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HMT forecast discussion for Wednesday 5 December 2007

Our approaching shortwave trough is near 45N/150W this morning, with the separate southern wave now a nice Kona low sitting just north of Hawaii. The upper low that was over Alaska has retrograded to join with one near Japan to form a large circulation in the northern Pacific, beneath which is an elongated strong jet with max speed near 170 kts near 35N/155W. Another jet extends north on the east side of the Kona low and joins with the jet that is still present from the big plume into the Pacific Northwest earlier this week. This jet is also quite strong also with a 170 kt max near 44N/140W, with the plume of moisture just south of this jet and then extending back to the Kona low. This configuration is feeding some moisture into the approaching northern shortwave trough, and at least some connection, albeit increasingly bent and certainly nothing like what we saw in the Pacific Northwest, is forecast to be maintained until the system hits the coast later Thu. It would seem that how much of this moisture gets entrained into the approaching system is one of the wild cards. SSMI imagery shows some pretty hefty PW values still present in the plume and the trend in the latest satellite imagery loop shows some of this moisture is pumping northward just ahead of the northern shortwave. The models are coming into better agreement overall, both for 0000 UTC and 1200 UTC, and the system can be characterized as a fast-mover, not particularly large, but taking a pretty ideal track to give the most precip into the HMT area. The chance of the major precip and the system passing too far south seems unlikely based on the current jet configuration which will be driving the shortwave more eastward than southward until fairly late in the game, when the upper level ridge then builds behind the system off the West Coast, plunging the wave into the Southwest after it moves past the HMT area on Friday.

As for the model details. Good overall consensus in the 0000 UTC runs with the exception of the NAM, which was rather far south. The 500 mb upper low passes right over the HMT area in most of the runs near 1200 UTC/Fri/7 Dec, with the 700 mb low either near the HMT or to the north of the HMT. As long as the upper low is not too far north a slightly more northern position actually brings a bit stronger maximum 700 mb flow, up to 40 kts briefly in the latest 1200 UTC GFS at 0600 UTC on Friday. Pretty much everything is over by 1200 UTC Fri except for some light precip. Start time right now looks like 1500 UTC/Thu with light precip but most models have hefty precip from ~1800/Thu through ~0800 UTC/Fri. Some numbers from the 0000 UTC runs include maximum in the HMT area of 1.8" in the ECMWF, 0.7" Global Canadian (which has more of an open wave farther to the north), ~1.4" GFS. The 1200 UTC runs are looking roughly similar to the 0000 UTC runs, and are all in good agreement, even now the NAM. Timing is about the same, total values include 1.75" for the GFS, ~an inch for the Canadian Global, 1.2" but not too far away up to 4" in the Canadian Regional, over an inch in the NOGAPS, and 1.2 at BLU in the NAM but just to the south of HMT 1.7", while the UKMET has 1.2" max in the HMT, and the ECMWF just in has 1.24 at SAC, 1.32 at BLU, and a max close to 2" in the HMT area. One note on the latest ECMWF run compared to the GFS is that the 700 mb low in the GFS passes just north of the HMT, while it passes just south in the ECMWF. This brings the strongest wsw-sw flow into the HMT in the GFS, but a bit south of the HMT area in the ECMWF, which has its overall precip max of close to 3" to the south of the HMT. It would be interesting to see the NCEP window runs from the WRF at higher resolution but they have stalled out at 24 h. A look at the 0900 UTC SREF indicates a min of 0.75" and most members higher, with the Eta members most generous and ranging up to 2-3". SREF mean comes in at 1-1.25" for max precip in the HMT area. Just saw the 1500 UTC SREF and it is a bit higher in the mean, with a max more in the 1.25-1.5" range for the HMT area, and the Eta members still the wettest. Our GSD high-resolution 3 km runs are partially in. The 1200 UTC run with Schultz microphysics and simulated radar reflectivity output shows a few echoes in the higher terrain showing up by 1200 UTC, moreso by 1800 UTC, then echoes really increasing thereafter through the end of the available run at 0000 UTC/Fri, by which time max precip amounts are in the 0.2-0.5" range. Freezing levels start out pretty high, dropping to a min of 4500' by ~0600 UTC/Fri as the trough/upper low nears. Consensus on the conference call was rain shower activity in the intial phase at BLU, with a changeover to snow by 0000 UTC or perhaps a tad earlier. There was not great consensus on when the heavier precipitation will start, with some thought that it could be later than 1800 UTC. Pretty good agreement that most will be over by 0900-1200 UTC.

Longer range about as thought before. A break for a few days over the weekend into next week as the upper level ridge builds off the West Coast and Thursday's storm system digs into the Southwest and then fiinally lifts out by about Tue/11 Dec. Then a fairly active pattern with a mean trough into the Intermountain West and a threat from waves progressing across the Pacific and then diving into this trough. The first threat is by midweek next week though consensus keeps this too far north. A wild card though with this is that the Kona Low hangs out near Hawaii and in the GFS and ECMWF and then lifts north on the backside of the building ridge to become entrained into the system that hits the Coast midweek. By the end of the week another stronger wave digs more off the West Coast so bears watching. Other threats then into the following week as well, with an increasing number now of the members in the latest GFS ensemble from 1200 UTC having a potential deep and slower moving trough off the West Coast in the Wed/19 Dec time frame...certainly a long way off, but it would be a good way to end the last HMT week before Christmas!

ed szoke NOAA/CIRA/GSD