So It Goes

Things have continued mostly dry except for some storms along the borderlands. Around 4 pm yesterday afternoon a high-based storm popped up west of Green Valley. The TWC 00Z sounding, again way too dry, indicated that there was likely no CAPE at the NWS building on campus. A weak outflow from storms near Nogales may have triggered this surprising little storm, which completely vanished as quickly as it popped up. However, this storm blip illustrates the “tipping point” nature of the stand-off between the wet and dry air masses along the border. There was a large MCS over Mexico last evening (see 03Z image above) that had tremendously cold cloud-top temperatures. The anvil and convective updrafts were undoubtedly venting directly into the stratosphere! Some portions of the MCS dissipated over the southern GoC, but essentially the event remained quite far to the south. Upturns in the surface dewpoints from Douglas west to Yuma this morning may indicate a northward push of outflows from this MCS.

Soundings at Phoenix and Tucson this morning are very dry and stable – as if it were still June. The anticyclone at 500 mb remains very large but unorganized with weak circulation centers and embedded trough lines. Models continue to forecast that it will consolidate toward the Four Corners region. The NAM strengthens the 500 mb flow after 36 hours and brings a nice inverted trough across southeastern Arizona on Monday afternoon – first real hopeful sign in the models for several days! Of course this time of year the reliability of model predictions falls off very rapidly after 18-24 hours, so we’ll have to watch to see how this actually plays out.

This said, I might as well dig a deeper hole and mention some aspects of the longer range forecasts of the global models. The 00Z runs of the operational members of the ECMWF and the GFS have markedly different forecast solutions for the 500 mb pattern at 168 hours. The ECMWF brings the cutoff low that is currently west of California onshore and establishes a trough over the western U.S. In contrast, the GFS moves the cutoff slowly to the north. The ECMWF has a flat ridge across the eastern U.S. with the anticyclone over Arkansas and the persistent great Lakes/East Coast trough pushed away. The GFS keeps the strong trough over the east and thus blocks the anticyclone far to the west along the Arizona/New Mexico border. If the ECMWF proves to be the better long-range forecast, our first week or so in August would go down hill after next Monday. Something else to watch during the coming week.