500 mb Squeeze Play

Yesterday - was yet another day with thunderstorms around but little rainfall. Above image at 1 pm MST shows buildups starting over the Catalinas. Around 4:30 pm there was a strong outflow here from the south with gusts 25 to 30 mph, also some thunder and slight spits of rain (another Trace day). This was from a weak, dissipating storm near the airport that produced a microburst and gusts to 61 mph at TUS. Later in the evening there was an outflow from storms to the west that brought in some dusty air. Plot of CGs through 5 am this morning (below) shows, again, little activity over southeast Arizona with storms mostly over south-central part of state. The ALERT data this morning indicate 12 sites, mostly in Catalinas, with light rainfall during past 24-hours.

Today - the synoptic pattern is very chopped up this morning. The 500 mb analysis at 12 UTC from the NAM is shown below. The anticyclone is being squeezed between the closed-low along the California coast and the large trough over the central US. One or two weak troughs have penetrated into the anticyclone, which seems to have two circulation centers - one near Raton, New Mexico, and one just west of El Paso, Texas. Generally an ugly mess.

The morning sounding from Tucson (above) illustrates this well with south to southwest winds in layer 700 to 500 mb, a layer with very light winds, and then southwesterly winds around 200 mb. CAPE is small and moisture remains quite sparse below 600 mb, so strong downbursts remain a threat, wherever storms do develop.

This morning the early WRF-GFS seems to have run, while the NAM version did not. The forecast composite radar chart above is valid at 2 pm this afternoon (18 August) and shows the most active time for eastern Pima County in the model forecast. The model forecasts a brief period with higher CAPE over the Tucson metro area that vanishes quickly during the early afternoon. Strong outflows run around all afternoon from Pima to Maricopa County in the model forecast. Forecast of total rainfall through midnight tonight is shown below, with little rain forecast on the ground in eastern Pima County and most activity, again, out to the west and in northern Sonora. The new NAM forecast this morning indicates considerably more rainfall in southeast Arizona than does the midnight WRF-GFS, so a check of the morning WRF runs is needed also.