After a rather warm and very humid end to the week, many of you are probably waiting for much cooler and drier weather. I can tell you, that looks to be on the way by the end of the week. Right now, the remnants of the tropical system are slowly weakening just to the south of the area while sliding east at the same time. This is the reason for the extreme humidity and at times rain. On the upper air analysis this morning (Monday), a closed low was sitting over Canada, causing a weak trough to form across the Pacific Northwest. As upper level ridging builds across the far eastern Pacific near the West Coast, this will eventually push the closed low southeastward towards southern Canada and the eastern United States. As the trough moves this way, by Friday and Saturday, it will force a cold front through the area, with a low pressure center forming along the weakening front. At the time, it will be hard to resolve the details on what exactly will happen. The GFS ensembles are showing that a broad trough originating from the closed low will deepen by Saturday, allowing a rather strong high pressure region to cause a much colder air mass to become entrenched across the region. In fact, according to the GFS, temperatures could struggle to make it to the 70's by Sunday and Monday.. After that, warmer temperatures will return, but they will probably still struggle to reach into the 80's.
What this indicates in the long-term is that the weak El Nino phase occurring in the equatorial Pacific is finally starting to have an effect on the world's weather. An El Nino phase usually favors troughing (which causes below normal surface temperatures) across the south and east. It also favors blocking across Canada, which is quite evident on the GFS ensemble forecast for the weekend. Already, this could be a precursor of what we see during the winter. I am not releasing a winter forecast yet, due to the uncertainty involved this year. In fact, this year's weather conditions are very similar to last fall. So, in fact, weather conditions during the winter could go either way. Most El Nino's across the area favor warmer than normal, and drier than normal conditions. Some El Nino's have been known to cause extensive troughing across the eastern and central United States during the winter, which suppressed the warm than normal conditions across the north and allowed for cold, arctic air to filter into the United States. This kind of pattern is evident when a certain long-term climate variable called the PDO is in its negative phase. While the PDO index has risen over the past few months, it is still negative, which should support a winter of colder than normal weather across most of the eastern United States. As for precipitation, that will be a bit trickier to forecast. It will all depend on whether the southern branch of the jet stream becomes dominant like it was last year. If so, most of our storm systems will come up from the southwest into the region, or they will swing in from the east as they are strengthening along the east coast. Right now, this is just speculation as to what is coming.
As for the months of September and October, based on trends from what happened in August, I expect that September will likely see slightly above normal temperatures (our average high temperature in September is in the middle 70's.), especially every time the ridge breaks down in the west, which will send a heat surge into the Midwest. In October, if I base it on our last El Nino, then it will likely turn out to be cooler than normal, and wetter than normal. In fact, in October of 2009 (the last El Nino/negative PDO phase), the Midwest was hit by a snowstorm towards the end of the month! While I can see this happening, this October will probably not be characterized by such cold temperatures as it was then. This is because of the fact that El Nino is still struggling to develop across the U.S., and with a still large area of severe drought across the Central Plains, one or two day waves of extremely warm temperatures will still be possible. As for precipitation, with the jet stream dropping southward, and a better connection with Gulf moisture than we did last year, showers and thunderstorms will likely be just as frequent than last year. The only reason last fall was so wet, was because of the predominance of the southern jet stream, and the predominance of cut-off lows that continually formed. So I do NOT expect a wetter fall than last year, but it could still be fairly wet, especially in September.
What this indicates in the long-term is that the weak El Nino phase occurring in the equatorial Pacific is finally starting to have an effect on the world's weather. An El Nino phase usually favors troughing (which causes below normal surface temperatures) across the south and east. It also favors blocking across Canada, which is quite evident on the GFS ensemble forecast for the weekend. Already, this could be a precursor of what we see during the winter. I am not releasing a winter forecast yet, due to the uncertainty involved this year. In fact, this year's weather conditions are very similar to last fall. So, in fact, weather conditions during the winter could go either way. Most El Nino's across the area favor warmer than normal, and drier than normal conditions. Some El Nino's have been known to cause extensive troughing across the eastern and central United States during the winter, which suppressed the warm than normal conditions across the north and allowed for cold, arctic air to filter into the United States. This kind of pattern is evident when a certain long-term climate variable called the PDO is in its negative phase. While the PDO index has risen over the past few months, it is still negative, which should support a winter of colder than normal weather across most of the eastern United States. As for precipitation, that will be a bit trickier to forecast. It will all depend on whether the southern branch of the jet stream becomes dominant like it was last year. If so, most of our storm systems will come up from the southwest into the region, or they will swing in from the east as they are strengthening along the east coast. Right now, this is just speculation as to what is coming.
As for the months of September and October, based on trends from what happened in August, I expect that September will likely see slightly above normal temperatures (our average high temperature in September is in the middle 70's.), especially every time the ridge breaks down in the west, which will send a heat surge into the Midwest. In October, if I base it on our last El Nino, then it will likely turn out to be cooler than normal, and wetter than normal. In fact, in October of 2009 (the last El Nino/negative PDO phase), the Midwest was hit by a snowstorm towards the end of the month! While I can see this happening, this October will probably not be characterized by such cold temperatures as it was then. This is because of the fact that El Nino is still struggling to develop across the U.S., and with a still large area of severe drought across the Central Plains, one or two day waves of extremely warm temperatures will still be possible. As for precipitation, with the jet stream dropping southward, and a better connection with Gulf moisture than we did last year, showers and thunderstorms will likely be just as frequent than last year. The only reason last fall was so wet, was because of the predominance of the southern jet stream, and the predominance of cut-off lows that continually formed. So I do NOT expect a wetter fall than last year, but it could still be fairly wet, especially in September.
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