Pesky clouds
Overview:
Widespread activity from yesterday and overnight ahead of the inverted trough axis has left the region with a tropical looking sounding with near moist adiabatic lapse rates and precipitable water values of 1.8". The moisture gradient that was situated in eastern Pima county yesterday has shifted eastward into New Mexicio allowing for the return of thunderstorm activity into Cochise county. Normally, I would be very reticent to forecast thunderstorm activity in the region given that we are on the eastern flank of an inverted trough in a relatively worked over atmosphere. However, given the unusually deep boundary layer moisture, surface temperatures in the mid 90's F would still be sufficient to create CAPE values in excess of 1000 J/kg. The fly in the ointment this morning is cloud cover that appears to be associated with a subtle secondary trough axis rotating northward around the larger cyclonic circulation. Given clearing morning and sufficient afternoon surface heating, CAMs are suggesting that convection will once again develop in the higher terrain of Cochise and Santa Cruz counties as well as the Sky Islands near Tucson. Mid-level flow will be weaker than yesterday with the strongest easterlies confined to regions north of Pima county. Given the relatively moist soundings, weaker mid-level flow, and less than typical surface dewpoint depressions, outflows strengths should stay below severe criteria.
Somewhat more typical Monsoon conditions return to the area tomorrow as the upper-level high over the AZ/NV today border shifts eastward while the inverted trough shifts slightly westward over Baja. Flow over the area will shift to southeasterly while moisture should remain on the high side. Expect convection to develop in the mid to late afternoon over Cochise county and gradually spread to the northeast. The shift in the flow to southeasterly will mean that Phoenix will be dependent on storms popagating northwestward from Pima and Pinal counties in marginal flow, reducing the potential for storms relative to today.
Today:
Large Scale:
A broad area of high pressure extends from near the AZ/NV border east-southeast into north central Texas. The main inverted trough can be seen near the Gulf of California. The low-level water vapor shows that near surface moisture has pushed into southwestern New Mexico supporting the potential for convection to return to Cochise county today.
Zooming in:
Cloud cover this morning in western Arizona associated with last night's activity is located just west of the main inverted trough axis. Further east, a secondary and more subtle tough axis is rotating northward in the southerly flow on the east side of the broader cyclonic circulation. Just ahead of it, cloud cover this morning in eastern Pima and western Cochise counties are slow to burn off.
The visible satellite loop shows a small MCV moving southwestward across Yuma county as well as developing mid-level clouds ahead of the weak eastern trough axis. Further north, a cumulus field is already developing over eastern portions of the Rim and I expect the strongest activity today to be in the higher terrain East of Phoenix in the stronger easterly flow.
As mention in the overview, the sounding from Tucson is on the tropical side of the spectrum with precipitable water values of 1.8" and lapse rates on the order of 6.5 C/km. However, the marginal lapse rates are somewhat balanced by unusually high mixing ratios from 700 hPa down to the surface which suggests that afternoon dewpoints should remain near or above 60 F. The deep mixed layer moisture should allow CAPE values to exceed 1000 J/kg this afternoon in spite of the marginal lapse rates given sufficient clearing takes place.
This Afternoon:
The trough axes from this morning are forecast to weaken and become difficult if not impossible to identify north of the international border by this afternoon. Consequently, I don't really see any focus for convective initiation outside of elevated terrain.
Instability by this afternoon should be approaching or exceed 1000 J/kg although the unusually moist conditions will mean DCAPE values of 800-900 J/kg. Flow will be uniform from the southeast at about 10 knots above the boundary layer and convective organization should not be a factor in todays's forecast.
When and where convection develops will largely depend on the evolution of the cloud field this morning. I would expect that any clearing over the higher terrain will quickly lead to the development of towering cumulus. However, I am not overly confident in the timing and location of convective development locally as cloud cover north and east of the I-10/I-19 junction is likely to be slow to burn off.
By 3 PM, the UAWRF shows convection underway to the south of Tucson as well in the Mongollon Rim region as well as over the Galiuro mountain range. I am most confident in the northern and southern sections of this picture due to the cloud cover north of I-10. The storms over the Rim should see significant westward propagation, while storms closer to the international border are more likely to pulse slowly northwestward along outflow boundaries.
Given the weak flow over southern Arizona and the exceptionally high moisture values, I expect that heavy rain with localized flooding to be the biggest issues in Santa Cruz, eastern Pima, and western Cochise counties. Further to the north, stronger flow and larger DCAPE values will produce a better chance of strong winds although they are likely to stay below severe criteria. The HREF indicates about a 70% chance of local precipitation amounts exceeding an 1" centered south of Tucson near the Pima/Santa Cruz border.
My Take:
I believe that CAMs this morning are under-done with the persistence and extent of cloud cover ahead of that weak secondary inverted trough. Consequently, I think that convective coverage in the immediate Tucson area is likely being overdone. I more more confident in the forecast activity south of Tucson where skies have been clear for a couple of hours already. Should the skies clear, thunderstorm development should occur fairly quickly once temperatures climb into the 90's. As there is no mechanism for organization, storm evolution is likely to be pulse like with propagation mostly associated with redevelopment along outflow boundaries.
Tomorrow:
Expect a more typical Monsoon day as flow at 500 hPa will become more uniformly southeasterly as the upper-level high shifts eastward and the weak upper-level cyclone remains situated over the Gulf of California. In the absence of organized activity overnight, morning cloudiness should be less of a problem.
CAPE values should once again be near or exceed 1000 J/kg as a slightly drier mixed layer is compensated by increased surface heating.
Expect convection to initiate in the early afternoon over the Rim and mid to late afternoon over the higher terrain of Cochise county. Activity will propagate slowly west-northwestward although larger DCAPE values relative to today will support stronger and deeper outflows.
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