August 31st: Better Storm Chances and Much Cooler Temps Today and Tomorrow

Synopsis

Better synoptics combined with sufficient moisture and instability will promote a much greater chance for showers and thunderstorms across Arizona today and tomorrow with tomorrow being the most active day. With lower mid level heights and decent low level moisture, high temperatures will be significantly cooler as well.


Current Conditions 

A strong thunderstorm cluster impacted the Tucson vicinity yesterday evening with strong winds, blowing dust, and even a bit of rainfall. 


NWS 24-hour accumulated precipitation map.


A severe storm over northeast Tucson dropped quarter sized hail via SPC storm reports too. 


SPC Hail reports from yesterday.



The mid level anticyclone which has been providing Arizona with extreme heat has finally weakened significantly and is now centered over central New Mexico. This puts Southern Arizona under easterly/southeasterly flow aloft.


SPC 500mb analysis as of 1700z.


An overnight Gulf of California moisture surge has increased moisture across southwestern and south-central Arizona with pwats between 1.25 in and 1.60 in from Phoenix southwestward. Much drier in southeastern AZ with pwats around 1.20 in in Tucson and decreasing rapidly eastward. 


SPC mesoanalysis of total precipitable water as of 17:00z.


The 12z TUS sounding displayed 285 J/kg of CAPE, -564 J/kg of CIN, and 1824 J/kg of DCAPE. Moisture is marginal at best with only 1.21 in of pwat. 



12z TUS sounding courtesy of the SPC.




Today's Forecast


Synoptics 

Much better synoptic conditions today with a weakened mid level anticyclone centered over central New Mexico and 20 to 25 kt mid level southeasterly/easterly flow over southern and Central Arizona. Only a weak bit of subsidence in the 500mb to 400mb layer which should not be an issue for storm development today. 

Also, an upper level low currently centered off the west coast of Central Baja will begin to lift northward today and tomorrow which will provide some synoptic scale support. The best synoptic scale ascent will likely be tomorrow (mentioned in tomorrow's forecast section below). 


12z GFS 2 PVU Pressure Map Courtesy of Tropical Tidbits.

As this feature approaches Arizona this afternoon and evening, synoptic scale ascent will slowly increase which is evident in the weak mid and upper level veering wind profiles on the 12z KTUS WRF-HRRR model sounding.


12z KTUS WRf-HRRR sounding for 3:00 PM MST today.


Moisture and Thermodynamics


The best moisture will remain in Central and Western Arizona today with southeastern AZ on the drier side. As shown in the pwat map above, there is a very tight zonal gradient in moisture  between western AZ and eastern AZ. The 12z WRF-HRRR maintains pwats between 1.1 and 1.25 in in the Tucson vicinity with pwats between 1.25 and 1.50 in for Phoenix westward. 


12z WRF-HRRR total precipitable water for today at 3:00 PM MST.


As shown in the KTUS model sounding above, the 12z WRF-HRRR forecasts CAPE to reach around 600 J/kg by this afternoon which is marginal at best. However, DCAPE is expected to nearly reach 2000 J/kg! 


A bit more favorable thermodynamics in Phoenix by this afternoon with the 12z KPHX WRF-HRRR sounding forecasting CAPE around 700 J/kg, DCAPE over 2000 J/kg, and pwat just below 1.50 in. 



12z  KPHX WRF-HRRR sounding for today at 4:00 PM MST.



What to Expect/Potential Impacts


Both the 12z and 15z WRF-HRRR runs keep most of the widespread, intense activity west of Tucson due to better moisture, instability, and synoptic scale support in that region. However, there will be enough moisture and instability for a chance of storms in the Tucson vicinity this afternoon and evening. All CAM solutions have convection initiating by the middle of the afternoon which seems reasonable considering that the atmosphere is going to need a few hours of insolation to mix out the large amounts of CIN from this morning. The 15z WRF-HRRR has been performing better than the 12z lately so am leaning toward that solution for today's forecast. The 15z WRF-HRRR forecast of the evolution of convection today is shown below.


15z WRF-HRR simulated maximum reflectivity for today at 3:00 PM MST.


15z WRF-HRR simulated maximum reflectivity for today at 5:00 PM MST.


15z WRF-HRR simulated maximum reflectivity for today at 6:00 PM MST.


15z WRF-HRR simulated maximum reflectivity for today at 8:00 PM MST.



15z WRF-HRR simulated maximum reflectivity for today at 10:00 PM MST.



15z WRF-HRR simulated maximum reflectivity for tonight at 12:00 AM MST.



Even though both the 12z and 15z runs keep the majority of storms outside of the major population centers today, it is interesting to note that recent runs of the HRRR (16z and 17z) are hinting at storm clusters making it into the Phoenix metro by late this evening. 


16z HRRR simulated reflectivity for tonight at 11:00 PM MST courtesy of Tropical Tidbits.


This scenario is very possible considering the southeasterly mid level flow and DCAPE around 2000 J/kg which would help develop strong outflow boundaries moving from southeast to northwest. 


SPC has Central Arizona in a marginal risk for severe storms today with the primary impact being strong, gusty outflow winds associated with the large values of DCAPE. 


SPC Day 1 Severe Weather Outlook.



The 12z HREF has a 70% chance of winds in excess of 30 kts in Central Pima and southern Pinal counties by this evening. 


12z HREF 4-hr max 10-meter wind speed probabilities courtesy of the SPC.



Blowing dust will also be a major concern making driving conditions extremely hazardous especially along the I10 corridor between Tucson and Phoenix as well as the I8 corridor between Casa Grande and Gila Bend. Severe grade hail is possible in the strongest updrafts but likely will remain isolated at best. WPC has central Arizona in a marginal risk for excessive rainfall (<= 5%) which is reasonable considering the dry sub cloud layer and brisk steering flow. However, as is always the case localized flash flooding is always possible.


A few degrees of relief in high temps today with temps likely reaching just around 100 degF in Tucson and temps between 100 and 105 degF in the lower deserts between Phoenix and Yuma.


12z WRF-HRRR 2-meter temps for today at 3:30 PM MST.





Tomorrow's Forecast

Synoptics

The synoptic pattern will be significantly favorable for widespread, strong storms tomorrow across Arizona. A trough will deepen and dig southward along the west coast and in response the mid level anticyclone will amplify over the Central Plains. This will generate a strong gradient in mid level heights and therefore enhancing mid level southerly flow across the state.



12z GFS 500mb absolute vorticity for tomorrow at 18z.

This enhanced southerly flow will transport deeper moisture from the south into Arizona increasing precipitable water and instability. 


12z GFS total precipitable water for tomorrow at 18z.


Meanwhile, the upper level low off the west coast of Central Baja will become embedded in the flow and move through Arizona tomorrow (denoted by the red X in the map below) providing additional synoptic scale ascent. This feature will enhance upper level divergence as well as provide a slight bit of cyclonic vorticity advection. 



12z GFS 250mb divergence for tomorrow at 18z.






Moisture and Thermodynamics 

The 12z WRF-HRRR forecasts pwats to sky-rocket by tomorrow afternoon with values between 1.50 and 1.75 in across most of southern Arizona.


12z WRF-HRRR total precipitable water for tomorrow at 1:00 PM MST.


Very sufficient instability for tomorrow with the 12z KTUS WRF-HRRR sounding forecasting around 1000 J/kg of CAPE in Tucson by tomorrow afternoon. 


12z KTUS WRF-HRRR sounding for tomorrow at 4:00 PM MST.


The 12z KPHX WRF-HRRR sounding is significantly more impressive with over 2000 J/kg of CAPE forecast for tomorrow! 



12z KPHX WRF-HRRR sounding for tomorrow at 5:00 PM MST.




What to Expect/Potential Impacts

Tomorrow is looking to possibly be the highest impact and most widespread event of the monsoon season thus far. Impressive moisture and instability combined with sufficient synoptic scale support will likely generate strong, organized storm clusters tomorrow afternoon and evening across much of Arizona. There are some differences in CAM solutions regarding the timing and location of storm coverage though. The 12z WRF-HRRR is by far the most aggressive with the numerous strong storms in the Tucson vicinity tomorrow afternoon and evening. 



12z WRF-HRRR simulated maximum reflectivity for tomorrow at 2:30 PM MST.


This is by far the most bullish solution, so need to take this with a grain of salt as a lot can change in the next 24 hours. Will have a better idea by tomorrow, but for now expect a very active day across Arizona. 

Considering the ample amounts of moisture and synoptic scale support, flash flooding is possible tomorrow with the WPC having Tucson and Phoenix in a slight risk for excessive rainfall. 



WPC Day 2 Excessive Rainfall Outlook.



Temperatures will be quite comfortable tomorrow with highs likely remains below 100 degF across the lower deserts. 



12z WRF-HRRR 2-meter temps for tomorrow at 12:30 PM MST.



Will have more details regarding tomorrow's forecast in tomorrow's discussion. Stay Tuned!!!





































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