Saturday, October 30, 2010

Significant storm likely Thursday

In the last few days, model guidance has been keying in on a potential nor'easter on Thursday (11/4). This storm potential coincides with a large scale pattern shift early next week.

General overview:

The strong MJO wave that traversed Indonesia in the last two weeks has been responsible for a surge in tropical cyclone activity in the Western Pacific. The most recent of which is Typhoon Chaba which reached category 4 intensity near 25N, and has since recurved just east of Japan. Upper level divergence and latent heat release has pumped up ridging ahead of the cyclone, triggering a wave train across the Pacific into North America.

In addition, the AAM relative maximum is over, with a strong negative tendency appearing in the northern subtropics. This will propagate northward, having the effect of temporarily weakening the westerlies over the north Pacific. As a result, perturbations in the jet can become more amplified.

Induced ridging over the West Pacific will shift into the central Pacific, supported by La Nina's weakened hadley cell. Downstream, this will amplify a trough over the Gulf of Alaska and ridge over western North America into early next week, with the positive EPO becoming more neutral. Between the ridging to our west and departing storm system on Monday, strong cold air advection will keep temperatures around 5 to 10 degrees below normal over the region.

On Tuesday, model guidance shows some energy from the GOA trough ejecting over top of the ridge into the Northwest Territories. This energy subsequently carves out a trough over central Canada which then digs into the Great Lakes region Wednesday Night and triggers surface cyclogenesis south of New England.


Difficulty in the forecast arises with the interaction of a number of individual short waves.

First to the show is a piece of energy that gets trapped underneath the developing ridge early in the week. Model guidance agrees on developing a weak cut off low over Texas, and most agree on the ridge shoving the low south into the Gulf of Mexico, and not being a significant player. However, the GGEM has been persistent in shifting this low east and phasing it with the trough digging into the eastern US, to make one extremely amplified disturbance displaced to the south. Ultimately this would mean a very warm moist southeast flow over the region with low pressure tracking inland over New York. This solution is being discounted ... for now. The latest run of the ECMWF is leaning in this direction, so unfortunately it cannot be completely ignored.

Next we must deal with the energy within the trough. The operational GFS continues to show very strung out vorticity, with surface cyclogenesis delayed until northern stream phasing occurs. As a result, low pressure develops east of the region and moves northward into Nova Scotia, giving New England only showery precipitation. This solution has little support, and may be less likely than the GGEM.

The ECMWF and UKMET on the other hand phases earlier, resulting in a more amplified solution, with surface low pressure developing east of Delmarva and moving over SE New England. The GFS ensemble mean is also closer to this solution versus the operational GFS, though with notable spread. Both the ECMWF and UKMET show a very dynamic situation with cooling temperatures aloft despite strong southerly flow. This would suggest heavy precipitation rates, falling as snow for much of the interior. In addition, this storm may have some tropical connection depending on the interaction of the trough with Tomas. So moisture will be plentiful.

The possibility does exist for notable snowfall over the interior higher elevations, dependent on the track of the nor'easter. One way or another, we will have a significant storm on our hands this week.

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