Tuesday, July 08, 2014

Significant Outbreak Of Severe Thunderstorms Likely This Afternoon

It appears that a number of classic features associated with significant severe thunderstorm events over southern Arizona are going to come together this afternoon. The 12 UTC CIRA PW analysis (above) shows very high values across southwestern Arizona. Anticyclonic, low-level flow this afternoon will be blowing from the north end of the GoC across the low deserts and then southeastward up the Santa Cruz Basin. This will bring increasing amounts of CAPE across Tucson and Pima County by mid-afternoon. Surface pressures are low, and mostly clear skies will allow afternoon temperatures to push into upper 90s to lower 100s across much of southern Arizona. Strong afternoon boundary layer winds blowing from the low deserts, southeastward across eastern Pima County will allow strong convergence to develop when storm outflows move off elevated terrain.

The morning skew-T plot for Tucson (below from SPC) shows small CAPE present but with forecasts for afternoon going over 1500 m2/s2. Anvils should stream off to the east to southeast today and wind shear should be favorable for severe storms by afternoon, assuming that steering level winds increase some during the day. The upper troposphere will be strongly difluent this afternoon and evening over southeastern Arizona.

The early WRF-NAM forecast of the TUS sounding is plotted above valid for 5 pm MST. The model forecasts CAPE to increase to well over 2500 m2/s2. The WRF wind profile is not as favorable as that forecast by the 12 UTC current NAM forecasts. The WRF forecast of composite radar echoes valid at 7 pm MST this evening is shown below, indicating numerous heavy storms over southeastern Arizona.

The storms are forecast to produce severe macrobursts and intense mesoscale outflow gust fronts. The forecasts of 10-m winds are shown in the three panels below valid at 7 pm, 9 pm, and midnight. Outflows from Pima County converge with several other outflows over eastern Maricopa County. This results in a large MCS and very significant storms for the Phoenix area in the model forecasts. The resultant, very large outflow and gust front then races westward across southwest Arizona and into the California deserts during the night.

The model forecasts indicate that the combination of severe winds and locally heavy rains will likely produce a variety of hazardous weather and driving conditions across southern Arizona - including haboobs and low visibility in dust, urban flooding, and washes flowing. This is a day to stay weather alert and for considerable caution outdoors.

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