Severe threat in the Plains Friday-Saturday

Sitting in the 40's in Oklahoma, and poised to drop into the mid 30's tonight, it may be difficult to imagine a return to warmth and regular thunderstorm threats. This blast of arctic air will have one last night with us, before giving way to a warm southerly wind upstream of the departing high pressure. By Tuesday, the southern Plains will be returning to temperatures closer to normal for mid October. 

As I discussed in the last post, short wave disturbances rounding the base of the Canadian trough will gradually lift the trough to the northeast through the week. As a result, the Midwest, Great Lakes, and New England will stay on the cold side while the rest of the nation returns to normal temperatures.

Meanwhile, as the block over the eastern Pacific breaks down, a closed low near the California coast will move onshore through the Southwest on Thursday, and approach the lee of the Rockies on Friday. Classic lee-cyclogenesis begins in eastern Colorado, with increasing southeasterly winds downstream across the southern Plains converging toward the surface trough.

12z GFS forecast for 7pm Friday evening

Aloft, the closed low is projected to stay sharp, with strong downstream upper level divergence. Combined with increasing low level moisture, this forcing should be enough to trigger widespread showers and thunderstorms. By Friday afternoon, shear profiles are looking very impressive. The limiting factor for severe weather will be marginal lapse rates, with lackluster CAPE.

The upper level low will continue northeastward on Saturday, with intensifying low pressure at the surface over the central Plains states. Once again, CAPE is limited, but shear is impressive. Any thunderstorms that develop strong updrafts will easily become rotating.

12z GFS forecast for 7pm Saturday evening

As the upper level low moves northward, it will interact with the strong baroclinic zone over the Great Lakes and Northeast, under the upstream side of the Canadian trough. Low pressure at the surface will strengthen in this environment. As it runs into cold high pressure to the north, it will also enhance frontogenesis from across the northern Great Lakes into New England. This could lead to a band of heavy rain impacting the aforementioned areas next weekend.

So we can expect some eventful weather from this system by the end of the week. There are still several details to be ironed out in order to get the full picture of the specific threats we may be facing. The picture will get a little clearer once the storm comes onshore and is fully sampled by sounding data, etc.

Canadian trough

The next ten days will be all about the long wave trough anchored over Canada. As a review from the last post, in the few days we have witnessed Pacific Jet energy dump into western North America and amplify over the northern Plains. Meanwhile, ridging originally suppressed south of the jet has exploded poleward over Alaska. This has formed a very strong block upstream of the continent. Downstream of the block, disturbances dive south and amplify. 

The first in a series of these disturbances is responsible for the shot of unseasonably cold air into the southern Plains and the early season snowstorm across North Dakota and Minnesota yesterday. More of these disturbances are poised to dive south into the CONUS over the next week. This cycle of shortwaves act to carve out a broad trough extending from northern Canada into the northern Plains.

What does this mean for you?

Expect much below normal temperatures to continue across the Midwest, Great Lakes, and Northeast through October 15th. Disturbances in the trough will also bring stormier conditions to Mid Atlantic and Northeast. There will be at least a couple of *cold rain* events in New England; the first coming this Sunday-Sunday night, and another possible at the end of next week.

What about beyond next weekend? Looking ahead, the block over the Gulf of Alaska does eventually break down, and the Canadian trough starts to lift northeast. New England stays on the cooler side but the rest of the nation should be warming up by next weekend.

GEFS projected upper level pattern by next weekend

My next point of interest is what happens once the block breaks down. The configuration of this block (which evolved from an omega to rex formation) includes a disturbance locked south of the massive ridge. Once the block erodes, this disturbance is ejected eastward into the southwest U.S.

At the same time we must consider the Canadian trough again and the strong upper level confluence upstream of the trough, over the northern Plains and Midwest. This confluence prevents the southwest U.S. disturbance from amplifying much and breaking northward. Instead, it slides eastward toward the southern Plains. In addition, with the block gone, the doors open to more shortwaves taking a similar track into the southwest and southern Plains.

What does this mean for you?

We could be looking at a period of enhanced storminess in the southern Plains including thunderstorms and severe weather, beginning next weekend and continuing through the following week.

Cold and stormy for the next few weeks

As I discussed in my last post, there is a major pattern change upon us. The storm system that brought clouds, rain, and cool weather to much of the southern Plains into the East is starting to wrap up and lift out to the northeast. The upper level trough will be responsible for one last round of rain for New England on Thursday and Thursday night as low pressure redevelops near the triple point and tracks south of Long Island.

In the wake of this disturbance, the first in a series of Canadian troughs will be diving southward into the Plains. Here's the surface forecast for Thursday morning:

You can see the storm system still affecting the east coast, but draw your attention to the Plains where a very strong cold front is cutting southeastward. Also notice the deep low over Minnesota, and that's right, that is snow in the forecast from North Dakota into Minnesota Thursday through Friday. Winter storm watches have been issued for these areas for accumulations greater than 6 inches! By Friday morning, this cold front will surge forward through the Ohio Valley with as much as a 20 to 25 degree drop in temperature from Thursday afternoon to Friday afternoon.

In the long range, this pattern looks like it will become stable for the next few weeks, as a large blocking ridge over the North Pacific creates cross polar flow, sending reinforcing shots of cold air from Canada into the U.S.

If this were in the winter, we would be talking about a notable arctic outbreak, but because it very early in the season, these shots of cold air get modified as they travel south. For instance, the lack of snowcover allows arctic airmasses to warm as they slide across the still warm ground.

First blast of Canadian air

From the glimpses I've taken at the developing pattern I'm becoming more and more interested in how fall 2012 is shaping up for North America.

This week will feature a significant pattern change beginning in the Pacific and extending across North America by next weekend.

Pacific jet energy is poised to dump into western NA by the middle of next week. The low in the deep south, responsible for all the heavy rain from Texas through Mississippi will provide a path for this energy to dig into one of the first large full-latitude troughs of the season. Upstream, the previously suppressed flat ridge over the eastern Pacific explodes poleward into Alaska.

Meanwhile, we can't forget the train of monster tropical cyclones recurving in the Western Pacifiic. These recurving typhoons can trigger a wave response downstream across the Pacific into North America. Upper level ridging arises over the northwest Pacific in response to the huge latent heat release and upper level divergence associated with these storms. This ridging translates to a trough around the Aleutians and to another ridge over the eastern Pacific into Alaska.

Combine forces and all of a sudden we have this massive blocking ridge forming along the west coast of North America.

What does this mean for you?

Expect a strong cold front to plow through the Plains during the middle of this week. A line of thunderstorms, some strong to severe will accompany the front across the Midwest. Meanwhile parts of the northern Plains could be looking at their first snowfall of the season behind the front as the storm intensifies over the Great Lakes. The cold front will continue eastward, through New England and the Mid Atlantic region by next weekend. In the wake of this storm system, temperatures will stabilize around 10 degrees below the normal for this time of the year.

Here's the GFS ensemble mean 6-10day 850mb temperature anomaly forecast (degrees C):

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