Friday, July 25, 2014

Morning Of 25 July - Big Changes Overnight

The large MCS of yesterday (as per earlier posts) did push low-level moisture all the way north up the GoC and into the Southwest. This was a fairly rare MCS (based on my recollections of past 20 years or so) and it apparently produced substantial damage in southern Baja. The MCS outflow was enhanced by additional outflows from storms out to our west. The result at sunrise (above) is very dirty skies with heavy, suspended dust, increased moisture and humidity, and very warm morning temperatures. The observations below are from the airport (TUS). Dewpoints across southern Arizona from Tucson westward are 20 to 25 F higher than the minimums of yesterday afternoon/evening.

The CIRA blended PW analysis (above) for 3 am MST indicates values of 1.5 to over 2.0 inches over south-central Arizona. The plot of CG flashes through midnight last night (below) shows where storms occurred in a narrow south-to-north zone of moisture convergence between the easterly winds we experienced and southwesterly winds in the lower Colorado River Basin. There were no storms or rainfall across southeastern Arizona yesterday, just some cloud streets of shallow cumulus.

I want to take a quick look the TUS morning sounding plot (above) versus the early WRF-NAM forecasted sounding (below) for essentially the same time and place. I'll look at the morning model runs later, after a morning appointment.

First, the increase in low-level moisture has produced a dramatic increase in CAPE likely this afternoon. There is some cooling near the surface, but daytime heating should easily over come this.

Observed PW was approximately 1.5 inches both for the observed and predicted soundings. Note that the model forecasts lifted parcels having theta-w of >24C ~ values greater than 24C often are associated with severe thunderstorms in this area. Now a bit of a negative - the time series of GPS PW from Tucson westward are indicating some decreases already, back toward 1.25 inches - so need to watch how this strange airmass evolves this morning.

Finally, the observed vertical wind profile is very chopped up and essentially light and variable below 400 mb. The observed wind shear profile is not good at sounding time - indicating anvils spreading out rapidly ahead of storms that would be trying to move to the northwest. The forecast wind profile and shear is not as negative as that observed - my quick assessment is that a special sounding would certainly be nice to have today. Guaymas remains out of service - not good since that leaves a big data void exactly where we'd like to know more.

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