It's been a long time in coming, but signs are pointing to a major change in the weather pattern across the northern Hemisphere. So far this winter, the coldest air has been largely confined to the Arctic, with an absence of strong storm systems to pull that cold air southward. Along the West Coast, a strong ridge of high pressure has been the dominant feature. The first image is a computer model forecast for tomorrow (Jan 12), showing the position of upper-level features (about 5 to 6 km above the surface). The colors indicate vorticity, or spin. And in general, wind follows the black lines. The ridge is noted by what looks like a "hill" in the black lines stretching from California northward almost to the North Pole. This has steered incoming Pacific storm systems northward, usually into British Columbia, and leaving us with dry, mostly sunny weather under northerly winds.
Finally, this high pressure ridge is forecast to break down and shift northward starting this weekend. The second image shows the forecast for next Wednesday (Jan 18). The big high pressure ridge that was along the West Coast strengthened and moved northwestward (or retrogressed, because its position moved opposite the usual west to east motion). It's forecast to be a huge high pressure system over eastern Russia. While this happens, strong westerly flow from Asia undercuts the high pressure system, stretching from Japan eastward and aimed at the Pacific Northwest. And, embedded in that westerly flow are several storm systems, noted by the vorticity (colored) regions. The first one will move into Washington and Oregon on Wednesday, but will drag a cold front across northern California, and finally bring us some rain. Then, several more systems will follow. It's still way too early to tell how much rain we will get. Much of the heaviest rain may be focused north of here, but we can at least bet on some decent rainfall totals, and accumulating snow in the Sierra above 4000 to 5000 feet. It's also uncertain how long this new pattern will last. We could be in for an active few weeks, or the atmosphere could reset back to a pattern similar to what we've had most of this winter.
It's been a long time in coming, but signs are pointing to a major change in the weather pattern across the northern Hemisphere. So far this winter, the coldest air has been largely confined to the Arctic, with an absence of strong storm systems to pull that cold air southward. Along the West Coast, a strong ridge of high pressure has been the dominant feature. The first image is a computer model forecast for tomorrow (Jan 12), showing the position of upper-level features (about 5 to 6 km above the surface). The colors indicate vorticity, or spin. And in general, wind follows the black lines. The ridge is noted by what looks like a "hill" in the black lines stretching from California northward almost to the North Pole. This has steered incoming Pacific storm systems northward, usually into British Columbia, and leaving us with dry, mostly sunny weather under northerly winds.
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