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HMT Forecast Discussion 1945z F 29 Feb 08

Overall outlook for significant pcpn in the ARB continues bleak. Low center is heading for the Queen Charlottes at present, and frontal band of middle and high clouds intersects the Coast near CA-OR border, and thence southwest to west of KSFO. Prospects are for possibility of some light pcpn tonite in the upper elevations of the ARB, but air mass is mainly dry and the frontal band may well come across trailing only virga.

Behind this first system, high pressure is building smartly and will move into the Northwest on Saturday 1 March. The next upstream system is forecast to stay well to the N as it crosses the Coast late M - Tu 3 and 4 March. Indications are that this system will then dig swd and quite likely cutoff over the Southwest or Sonora. This morning's hi-res GFS run is not as serious about this as are many of the GFS ensemble members or the ECMWF: many of the ensemble members as well as the ECMWF build the ridge more strongly just off the Coast early next week, both creating a strong Nly flow down the Coast and effectively blocking future upstream systems from affecting California. As a consequence, the 12z today GFS hi-res is an outlier in its setting up a strong zonal flow across the central and eastern Pacific by M 10 March with significant pcpn into the ARB. Partly for this reason, and also partly because the GFS often is too conservative on West Coast digging trough scenarios that lead to cutoff low formation, consensus is that we are not going to see any significant pcpn in the ARB till at least 10 March. For reference, the Penn State E-wall web site GFS ensembles from the 00z 29 Feb runs (last night) keep the ARB 100% dry (after tonite's whatever) through 12z F 7 March, and the 12z ensembles are all dry through 12z M 10 March except the aforementioned hi res GFS.

John B.