Synopsis
A shortwave trough moving east across Southern Arizona combined with sufficient thermodynamics will generate showers and thunderstorms across Southeast Arizona with the greatest coverage remaining south and east of Tucson today. Main impacts from storms will be locally gusty winds, blowing dust, and brief heavy rainfall.
Dry and stableconditions are expected across Arizona Wednesday through at least Thursday due to a ridge rebuilding in from the west and 500mb heights rising. This pattern willgenerate strong subsidence and suppress convection across the majority of the state with high temps increasing a few degrees through the period.
Current Conditions
Theupper level low which has been anchored over Northern Baja for several days has finally opened up into a shortwave trough and is moving east across Southern Arizona this morning. Mid level water vapor imagery overlaid with 500mb RAP analysis and lightning detection displays theaforementioned shortwave over Western Pima County with an isolated storm east of Gila Bend ahead of the shortwave axis.
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| Mid level water vapor imagery overlaid with RAP 500mb analysis and glmflashes as of 9:30AM MST courtesy of the College of Dupage. |
Behind the shortwave axis, there's quite a bit of subsidence due to differential AVA and drier air aloft.
Moisture and instability aren't bad this morning with the 12z TUS sounding measuring 1.61 inches of precipitable water, a surface dew point of 62 degF, and 558 J/kg of MLCAPE.
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| 12z TUS sounding this morning courtesy of the SPC. |
However expecting these thermos to deteriorate by this evening from west to east as drier and more stable air invades the atmosphere behind the shortwave trough.
Today'sForecast
The extent of convection and storm chances in the Tucson vicinity will be influenced by the timing of the passage of theaforementioned shortwave trough. The region of differential CVA (mechanism for ascent) is quite narrow and currently (as of 10AM MST) in Central Pima County. Behind the shortwave axis, subsidence induced by differential AVA and dry air aloft will rapidly suppress any chance of storm development.
Both the 12z and 15z WRF-HRRR forecast the shortwave axis in Central Pima County during the early and mid afternoon hours as shown below.
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| 12z (left) and 15z (right) WRF-HRRR 500mb temps and wind barbs valid for 3PM MST this afternoon. |
Even though the magnitude of CVA ahead of the trough axis isn't real impressive just due to the weak 500mb winds, there still will be enough ascent to aid storm development.
Both the 12z and 15z WRF-HRRR forecast the maximum in buoyancy in Tucson to occur during the early afternoon hours (MLCAPE between 500 and 1000 J/kg).
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| 12z (left) and 15z (right) KTUS WRF-HRRR sounding valid for 1PM MST this afternoon. |
Thermodynamics are expected to deteriorate throughout the afternoon as CAMs forecast drier mid and upper level air to invade the atmosphere and generate subsidence.
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| 12z (left) and 15z (right) KTUS WRF-HRRR sounding valid for 4PM MST this afternoon. |
DCAPE is forecast to be around 1200 to 1400 J/kg this afternoon, so storms should be able to produce strong(ish) outflowboundaries.
Both the 12z and 15z WRF-HRRR keep the majority of storm activity to the south and east of Tucson this afternoon and evening as shown below.
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| 12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 2PM MST this afternoon. |
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| 12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 4PM MST this afternoon. |
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| 12z (left) and 15z (right) WRF-HRRR simulated reflectivity valid for 6PM MST this evening. |
CAMs show the best chance for storms in the Tucson vicinity this evening when they forecast outflow from convection to the south and southeast to move into the area.
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| 12z (left) and 15z (right) WRF-HRRR 10-meter winds valid for 6PM MST this evening. |
My Opinion forTucson:It's a tough call today. Themaximum in dynamic forcing should occur during the early/mid afternoon hours which is also coincident with the forecast maximum in buoyancy. CAMs don't seem too excited with storms during that time frame, but rather forecast the best chances later due to outflow boundaries. My probabilities would be 20 to 30% in the Tucson area, with increasing chances farther south and east,especially over the higher terrain.
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From an ensemble perspective, the 12z HREF forecasts thegreatest probabilities for precipitation mainly south and east of Tucson, especially in Santa Cruz and Southern CochiseCounties.
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| 12z HREF 3-hour mean QPF and probabilities of precip in excess of one inch valid for 5PM MST this evening courtesy of the SPC. |
Main impacts from storms today will be locally gust winds, blowingdust, and brief heavy rainfall. Traveling conditions could be hazardous along the I10 corridor between Tucson and the AZ/NM Border as well as along the I19 corridor between Tucson and Nogales.
Overall expect showers and storms across Southeast Arizona, mainly south and east of Tucson this afternoon and evening.
Wednesday and Thursday
By late tonight, the aforementioned shortwave will move east into New Mexico with drier, more stable air spreading across the region. In addition, high pressure aloft will rebuild from the west across Arizona tomorrow. This will cause mid levelheights to rise reinforcing subsidence as well as preventing moisture advection and therefore suppressing convection across Arizona.
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| 12z GEFS 500mb cyclonic vorticity loop valid forWednesday at 06z through Friday at 06z courtesy of Tropical Tidbits. |
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| 12z GEFS precipitable water normalized anomaly loop valid forWednesday at 06z through Friday at 06z courtesy of Tropical Tidbits. |
With rising heights and drier air, high temperatures are forecast to increase to a few degrees above climo with high temps between 100 and 105 degF in Tucson and near 110 degF in Phoenix.
Some isolated storm activity is possible Wednesday and Thursday over the highest peaks of the SkyIslands in SE AZ, near the International Border, and over the White Mountains. In general, expect dry andstable conditions through atleast Thursday.
This discussion will be valid through Thursday due tolimited activity during the period. Next discussion will be on Friday.
