also of note was snow falling at locations not accustomed to seeing the white stuff during October, such as central Nebraska and the high plains of Texas, where as much as 6” of snow fell. A reason for these cold Octobers can be traced to El Nino, the periodic warming of the equatorial waters of the Pacific Ocean. Many Octobers falling on El Nino years tend to be much cooler than average across the US as a higher incidence of high-latitude blocking occurs, forcing a lowering of heights across the mid-latitudes helping to draw in cold air from the north. Climatology of El Nino Octobers would tend to favor at least one more outbreak of cold air during the second half of the month which could keep temperatures low enough to yield one of the coldest Octobers on record considering the very cold start to the month already seen on top of the record cold that’s expected to persist over the next 7-10 days. References and further reading. James Wagner, A., 1977: Record Cold over the South and Midwest. Mon. Wea. Rev., 105, 121–127. - http://ams.allenpress.com/archive/1520-0493/105/1/pdf/i1520-0493-105-1-121.pdf SAMUELS, L.T., 1925: FREE-AIR SUMMARY. Mon. Wea. Rev., 53, 457–459. - http://docs.lib.noaa.gov/rescue/mwr/053/mwr-053-10-0457c.pdf DAY, P.C., 1925: THE WEATHER ELEMENTS. Mon. Wea. Rev., 53, 459–461. - http://docs.lib.noaa.gov/rescue/mwr/053/mwr-053-10-0459.pdf , 1925: Chart VII. Total Snowfall, Inches, October, 1925. Mon. Wea. Rev., 53, c7. - http://docs.lib.noaa.gov/rescue/mwr/053/mwr-053-10-c7.pdf strong>Current watches, warnings and advisories.  Fig.9 - Current watches, warning and advisories issued by the National Weather Service. Courtesy of NOAA. -------- Forecast Discussion Coming later... ___________________________________________________________ Radar: Northeast Region Loop Fig.10 - Radar loop of the Northeast region. Courtesy of Weather Underground. ___________________________________________________________ Local SST's  Fig.11 - Sea-surface temperatures off the Northeast Coast. Courtesy of NOAA. ___________________________________________________________ All hits. Unique hits.  My weather dragon!
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