Wednesday, September 03, 2014

Watching Norbert

This morning at sunrise there were ACC far off to the south - view above is from Kitt Peak looking south at about 7:30 am MST this morning. The clouds are at the leading edge of a very large mass of moisture and cloudiness that extends north-northeastward from Tropical Storm Norbert, which is close to 110 W and 20 N this morning (IR image below is from 6:30 am MST).

The track and evolution of Norbert will essentially be the most important weather factor for the Southwest for the next 5 days or longer. The current NHC forecast for Norbert this morning is shown above. The NHC forecast is for Norbert to become a hurricane and to track west of Baja as it moves northward. If Norbert behaves as forecast above, the surge of associated deep mT moisture into the Southwest will occur late Thursday night and Friday morning. However, the NHCs track forecasts for Norbert have been shifting eastward with time and the NAM this morning forecasts the storm to approach the mouth of the GoC. If this were to happen, the moisture advection fields would be strongly impacted by the falling pressures at the south end of GoC. So, it is a very tricky situation and I'll be watching the storm and the NHC forecasts closely.

As for today, PW has already increased to over an inch along the Borderlands, and surface dewpoints are up 5 to 10+ F relative to 24 hours ago. This trend will continue, however, CAPE will be slow to develop as mid-level temperatures are very warm. The early run of the WRF-NAM forecasts isolated thunderstorms this afternoon to the southwest of Tucson - above forecast of composite radar echoes is valid at 6 pm today.

The WRF-GFS 5.4 km grid forecast of PW valid at 11 pm MST on the 5th is shown below. Note that broad plumes of high PW are affecting Arizona both from Norbert and from the southeast up the Rio Grande, from the remnants of Dolly. 

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