An active 48 hours

Overview:

Southeastern Arizona is still being impacted this morning by relatively dry air on the back side of a weak but latitudinally elongated trough. This has left southern Arizona with a significant moisture gradient with deeper moisture this morning confined to the western portions of Pima and Maricopa counties. Moisture return into the Tucson metro area last night was fairly shallow and I would expect surface dewpoints to drop into the 40's late this morning or in the early afternoon. Later in the day, as surface westerlies develop and deepen throughout the mixed layer, moisture advection from the west should allow for precipitable water values to increase with surface dewpoints climbing back into the low 50's. With less moisture than advertised yesterday, CAPE will be somewhat limited with values around 500-700 J/kg in Tucson with values closer to 1000-1500 J/kg in the Phoenix metro area. The limited moisture environment in Tucson will likely preclude any early development of convection and initiation will likely depend on help from moist outflow boundaries from storms propagating southwestward off of the higher terrain which should reach the area between 5-8 PM MST. This activity should encounter a more favorable thermodynamic environment in central Pima county where the SPC has issued a slight risk for severe weather today. For tomorrow, expect conditions largely similar to day with a slightly better moisture profile. The increase in moisture could result in convection developing in the higher terrain of the Rincon or Catalina mountains by mid afternoon although more widespread activity will once again have to wait for help from southwestward propagating storms in the evening.

This Morning:

Large Scale:

This morning, a weak positively-tilted trough axis evident in the 500 hPa wind field extends southwestward from the Great Lakes into West Central Texas. High pressure continues to be situated near the Nevada/Arizona border. The gradient between the high-system and the positively tilted trough is resulting in northeasterly flow over most of southeastern Arizona with flow diminishing in central and western portions of the state closer to the center of high pressure.


GFS Initialized 500 hPa Hights/Vorticity valid at 5 AM MST


The broad subsidence on the western side of trough axis has resulted in below normal moisture conditions over most of the Front Range all the way south into Mexico.

GFS Initialized PWTR anomaly valid at 5 AM MST


Zooming In

This surge of dry air to our east has left eastern Arizona in a significant moisture gradient with values ranging from 0.8" along the AZ/NM border to 1.8" in western Pima county. Convective activity later today will depend on the onset of moisture transport from the west, the beginnings of which are starting to be seen in the moisture transport vector on the Maricopa/Pima county border.

PWTR Mesoanalysis Valid at 10 AM MST


The lower PWTR values to the east hint at the depth of moisture return overnight as evidenced by the Tucson and Phoenix soundings from this morning.

5 AM MST Tucson Sounding


At Tucson, the moist layer is "ankle deep" with implied mixed layer values in the upper 40's F. With such a shallow moist layer, the MLCAPE is essentially nonexistent and CINH values are generally around -400 J/kg. PWTR values are on the low end of climo with a value of 1.24" and DCAPE values of 1500 J/kg would support significant outflows wind speeds. Lapse rates of 7-7.5 C/km are decent and would support the development of CAPE provided that we were able to increase moisture this afternoon. 20 knot easterly winds in the mid-levels of the atmosphere would support marginal shear conditions provided that mixed layer westerlies of 10 knots or greater develop this afternoon.

5 AM MST Phoenix Sounding


Over at Phoenix, a deeper moist layer (>100 hPa) corresponds to a significantly larger PWTR value of 1.67".  Thermodynamic profiles are slightly better as compared to Tucson, with a DCAPE value of 1600 J/kg and significantly larger forecast surface based CAPE. On the negative side, the proximity of Phoenix to the upper-level anticyclone means that winds in the mid levels are relatively weak and convective organization to aid propagation into the Phoenix metro area is unlikely.

The Forecast:

Mixed layer moisture transport from the southwest is forecast by the CAMs to increase in the afternoon/evening in response to strengthening westerlies. Meanwhile, moist outflows boundaries are likely to be approaching the major metropolitan regions in the early evening.

12Z UAWRF HRRR 60m Moisture Transport Valid at 3PM (left) and 6 PM (right) 


The convergence of these features over central Pima county west of Tucson should provide the greatest threat for severe weather heading into the 7-9 PM timeframe.

By 6 PM, the forecast soundings at Phoenix and Tucson are similar with somewhat better thermodynamic conditions at Phoenix.

12Z UAWRF HRRR Soundings at Tucson (right) and Phoenix (left) Valid at 6 PM 



CAPES of approximately 500 J/kg at Tucson increase to close to 1000 J/kg at Phoenix with a similar difference in magnitude of the DCAPE (1300 to 1800 J/kg respectively). Despite the better profiles at Phoenix, the closer proximity of Tucson to the elevated terrain will be a significant aid to the ability of outflows to intiate convection in town. As the convection propagates westward into the better mositure and the afore mentioned convergence between moisture transport and outflows, the potential exists for a significant increase in storm intensity as indicated by the UAWRF.

12Z UAWRF HRRR 60m reflectvity Valid at 6PM (left) and 8 PM (right) 




My Take:

Although conditions are not quite as conducive as I had expected given yesterday's forecast, chances for convection should remain in the 50% range around town given that the promised mositure transport from the west actually occurs and surface dewpoints can climb back into the low-mid 50's F by evening.

Tomorrow:

I expect somwheat of a repeat to today except with slightly better moisture and instability, particularly in southeastern portions of the state as the influence of the positively tilted trough to our east gradually dissipates. 

The forecast 2m dewpoints from the UAWRF show significant increases relative to day with 11 AM dewpoints near 60 F tomorrow as opposed to near 50 F today. 

12Z UAWRF HRRR 2m Dewpount Valid at 11AM-Wednesday (left) and 11AM-Thursday (right) 



Consequently, CAPE values are forecast to be 500-700 J/kg higher tomorrow

12Z UAWRF HRRR Forecast KTUS Sounding Valid at 4 PM Thursday


Perhaps setting the stage for multiple rounds of convection throughout the day.

12Z UAWRF HRRR Reflectivity Valid at 4:30 PM (left) and 9 PM (right) Thursday





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