A Look At The 500 mb Pattern From Asia To US

The 500 mb pattern across the north Pacific has been rapidly evolving during the past few days ( above is the GFS analysis from 00 UTC last evening). The rapid development of this high amplitude pattern has been causing spasms within global forecast models. There is a nice discussion of the model vagaries impacting the forecast through the weekend at the NWS Tucson webpage - see the 4:20 am MST forecast discussion (FD). Graphic below is the MIMIC PW analysis for 11 UTC this morning (from CIMSS at University of Wisconsin). The southeast tail of the intense cyclone south of the Aleutians evolves into the late week system that will impact the Southwest. Both of these analyses indicate that the remnants of once Hurricane Olaf are being picked up by the system to its northwest - the evolution of the strong PW plume into Olaf from the tropics may be the wild card in our events come the end of the week.

By early yesterday the ECMWF and GFS operational forecasts had come into fair agreement - with GFS trending a closed low over the Southwest and the ECMWF becoming a bit more progressive with the low. But the 00 UTC 120-hour forecasts have again diverged, illustrating the difficulty of the current situation. Forecast above is for 500 mb from the GFS and is valid at 00 UTC on the 31st. The ECMWF forecast below is for the same valid time as GFS above. The ECMWF continues with a closed low over the Southwest and is still 100 m or so deeper over parts of southern Arizona. The ECMWF has been more stable in its forecasts over the past few days than has the GFS.

The October pattern for the Southwest has featured two, and now perhaps three, closed lows in the southern branch of the 500 mb flow pattern. If this is a harbinger for our coming El Nino winter, then it will be an exciting season for us.

Finally, the forecast challenge is well illustrated by the rapidly changing precipitation forecasts from the GFS. Above is total precipitation forecast from the operational version (168-h valid through 06 UTC on November 1st) indicating a significant precipitation event for all of the Southwest. However, the current 00 UTC 120-hour forecast of total precipitation (valid through 00 UTC 31 October) is very much different. The heavier precipitation has shifted northward, while Arizona and New Mexico get significant precipitation mainly in the White and Mogollon Mountains, with little elsewhere.

What will tomorrow morning's forecasts bring?