February 27th, 2024: Elevated Showers and Even a Few Thunderstorms Tomorrow Afternoon and Evening Across Southern AZ

 Synopsis

Mid and high clouds will continue to move eastward throughout the day today with mostly clear skies expected across the majority of AZ by late this afternoon through tomorrow morning. A closed low, currently a few hundred miles west-southwest of San Diego, will move rapidly eastward today and eventually impact Arizona tomorrow. Widespread bands of elevated convective showers and even a few embedded thunderstorms will be possible, mainly across the Southern half of the state. Precipitation amounts will remain mainly light with only a few hundredths to 0.25 in expected across lower elevations with highest amounts expected in Cochise County and over the higher terrain. This is a warmer system, especially for this time of year, so snow levels will remain above 8000 ft with only a few inches of accumulated snowfall expected over the highest peaks of the Sky Islands and the White Mountains. 



Current Conditions


Visible satellite imagery this morning displayed a rapid moving stream of mid and high clouds over far Southern AZ and Northern Mexico. 


Visible satellite imagery this morning courtesy of College of Dupage.


This stream of mid and high clouds is associated with the brisk subtropical jet overhead as shown on the SPC 300mb analysis as of 18z this morning.


SPC 300mb analysis as of 18z this morning.


Throughout the day today and into tonight the subtropical jet will move eastward and a closed low off the coast of Southern California (shown below) will move onshore toward Southern Arizona.


SPC 500mb analysis as of 18z this morning.


This morning's 12z TUS sounding displayed an impressive vertical wind profile with 120 knots of shear between the surface and 8km as well as plenty of mid and upper level moisture.



12z TUS sounding courtesy of the SPC.


However, the lower levels remain quite dry which explains the lack of instability.  



Today Into Late Tonight


Mid and high clouds will slowly move east throughout this afternoon and evening with mostly clear skies across the majority of the state with the exception being far SE AZ where some clouds will hang around into the evening hours. Expect warm and tranquil weather across the state this afternoon and into tonight with afternoon temps in the low to mid 70s across the lower deserts. 


15z WRF-HRRR 2-meter temps for this afternoon at 3:00PM MST.


By tonight the closed low off the SoCal coast will move onshore into far Northern Baja California and the conveyor belt of mid and high clouds will move into New Mexico. 


12z GFS 500mb absolute vorticity for tonight at 11:00PM MST.


Will be much cooler tonight compared to the last couple of nights due to clearer skies with low temps in the low to mid 50s across the lower deserts.



Tomorrow Morning Through Tomorrow Evening


Synoptics

By tomorrow morning, the aforementioned closed low will move into Southern Arizona, with the center moving almost directly along the International Border. By midday and into the late afternoon, PVA ahead of the vort max as well as some upper level divergence will induce synoptic scale ascent. 


12z GFS 500mb Absolute Vorticity for tomorrow at 11:00AM MST.


12z GFS 500mb Absolute Vorticity for tomorrow at 5:00PM MST.


12z GFS 250mb Divergence for tomorrow at 5:00PM MST.


Current guidance suggests the best synoptic scale support will remain in SE AZ where the maximum in PVA and associated upper level divergence is expected. The synoptics aren't real impressive with this system as the closed low is quite compact and there is only a relatively small region of significant synoptic scale lift. Albeit there will still be sufficient ascent to support bands of showers with a few embedded thunderstorms.



Moisture and Instability


The main issue with this system is that it is relatively moisture starved, especially in the lower levels. The 12z and 15z WRF-HRRR forecasting pwats only around 0.50 in across SE AZ tomorrow afternoon.   

 

15z WRF-HRRR total precipitable water for tomorrow at 3:00PM MST.


This appears to be more of an elevated convection event which is reflected by the 12z KTUS WRF-HRRR sounding which forecasts surface RH between 30% and 35%, an LCL just below 700mb, and DCAPE of around 600 J/kg. There is some marginal instability forecast for tomorrow though (~300 J/kg of CAPE) due to steep lapse rates between the surface and 600mb. 

12z KTUS WRF-HRRR sounding for tomorrow at 2:00PM MST.


A couple things to point out here. First, the relatively well-mixed profile from the surface to 600mb is attributed to heating of the surface during the morning and early afternoon hours. In fact, CAMs are in good agreement that we should have several hours between sunrise and around 1PM of mostly clear skies. Heating of the surface combined with cooling mid level temps will aid in destabilization prior to convective initiation. Another thing to note is that the the UA WRF tends to have a dry boundary layer bias which was something I took note of during the 2023 Monsoon Season. Of course different time of year and different pattern, but slightly more low level moisture could mean much better buoyancy especially combined with several hours of surface heating so just something to keep an eye on.



Timing and Potential Impacts


CAMs are in good agreement regarding the timing of precipitation across Southern AZ. Bands of showers will initiate around midday (12 to 1PM) in Pima, Santa Cruz, and Cochise County then move northward throughout the afternoon. Should be enough instability and synoptic scale ascent for a few embedded thunderstorms within these bands of showers. The majority of the activity will likely remain south and east of the Phoenix metro, but a few showers are possible with even an outside chance for a thunderstorm. Below is the 12z WRF-HRRR simulated reflectivity for tomorrow afternoon and evening.


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.


Rainfall amounts will mainly be light due to the high-based nature of the precipitation and fast moving cells. Total rainfall amounts will be highly variable but the safe forecast is anywhere from a few hundredths to 0.25 in in the lower deserts with higher amounts possible over the higher terrain and in Cochise County where the most significant synoptic scale support is expected at this time. 


Thunderstorms will be able to produce brief, locally heavy rainfall, occasional lightning strikes, and gusty winds. Gusty downdraft winds appear to be the main risk at this time due to high DCAPE values.


Since this is a warmer system, snow level will remain quite high with only a few inches of snow accumulation expected over the highest peaks of the Sky Islands and the White Mountains above 8000 ft. 


Travel conditions will be hazardous, especially along I10 between Casa Grande and Tucson as well as between Tucson and the AZ/NM Border. The evening commute will also be impacted, particularly in Tucson where showers and storms are expected to persist into the early evening hours. 


Will be keeping an eye on CAM runs this afternoon and evening as well as early tomorrow morning to see if any significant changes are detected. Will likely provide a brief update tomorrow morning to make sure the forecast is on track. Stay Tuned!

 



 


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