Tuesday, July 27, 2010

Lower Half of Troposphere Very Tropical At TWC

Yesterday - Had a light shower here at house around 11 am that left 0.01" in the gauge. Around eastern Pima County there was rainfall at 21 of the 93 gauges, almost all at higher elevations. Notable rain amounts last 24 hours - 1.34" at Manning Camp and 0.91" at Coronado Camp. At the routine reporting stations in southeast Arizona, Safford had 0.61" and Nogales had 0.78". At the RAWS stations, Carr had 1.27". So, yesterday was definitely an upturn relative to Sunday.
Today -Another heavily overcast morning at Tucson (see top image). Dewpoints are quite high in the southeast, with a drier slot of air across south-central Arizona (just as the U of A versions of WRF model had predicted!). Cloudiness is very extensive over Arizona and all of northern Mexico this morning (see visible satellite image, middle above), and the middle and upper cyclone over Texas is apparent, having moved little during past 24-hours. The morning sounding at TWC (bottom image) is quite interesting. The troposphere is very moist and tropical below 400 mb, while the upper troposphere is quite dry. With a little sunshine the lifted parcel has theta-w of 24 or 25C, which usually favors strong storms. Even with the very warm mid-level temperatures, there is moderate CAPE at low elevations today. The sounding appears favorable for wet microbursts. Note the very high TWC 850 mb Td of 19C, while Yuma has an 850 mb Td of 0C - reflecting the push of dry, low-level air off the Pacific, again as per WRF forecasts yesterday. PW values are high in the southeast with 50 mm at PHX; 47 mm at TUS (note the sounding is a couple of mm too moist this morning), while Yuma has only 32 mm. However, very moist surface air has surged north again across Yuma, with current Tds in middle 70sF.
Large-scale setting remains stagnant with mostly L/V winds in troposphere. The TWC sounding had 20 kts from northeast at 700 mb, but this seems to be an outlier when compared with the winds at Marana and Ft. Huachuca profilers. The morning NAM moves the weak 500 mb inverted trough slowly westward today along the borderlands, increasing the 500 mb winds from the east. Favorable for letting storms move from mountains into lower elevations - if it happens. The 500 mb height gradients are very weak across a huge area of the Southwest and Mexico. Regardless, it appears that today will see another uptick in activity, with some locally heavy rains and the threat of damaging wet microbursts.
Rest of Week -the NAM forecasts continue to dislodge the Texas cyclone and move it westward as an inverted trough, that affects the borderlands and southern Arizona on Thursday and Friday - keeping the rest of the week quite interesting. The Texas system has cooler air in middle-levels, and so it may help to increase CAPE, especially Friday when the NAM forecast indicates a nice setup for severe storms.

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