Thursday, July 23, 2009

Very Complex Day

There was a definite up-tick in activity over southeastern Arizona yesterday. For example, 41 of the 93 Pima County ALERT stations recorded measurable precipitation. Highest amounts exceeded an inch at two locations in the Catalinas. However, rains weren’t heavy enough to produce flow in the Rillito Wash, which was dry this morning. A storm drifted south from the west flanks of the Catalinas last evening and produced lightning and thunder here at house along with a moderate to heavy shower with 0.44” of precipitation.

At sunrise this morning there were showers to south and west (one of these was over the airport producing nearly a quarter inch of rain and apparently impacting the morning TWC sounding at the University), and the skies were heavily overcast. Indeed, heavy cloud cover affects the entire southwestern 2/3rds of Arizona this morning – one of many factors complicating the forecast.

A very large MCS affected western Mexico and the central GoC for much of the past 12 hours – see the IR image from 0330Z on the blog. This event will keep moist, subtropical air flowing up the Gulf into southwestern Arizona and southeastern California. The MCS and intense activity over Mexico has apparently interacted with several weak inverted troughs, with the result that there appears to a strengthened and significant inverted trough present at 500 mb this morning – this feature stretches from just west of El Paso into the south-central GoC where there is undoubtedly a fairly strong, convectively enhanced vorticity center. The north end of this inverted trough will move westward this afternoon affecting southern Arizona.

Additionally, although the main 500 mb anticyclone has shifted inland to the Nevada/Utah border, the weak trough of the last several days over southern Arizona has managed to persist and extends from between Phoenix and Tucson westward to north of San Diego. This feature has been keeping the winds aloft light and variable; whereas, 500 mb winds at Phoenix and Flagstaff are now from the east-southeast at about 20 kt. How exactly will the wind fields aloft change today along the borderlands? The NAM keeps the speeds light as the inverted trough passes by. The NAM also moves the upper-tropospheric anticyclone right overhead by midnight so that upper-level winds decrease during the day.

Showers and convection continue to bubble this morning, particularly from the south to west of Tucson. So, convection may be quite early today suppressing afternoon heating and keeping CAPE from building to higher levels. The morning sounding was modified by the cloud and shower activity, so it is not representative of the larger environment. IF, the boundary layer mixed at 12 g/kg, with afternoon breaks in the cloud letting enough heating to occur so that cloud base was at 700 mb, there would be a moderate amount of CAPE and a substantial threat of severe storms. But, continuing cloud and early convection would mean that the day would primarily pose a threat of locally heavy rains.

Given that the inverted trough to the south will be butting heads with the middle-level high over Utah, some mix of all the above will likely occur over the south half of Arizona. Can it get anymore complicated?

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