August 16th: Scattered Showers and Storms Today and Tomorrow; Keeping an Eye on Tropical Storm Hillary

Synopsis

Better moisture and instability today and tomorrow will promote a greater chance for showers and thunderstorms, mainly over the higher terrain and west of Tucson. The 500mb anticyclone will remain centered in New Mexico today and tomorrow with 500mb heights around 592 to 594 dm. Temperatures will be a couple degrees warmer today and tomorrow with temps between 100 and 105 degF in southeast Arizona and temps between 105 and 110 degF between Phoenix and Yuma (slightly warmer tomorrow). Deterministic and Ensemble guidance has slowly become better in agreement regarding the influence of Tropical Storm Hillary on the Desert Southwest this weekend. Still plenty of uncertainty regarding locations, timing, and intensity of rainfall, but for now an increase in deeper, eastern Pacific tropical moisture is expected in our region. 

Current Conditions

A few thunderstorms developed over Phoenix yesterday evening due to a strong outflow boundary propagating off the Mogollon Rim. The Sky Harbor only reported a trace, but a few stations, particularly in Northwestern Phoenix reported some measurable rainfall. It wasn't much but I'm sure Phoenix residents were happy to finally see some storms in the metro!

SPC 500mb analysis this morning displayed the mid level anticyclone centered over central New Mexico at 594 dm, a cutoff low a few hundred kilometers off the coast of California (cannot see it on the map below), and southeasterly mid level flow across southern Arizona. 

SPC 500mb analysis as of 17:00z this morning.

The 12z PHX sounding (TUS was unavailable) measured 1.79 in of pwat and 543 J/kg of CAPE which is a much better thermodynamic profile compared to yesterday. 

12z PHX sounding courtesy of the SPC.

SPC analysis of pwats show values between 1.4 and 1.6 in across south-central and southwestern AZ with southeastern Arizona on the drier end with pwats between 1 and 1.4 in, decreasing from west to east. Surface obs also measured dew points in the mid 50s to low 60s across the area with highest values in south-central and southwestern AZ.

SPC analysis of precipitable water as of 17:00z

Surface Observations as of 17:15z this morning courtesy of the NWS.

Visible imagery shows mostly clear skies this morning, so ample amounts of sunshine today will aid in convective initiation by early this afternoon.

Today's Forecast

Synoptics

Arizona will remain between two mid level features today with the subtropical high centered over Central New Mexico and a cutoff low off the coast of CA. Mid level flow has weakened a bit due to the weaker height gradient between these two features. Steering flow will remain southeasterly throughout the day between 10 and 20 kts. 

12z GFS 500mb absolute vorticity at 18z today.

The GFS is indicating that a weak mid level disturbance could move through the Phoenix metro late tonight into early tomorrow morning. It appears this disturbance could be associated with convection over far southern Arizona this afternoon and evening so definitely something to look out for.

12z GFS 500mb absolute vorticity for tomorrow at 5:00 AM MST.

Thermodynamics

Plenty of moisture will remain available this afternoon and evening for storm development with the WRF-HRRR maintaining pwats between 1.25 and 1.75 in across the region with the highest values remaining confined to south-central and southwestern AZ. 

12z WRF-HRRR total precipitable water at 3:30 PM MST.

Instability is also much better today, but still somewhat marginal with the 12z KTUS WRF-HRRR sounding forecasting MUCAPE to reach around 600 J/kg this afternoon. DCAPE is slightly higher today as well with the WRF-HRRR forecasting DCAPE to reach 1425 J/kg by this afternoon. The 12z WRF-HRRR KPHX sounding is almost identical but with higher pwats as shown below. 

12z KTUS WRF-HRRR sounding for 1:00 PM MST.

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

What to Expect

With plenty of moisture, marginal instability, and DCAPE > 1000 J/kg, scattered showers and thunderstorms can be expected across the region today. The most widespread, intense activity will be focused west of Tucson where better moisture and instability are in place. A few showers and storms are possible in the Tucson area this afternoon and evening, but as mentioned above, most of the activity will remain west of the city as well as over the higher terrain. This is reflected in the 12z WRF-HRRR simulated reflectivity this afternoon as shown below. 

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

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

Regarding the potential for overnight convection in Phoenix, the 12z Phoenix region simulated reflectivity and station plots forecasts a few cells to develop in the metro around 3AM MST tomorrow so am thinking the best chance for storms in Phoenix will be late tonight into early tomorrow morning.

12z WRF-HRRR Phoenix Area station plots and simulated maximum reflectivity for tomorrow morning at 3:00 AM MST.

With plenty of low level moisture and mid level heights around 592 to 594 dm, temperatures will remain below 105 degF in southeastern Arizona (including Tucson), but locations between Phoenix and Yuma will range between 107 degF and 110 degF by this afternoon.

12z WRF-HRRR 2-meter temps at 4:00 PM MST.

Tomorrow's Forecast

By tomorrow the low off the CA coast deepens and digs a tad farther south. In response, the mid level anticyclone over New Mexico increases in amplitude putting Arizona under weak southerly flow aloft. 

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

Models forecast pwats to decrease slightly by tomorrow, but there will still be plenty of moisture with the WRF forecasting pwats to range between 1 and 1.5 in across southern Arizona. 

12z WRF-HRRR total precipitable water for tomorrow at 11:00 AM MST.

Instability is looking to be slightly better tomorrow with the 12z KTUS WRF-HRRR sounding forecasting around 700 J/kg tomorrow afternoon and DCAPE near 1700 J/kg. 

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

In general am expecting today and tomorrow to be very similar with the main focus for showers and storms to the west of Tucson as well as over the higher terrain with a few storms possible in the Tucson and Phoenix metros. The simulated maximum reflectivity demonstrates this scenario tomorrow as shown below. 

12z WRF-HRRR simulated maximum reflectivity for tomorrow at 2:00 PM MST.

12z WRF-HRRR simulated maximum reflectivity for tomorrow at 6:30 PM MST.

A slightly stronger mid level high and a tad bit of drying will result in temperatures a couple degrees warmer tomorrow. Southeastern Arizona is expected to remain at or below 105 degF with the lower deserts between Phoenix and Yuma reaching temps at or a tad over 110 degF. 

12z WRF-HRRR 2-meter temps for tomorrow at 3:00 PM MST.

This Weekend

Tropical Storm Hillary, currently several hundred kilometers off the coast of Southern Mexico, is forecast to strengthen into a major hurricane over the next few days and move northward just off the coast of the Baja California Peninsula by this weekend. The deterministic GFS and ECMWF as well as their ensemble members have slowly become better in agreement regarding the track of the tropical cyclone and the influence on the southwest. Below is the most current NHC forecast track of Hillary.

NHC forecast track for Tropical Storm Hillary.

There is still plenty of uncertainty though, so need to wait a day or two to really start analyzing the details as a lot can change over the next couple of days. For now we can at least expect a significant increase in moisture as well as an uptick in coverage of convection across Arizona this weekend. Stay Tuned!!