A Complicated Day As Summer Draws Down

First - a brief summary of yesterday. Storms occurred over a wider area of south-central and southeast Arizona than either the WRF (06z run) or the morning NAM forecast. Amounts were heavier than forecast by the NAM and a bit lighter than indicated by WRF. The WRF forecast again appeared to be more useful than the NAM. The top graphic shows the TUS radar rainfall estimate for period ending at 5 pm yesterday. Most of the storms and rain occurred prior to 5 pm. Coverage across the ALERT network was about 30%. The storms yesterday occurred mostly at higher elevations, although there was thunder but little rain at lower elevations (may have thundered here at house but I wasn't able to hear any). Notable amounts of rain: Manning Camp in Rincons 0.74"; Marshall Gulch 0.79" and Samaniego Peak (both in the Catalinas) 0.83"; Columbine on Mt. Graham 0.99" and Mt. Hopkins 1.01". It was an interesting contrast to last Friday, when the storms avoided the high elevations and produced heavy rains at lower elevations!

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Very complicated situation today. The TUS morning sounding (middle image) indicates a small amount of CAPE at low elevations and moderate amounts possible at higher elevations. Winds above 400 mb are westerly, where a large, higher-latitude trough dominates. In middle levels, around 700 to 400 mb southerly winds prevail with some speeds up to 20 kts or so, while low-level winds continuing light and variable. The sounding is about 3 mm too moist wrt to GPS data. Precipitable water amounts are highest in a band from southeast Arizona northwestward almost to Las Vegas - amounts around an inch and a half. However, drier air is already intruding from the southwest, with downward trends of PW at Yuma, Organ Pipe and Tohono O'odham CC.

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A backdoor cold front on the Plains has intruded into central New Mexico - but it appears that it will be stalled by pressure falls associated with Tropical Storm Hermine. Hermine is moving north-northwest toward west-central Texas. Note that air behind this particular front is somewhat drier than air over southeast Arizona, so it's evolution should be watched.

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A weak trough at 500 mb (see NCAR RAP 500 mb chart for 12z this morning) from low latitudes is being kicked eastward by the digging Pacific coast trough. This morning's data indicte that the southern short-wave trough has pushed just east of Yuma this morning. This trough has left behind a remnant low below 500 mb that is stagnant just west of San Diego -this cyclone will be keeping low-level winds southerly to southeasterly over southern Arizona until it is absorbed by the larger system to its north. Finally, middle-level air coming in from the west is VERY warm and dry - Note the -2 and -3C 500 mb temps at Yuma and San Diego this morning.

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Both the WRF (06Z) and the morning NAM indicate another active day over southeastern Arizona. I expect that this will be somewhat similar to yesterday, with the heaviest cores and highest rain amounts most likely over higher elevations. Once the southern short-wave trough passes by, it looks like dry, Fall-like weather for a number of days!