Wednesday, July 12, 2017

Interesting Situation Today With Pros And Cons

Edited to add - visible image for 9:30 am shows very nice clearing out of the debris clouds from earlier this morning, which is definitely a positive.

Yesterday was nearly a totally down day in Santa Cruz, Pima, Pinal, and Maricopa Counties. The 24-hour rainfall in ALERT network went from 100% areal coverage yesterday morning to 0 % coverage this morning (after yesterday's light morning showers moved away). This is often the case after a big event works over the BL. This early morning there were storms along the border to our south (TUS radar below from 6:30 am) and they have moved southwestward into Sonora.

It was VERY humid at sunrise this morning and TUS had the highest dewpoint anywhere in the Southwest at sunrise - was like walking in a sauna. The visible satellite image above shows that cloud cover extended from Sonora all the way north to Four Corners at 6:15 am - so one issue today will be how quickly the clouds break, allowing heating.

The 12 UTC 500 mb analysis (below from NCAR) shows that troughiness in the West has complicated the anticyclone situation - part of the high has shifted eastward and the Great Basin high has shifted southward and is about over Yuma. There continues to be a weak trough separating the Yuma high from the west end of the Atlantic subtropical high. For the rest of the week a more substantial inverted trough (currently over south Texas) will move our way. Temperatures at 500 mb have warmed several degrees C as all the synoptic re-arranging has occurred.

The 12 UTC TWC sounding plot, second below, has high PW (1.74") and substantial CAPE - although winds are light and totally chopped up. The WRF-GFS does forecast slight strengthening and improved steering flow from the northeast by late in the day. The temperature inversion above 500 mb is very nasty and will affect available CAPE, especially if were to strengthen during the day.

The 06 UTC run of the WRF-GFS forecasts significant thunderstorms for eastern Pima County, with these occurring late afternoon and evening. The forecast of composite radar echoes above is valid at 9:00 pm. The forecast of total rainfall through midnight is below. The NAM version of the model is less enthusiastic in its 06 UTC run.

Many pros and cons present in the situation, but a strong chance for evening thunderstorms lingering into early morning hours appears quite reasonable at this time. The 12 UTC forecasts should be very interesting.

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