HMT-West 2007: Forecast

Snow in Malibu, but not much going on in the ARB. The 00ut GFS12 hour forecast run shows a good water plume connected to the tropics at roughly 180W bringing moisture up along the western side of the synoptic ridge that sits atop the forecast area. The future model runs have this plume detach from the tropics and begin an inland circulation in northern latitudes (Canada) only to follow an anti-cyclonic circulation under that ridge and become redirected out to the Pacific by the time the plume makes its way south into the CONUS latitudes. With time, the ridge continues to dominate the circulation pattern bringing in drier continental air east of the forecast area further west which probably obliterates any chances of a good precipitation event in the near term and possibly through the remainder of the upcoming week.

This morning's NAM has a 500 hPa cutoff just off shore of Southern CA and with time this ejects which modifies the flow over the ARB area from weak westerly flow at 12ut this morning to NNW in 84 hours (Monday 00ut) as the ridge increases in amplitude with an axis that is parallel and slightly offshore of the western coast.

The GFS 90h forecast from 06ut shows about the same upper level pattern as the NAM with the presence of a closing off low over western Colorado. This feature briefly cuts off and drops south in to New Mexico. Meanwhile the ridge over the ARB area stays in place with possibly a relocation directly over the ARB. In spite of the passage of numerous short waves all the way out until Thursday of next week when another off shore cut-off forms under the ridge threatening a split flow kind of pattern, the ridge dominates. Regardless of whether a split flow pattern forms, the ridging remains, directing Pacific flow well to the north of the forecast area and puts the ARB region in a weak flow regime at 500 hPa. Both model solutions are grim in terms of potential events, I spent the remaining part of this morning examining the possibility of the next potential system that will probably occur beyond this week using GFS ensembles.

Last night's ensembles have the ARB dry with the exception of 2 model runs, one 00h Sat 27 UTC and a different run (not the same forecast that produced the evening precip) 12ut Sat 27 -- both very weak. Other than that, the ensembles look dry for the ARB to the end of the ensemble forecast run (384 hours). Given this result, I would say that the distant future looks poor as far as additional precipitation opportunities.

Dan Birkenheuer - ESRL/GSD