Impressively Hot and Humid Morning Paves Way for Scattered Thunderstorms

 Overview:

Record or near record warmth greeted the Old Pueblo today as a combination of impressive humidity, winds, and cloud cover kept temperatures near 90 F overnight. Moisture will continue to be well above normal and slowly spread eastward today, making its way into western Pima county as we head into the weekend. Cloud cover is the fly in the ointment of today's forecast as 1) temperatures from Tucson eastward may be somewhat surpressed and 2) a sharp boundary in the cloud cover to the west may serve as a focus for convective development later today. Model forecasts however are suggesting that enough sunshine will work its way through the clouds around Tucson that we should expect convection to initiate along the higher terrain 'of the Catalina and Santa Rita ranges. As we move into Saturday, the threat of convection will continue with increasing coverage to the south and west of the greater Tucson area.

This Morning:

Tucson was greeted with an all time record low minimum temperature for June of 89 F, besting the previous record of 87 F set in 2017. Warm minumum temperatures have been shown to have a significant impact on human health for individuals that lack access to sufficient air conditioning. The extremely warm overnight lows were due to a combination of high clouds moving in from the east...


Visible Satellite Imagery ending at 7 AM MST with 500 hPa Flow

(weather.cod.edu)


Strong southeasterly surface winds winds that moved into the area at about 1 AM MST

                                             Surface observations at 1 AM MST

                                          (https://weather.ral.ucar.edu/surface/)





As well as precipitable water values of close to 1.3"

PWTR with 850 hPa Moisture Transport Vector 9 AM MST

(spc.noaa.gov)


Note that the moisture transport vectors are indicative of precipitable water values continuing to increase and spread westward throughout the day. From a historical perspective, these values are well over the 90th percentile (1.16") although it falls short of the daily record of 1.46"

PWTR sounding climatology

(spc.noaa.gov)


An analysis of the the 12Z sounding from this morning shows that most of the water in the column is situated in the lowest 3 km of the atmosphere.

12Z Sounding from Tus

(spc.noaa.gov)


The relatively dry layer from approximately 500-250 hPa should provide for plenty of evaporative cooling for any storms that develop today and is supported by an analyzed downdraft CAPE of over 1900 J/kg. This will mean that convection will be capable of producing strong outflow winds and as a consequence the Storms Prediction Center has indicated a marginal risk for severe winds in from the I-10 corridor and east to the New Mexico border for this afternoon. A veering wind profiles and still sufficient mid-level flow may aid in convective organization this afternoon although the strength of the 700-500 hPa flow is expected to weaken throughout the day.

Day 1 convective outlook (wind)

(spc.noaa.gov)



The Weather Prediction Center in DC has also indicated a marginal risk of excessive rainfall for the region

Day 1 excessive rainfall outlook

(wpc.noaa.gov)




based on the plume of moisture of Alberto in conjunction with CAPES that should be approaching 1000 J/kg provided sufficient sunshine/heating.

This Afternoon:

Most Convective Allowing Models (CAMS) are indicating the presence of near surface convergence developing early this afternoon along the I-10 corridor between southeasterlies east of I-10 and southerlies or southwesterlies west of I-10. I expect this line of convergence in combination with the higher terrain serving as a focus for convective initiation this afternoon.

UA WRF HRRR initialization 10 m Wind valid at 3 PM MST

(https://weather.arizona.edu/)




UA WRF HRRR initialization Simulated Reflectivity 4 PM MST

(https://weather.arizona.edu/)





Instability is forecast to peak between 4-6 PM with CAPE values exceeding 1500 J/kg. It should be noted that should those values of instability would support a threat of excessive rainfall. I

UA WRF HRRR forecast sounding valid at 5 PM MST

(https://weather.arizona.edu/)




Note that the SPC mesoanlysis indicates that values of CAPE are already around 1000 J/kg as of 10 AM. Given the humid conditions, convection that does develop should persist past sunset.

UA WRF HRRR initialization Simulated Reflectivity 8 PM MST

(https://weather.arizona.edu/)




Rainfall accumulations in the UA WRF HRRR approach an inch or more along the southwestern slopes of the Catalina mountain range. The HRRR ensemble indicates the highest threat of 1"+ rainfall will be Northeast of Tucson an extending in eastern Pinal County with a 30% chance of greater than an 1" of rain indicated.

HRRR ensemble max 3 hr rainfall and Probability of greater than 1" 5 PM MST

(https://weather.arizona.edu/)




Outlook for Saturday:

While details of the outlook for Saturday will heavily depend on the evolution of storms today and overnight, I generally expect the threat of convection to spread westward as the axis of greatest precipitable water shifts into western Pima county and into Maricopa county. 

UA WRF HRRR inititilaization PWTR valid 4:30 PM MST Saturday

(https://weather.arizona.edu/)



The threat of severe weather or excessive rainfall will be somewhat mitigated by a less favorable synoptic-scale setup as the western trough dissipates and the upper-level high continues to build westward.

HRRR ensemble 500 hPa heights/wind valid at 5 PM MST Saturday

(spc.noaa.gov)


One thing to keep an eye out for is a signal in the CAMS of convection initiating in the higher terrain tomorrow sending an outflow boundary southwestard through Phoenix and Pinal county. This is indicated in both the UA WRF

UA WRF HRRR initialization 10 m Wind valid at 7 PM MST Saturday

(https://weather.arizona.edu/)



As well as suggested by the HRRR ensembles

HRRR ensemble max 3 hr rainfall and Probability of greater than 1" valid 5 PM MST Saturday

(https://weather.arizona.edu/)





 HRRR ensemble max 3 hr rainfall and Probability of greater than 1" valid 8 PM MST S

(https://weather.arizona.edu/)


At the very least, this suggests the threat of Dust across the I-10 corridor north or Tucson. Another region to keep an eye out on is Sierra Vista and portions of Pima County bordering Mexico where mid-level easterly flow will be better established.



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