Monday, September 07, 2015

What, Exactly, Is A "Gulf Surge"

First, brief summaries of yesterday and a look at this morning's situation. Yesterday forecasts and expectations again exceeded what the environment could produce. Thunderstorms were mostly restricted to Santa Cruz County, The Santa Rita Mountains and the flanks of the Huachuca Mountains. Model forecasts were significantly overdone for northeastern Sonora and Cochise County. Very blah and down days here in eastern Pima County for both Saturday and Sunday. Across the ALERT network only 3 sites on the north side of the Santa Ritas had rainfall and only amount worth mentioning was 0.35" near Elephant Head Butte.

This morning light showers have developed over a fairly large area to our west and northwest - visible image below is from 7:00 am MST. These are moving northeastward and were forecast in the 06 UTC run of the WRF-GFS, which is interesting, since the WRF-GFS forecasts for Cochise County yesterday were seriously out-of-whack. Neither version of the WRF forecasts much in the way of thunderstorm activity this afternoon. However, the morning TWC sounding has high PW and afternoon CAPE of 1000 J/kg or more, so a look at the morning model forecasts would be prudent.

Yesterday I mentioned that Hurricane Linda, which is moving fairly rapidly north-northwestward, might move close enough to south Baja to force a GoC moisture surge This morning I took a closer look at the moisture situation over the eastern Pacific, which led to the question posed in the heading for this post. The 09 UTC MIMIC PW analysis (above from CIMSS at Univ. of Wisconsin) shows that PW values are already very high across the entire GoC region and into the lower Colorado River Basin. The GPS time-series for PW at Yuma (below) shows that PW has been increasing gradually for the past five days - this increase was associated with the southerly flow to the east of TS Kevin (which has completely dissipated). 

My basic perception of what a Gulf Surge is goes as follows - after a period of hot and dry conditions over northwest Sonora and the lower Colorado River Basin, an impulse of much increased low-level moisture, cooler temperatures, and distinctly stronger low-level winds pushes rapidly northward up the GoC into the Southwest. So, according to my subjective definition, we can't have a GoC surge due to Linda because high moisture is already in place. However, there are differing perceptions of what exactly a surge is. Since Jack Hales (who was at the Phoenix Forecast Office) first identified the phenomena in 1972, there have been continuing publications about GoC surges in the formal literature, but I don't think there is a generally accepted, quantitative definition of the phenomena - putting it somewhat in the category of "I know one when I see it!" I'll do a quick literature survey to see what I can find - more later.

Finally, a Thomas Hart Benton mural for Labor Day -

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