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HMT Forecast Discussion for Monday, 10 December 2007

Guidance continues to point to the next 5 days being dry over HMT domain, but big changes are in store starting the weekend as a more favorable large-scale flow is firmly established by no later than 2007121600, and it should last until the Holiday shutdown around 2007122200 (1600 PST Friday DEC 21)

Latest 2007121012 NAM-GFS-ECMWF runs keep the ARB dry through 2007121512. Interestingly, the GFS digs the 500 mb trof, which has consistently looked innocuous in its fcst arrival over ARB this Wednesday, well offshore and closes it off; systems that close-off has been the scenario the past ~10 days. Again, no precip over HMT domain with the mid-week system.

The GFS has precip reaching BBY-CZC btw 2007121600-2007121606 (1600-2200 PST Saturday), and ARB 6 hours after that. It is considerably faster than yesterday's run which was 24 h slower, and it was a tad faster than the prior day's run. I smell trend. Precipitation lasts 18 h over the coast, and longer over ARB with max GFS accumulations around 1.0 in. there. There is little break before the next impulse, as the GFS brings a second juicier system onshore 24 h later 2007121706 (2200 PST Sunday) which deposits a comparable amount over ARB. Finally, ARB finds itself at the southern end of a third event toward the end of the run (2007121912-2007122012).

The ECMWF has precip reaching BBY-CZC btw 2007121500-2007121512, or faster than 2007120912 cycle, so the run-to-run trend continues. Amounts look light, with 0.10-0.20 in. near BBY-CZC and around 0.50 in. over ARB. The ECMWF's next system arrives onshore 2007171200 (0400 PST Monday) and is wetter, with 3/4 in. on coast and 0.5 in. over ARB. It appears ARB never really clears out before a third system comes over basin.

The latest 2007121012 GFS ens. guidance (http://www.meteo.psu.edu/~gadomski/ewallmref.html) has the majority of members with 500 mb S/W troughs hitting the HMT domain starting around 20071211600 and continuing up to the holiday break. The S/W troughs are embedded in WNW flow whose westerly component should be faster than normal according to the ERSL/PSD EPO refcst teleconnection (http://www.cdc.noaa.gov/map/wx/images/epo.gif). The NAESF plumes for the latest 2007121012 run continue to show the period 2007121712-2007122012 as the most likely window for rain over KSMF, but 1 or 2 outliers appear to start the precip as early as 2007121512, consistent with the fast 2007121012 ECMWF solution. The 2007121012 CMC calibrated PQPF's suggest the 24 h period 2007121800-2007121900 will be the wettest.

The overall message from the latest GFS, ECMWF, NAEFS guidance, and ESRL/PSD refcts ensembles and ESRL/PSD teleconnections seems to be a wet pattern beginning no later than Saturday PST. The pattern will probably stay wet thereafter, with rapidly moving systems hitting the HMT domain every 24-36 h and little clearing likely btw systems over the ARB. Their timing is impossible to pinpoint 5+ days out, however. There is one BIG caveat, however: HMT is at the southern end of the significant precip associated with some of the systems, so a subtle poleward shift in the jet axis shunts the rain to the north, so the situation will need to be updated.

Steve Mullen