HMT Forecast Discussion for Wednesday 27 Feb 08 (entered 1950z)
The Pacific continues to be quite active, but as discussed yesterday a mean ridge along or near the West Coast could keep the ARB out of action for quite a while. Two strong systems currently in the Pacific, one near 45N/150E, and the other near 45N/150W, with this one stretching from the Gulf of Alaska way back to the sw with a nice tap into tropical moisture. The main part of the strong system that was east of this wave is now hitting far northern British Columbia with a weakening trough stretching back to off the CA coast, but this continues to weaken and will not produce any precip in the ARB tomorrow. So the next potential system is the strong eastern Pacific one noted above, but this system will also head to the northeast with the main impact in the Pacific Northwest late Friday and Saturday. This leaves the ARB at the extreme southern end of the system. The deterministic 00z runs were overall similar to what we saw yesterday except maybe a tad farther north with the wave on Sat/1 March. As a result the 00z/GFS has only a trace of precip near KBLU on Saturday, while the ECMWF has less as well, with maybe a tenth of an inch or so. The 00z/Canadian Global run has a little more, maybe 0.25" in the ARB higher elevations, and the NOGAPS around 0.2 or so. All the models have more of course not too far away to the north, but all also keep this as a fast-moving wave. The 00z ensembles are also favoring a near miss, though there are a few GFS members with more precip than the deterministic run, and this is even more the case for the Canadian Global ensemble members. The 09z/SREF is like the larger scale ensembles, most do not have much but a few members have up to 0.25" or so. The 12z models coming in paint a similar picture; the 12z/NAM produces only ~0.10" at KBLU, with a similar amount from the 12z/GFS, and maybe a slight bit more from the 12z/Canadian Global model, while the 12z/ECMWF just in has only 0.01" at KBLU. So this remains a close call but not a system to be IOP-worthy, unless things change considerably.
The next wave, a much weakened version of the system now in the western Pacific, is forecast to pass mostly to the north and over the upper level ridge, barely brushing the ARB on Mon/3 Mar. There are only a few of the 00z Canadian ensemble members that bring any precip to the ARB with this system, but quite a bit does fall over the Pacific Northwest where the remnants of its tropical connection make the most impact. The 12z/ECMWF forecasts almost a tenth of an inch at KBLU, but most other models have nothing for the ARB. Overall there continues to be general agreement in the 00z deterministic runs and in the ensembles that this wave dives into the CONUS resulting in a longwave trough by mid next week, with an upper level ridge to its west along the West Coast that then keeps things dry through most of the 10-day forecast period ending on 8 March.
Forecasts for days 10-15 ending Thu/13 Mar, available from the Global Canadian and GFS and their ensembles, continue to show some potential for action in the ARB with a long fetch of more zonal flow across the Pacific breaking down the West Coast ridge. But overall the 00z ensembles are slightly less favorable than those from yesterday in agreeing on a big change, with a number of members, especially from the GFS, keeping a ridge that is retrograded but still close enough to the West Coast to keep things dry in the ARB. Basically by 8 Mar a couple of Canadian ensemble members have some precip, and then the number of members grows each day farther out, including a few from the GFS, though at no day do the majority of members support a more zonal flow into the West Coast. This is seen nicely in the NAEFS meteogram presentations, which for KSAC show very little precip for the median of the ensembles, but a trend towards greater potential from a few of the members, beginning on 7 March. Moving up the coast to Portland, the same presentation has greater median precip each day from 7-13 March, as well of course as for the next two storms that are forecast to pass to the north of the ARB. This indicates the active nature of the Pacific, which means that a southward shift to the storm track would put the ARB back in action by the second week in March, but that such a shift currently does not have a high probability of occurring, according to the 00z ensembles. A look at the 12z deterministic GFS provides more optimism for the 8-14 Mar period, with a series of waves chrashing the coast much farther to the south than shown by the 00z GFS. The 12z/ECMWF takes a few days longer before it looks like things could break down, so that by the last day of the ECMWF forecast (day 10, 12z/Sat/8 Mar) the ECMWF is still dry in the ARB but has a potent system approaching, while the 12z/GFS already has a sharp wave about to produce precip in the ARB. The 12z/GFS and Global Canadian ensembles also have more members now with potential systems hitting the ARB for the 8 to 13 Mar period, moreso than was seen in last night's 00z ensemble set. Not sure why the 12z runs should necessarily be more favorable-looking than the 00z runs, but basically this supports the notion that things could get more interesting again as early as 8 March and certainly by the second week in March for the ARB.
ed szoke NOAA/GSD/CIRA