The latest model runs of next week’s frontal system are appear to be kind of/sort of converging toward a solution that would place the strongest deep layer forcing on the cool side of the frontal zone, although I wouldn’t rule out a discrete cell or two developing in the warm sector in this scenario. However, we’re still 3 days out, so the usual “still considerable uncertainty” disclaimer applies.
This post is more of a follow-up to Matt’s comment from yesterday regarding SHERB values over the high terrain. Here’s the SHERB GFE smart tool output from the NAM:
And here’s the GFS (this is the 06Z run):
The higher values correlate very well with the high terrain. The main culprit appears to be the low level lapse rate (although the 0-3 km shear is slightly higher over the high terrain as well).
Here’s a NAM image of low-level (0-3 km) lapse rate overlaying the temperature forecast at 3 km AGL.
If we focus on the higher elevations (i.e., above 4000 ft), we can see that the 3 km AGL temps are considerably cooler (2-4 degrees C) than the surrounding valleys, as one would expect.
Now look at the surface temp forecast for the same time:
The high elevation surface temps are only about 5 degrees F cooler than the surrounding valleys. So (relatively) warm surface temps w/ cool 3 km AGL temps = steep 0-3 km lapse rates. Is this realistic? Due to the coarse resolution of the model terrain, the NAM appears to be too warm with its high elevation surface temps (even for this WAA regime), so probably not, although during very strong WAA events, temp differences between the high elevations and the valleys can be quite small, and this scenario is possible. I suppose the bigger question is: is this meteorologically significant, or simply a limitation of using the fixed layers?