Thursday, August 30, 2012

Low-Level Moisture Inches Upward

It was noticeably more humid this morning, as low-level moisture increased during the night from the south to west. There is a pronounced PW gradient across southern Arizona, with values approaching 2.00" in the west, while in the Tucson area values are around 1.25". The morning sounding from Tucson (skewT plot above), shows the cool and more moist low-levels, but with a dry layer remaining from around 600 to 500 mb. The easterlies below 700 mb are essentially gone; steering flow is apparently from the southeast; and upper-level winds are from the west. The SPC diagnostics forecast an afternoon surface-CAPE of nearly 2000 J/kg. So, the day starts with more favorable conditions for storms in eastern Pima County than we've seen for a number of days.

Hurricane Ileana is southwest of the southern tip of Baja (visible image below is for 7 am MST), but is forecast to move slowly to the northwest.

Latest blended PW (above from CIRA at 6 am) indicates that high PW already is in place, so the possible role of Ileana would only be to broaden the plume of moisture over the Southwest. Moisture is high into southern California and northward into southern Nevada and Utah. The 500 mb data indicate several weak troughs moving northward along the GoC. The large-scale setting is favorable for more storms to our west and northwest (note that Yuma had a thunderstorm at sunrise this morning and there were a couple of severe thunderstorm reports yesterday in California and along the Colorado River). Once again, the early WRF-GFS model run has failed, so no high-resolution forecast data available.

It appears that low-level winds (while weak) will be from the southwest to west today, which will keep the PW values steady, or on the increase. It appears that we should see thunderstorms in central and eastern Pima County this afternoon and evening. It remains to be seen if storms will move into lower elevations. But, an upturn appears to be underway - check the 12 UTC runs of Atmo's WRF later this morning.

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