Alberta Clipper passes tonight/Weather action moves to the coasts/Quiet here Thanksgiving and beyond

We’ve had typical weather for late November today and much warmer than last week.  Overall, not much of significance will be going on over the next week or so in the local area which is good news for holiday activities and for travels. The coasts will be a different story — snow in the east is imminent and the west will get its first, big wet storm in many months NEXT week (more on those later).  For the local area, we appear to be locked in a mostly dry, uneventful and more seasonable pattern for the time being and that may last into early December.

Up first, though, an Alberta Clipper is on track to pass the area overnight and it may bring some light rain, rain & snow mixed and/or very light snows to the area.  We’ll likely see the Alberta Clipper as a frequent feature this winter (see yesterday’s winter outlook discussion), so we’ll take a look at how these systems come together .

Clipper lows form when Pacific storms move ashore over western North America.  The original storm and frontal system will dissipate over the high terrain of the Canadian and U.S. Rockies, but the cross barrier (the barrier being the Canadian Rockies) flow of air warms as it heads downslope on the eastern side of the mountains.  This lowers air pressures, increases the temperature contrast between the warmed air and neighboring Arctic air and begins to set up a spin in the atmosphere.

This is a cross section of the air cresting the Canadian Rockies and moving downslope into the high plains of Alberta.  Downslope flow begins to warm the air near the eastern slopes (greenish-blue area), with an area of spinning air forming along the low pressure trough on the Alberta Plains.

This is a cross section of the air cresting the Canadian Rockies and moving downslope into the high plains of Alberta. Downslope flow begins to warm the air near the eastern slopes (greenish-blue area), with an area of spinning air forming along the low pressure trough on the Alberta Plains.

This spin evolves into a low pressure area which begins to wrap Arctic air around it to the north and west and warm, dry air to the south and east.  The low then follows the jet stream southeast into the northern U.S.

The Pacific storm and frontal system weakens and dissipates over the high terrain of the Rockies.  The cross-barrier westerly flow continues and a low pressure area forms where the "spin" began in Alberta.  Ongoing downslope flow spreads southward into the U.S., with continued warming of the air mass against the eastern slope of the Rockies.  This causes the low pressure trough (lee trough) to transition to a warm front.  The new low pressure area forces Arctic air to its north to the west, which will cause the old frontal boundary to strengthen and become a cold front.

The Pacific storm and frontal system weakens and dissipates over the high terrain of the Rockies. The cross-barrier westerly flow continues and a low pressure area forms where the “spin” began in Alberta. Ongoing downslope flow spreads southward into the U.S., with continued warming of the air mass against the eastern slope of the Rockies. This causes the low pressure trough (lee trough) to transition to a warm front. The new low pressure area forces Arctic air to its north to the west, which will cause the old frontal boundary to strengthen and become a cold front.

This is a cross section of the air cresting the Canadian Rockies and moving downslope into the high plains of Alberta.  Downslope flow begins to warm the air near the eastern slopes (greenish-blue area), with an area of spinning air forming along the low pressure trough on the Alberta Plains.

This is a cross section of the air cresting the Canadian Rockies and moving downslope into the high plains of Alberta. Downslope flow begins to warm the air near the eastern slopes (greenish-blue area), with an area of spinning air forming along the low pressure trough on the Alberta Plains.

As the low moves southeast, light snows will typically fall along and north of the track of the low, mixing with rain just to the south and west.  Further south and west, conditions are dry with a rapid (sometimes windy) warm-up as the warm front passes and a rapid (sometimes windy) cool down as the Arctic air flows in behind the cold front.

Northwest flow aloft carries the new storm system into the U.S. A now well-defined warm front swings out into the Plains, with very warm and dry air, heated and dried by the downslope flow, causing windy and warm weather to the south of the clipper low.  To the north, Arctic air is flowing southward into the northern Plains. Left over Pacific moisture is squeezed out in the cold air causing light snows or light rain to develop along and north of the low pressure area's path.

Northwest flow aloft carries the new storm system into the U.S. A now well-defined warm front swings out into the Plains, with very warm and dry air, heated and dried by the downslope flow, causing windy and warm weather to the south of the clipper low. To the north, Arctic air is flowing southward into the northern Plains. Left over Pacific moisture is squeezed out in the cold air causing light snows or light rain to develop along and north of the low pressure area’s path.

Coming from the north, these systems do not have access to a moisture source, so whatever precipitation falls is from left over Pacific moisture which survived the trip over the mountains, or from the lift and forcing of the storm itself being able to wring out any pre-existing moisture in the air (easy to do in cold air).  Resulting rains and snows are usually light to moderate and quick moving, which also really reduces time for accumulation.  On the south side of the system, the downslope air is warmed, and as air warms, it dries out, so there is no moisture available for anything but few to scattered mid or high level clouds.

Tonight’s clipper is taking a track far enough south and west to bring some light rain and snow to parts of the district.   As you can see in the forecast snow accumulation graphic, a dusting to an inch is the most that can be expected with slightly better snows from Iowa to western Illinois and in northeast Missouri. Some areas, including SEMO/SW IL and KC might not see much of anything at all in terms of even light snow.

2014-11-25-2052-72HSNWQPF

Thanksgiving Day itself looks uneventful locally as high pressure will be over the area and will bring mostly sunny skies and highs in the upper 30s to lower 40s, just a few degrees below average.

Wednesday and Wednesday night look to be a major issue from the Mid Atlantic through New England, as the first major snow impacts the region.  Take a look at the current winter storm headlines (pinks are warnings, blue watches and purple advisories) and snow amounts forecast:

2014-11-25-1454-EDDWSWNEW 2014-11-25-1248-72HSNWEDD

If you have plans to drive out that way, fly out that way or travel through any of those airports, check local forecasts and consider adjusting plans if needed.  Flights across the country may be impacted if any of the flights to or through major eastern hubs are delayed or cancelled.

Going forward for the next seven days, these model graphics show total precipitation forecast through next Tuesday morning. This includes any from tonight’s clipper, and its easy to see all the action is well north, east or southeast of us. Parts of KS may not see anything measurable at all.

2014-11-25-1122-12ZCMCTTP 2014-11-25-1122-12ZGFXTTP

Meanwhile out in drought-parched California, they may see their first heavy rain in 8 months or so with 5″+ on the peaks of the Sierras and 3″ possible for Los Angeles, San Francisco and an inch for San Diego.  Totals drop of dramatically in the rain shadow, but measurable rain of 1/4 to 3/4″ make make it as far east as Arizona.  Rains don’t make it east of the front range however.

2014-11-25-1203-WPCQPFSW7

The reason for the reversal of fortune -for them and us- is change in the overall jet stream pattern. We discussed the likelihood of an early December pullback of the cold last week as the persistent drought enhancing western ridge weakens and shifts over us and a deep, cool and moist trough moves in out west.  This locks up the Arctic air in northern Canada and won’t let it out.  The flow aloft is not a favorable one for getting a lot of moisture into the local area and so we’re left with a mild and (mostly) dry pattern.

2014-11-25-1210-12ZGFX500

This mild pattern will continue well into next week and perhaps into the week beyond, take a look at this model trace forecast for STL over the next 10 days, showing outside of a dip early and then late next week, temperatures are not all that bad, at least according to this model.

2014-11-25-1331-12ZECMLSX

There are signals present indicating that stronger and more persistent cold will return by mid December at the latest, this will happen as an area of thunderstorms and rising air currents pushes out of the tropical Indian Ocean into the tropical western Pacific Ocean.  This distant, but powerful atmospheric signal helps rearrange the jet stream across the hemisphere and operates over a timescale of several weeks.

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