Aug 24th - Deep upper-level trough continues storm threat across the southeastern corner of AZ

 Overview:

The area continues to be under an unusually amplified flow pattern with a deep upper-level trough centered over California and a strong ridge of high pressure stretching from the Southern Plains to the Canadian Prairies. Broadly speaking, this places the Intermountain West in a region of upper-level cyclonic vorticity advection which should support scattered thunderstorm activity from southeastern Arizona into central Colorado. While southern Arizona is close to the southern extent of the West Coast trough, the right entrance region of an upper-level jet streak should aid in upper-level divergence over southeastern Arizona today and to a lesser extent tomorrow. In the meantime, drier air in the southwesterly flow ahead of the upper-level trough is settling into Arizona from NW to SE over the next 48 h which should limit convection to areas east of the Tucson Mountains. Destabilization in the valley locations of the Tucson metro area today will be somewhat problematic due to modification of the environment from activity yesterday in combination with expected high temperatures of less than 100 F. While synoptic-scale forcing will be on the decline tomorrow, the thermodynamic environment is forecast to be better early tomorrow ahead of the encroachment of the aforementioned dry air. 

Saturday:

Large scale picture:

The 500 hPa mesoanalysis overlayed on the mid-level water vapor image from this morning provides a nice overview of the features in play today. The trough axis (highlighted by the thick black line) has a slight negative tilt and stretches from the Pacific Northwest southward to just off of the West Coast of the Baja Peninsula. The ridge axis (highlighted by the jagged line) stretches from near Dallas,Texas northward into the Canadian Prairies. A plume of dry air embedded in the southwesterly flow ahead of the upper-level trough stretches northeastward from the AZ/CA border into eastern UT. This plume of dry air should define the western extent of the threat of convection as it slowly pushes eastward over the next 48 h.

Zooming In:

Cloud cover this morning is associated with remnant cyclonic vorticity from overnight convection. The cloud cover should continue to dissipate from west to eat this morning as the cyclonic vorticity lifts northeastward, clearing the way for significant sunshine over the greater Tucson metro area.

While the Tucson sounding remains unavailable, this morning's sounding from Pheonix clearly shows relatively dry lower-layers above the surface and a PWTR value of 1.14" which is near the climatological average for this time of year. Lapse rates and the potential for destabilization are somewhat mitigated by a temperature inversion near 500 hPa. Wind shear is linear in nature but sufficient to indicate the potential for convective organization. The presence of the upper-level jet streak is indicated by 60 knot southwesterly flow at 200 hPa.

Using the 12Z UAWRF intitial analysis as a stand-in for the Tucson balloon sounding, we can see lightly better (but not great) moisture and weaker flow relative to Pheonix. Also, the inversion appears weaker an higher up in the atmosphere at 400 hPa. As the upper-level trough pushes eastward, we will likely see stronger upper-level flow and better dynamics overspread the area, but mixed layer moisture looks likely to continue to be problematic.

 The moisture gradient between Pheonix and Tucson is supported by this morning mesoanalysis, although moisture transport vectors suggest that some northward moisture advection is possible throughout the day.

The Forecast for this Afternoon:

The UAWRF shows that the greater Tucson metro area should be in the right jet-entrance region by 2 PM this afternoon, suggestive of upper-level divergence.

At the same time, surface winds indicate a region of convergence just to the east of the Tucson metro area.

Meanwhile, CAPE values in Tucson are poor with values of 300 J/kg although larger CAPE values are situated to our southwest....

on the heels of low-level moisture transport in southwesterly boundary layer flow.

The favorable dynamics result in convective initiation in both the 12 and 15Z runs of the UAWRF over the Tucson mountains by 2 PM.

Both runs indicate marginal instability over Tucson at this time although slightly better mixed layer moisture in the 15Z run results in slightly larger CAPE values. Both models show unidirectional shear with increasing wind speeds with altitude  which would suggest the possibility of anvil contamination in the direction of convective propagation from SW to NE.

Storms should continue to develop to the southeast, although the marginal instability and anvil contamination will influence convective evolution in the Tucson metro area as indicated by the 4 PM reflectivities.

Convection is then forecast to dissipate rapidly after sunset.

Broadly speaking the HREF members are in agreement with this scenario with members showing scattered > 40 dBz reflectivelties across the Tucson metro concentrated along the higher terrain.

My Take: I find it difficult to argue with I view as favorable dynamics. However, I expect the significant anvil activity to limit direct propagation. Instead, I would look for new storms initiating to the east of original updrafts along outflow boundaries which will give the appearance eastward as opposed to northeastward propagation. Limited instability should minimize the severe threat, although DCAPE values of around 1300 J/kg and favorable synoptic-scale forcing and wind shear will mean that localized heavy rains and strong winds are possible.

Tomorrow:

The deep upper-level trough should slowly weaken as it creeps eastward. As it does so, the right entrance region of the upper-level jet should move into northeastern AZ by tomorrow afternoon.

Dry air will continue to overspread Arizona, although a axis of deeper moisture looks to consolidate early over eastern Pima and Cochise county with a sharp boundary just to the west of the Tucson metro area.

The boundary is forecast to move across the Tucson metro area between 1-4 PM MST 

The passage of this boundary/front should limit convective activity across the northwestern side of the Tucson metro, with more widespread and intense activity in an axis from Santa Cruz northeastward into Cochise and Graham counties.