Wednesday, August 10, 2016

Unusual Large-Scale Setting Today

The large-scale setting today is somewhat unusual and is similar to transition situations often seen at the end of the summer. Graphics above are 12 UTC analyses from NCAR RAP of 300 mb (top) and 700 mb (just above). At 300 mb a fairly strong trough is located out west near the Colorado River. However, the 700 mb trough has lagged behind, or been overrun, by the features in the upper-half of the troposphere. The 700 mb flow is much weaker and with a fetch from the subtropics and high moisture content.

Below are morning sounding plots (skewTs from SPC) for Tucson (top) and Yuma (bottom). The Tucson sounding indicates a two layer troposphere - moist and tropical airmass below 500 mb and a dry, westerly trough above. The Phoenix sounding is similar. However, the Yuma sounding is more typical of a Plains severe thunderstorm environment, as is the Flagstaff sounding.

This meteorological split personality results in a situation today where the main threat for the southeastern third of the state remains heavy rain storms, while the northwest third could experience severe thunderstorms. The middle third has a poissiblity for a bit of both.

A very interesting day on tap.

1 comment:

  1. Bob, could not agree more about both your assessment of the atmospheric structure and the threat of both severe weather and flash flooding! Special YUMA 15Z and 16Z soundings showed MU CAPE has increased above 2600! Looks very impressive for a severe weather outbreak along the Yuma-Flagstaff line. Depending on how far south the trough sags north portions of Maricopa County could get into both severe weather and flash flooding events over the next 12-24 hrs. Satellite suggests a 300mb jetlet could edge across central AZ overnight. From the small sample of last year's tornado study I would not be surprised to see a tornado mixed in with hail and high winds in the prime severe weather area. Severe wx notwithstanding the flash flood threat is also quite high with a wide tongue of 850Td's>14C. Son Dan at FCDMC in Phoenix noted the threat of training thunderstorms adding to the flash flood threat. While envious of your recent 2in+ event, my gage along the foothills here has reached 2.36in for month. Storm spotters are already headed up to Northern AZ. As you've said, "A very interesting day on tap". Jack