July 20th-Low grade Monsoon conditions for today, steadily improving into the weekend
Overview:
After an unexpectedly active pattern in the vicinity of Tucson last night, conditions have dried out a bit and it is still my thinking that the next 48 h will be relatively quiet with probabilities of convection at about 20 percent in the immediate Tucson vicinity and increasing as you move towards the border. Activity today should still be focused in the higher terrain near the AZ/NM border, shifting to the international border as we get into the nighttime hours. Temperatures should remain at or near record highs for the next 48 hours before cooling slightly as the upper-level high consolidates in the 4 corners area, allowing upper-level heights to drop slightly. That combined with increases convection over the weekend will hopefully provide at least marginal relief from the continued heat wave.
Today:
This morning's sounding from Tucson shows lower grade thermodynamics relative to yesterday.
12Z TUS Sounding
The vertical structure of the dewpoints suggests that dewpoints at the surface should drop to 50F or maybe below as the day progresses and the boundary layer becomes more well mixed. As a result, forecast CAPE values are 622 J/kg as compared to over a 1000 J/kg from yesterday. Precipitable water values have also dropped to 1.19". They lower available moisture is seemingly limiting instability even though lapse rates remain favorable up through approximately 480 hPa. We continue to see offset locations in the uppe-level high pressure system with 500 hPa winds from the south and 300 hPa winds from the east. Much like yesterday, any convection that does develop will likely propagate very little and instead will have to rely on the movement of outflow boundaries for the creation of new updrafts. Downdraft CAPE values remain on the high side at 1600+ J/kg, which suggests that if convection does develop, surface wind speeds could approach 50 mph and will be the primary hazard associated with convection today.
This morning's low-level water vapor imagery from weather.cod.edu, does a good job oh highlighting the area of that guidance is developing relatively early day convection.
12Z HREF Wind >30 knot Valid at 21Z
I am using wind speed probability to define the primary axis of convection since heavy precipitation probabilities are uniformly low. As the night progresses, an inverted trough that moving eastward south of the border should result in relatively robust convection in Mexico overnight.
12Z UA_WRF Wind and PWTR Valid at 00Z
The inverted trough can be seen highlighted by the white line in the wind field, while the black line highlights a region of convergence ahead of the inverted trough that should serve as a focus for convective initiation. The expected complex that will develop is well reflected in the HREF wind probability map from overnight.
12Z HREF Wind >30 knot Valid at 21Z
The primary impact of this complex on Arizona should be a relatively robust moist outflow increasing low-level dewpoints as shown in the 2m dewpoint maps below.
12Z HRRR Wind and 2m DWPT Valid at 07Z/21st
12Z HRRR Wind and 2m DWPT Valid at 09Z/21st
Precipitable water values around an inch with LCLs up over 600 hPa suggest strong and deep outflows with enhanced eastward propagation. As these eastward moving outflows encounter a NW'ly environment near surface wind field, the enhanced convergence should help initiate new updrafts. The timing on this scenario looks to be late afternoon.
We set the stage for potential Rim activity....
Thanks for the updates!
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