Near record moisture continues to support the threat of storms

Overview:

Moisture continues to be abundant with precipitable water levels at or near all time highs for this time of year. However, with the abundant moisture comes a significant cloud deck and a "worked over" atmosphere from activity yesterday which will act to limit instability. The deep moisture will also diminish the potential for evaporative cooling meaning that the types of winds recorded yesterday at the Tucson Airport (gusts as high as 72 mph) are unlikely to be realized today in eastern Pima and/or Cochise counties. Models still indicate the potential for stronger outflow winds further northwest with the chance of gusty winds with blowing dust extending into the Phoenix area later today. For Sunday, focus will shift a bit further south with the possibility of heavy rains in parts of Sierra Vista and extreme southern Pima counties although the general threat of convection looks to continue for the next few days throughout the area.

This Morning:

The impact of yesterday's convection is evident in the sounding from Tucson this morning.

12Z Sounding from KTUS

(spc.noaa.gov)

The good: 

Precipitable water values are near at an all time high for 12Z on this date with a value of 1.66". 

PWTR Sounding Climatolgy for KTUS

(spc.noaa.gov)

The previous record for 12Z on this date was 1.58"

The lack of any significant inversions means that the temperatures needed to overcome any inhibition are only in the mid-upper 90's

The bad:

Lapse rates are generally moist adiabatic above 700 hPa, with near saturated conditions above 500 hPa. Consequently the potential for evaporative cooling is significantly reduced as compared to yesterday with values of downdraft CAPE this morning about 1000 J/kg less than yesterday. Also, the wind field from 1000-500 hPa is weaker as the high-pressure system that is dominating the southern tier of the United States continues to build westward. The lack of any synoptic-scale forcing will mean that triggers for convection will be relegated to terrain and any surface convergence that may develop (more on this later).

In general, lapse rates are moist adiabatic in the 700-500 hPa layer over eastern Pima and all of Cochise counties.

Mid-level Lapse Rates mesoanalysis 7 AM MST

(spc.noaa.gov)

Broadly speaking the near saturated conditions above 500 hPa are also indicative of significant cloud cover in the area.

Visible Satellite Loop Ending at 10 AM MST

(weather.cod.edu)

Note that there is some clearing in parts of western Cochise county and extreme eastern Pima county although thick cloud cover and continued shower activity are evident across west-central Pima county. Expect convective buildups to occur quickly over the higher terrain of the Catalina and Rincon mountains should the clearing persist. 

This Afternoon:

I am showing the broad 500 hPa pattern valid for this afternoon just for the sake of reference as there is nothing particularly notable about it.

HRRR ensemble mean 500 hPa Flow 12 PM MST

(spc.noaa.gov)

The 500 hPa pattern indicates a broad and diffuse area of high pressure dominating the southern tier of the United States. A specific center is hard to identify, but is likely situated over the parts of southern Plains or lower Mississippi Valley. The trough that had been evident over the West Coast has lifted Northeast and is now located over the northern Plains/Midwest. All this is to say, that we are not getting help and or significant hindrance from the large scale pattern, so we can turn our attention to boundaries and destabilization.

While a zone of convergence in the near surface wind field is still forecast this afternoon, the overall wind field is weaker than yesterday and the convergence is not expected to be as pronounced.

UAWRF 10 m wind valid at 3PM MST

(weather.arizona.edu)

However, the convergence that does exist is expected to be alligned with and axis of CAPE values that in the ensemble mean are expected to be around 500-750 J/kg in eastern Pima and Pinal counties although values will approach 1000 J/kg in the Mogollan Rim. 

HRRR ensemble mean MUCAPE valid at 1 PM MST

(spc.noaa.gov)

The 12Z run of the UA WRF however produces enough persistent cloud cover that the CAPE at Tuscon fails to exceed 200 J/kg and the situation is not much better for Phoenix

UAWRF forecast sounding for KTUS valid at 5PM MST

(spc.noaa.gov)

However, the UAWRF depiction of the CAPE appears to be underdone as it forecasts surface dewpoints to fall into the mid 50's F. (Note this image depicts the greatest forecast CAPE at KTUS) Current mesoanlyses indicate that the current dewpoints across the region remain in the low-mid 60's which supports significantly higher CAPE values of 1000+ J/kg as indicated by the current mesoanlysis.

11AM SBCAPE mesoanaysis

(spc.noaa.gov)

Because of this discrepancy, I think that the UAWRF is underestimating the threat of convection locally, I will largely focus on the wind and precipitation threat as outlined by the HRRR ensemble forecast.

As mentioned earlier, I expect the greatest potential impact for wind to be associated with activity that develops along the Mogollon Rim which indicates a 50% chance of winds exceeding 30 knots stretching into parts of Maricopa county.

HRRR ensemble max wind and probability of gusts exceeding 30 knots

Valid at 5 PM MST

(spc.noaa.gov)

This is also supported by the UAWRF which shows an area of strong winds in northeastern Maricopa county this evening in conjunction with Rim convection.

UAWRF forecast Reflectivity (right) and 10 m wind (left) for KTUS valid at 6PM MST

(weather.arizona.edu)

On the other hand, the precipitation threat appears to also extend south into Sierra Vista and southern Pima counties where there is a 50% chance of precipitation amounts exceeding an inch.

HRRR ensemble max 3 hr QPF and probability of > 1" forecast valid at 5PM MST

(spc.noaa.gov)

Of course all of this is predicated on enough clearing and surface heating to occur that convection at least initiates in the higher terrain. 

Outlook for Sunday:

I do not expect much to change on the synoptic scale side of things for Sunday. The center of the upper-level ridge should shift westward into Texas with not much change in the upper level flow over southern AZ.

HRRR ensemble mean 500 hPa Flow 12 PM MST

(spc.noaa.gov)

However, cloud cover is not expected to be as extensive or dense as it is today.

HRRR ensemble mean cloud cover valid at 1 PM MST

(spc.noaa.gov)

Precipitable water values will remain high especially in western Arizona, but will decrease from the levels of today in eastern Pima county.

HRRR ensemble mean PWTR Valid at 2 PM MST

(spc.noaa.gov)

The decrease in cloud cover should allow for greater destabilization with CAPE values in the ensemble mean approaching 1000 J/kg locally, and approaching 1500 J/kg along the southern border.

HRRR ensemble mean MUCAPE valid at 1 PM MST

(spc.noaa.gov)

This should support a heavy rain threat with a greater than 70% chance of an 1" or more occurring in southern Pima county SW of Tucson.

HRRR ensemble max 3 hr QPF and probability of > 1" forecast valid at 5PM MST

(spc.noaa.gov)

The higher CAPEs in conjunction with some mid-level drying will also support more evaporative cooling with the potential for strong winds roughly colocated with the precipitation threat.

HRRR ensemble max wind and probability of gusts exceeding 30 knots

Valid at 5 PM MST

(spc.noaa.gov)