Increasing Dewpoints

Pat Holbrook wrote: Monsoon start still looks on schedule for this weekend. Strong 250 mb westerlies aloft recedes as ridge builds in, only question is how fast the moisture will return I will be watching GYM next few days for a surge signal.
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There were actually a few cumulus around southeast Arizona yesterday afternoon and the trend should be for increasing moisture and cloudiness as the week progresses. The flow regime is quite chopped up in lower and middle levels over the Southwest and much of Mexico, so changes will be mostly associated with the westward building of the middle-level anticyclone later in the week.

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The graphics shown above are from the Tucson NWS monsoon pages - top graphic shows Tucson Td time series to date for this summer; middle graphic shows summer 2003; and lower graphic shows summer 2002. The onset of higher moisture (as indicated by the surface dewpoints) is often either very gradual (2003) or very sudden and implusive (2002). The sudden increase in 2002 was associated with the approach of a pronounced upper-level cyclone/inverted trough from the east. This feature moved across northern Mexico and southern Arizona triggering a northward push of subtropical air into southern Arizona. The top graphic seems to indicate that we're experiencing a gradual onset of higher moisture content in the low-levels this year. Some years do exhibit a mixed mode, and so the events of the next week or so will be interesting to observe.