Friday, August 19, 2016

Westerlies Dominate Today & TS Kay Develops South of Baja

Thunderstorms occurred yesterday mostly over western half of Arizona. There were some thunderstorms off to the southwest of the metro area before sunrise this morning, and cool outflows prevailed during my early am walk. Plot of detected CG flashes above is for 24-hours ending at 6:00 am MST this morning (from Atmo and Vaisala).

The larger-scale pattern is very complex this morning. The 500 mb analysis above is from the 12 UTC NAM model. There are two weak troughs affecting Arizona today: one extends southwestward from northern Colorado into the Pacific west of southern Baja; while the other extends westward from the Four Corners across California and well out into the Pacific. Regardless, all of the Southwest is in the westerlies, with little left of a monsoon pattern, except for residual low-level moisture. The short wave across eastern Arizona rapidly weakens and vanishes, while the California trough persists. 

The morning sounding plot below from SPC shows the westerlies now prevailing through the entire troposphere, in a sounding with minimal CAPE. The situation would be very unstable with an increase in PW from the west. Clearly a Fall-type transition situation. The 06 UTC WRF forecasts differ, with the NAM version again keeping afternoon storms out to the west, while the GFS version forecasts more widespread storms affecting all of Pima County. 

Meanwhile, Tropical Storm Kay has developed south of Baja, as per the morning graphic above from NHC. Kay is forecast (below from NHC) to track southwest of southern Baja, along a path that would be good for GoC surge initiation, if the storm were larger or stronger. So, it appears to be a situation that's unclear, but worth monitoring.

I looked at the 06 UTC WRF forecasts of PW on the 5.4 km grid and found that the two models have quite different forecasts for the track of Kay. Both forecasts shown are valid at 11:00 am on August 22nd - WRF-NAM top and GFS version below.The storm is located much closer to Baja in the NAM version forecast, while the GFS version is more similar to the NHC forecast. A puzzling difference, since the NAM is essentially embedded within the GFS. My guess is that the NAM version is reflecting the TS location within the 00 UTC GFS forecast, while the GFS version reflects the new 06 UTC GFS forecast. The difference leads the NAM version to forecast a significant GoC surge, while the moisture intrusion is not nearly so strong on the GFS version.

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