Friday, September 20, 2013

Thundershowers This Afternoon?

There are a number of interesting things evolving in the weather situation as a fairly strong, Fall-like trough approaches the west coast. Once Hurricane Manuel weakened, the storm moved rapidly eastward into north-central Mexico and on toward southwest Texas. Manuel has produced a significant disaster in Mexico, with over 100 deaths being reported now and many more missing in the floods and landslides. Manuel moved off in exactly the opposite direction of the early NHC forecasts after the storm re-formed south of the GoC. Graphic below shows CG lightning strikes for the 12-hours ending at 11 pm MST last night - storm activity north of Manuel and from near the Continental Divide eastward.




This morning's 12 UTC sounding at TUS (above) indicates that PW is currently less than an inch; however, the SPC diagnostics forecast some CAPE this afternoon. The ugly, strong inversion just above 600 mb remains and is a significant deterrent for deep convection, as is the very dry upper-troposphere. The early WRF forecasts are quite similar for both versions this morning - below is PW forecast by WRF-NAM on the 5.4 km grid valid at 7 pm this evening. Note that the model brings higher PW in from the east and south, with values well over an inch by evening.



The models weaken the inversion very substantially (may be over-done) and forecast storms for eastern Pima County this afternoon and evening. Above is WRF-NAM forecast on the 1.8 km grid of composite radar echoes valid at 2 pm MST this afternoon. The WRF models forecast today to be the most active day, with only some light shower activity for eastern Pima County tomorrow. The outcomes will depend upon the low-level moisture advection that actually occurs and whether the inversion weakens later today. On Saturday night and Sunday the models forecast a significant dry out from the south-southwest to march across the state in advance of the Pacific trough - below is PW forecast on the 5.4 km grid valid at noon on Sunday the 22nd. The color bar to right indicates that PW is forecast well below 10 mm across most of Arizona on Sunday. The models also forecast a very significant wind storm with the Pacific trough, and I'll cover that in a separate post.

1 comment:

  1. Sorry, but this is a pet peeve - the following is from my Pet Peeves webpage:

    The word "thundershower" is used often to imply a weak thunderstorm, often in radio and TV weather broadcasts. I have heard (via the Internet) such pathetic rationalizations as "The public might panic if they heard they were going to experience a thunderstorm, and so I use thundershower for ordinary events." Without going into details, I find it monumentally unlikely that the public would panic over the use of the right word: "thunderstorm." They don't panic when hearing "tornado" in a weather broadcast, either .. contrary to the discredited policy imposed on the Weather Bureau (before it became the National Weather Service) for many decades ... and the word "tornado" ought to be a lot scarier than "thunderstorm"!

    Strictly speaking, any occurrence of thunder, with or without precipitation, is defined to be a thunderstorm ... the intensity of the "storm" is not really implied at all by the word "thunderstorm." Hence, the intensity must be indicated by including an adjective (from "weak" to "severe"). The occurrence of a thunderstorm ("T" in the pre-METAR format) accompanied by showery precipitation (either "RW" for rain showers or "SW" for snow showers in the pre-METAR format) might be considered a thundershower, of course. However, that's not the typical context in which the word is used ... or perhaps misused. Presumably, as the word is often misused, a thundershower could be accompanied by tornadoes, large hail, and damaging winds ... a "severe thundershower" ... a term that is virtually never heard. Since a "thunderstorm accompanied by showery precipitation" says nothing about intensity of either the thunder or the showery precipitation, you would have to use an adjective in addition to the word "thundershower" to convey properly any sense of intensity. Now, what is the difference between, say, a "weak thunderstorm" and a "weak thundershower"? Presumably, it could be argued that one implies the presence of showery precipitation and the other doesn't ... again, however, this is not the way the word is used (abused?). I vote for expunging "thundershower" from our vocabularies.

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