A Tale of Two Noras

A Tale of Two Noras

During the late afternoon hours of this past Tuesday the precipitable water values around Tucson and in all directions away from Tucson were about as high as they ever get. For good reason the National Weather Service had a flash flood watch in effect for Tucson. Below is what about as high as precipitable water values ever get around Tucson looks like.

I use a color range maxed out at 1.7 inches because at Tucson's elevation it doesn't get much higher, and attention is usually on the transition from slightly below to slightly above one inch. Obviously on Tuesday in order to get to the one inch neighborhood you had to go a long distance, either to the highest elevations in the sectors north through southeast of Tucson, or to the moderately high elevations of the Baja peninsula.

By Tuesday Nora's circulation had completely dissipated at all levels, leaving a broad and deep southerly flow at low to mid levels that was continuing to transport moisture northward. The mid level southerly flow veered to a 70+ knot southwesterly upper level jet extending across northern Baja just south of the California border. I picked up about half an inch of rain during the evening hours of Tuesday tapering off into the first few hours after midnight on Wednesday. Most of the eastern part of Tucson, east of about Swan Road, picked up about an inch. By sunrise on Wednesday the threat was past Tucson. The sky was clearing from the southwest as precipitable water levels were already dropping. It was obvious from the large scale radar composite that the upward motion associated with the upper level jet had shifted north and east of Tucson. The radio station that I listen to in the morning continued to report the forecast of a 40-50% chance of rain, but the on-air personality simply ignored the fact that officially the flash flood watch was still in effect for Tucson. The local paper took the opposite tack. The following morning, 24 hours later, the Thursday online edition still featured a story, last updated 21 hours earlier, detailing the flash flood preparations, without noting that the watch had finally been cancelled 18 hours earlier.

One week ago, when the final details of Nora itself were still uncertain, but the threat of flooding for the Southwest was already being publicized, the local paper recalled its snarky coverage of the 1997 edition of Nora, which produced only a few drops of rain in Tucson instead of the 6, 4, 2 inches that had been forecast. The 2021 paper claimed, It's possible the same thing could happen with Nora 2.0.

Here is what the precipitable water looked like on the afternoon of September 25, 1997, the day before the snarky 1997 newspaper article.

By that afternoon the low level circulation of 1997 Nora had had a complicated interaction with the Baja peninsula as Nora-1997 moved rapidly north toward the north end of the Gulf of California. Thick high level outflow debris clouds had overspread Tucson, darkening the sky dramatically compared to the morning sunshine. The wind had become a bit gusty from the south-southeast. I recall walking across the U of A campus with a colleague, a hydrologist, and he mentioned the anticipated (by anyone following the official NWS forecast) heavy rain. I said, "I wouldn't be surprised if we don't get anything." My colleague looked around at the overcast sky and at the effects of the wind gusts and sniffed, "Well obviously the hurricane is coming." The problem is that a hurricane and its environment can't be conceived as a system analogous to a baseball traveling through the air. If the hurricane and its far-flung moisture field were like a solid body, then the precipitable water would have increased dramatically at Tucson as the center of the hurricane raced north. In fact precipitable water did increase during that day over the western half of Pima County, but remained almost constant around Tucson from morning to evening, consistent with what the operational numerical models of the day had been predicting. Even scientists, who should know better, will stick with a conceptual model that is wrong long past the point when reality has given them an answer they didn't want to hear.

Access to the reanalysis dataset through NCAR is gratefully acknowledged:
National Centers for Environmental Prediction/National Weather Service/NOAA/U.S. Department of Commerce. 2005, updated monthly. NCEP North American Regional Reanalysis (NARR). Research Data Archive at the National Center for Atmospheric Research, Computational and Information Systems Laboratory. https://rda.ucar.edu/datasets/ds608.0/. Accessed 28 Aug 2021.

September 17: Corrected paragaraph before second figure, September vs. November.