The forecasts for Saturday by most model runs were quite good as widespread storms formed to the north and east of Phoenix and moved towards the valley. CAPE was limited thus the storms died out as they moved into the lower elevations, but produced strong outflow winds and blowing dust.
A transition type event is underway as a large trough is moving onshore over the western CONUS. Arizona's winds are now mainly from the southwest and temperatures have cooled to a more favorable -8 to -9C. There is a 250mb 60 knot jet over southern California associated with a weak upper shortwave which is likely to play a part in enhancing storms over Arizona today. There is a band of high-level clouds associated with the jet. The RR and NAM appear to have the main features initialized well. PW has also been initialized well. Model forecasts confidence is high today.
Day 1
It looks like a textbook transition event today as all ingredients are in place for a few mini-supercells, including this: (hopefully, SPC has a sense of humor)
Low-level moisture is quite high as most stations in southern Arizona, including mid-elevations, have dewpoints in the 60s. Yuma is 76! However, the moist layer is quite shallow as PW in Yuma is only 36mm. By early afternoon, the deeper moisture is mainly over southern and central Arizona. It is quite moist with dewpoints in the 12 to 14C range which is plenty to support storms.
This is a textbook severe weather Skew-T plot for Phoenix this afternoon. Excellent veering with height vertical wind profile in the lower troposphere plus an upper jet. CAPE is high at around 1600 J/kg, plus a deep mixed layer with only a minimal cap. However, the storm motion is in the same direction as the upper winds thus anvil shading could kill off low elevation storms as they move towards Phoenix and Tucson.
The upper trough and associated jet streak move eastward across the state this afternoon, providing dynamical support for storms.
There is also a bit of cold air advection in the mid-levels as cooler air is present over western Arizona.
The model consensus is to develop storms first on the higher terrain of central Pima, southern Maricopa, and SW Pinal Counties by early to mid-afternoon.
At this point, the forecasts diverge somewhat. The 6Z WRFGFS, 12Z WRFNAM, and 15Z WRFRR are more active and move scattered strong to severe storms into lower elevations of mainly Pinal County. some of the runs have very high reflectivities, so large hail is a possibility with these storms.
Outflows move into the Phoenix area by late afternoon and they appear strong enough to trigger additional storm development in and around the valley.
Day 2
Some moisture remains, but no activity is expected due to strong subsidence and mid-level warming as the trough moves out.
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.