Summer Solstice And Look Ahead

The summer solstice for the Northern Hemisphere occurred last night, so summer is "officially" here, and the sun will now start retreating southward for the next six months. Here in Tucson real summer started weeks ago and we have now had 21 (counting today) straight days with highs over 100 F. The graphic below (from CIRA at Colorado State Univ.) shows blended PW at 5 am MST this morning (21 June 2013). The important mT air mass covers the south half of the Gulf of California (GoC), with PW amounts ranging from 30 mm (bright green) to over 50 mm (reds). To get the deep moisture up into Arizona requires some weather feature to push this air mass northward up the GoC (referred to as a Gulf Surge). So this is what we'll be watching for the next week or two.

The global models strengthen the subtropical anticyclone northward during the latter part of next week - the ECMWF 500 mb forecast valid at 5 pm MST next Thursday (the 27th) is shown above. The ECMWF forecasts a large area of the West to have 500 mb heights over 5940 m - definitely summertime heights. However, the model also forecasts the air at 700 mb at that time to remain quite dry over the Southwest - see below.

The same forecasts from the GFS are shown above and below. The GFS 500 mb forecast is very similar to the ECMWF, with the strong anticyclone at 500 mb centered over Grand Junction, Colorado. The winds at 500 mb are forecast to be easterly to northeasterly over southern Arizona and New Mexico - this easterly flow regime would probably bring in some mid-level moisture and fuel more high-based, mountain thunderstorms with the threat of more lightning but little rainfall. The GFS forecast at 700 mb for the same time also forecasts continued dry air over the Southwest and northern Mexico. Note, however, that the models don't forecast moisture changes well, especially at this long time range.

The important unknown at this time is whether or not the forecasted tropical storm southwest of Baja (if such a storm actually develops) will come close enough to Baja to initiate a Gulf Surge of low-level moisture into southern Arizona. Events like this forecast tropical storm are often the trigger for the onset of the actual monsoon at lower elevations of the Sonoran desert - so we will have to watch all of this closely.