HMT Forecast 1/24/07
The models continue to mess with my mind, this is like a bad dream. I guess handling 4-5 closed lows at 500 is just too much to expect any model to do well with. Solutions today again differ greatly from yesterday's however the bottom line remains the same. The one element that this week's forecasting has demonstrated to me is the effectiveness of ensemble forecasts especially when the weather gets into a difficult-to-model regime as it currently stands. I think in spite of the fact that the offshore low pressure system is now progged to move N today instead of S, giving the ARB better flow aloft (W to SW flow) there is not enough moisture to work with and the end result is that both the NAM and GFS keep the ARB dry.
This morning's NAM analyzes a 500 hPa ridge over the NV CA border with the circulation center near Reno. The upper level low that we have been watching in old Mexico has been moving slightly to the east and now is positioned south of the CONUS between El Paso and the AZ NM border. The current analysis still does not have a closed low off in the Pacific, but the approaching wave is beginning to affect the ridge. Winds begin to pick up out of the SW 06utc Thursday as the approaching Pacific wave begins its split and the southern part begins to close off. By 00utc Friday the low has cut off in the NAM and is impinging on the OR CA coast and is in a better position to come inland for the ARB. Meanwhile at the same time the northern piece of the wave which carries more energy has closed off and is sitting atop the ridge depressing it while the Mexican closed low has slightly retrograded. This pattern looks more like the one forecast 2 days ago instead of yesterday with the exception of the better location of the offshore low. The NAM brings the 500 Pacific low a tad inland 06ut Friday into southern OR. The ARB at this time is experiencing SW winds around 25 m/s. By 00utc Saturday the low has two distinct vort centers (as we have noted might be the case in past runs) and its diameter increases and moves north off the central OR coast with a very strong vort center developing on the west side of the closed system, I would expect the low to dig after this point and sure enough 18utc Saturday has the low moving south and strengthening but still remaining offshore at about the OR CA border with a 548 height. In spite of the fact that the winds pick up more favorably over the ARB again the moisture remains right on the coast and dry conditions persist inland. It appears that the moisture plume has become wrapped up in the upper level circulation and is not in a position to be advected inland by the better winds south of the low; so the moisture goes where the low moves. By 00utc Sunday the low moves slightly north, still off shore but a rogue piece of moisture gets caught up in the circulation south of the low and appears to head for the ARB. This is certainly NOT the kind of scenario the HMT exercise is about. There appears to be no connection to a topical moisture plume with this small moist surge. By the end of this morning's run, the NAM has put down no precipitation over the ARB, even the higher terrain in spite of the late shot of moisture. This is out to Sunday 00utc (84h forecast). And as of this time the closed low is STILL off the coast beginning to interact with a new Pacific low that develops to its SW. Meanwhile the ridge axis has drastically retrograded (hats off to Klaus and Ed) now setting up over the Pacific near the AK coast. Perhaps this is the signature of the beginning of the MJO changes that Ed mentioned last week.
Today's GFS run shows no precipitation over the ARB until 06utc 31 January. So for this weekend, it is also dry. As far as the specifics in the treatment of the upper air systems, the GFS keeps the low pressure center of interest further off othe coast overall with about the same N-S excursions. However, the flow over the ARB from the GFS remains steady from the west but much weaker mainly due to the fact that the low pressure center does not come as close to the ARB.
The ensembles today are bleaker than yesterday with no precip forecast over the ARB for this system from the GFS. The Canadian ensembles are similar in that the only precip is either off shore or comes in north of the ARB (one frame).
In summary, despite poor model performance, the only reason I think that the forecast has remained dry for the ARB is the simple absence of water; certainly most of the other ingredients would have been in place had there been a good moisture plume to work with. I still see no reason for and IOP late this week.
Looking downrange, the ensembles are still indicating a possibility of something near the 2nd or 3rd of February at the earliest. Until that time the area looks dry.
Dan Birkenheuer - ESRL/GSD