Thursday, May 12, 2011

A Brief Break and Then Things Get Interesting

I don't want to get folks too excited, but there is a bonafide ridge axis building over Utah this morning.  Will miracles never cease!

Yes indeed, we are stuck between two troughs for a couple of days and won't that be grand.

That being said, the NAM model is quite excited about the potential for afternoon showers and thunderstorms on Saturday afternoon.  As shown in the analysis below, the name produces a broad region of positive maximum CAPE and precipitation from eastern Washington through western Colorado, including northern Utah at 6 PM MST Saturday. And, for you Northwest-types, how about >1000 J/kg in eastern Washington and Oregon.  Impressive by Northwest standards!

This area lies where there is strong deformation and weak flow between the upstream trough over the eastern Pacific and the downstream trough over the central United States.

I suspect the instability and showers are strongly related to surface heating and the presence of copious soil moisture and related evapotranspiration in the NAM model.  As such, I'm curious to see how well this forecast verifies since it is likely sensitive to the accurate initialization and simulation of land-surface processes.  Observed soil moisture is indeed quite high across much of the affected region and lapse rates in the middle troposphere are expected to be quite steep as we're more sandwiched between two troughs than beneath a high amplitude ridge.  On the other hand, land-surface processes and convection are heavily parameterized in the NAM, meaning we can't simulate them directly and have to make assumptions and shortcuts in order to incorporate them with existing computer horsepower and modeling capabilities.  Thus, this is the sort of situation where we're not sure how much to rely on the model guidance.

The bottom line is that this situation bears watching and it will be very interesting to see how it plays out.  We really don't need any gully washers given the already anticipated high streamflows this weekend.  I'm hoping the NAM is overexcited, but the forecast does provide me with additional incentive to do an early ski tour on Saturday.

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