HMT-West 2007: Forecast

Summary: still a potential IOP, latest GFS a little faster. Marginal system, with total precip around an inch or so but some uncertainty still given moisture plume lurking in the eastern Pacific.

The upper level ridge this morning is firmly established in the far west with the moisture plume way to the north and then extending into the Pacific where it meets an extensive north-south band of moisture near 145W that goes all the way south to the tropics just east of Hawaii. Some of the moisture in this north-south plume gets caught up ahead of next weeks trough and is likely what brings the initial warm precipitation later on Monday and Monday night. Our wave of interest is currently approaching the Dateline with an associated jet near 150 kts. The U of Hawaii site PW presentation from the 12z GFS indicates a very narrow plume of moisture from this system back to the southwest towards where the tropical convection discussed earlier in the week by Ed Berry has been flaring up, but it is a narrow stream and with time tends to diminish as the wave moves eastwards. Behind this system is another storm with a large jet streak now moving out of Asia near 30N that is forecast to increase in magnitude and keep our system moving. The 00z GFS has this jet near reaching the Dateline by 12z Wed/21 Mar at 180 kts, with the associated trough centered around 35 to 40N. As this system moves across the Pacific it is tapping much more into the tropical moisture, and in fact a bit of this moisture may be in the secondary wave that follows our Tue system, even though right now (as noted below) that second wave does not produce much precipitation in the HMT area. At first this jet drives into Vancouver and the Pacific Northwest late next week, but eventually (by 240 h/00z Mon/26 Mar) both the 00z GFS, ECMWF and Canadian models have a pretty good shortwave trough position just north of the HMT, although in the deterministic GFS run beyond 240 h it stays too far north to produce precipitation of note. The latest 12z GFS has this system much farther south for a potential IOP. Of course it is a long way out, and the 00z GFS and Canadian ensembles actually have quite a bit of spread by this time. The 12z GFS ensemble just coming in shows even more spread by 240 h, with no single member quite matching the deep trough shown in the deterministic 12z run.

As for the details of Tuesday's system, things are not largely changed from what we discussed yesterday. An open wave, fairly fast-moving, with a limited tap into any tropical moisture but some, including the band of moisture now sitting near 145W. This all adds up to around an inch of precipitation (about 0.80 in the 00z GFS) or so for the HMT area, with potential for more locally but still enough uncertainty that it could be less overall. The 00z model runs generally had similar timing with the trough axis reaching the California coast about 12z Tue/20 Mar, then passing the HMT area in the GFS around 18z, a little slower in the ECMWF and even a bit slower in the UKMET as well as the NOGAPS. The Canadian run is pretty close to the GFS but with less precipitation in the HMT area. The 00z GFS ensemble members are pretty similar, maybe a slight tendency to have a few more that are a little slower. The 12z GFS is a little (maybe 6 h) faster than the 00z GFS with the trough. As with the 00z solution, warm sector precipitation begins Monday after 18z apparently tapping into the plume now in place near 145W. The GFS40 forecasts only a few hundreths of an inch of precipitation by 00z/Tue in the HMT area, but close to 0.30 inches a little farther to the north. The main precipitation falls in the next 12 h, with near 0.70 inches in the HMT region but closer to an 1.25 inches to the north, all by 12z/Tue/20 Mar, with very little precipitation then falling after 12z Tue, owing to the faster trough movement. The 12z GFS ensemble forecasts are in excellent agreement for Tuesday's storm, with a minority of members a tad slower.

This latest GFS solution keeps the IOP potential on the marginal side, so suspect we will have to just update over the weekend. The wild card may be just how much moisture is left in that band now near 145W and whether the models have a good handle on this moisture. The other wild card is the trailing shortwave for Wednesday that seems to be tapping into some of the tropical moisture much farther to the west, and whether this will sag far enough south to produce anything in the HMT. The 12z GFS says no to the second wave, with enough ridge building to keep any significant moisture to the north. The 12z Canadian run is pretty similar to the GFS forecast. The 12z NOGAPS is a little slower, with the trough deepening as it enters California. For the HMT area, though, the timing of the precipitation is not a lot different than the GFS and Canadian models. Finally, the 12z ECMWF is just coming in. It is about 6 h slower than the 12z GFS with a little deeper overall trough, and then slows some as the trough axis goes east of California by 12z Wed. Trough axis passage though across the HMT area could be up to 12 h or so slower than the GFS. I don't get to see any precip output from the ECMWF but would think it could be more than the GFS given a slower movement and deeper system.

In the very long range beyond 240 h as noted earlier both the Canadian and GFS 00z ensembles and the latest 12z GFS ensembles have considerable spread, with potential for something but certainly no overwhelming agreement on any system. Hopefully Monday's long range discussion with Ed and Klaus will give us a better idea of the longer range.

ed szoke NOAA/GSD