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HMT Forecast 2/2/2008 19:44 UTC

This morning (16UT) the ARB is already seeing some signs of early precipitation with BLU reporting 23F and S-; stations to the north have light precip while stations mostly in and south of the ARB are still reporting partly cloudy conditions and no precipitation.

The satellite water vapor imagery shows a good N-S oriented moisture plume west of HI extending from 24N 173E to 47N 174W, there is also evidence of a low spinning up at 25N 164 W.

Yesterday the decision was made not to conduct an IOP for the system progged to start in the ARB this evening. Today's models have this system producing about the same amount of precipitation as in prior forecasts with this morning's NAM showing 2.4 inches consistent with earlier forecasts. Winds with this system also seem to be about the same but maybe a tad lower in intensity with 30 kts at low elevations and 50kts at higher elevations from the SW at 12UT 2/3 with the duration of the stronger winds less that prior forecasts indicated. This timeframe coincides with the GFS model's moisture plume impacting the area at its peak; furthermore the moisture plume appears to be stronger, at least out off the coast with TPW values today reaching 3cm. Precip at this time (12-15ut) is shown to be about 0.8 inches. The one thing that is different about this event in this morning's models is that it appears to be a bit warmer than prior forecasts showed with freezing level starting around 3500ft and then rising to 5000 ft at 12UT 2/3, about the same time of greatest windspeed and just before the trof passes at 18UT 2/3. The SREF ensembles show the event to end at 03UT 2/4 and this is consistent with earlier model runs.

Following this event the GFS shows a 1.6 cm TPW plume (small in size) sweeping by the ARB at 06UT 2/6, again consistent with prior forecasts.

In addition, today we see some "new" potential events in the GFS, one at 18UT 2/8 and the one we have seen for some time for Wednesday 2/13. Studying the upper level wave patterns it appears that as we move through this week the high pressure ridge off the Pacific coast retrogrades slightly so that moisture plumes are driven north to south on or near the ARB instead of a preferred W to E trajectory. So if "rivers" (I hesitate to call them this since they have no direct trajectory out of the tropics) make it into the area they move out swiftly and there appears to be a greater chance that they do not make it far enough inland, reducing any impact on the ARB.

Dan Birkenheuer ESRL/GSD