July 14th - Marginal risk of excessive rains and severe winds, but the devil is in the details.....

Overview: 

Upper-level high pressure has slowly shifted to the east northeast over the last 48 h and is situated over the Four Corners region as of this morning. This has shifted the mid and upper-level flow to easterly/southeasterly over southerneastern Arizona. Further west, deeper southerly flow around the western periphery of this high pressure system and (aided by a decaying upper-level low off of the west coast) has allowed for the advection of moisture  from the Gulf of California and eastern Pacific allowing for Monsoon conditions to become established in western Arizona and California. This moisture has slowly pushed eastward in the greater Tucson area aided by moist outflow from overnight convection in Sonora, Mexico. Models are in good agreement that convection will initiate in southern Pima and Santa Cruz counties early this afternoon. Outflow from this convection is forecast to spread northward into the southern Tucson area this afternoon. This is where model agreement ends as there is significant variations in convective coverage over the greater Tuscon area initiated by this outflow boundary. Thermodynamic profiles appear generally favorable across model solutions so I would lean towards more aggressive solutions. Areas most favored for convection in the Tucson metro would be the southeastern slopes of the Rincons and Catalinas where outflow and low-level upslope should coincide. Further to the north, warm air at 700 hPa may limit the ability of convection to work its way into the Phoenix metro area today. Most forecasts indicate convective inhibition associated with this warm air at 700 hPa to persist into the evening hours across the aptly named Valley of the Sun. 

As for tomorrow, conditions should be largerly similar to today with deeper moisture working its way eastward into Cochise county. As a result models are indicating storms initially developing further east before moving into Pima county. The greatest threat for heavy rain today (50% chance) should be along and south of the I-10 corridor with lesser chances (30% chance) across northern sections of town. Any storms that due develop over the next two days should be capable of supporting strong gusty winds in excess of 30 knots with a marginal risk of severe winds.

Today:

The large scale flow shows that the upper-level high influencing our weather for the past several days has shifted to just east of Four Corners Region with southeasterly to southerly flow becoming established across southern Arizona and the West Coast.

500 hPa Mesoanalysis Valid at 10 AM

This has allowed for moisture to become better established across the lower deserts in southwestern AZ and southern CA with a strong decrease moisture as you move eastward towards New Mexico.

PWTR Mesoanalysis Valid at 10 AM

A little closer to the surface, the 700 hPa temperature analysis shows temperatures increasing from +12 C just south of the international border to +18 C across parts of central Colorado.

700 hPa Mesoanalysis Valid at 10 AM

The plume of warm air may act as a capping inversion at it advects southwestward suppressing convective activity in valley locations north of the Tucson metro area. 

This morning's sounding from Tucson seems to indicate relatively favorable conditions for Monsoon activity with:

precipitable water values in excess of 1.5" 

a relatively deep moist layer with mixed conditions still supporting dewpoints in the mid to upper 50's

minimal convective inhibition

early morning CAPE in excess of 1500 J/kg

DCAPE in excess of 1500 J/kg

mid-level easterly flow of 20-30 knots

12Z KTUS Observed Sounding

While lapse rates are not as steep as they were last week, 700-500 hPa lapse rates in combination with surface heating should still support MLCAPE values in excess of 1500 J/kg. This is supported by forecasts of the from the 12Z UAWRF HRRR initialization which shows MLCAPE and DCAPE values near 1700 J/kg by 2 PM this afternoon.

UAWRF HRRR KTUS Forecast Sounding Valid at 2 PM MST

Meanwhile, further to the north in the Phoenix metro area, a capping inversion is evident with mixed layer CIN still in excess of -100 J/kg.

                            UAWRF HRRR KPHX Forecast Sounding Valid at 2 PM MST

There is general agreement among the CAMs that convection will initiate near the international border of Santa Cruz and Pima counties by 2 PM as indicated by the HREF reflectivity paintball image

                     HREF Ensemble Paintball of greater than 40 dBZ Valid at 2 PM

as well as the 12Z run of the UAWRF HRRR

12Z UAWRF HRRR Simulated Reflectivity Valid at 2 PM

This convection should develop into a large cluster of storms moving eastward in central Pima county and Sonora, MX

                           12Z UAWRF HRRR Simulated Reflectivity Valid at 5 PM

with a strong outflow boundary moving eastward ahead of the convection, but a weaker outflow boundary moving north into the southern Tucson metro area by 5 PM.

                           12Z UAWRF HRRR 10m Wind Valid at 2 PM

The part of the forecast that is in question is the impact of this outflow boundary on convective initiation in the Tucson metro area. My personal intuition is that this outflow boundary will encounter a favorable enough environment to initiate convection in the Santa Rita mountains with diminishing probabilities as you move north into the Tucson mountains as well as the eastern slopes of the Rincons and the southern slopes of the Catalinas. Given the thermodynamics, I would favor more aggressive model solutions. The sounding evolution indicates that any storms that do develop may quickly become dominated by downdrafts in the presence of DCAPE values approaching 2000 J/kg. This can bee seen in the difference in the UAWRF soundings for Tucson from 4:30 PM preoutflow sounding to the 6PM post outflow sounding.

                        12Z UAWRF HRRR KTUS forecast Sounding Valid at 4:30 PM

                        12Z UAWRF HRRR KTUS forecast Sounding Valid at 6 PM

Well after sunset, northward propagating outflows from Pima and Pinal counties and southward propagating outflows from Yavapai may converge on the Phoenix metro area although convective inhibition will continue to be a problem.

                        12Z UAWRF HRRR KPHX forecast Sounding Valid at 9 PM

Given the prestorm CAPE and DCAPE environments, I expect marginal conditions for severe winds and heavy rain. In comparison to last Thursday's event, I expect a slightly decreased probability of severe winds and increased probability of heavy rain. The highest threat for severe winds I think will be in southe central Pima county whereas the highest for excessive rainfall in Santa Cruz. As for the National Centers, the SPC has the region in a marginal risk for severe winds and the WPC has the region in a marginal risk for excessive rainfall.

Sunday:

This will be exceptionally brief given time constraints and the focus on today.

Large-scale conditions should remain largely unchanged

                                         500 hPa HREF Mean Valid at 12 PM Monday

although higher moisture values should continue to spread eastward towards the New Mexico border.

                                           PWTR HREF Mean Valid at 12 PM Monday

Cloud debris from convection today and tonight may be an issue for destabilization from eastern Pimae county westward. Consequently, expect the earliest destabilization and convective initiation to be in central Cochise county. Eastward propagation of outflows would suggest the greatest probability of storms locally along the eastern slopes of the Rincons and Catalinas although much will depend on the level and placement of convective actvitity today.