Recent work has shown that the Intermountain West is a fertile breeding ground for strong cold fronts, many of which develop over eastern Nevada and northwest Utah (e.g.,
Shafer and Steenburgh 2008;
Steenburgh et al. 2009). In particular, Shafer and Steenburgh (2008) show that the number of strong cold frontal passages, defined based on a temperature fall of at least 7ºC and pressure rise of 3 mb in 2–3 h in conjunction with a 700-mb temperature gradient of at least 6ºC/500 km, increases as from the Pacific Coast to Salt Lake City where there is a local maximum.
There's a minimum at Wendover, but some weird stuff happens there (meteorological and other) because of the mountains to the immediate north and northwest.
However, climate is what you expect and weather is what you get. Some fronts that move across the Intermountain West do not intensify.
For example, the NAM model predicts that a fairly healthy cold front will develop tonight over the Pacific Northwest. The cross-front temperature contrast is not large, but the gradient becomes quite concentrated by 1200 UTC (0600 MDT) tomorrow (Friday) morning.
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1200 UTC 15 Sep Initialized NAM model forecast of sea level pressure,
10-m wind, and 3-h accumulated precipitation (top) and 700-mb temperature,
wind, and relative humidity (bottom) valid 1200 UTC 16 Sep 2011. |
I suspect some of the frontal development is related to sub-cloud diabatic cooling from precipitation in the post-frontal environment.
Although many of these fronts intensify as they move into Utah, the NAM model forecast calls for the front to weaken by tomorrow afternoon.
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1200 UTC 15 Sep Initialized NAM model forecast of sea level pressure,
10-m wind, and 3-h accumulated precipitation (top) and 700-mb temperature,
wind, and relative humidity (bottom) valid 1200 UTC 16 Sep 2011. |
That doesn't mean that we won't have an interesting weather day tomorrow. Whenever a mid-latitude trough plays with monsoonal moisture, I tend to stay alert. Perhaps we'll get some decent outflow boundaries as well with a rapid temperature fall. But this won't be an event where we're in shorts one day and parkas the next.
There are probably a few reasons why this event does not intensify over the Intermountain West. First, the large-scale flow and cold air are well to the north. They are not interacting at all with the Sierra Nevada and southern Cascade range. Second, diabatic processes appear quite important in the frontal sharpening tonight, but also perhaps the decay tomorrow (note the well organized frontal precipitation band forecast for tomorrow, but the more scattered nature of the precipitation forecast for tomorrow afternoon). There is abundant moisture ahead of the front, which contrasts with our big spring cold fronts, which usually have a dry pre-frontal environment. Finally, I suspect the large-scale flow in this case is simply not favorable for strong frontal development, even in the absence of topography.
I've always thought an interesting extension of Shafer and Steenburgh (2008) would be to look at landfalling Pacific cold fronts with and without intensification over the Intermountain West. Tomorrow may provide an example of the latter. We'll see if the NAM forecast verifies.
And for you skiers out there, don't bother waxing the skis yet.
