Wednesday, July 18, 2018

Increased Chances For Storms In Metro Today

Above shows large thunderstorm northwest of Kitt Peak a bit before 8:00 pm MST yesterday evening. There was thunder at the airport, but from the north side of City we mainly watched some lightning at dark. Model forecasts were quite good from 06 UTC early yesterday, keeping storms mainly south and west of the metro area. The 24-hour CG flash density ending at 6:00 am (below from and Vaisala) indicates storms about where the model forecast. There were also strong thunderstorms and a number of severe wind reports in northwest Arizona and southern Nevada.

Second graphic below shows that a few ALERT gauges had rain (24-hours ending at 7:00 am this morning) through the southern part of the network, with heaviest amounts southwest of downtown.

The situation this morning appears more interesting than was yesterdays - note that drying occurred during the day (although not to degree that had been forecast) and combined with large-scale subsidence to suppress activity except along the Borderlands. The 12 UTC skew-T plot for TWC (above) indicates PW has increased again and is over an inch and three-quarters. CAPE is very large. An afternoon lifted parcel could follow the 25 C moist adiabat - a situation that usually brings severe thunderstorms here. The vertical wind profile is also very favorable, as upper winds would move anvils off to northwest of thunderstorm cores. But, if storms were to develop initially over the Santa Rita, then there could be anvil-shading problems in the metro area.

Today the upper-level IT has moved westward (12 UTC 250 analysis below from SPC), and there are two distinct anticyclone centers with one to our west-northwest and the other to our east-northeast. Between the centers there appears to be a large area of difluence over southwestern New Mexico and extending into southeastern Arizona. This situation is considerably different than yesterday's when strong confluence was occurring over the same region. The 06 UTC WRF runs are a bit different in their forecasts: the GFS version keeps storms again mainly south to west of the City; however the NAM version forecasts several severe thunderstorms to cross portions of eastern Pima County. The forecast skew-T at bottom is from NAM version and is valid at 2:00 pm this afternoon, and remains very favorable for severe thunderstorms - so hopefully it will be a much more interesting and active weather day today.

1 comment:

  1. The lightning flash density graphic is very interesting. There are two long and narrow swaths of lightning that extend from northeast Arizona into southern California. It's not common to see something of that nature during the summer season. It can probably be argued that the presence of the inverted trough exerted some influence resulting in several long-lived clusters of thunderstorms.