Monday, January 31, 2011

Back-Door Arctic Front

This morning's (1200 UTC) initialized NAM forecasts the movement of a back-door Arctic front from Wyoming into Utah and across the northern Wasatch Mountains at about 1200 UTC (0500 MST) tomorrow (Tuesday) morning.  I refer to this as a back-door Arctic front because it is approaching from the east.  Below are three perspectives of the NAM-forecast back-door Arctic Front as it moves into the Wasatch Front.

NAM 2-m temperature (color fill every 2C) and 10-m wind
(full and half barb denote 5 and 2.5 m/s, respectively) valid
1200 UTC 1 Feb 2011
Perspective view with potential temperature cross section (white)
More oblique perspective showing potential temperature
"wall" with arctic front merged with mountain wave
induced by the Wasatch Mountains
I suspect tomorrow and possibly Wednesday are going to be a brutally cold area along portions of the east bench from Parley's canyon north.  Not only are we dealing with very cold air, but easterly flow, a strong cross-barrier pressure gradient, and the presence of a stable layer near mountain top level (as indicated by the strong vertical gradient in potential temperature surmounting the arctic airmass) all indicate that there will be enhanced canyon winds that are stronger and more widespread than usual.

We are running an experimental numerical modeling system here based on the Weather Research and Forecasting (WRF) that features much finer (1.3 km) grid spacing than the operational NAM (12 km).  This is the first time we've been able to simulate a potential canyon wind event with it.  The run from last night (0000 UTC initial conditions) produces a modest canyon wind event by 1800 UTC (1100 MST) tomorrow.  At this time, both gap and downslope winds are produced in this simulation along the western slopes of the Salt Lake Valley, although these winds are weaker than those produced downstream of the Raft River range and over the western Great Salt Lake.

Purples and blues along the western slopes of the Wasatch
Mountains indicate canyon winds of 15-25 knots
What we don't know is if this model can reliably simulate the intensity of canyon wind events.  This is also a single simulation (and the snapshot above is of a single time), when one would really like a calibrated ensemble of high resolution simulations to examine the range of possibilities given the potential sensitivity of localized downslope flows to small changes in the environmental conditions.  That is the future - but we don't have those capabilities today.

No comments:

Post a Comment