Monday Morning - July 20, 2009

There were a surprising (when one examines the CAPEless TWC 00Z sounding) number of thundershowers around yesterday afternoon. Before 5 pm, 7 of the 93 ALERT gauges in eastern Pima County had recorded light amounts of rain. After 5 pm an additional 21 gauges had light rainfall from the thunderstorms that were drifting around. All amounts were less than a quarter of an inch, except for 0.55” at Florida Canyon. Here at the house there were several rumbles of thunder after 4:30 pm and the afternoon showers left 0.02” in the gauge, but another shower must have come by after dark, since there was 0.10” in the gauge this morning.

Precipitable water has inched up some and there is a distinct nose of higher values out to the west in the lower Colorado River Basin – values are around 1.5” in west and around 1.25” in the Tucson area. The morning TWC sounding agrees well with the GPS PW (for first time in quite awhile) this morning and indicates that there’ll likely be more CAPE available for afternoon storms today, and the NAM indicates this also.

The large 500 mb anticyclone that covers all of the southwest US has two separate circulation centers this morning – one over the central California coast and one over northern Sonora. The second center is bringing usually hostile west winds across southern Arizona, but speeds are light and there is higher moisture to the west and southwest. The NAM keeps the two circulation centers through tomorrow, with a weak trough between them. Flow in the upper-troposphere appears to be difluent over southeastern Arizona. The model forecast then brings a shortwave trough down the east side of the 500 mb ridge that affects New Mexico and Arizona – if the model forecast verifies, conditions for strong storms will be more favorable Wednesday and Thursday.

Today looks much like yesterday, but probably with an up-tic in storm coverage and rain amounts. The plot at top of this post illustrates that GPS PW values have been different than TWC sounding PW amounts for most of the past five days. This uncertainty in the RRS sonde data exacerbates an already difficult, summer forecasting situation.