January 22, 2024: Strong Pacific Storm to Impact Arizona Tonight into Wednesday Morning (Part 2)

  Synopsis

This is the second part of a forecast discussion regarding a strong Pacific Storm expected to impact Arizona starting this evening and persisting into Wednesday morning. The first round of moderate to heavy rain is expected to begin this evening and persist into the early morning hours tomorrow. A few sporadic lightning flashes are possible within the heaviest rain bands, but better instability tomorrow afternoon will promote a greater chance of thunderstorms particularly across Eastern Arizona. Snow levels will remain above 7000 feet with this storm.


 Current Conditions

Current radar and IR satellite imagery as well as lightning detection networks indicate a large area of showers with a few embedded lightning strikes across the western half of Arizona moving rapidly eastward. This area of precipitation is associated with PVA ahead of a shortwave and a region of strong upper level divergence due to left exit region dynamics as indicated by the current SPC 300mb and 500mb analysis shown below.


SPC 300mb analysis as of 01z this evening.




SPC 500mb analysis as of 01z this evening.




The 00z TUS observed sounding indicated some weak instability (<200 J/kg of CAPE), but increasing moisture and vertical wind shear. 

00Z TUS sounding courtesy of the SPC.





 Tonight into Early Tomorrow Morning

The discussion from this morning is still valid for tonight into tomorrow morning. See part 1 discussion for additional information.


 Tomorrow into Wednesday Morning

Synoptics

The most favorable aspect of this system is the synoptic scale induced ascent due to PVA and left exit region dynamics. There appears to be two shortwaves embedded in the long wave trough over the Southwest to impact the region with the first shortwave providing precipitation tonight into early tomorrow morning and the second impacting the region tomorrow afternoon and evening. Have provided the 18z GFS 500mb vorticity map to show these two features where the blue X is the first shortwave and the green X is the second shortwave. 



18Z GFS 500mb absolute vorticity valid for tonight at 11:00PM MST.



18Z GFS 500mb absolute vorticity valid for tomorrow at 11:00AM MST.




The primary difference between these two features is the instability associated with them. Both shortwaves will provide plenty of synoptic scale ascent, but the second shortwave will have slightly colder mid level temperatures due to its closer proximity to the trough axis. The KTUS 12z WRF-HRRR sounding depicts this which shows marginally steeper mid level lapse rates tomorrow compared to tonight as shown below. 


12z KTUS WRF-HRRR sounding for tonight at 11:00 PM MST.


12z KTUS WRF-HRRR sounding for tomorrow at 3:00 PM MST.


Slightly more instability will promote a chance for a few thunderstorms across the eastern half of Arizona (including Phoenix and Tucson) tomorrow afternoon and evening. Thunderstorms have the potential to produce lightning, small hail, gusty winds, and localized heavy rain with isolated flooding possible. Below is the forecast evolution of precipitation using the 12z WRF-HRRR simulated reflectivity.




12z WRF-HRRR simulated reflectivity for tomorrow at 11:00AM MST.



12z WRF-HRRR simulated reflectivity for tomorrow at 1:00PM MST.


12z WRF-HRRR simulated reflectivity for tomorrow at 3:00PM MST.




12z WRF-HRRR simulated reflectivity for tomorrow at 5:00PM MST.


12z WRF-HRRR simulated reflectivity for tomorrow at 7:00PM MST.



12z WRF-HRRR simulated reflectivity for tomorrow at 9:00PM MST.


12z WRF-HRRR simulated reflectivity for tomorrow at 11:00PM MST.


Overall, expect moderate with a few pockets of heavy rainfall and a rogue lightning flash or two tonight. Tomorrow will be a bit more busy with heavy convective showers and a few embedded thunderstorms. Some lingering showers can be expected early Wednesday morning, but should clear out by midday. 






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