Sunday, July 15, 2012

The Large Scale Matters

It is easy for a meteorologist to suffer from what some people describe as mesomyopia, a tendency to focus on weather phenomenon on scales of perhaps 200 km or less.  This is especially true in the western United States where topography plays such a dominant role in the weather.  However, to be a successful forecaster, you need to look at the big picture and always examine the atmosphere from the large-scale on down to the microscale.

Case in point is a comparison of two troughs, one that is influencing our weather this weekend, the other that will influence our weather in the coming days.  Both troughs dropped in from the Pacific Northwest, but will have dramatically different influences on the weather of northern Utah.

The first was located off the Washington–Oregon coast at 0000 UTC Friday.  At this time, substantial moisture was found over Arizona, southern California, and the adjoining eastern Pacific Ocean due to a prior surge of monsoon moisture.

As a result, as this trough moved southward and broadened, brought a fairly moist airmass into northern Utah, leading to showers and thunderstorms this weekend.

Forecasts for 0600 UTC Monday (late tonight), show a second trough dropping into the Pacific Northwest.  The position is virtually identical to the previous trough.

However, there are two substantive differences.  First, the airmass over southern California and the adjoining eastern Pacific waters is much drier.  Second, the flow and subsidence (sinking motion) behind the trough is much stronger.  Subsidence acts to dry out the atmosphere.

As a result, as this trough sags southward, it will result in drier air moving into northern Utah, an a decrease in the likelihood and coverage of showers and thunderstorms early in the coming week.

Thus, the two upper-level troughs have dramatically different impacts based solely in subtle changes in structure and intensity and the distribution of moisture as they move southward.

Interestingly, there a tropical cyclone forecast to develop over the tropical eastern Pacific early this week.  The moisture associated with that tropical cyclone can be seen in the bottom left panel below.  The trough eventually taps into that moisture and brings it into northern Utah later in the week, although we will have to see if that results in another increase in showers and thunderstorms.  Again, this illustrates the importance of seeing the big picture and knowing what type of airmass that the trough is tapping into.  This is particularly important during the monsoon when midlatitude systems can interact with tropical and subtropical systems and their associated moisture surges into the southwest.

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