HMT Forecast 1/10/2008 19:46 UTC
Current conditions as of this morning are light precipitation and fog in the ARB with temperatures 28-34F in the higher terrain and 40 degree temperatures in valley areas. Precipitation has been light snow in the high terrain and light rain along the coastal areas.
Water vapor satellite imagery shows a moisture plume well on shore to the north of the ARB extending back to the HI Islands. The imagery also shows a definite break in the moisture at the point with the tropical source south of HI. In other words, the plume is "disconnected" from the tropics. In addition, a good vorticity center appears near 163W 33N and a second circulation is noted further west at 186W 40N. The second one (186W) has a better moisture fetch to the tropics extending back well into the Indonesia area and would be well worth watching. The source nearer the CONUS does not appear to be that impressive at this time.
Near term model forecasts show that the current system should produce minor precip over the ARB ranging from 0.03- 0.17 in ending by 06utc Friday. The probability of precipitation (SREF) increases precipitation probabilities over higher terrain by 1/12 18utc (Saturday) through 1/13 06 UTC Sunday. However, all models show that the total precipitation expected during this interval will be light. The GFS shows no moisture over the ARB while the EC model produces only minor precipitation well under the threshold for an IOP. Also during this time a ridge builds over the coast in both the GFS and EC models. At 1/12 00UTC, a weak wave moves over the ridge coinciding with the higher precip probabilities in the SREF reaching the coast at 1/12 18UTC but the GFS keeps the precipitation north of the ARB. The EC model has this wave propagating faster and even though it brings in the prospect of some precipitation into the ARB, it moves through very rapidly.
Following the end of this weak precipitation potential 1/13 06UTC (Sunday) we enter what appears to be an extended dry period with the exception of the EC model (more on that below). The GFS ensemble runs show basically a dry CA and ARB from late Sunday (1/13) until the end of the ensemble forecasts ending 1/26 (Saturday) with maybe a hint of something on 1/21. A strong 500 hPa ridge builds off the CA coast by Tuesday 1/15 18UTC with a closed 584 high pressure center at its base centered at 128W 23N. With the exception of the EC model no precipitation is forecast to enter the ARB in this timeframe with the EC bringing a very fast, but weak precip event into the ARB 1/15 03UTC (Tuesday). However, the system might only result in increased cloudiness to the region and may not produce measurable precipitation. At this time it appears that the chances of this becoming an IOP worthy event is near zero.
If the large ridge builds as forecast off the coast by Tuesday, it will be very difficult for moisture plumes to find their way to the ARB until it breaks down or allows the circulation of moisture to be brought up under the ridge, but the closed high pressure center does not appear to strengthen after Tuesday, which would likely reduce the likelihood of the "undercut" possibility. The fundamental message at this time is that with the exception of some minor disturbances moving through the ARB in the near term, we will be entering an extended period of dry conditions for the ARB.
Dan Birkenheuer ESRL/GSD