Sunday, July 19, 2015

More Storms Yesterday But Low Elevations Skunked Again

The threats for storms and heavy rains for much of eastern Pima County were again overplayed yesterday, with POPs at airport grid of 70% yesterday and again last night. The NWS had the following as part of their weather story, which was up on their webpage for much of the day.
Widespread rainfall amounts of around a half to three quarters of an inch will likely occur today. However, some areas will receive higher amounts, potentially exceeding 1 to 1.5 inches. In areas where the strongest storms develop, expect an increased threat for flash flooding along with the subsequent road closures in the normally flood prone areas.
I think that yesterday was yet another example of a day where the observations needed to be carefully scrutinized before one began to evaluate the plethora of model forecasts available.

Thunderstorms developed early yesterday on higher elevations and drifted slowly around producing rains mostly on the mountains and spewing thick anvil clouds over lower elevations. The composite radar chart above is from just before 5:00 pm MST yesterday and shows that thunderstorm activity had basically ended over southeast Arizona. Many stations in southeast Arizona hit their maximum temperatures right around noon, with falling temperatures during the afternoon - for example, TUS was at 92 F at noon but was only 81 F at 4 pm.

The plot of detected CGs (below from Vaisala and Atmo for 24-hours ending at 06 UTC last night) shows much increased activity over higher elevation regions of southeast Arizona relative to the down-day experienced on Friday. The most significant thunderstorm activity was over western half of state as per WRF forecasts and also over northern Sonora. The Tucson metro donut hole is clearly apparent.

Across the ALERT network only 21 sites (23%) had rainfall from 5:00 am to 5:00 pm, with most of those in the Catalinas where there was one report of 0.83". From 5:00 pm through 5:00 am this morning 13 sites (14%) reported rainfall that was mostly very light. There may have been a sprinkle here, but I don't know for certain, so yesterday will go in my logbook as a goose egg.

The CG flash density plot above for period 06 to 12 UTC (from Weather.Graphics and Vaisala) shows almost no thunderstorm activity in Arizona during the early morning hours, but a highly unusual storm area along the California coast - southern California is where many July weather records fell yesterday, as Dolores brought rare summer storms and rain. Some severe reports scattered across the west yesterday, with 4 reports of severe winds in west Arizona, most along the Colorado River.

Today we have continuing high PW values, but CAPE is actually down relative to yesterday (skewT plot of TWC morning sounding above from SPC). The pathetic wind profile below 500 mb continues (this has been a serious nemesis during last several days), with northward streaming anvils coming off of storms at higher elevations. Only the 06 UTC WRF-GFS forecast was available this morning and its forecast for accumulated rainfall through midnight tonight is shown below - heaviest storm activity forecast by model to remain out to the west. Once again a large minimum area centers on Tucson metro area. 

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