January 27th, 2025: Unsettled Weather Across Arizona Through Wednesday Night
Synopsis
A low pressure system in Southern California will gradually move eastward into Arizona leading to unsettled weather through Wednesday night. Precipitation will mainly impact the higher terrain in Northern Arizona today and tomorrow with precipitation chances increasing in Southern Arizona on Wednesday. Rainfall amounts will be light and variable with a trace to 0.25 inches in the lower deserts and between 0.25 and 0.75 inches over the higher terrain with locally higher amounts possible along favorable southwestern facing slopes of the Mogollon Rim. Snow levels will remain mainly above 4000 feet today and tomorrow with total 72-hour snow accumulations ranging from a dusting to 4 inches, but locally higher amounts are possible over the tallest peaks of Northern Arizona. On Wednesday, snow levels will likely drop to 3000-3500 feet which means some higher valley locales could receive a dusting of snow.
Current Conditions
As of 11AM MST, visible satellite imagery and SPC 500mb RAP analysis display scattered showers across Southern California and western Arizona associated with a closed low gradually moving inland.
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| Visible satellite imagery overlaid with SPC 500mb RAP analysis valid at 11AM MST courtesy of the College of Dupage. |
Even though this system is relatively moisture-starved, precipitable water continues to increase across Arizona as indicated by SPC mesoanalysis this morning.
Today and Tomorrow
Dynamics and Moisture
In general, Arizona will remain within a favorable region of synoptic scale ascent due to differential cyclonic vorticity advection ahead of the 500mb vort max anchored over Southern California today.
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| 12z GFS 500mb cyclonic vorticity valid for 11AM MST today courtesy of Tropical Tidbits. |
As is typically the case with cold core lows this time of year, this system is quite moisture-starved as models only indicating precipitable water in the 0.25 to 0.50 inch range today and tomorrow.
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| 12z WRF-HRRR total precipitable water valid for this afternoon at 5PM MST. |
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| 12z WRF-HRRR total precipitable water valid for tomorrow afternoon at 5PM MST. |
However, there should be enough moisture for occasional showers across, mainly Northern Arizona.
Timing, Impacts, and Precip Amounts
The majority of precipitation coverage will remain in Northern Arizona through tomorrow afternoon due to better synoptic scale ascent as well as orographic enhancement. Thunderstorms are unlikely due to limited buoyancy, but an isolated rumble of thunder or two cannot be ruled out. As of 11:30AM, radar shows bands of shower moving into Maricopa (west of Phoenix), Yavapai, and Coconino Counties, so I would expect bands of showers to continue in these areas through tonight with greatest coverage still remaining over the higher terrain north of Phoenix.
By tomorrow, the 500mb vort max becomes centered in Northwest Arizona/extreme Southern Nevada. UA WRF ensemble members as well as NCEP guidance keeps most activity in Northern Arizona throughout most of the day with shower activity gradually shifting south into the lower deserts overnight. Total precipitation between 5AM MST this morning and 11PM tomorrow night will range from a trace to 0.10 inches in the lower deserts (including Phoenix) and between 0.10 and 0.50 inches in Northern Arizona with locally higher amounts possible over the tallest peaks.
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| 12z WRF-HRRR total accumulated precipitation forecast valid between 5AM MST this morning to 11PM MST tomorrow night. |
Wednesday
Dynamics and Thermodynamics
By Wednesday, the aforementioned closed low will shift east and become centered just south of the Four Corners region, and models indicate a couple of embedded vort lobes on the back side of the low to move through Southern Arizona tomorrow.
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| 12z WRF-HRRR 500mb temperatures valid for 10AM MST on Wednesday. |
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| 12z WRF-HRRR 500mb temperatures valid for 8PM MST on Wednesday. |
There is still some uncertainty as to the exact timing, however based on current guidance Wednesday appears to be the best opportunity for precipitation in the lower deserts (including Phoenix and Tucson).
Moisture will still be scarce on Wednesday, so buoyancy will also be very weak. However, model soundings do indicate steeper lapse rates on Wednesday due to cooler mid level temperatures. The 12z KTUS WRF-HRRR sounding indicates a maximum of around 150 J/kg of CAPE early Wednesday afternoon, so an isolated low-topped thunderstorm is possible.
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| 12z KTUS WRF-HRRR sounding valid for Wednesday at 12:30PM MST. |
Timing, Impacts, and Precip Amounts
There is still some uncertainty regarding exact timing and precipitation amounts on Wednesday since it is still 48 hours out, but based on current guidance Wednesday will be the best chance for precipitation in Southern Arizona. The timing of the greatest coverage of showers will be primarily dependent on the passage of the aforementioned vort lobes. At this time, I would expect periods of showers beginning early Wednesday and persisting into the early evening hours.
Rainfall amounts will still be light with preliminary 72-hour forecast rainfall amounts between a trace to 0.25 inches in the lower deserts, but locally higher amounts are possible dependent on trajectory, frequency and intensity of showers.
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| 12z WRF-HRRR 72 hour precipitation forecast between 12z this morning and 12z Thursday. |
With 1000-500mb thickness decreasing to 540dm, snow levels will drop to around 3000-3500 feet on Wednesday. Valley locations that could receive light snow accumulations include Oracle, Nogales, Sierra Vista, Benson, and Wilcox. At this time, snow in the Tucson valley is unlikely (<20%), but I cannot rule out some wet flakes with the heavier showers. Snow in Tucson would require heavy precipitation and/or showers after dark on Wednesday. Preliminary 72-hour snowfall will range between a dusting and 2 inches between 3000 and 4000 feet, and between 1 and 4 inches above 4000 feet with locally higher amounts possible over the tallest peaks.
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| 12z WRF-HRRR 72-hour snowfall forecast valid between 12z this morning to 12z Thursday. |
-Tyler
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