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HMT discussion for Thursday 13 December 2007

Active pattern in store with a series of fast-moving waves across the Pacific, beneath a quasi-stationary upper low over Alaska, that will be affecting the ARB beginning late Sunday and continuing probably right up to the Holiday break. First glance at the 00z models raised the question of which of these systems, if any, would be worthy of an IOP. Further review and the trend in the latest models, however, and consensus from the conference call, indicates a likely event with the main questions the timing of the start and the duration. As might be expected with such a pattern, there are model differences in timing and strength of the systems as they hit the West Coast. There are two main waves of note in this fast flow. The first of these arrives on late Sun into Mon/16-17 Dec and the second right behind it Tue-Wed/18-19 Dec. The first wave at this point looks to be fast-moving with limited amplitude, but potentially decent precip, while the second is stronger. An important question is whether there will be much of a true separation between these two in terms of precip ending/starting. Trend in the models is that there would be a break between the systems late Monday perhaps into Tuesday, but probably no real end to the precip. Another question is timing for the start of significant precipitation with the first wave. Latest indications favor an early beginning to the first wave, with the potential for precip amounts Sunday night ending 12z/Mon approaching an inch in the ARB in the latest ECMWF run.

Currently wave #1 is a potent system near the Dateline at ~50N, and it rapidly cruises across the Pacific and moves into the ARB on Mon/17 Dec. Wave #2 is just entering the western Pacific. The latest GFS forecast starts precip with the first system Mon ~15z, with close to an inch at BLU by 12z/Tue/18 Dec. There is a very limited tap to some moisture that is pooled way in the sw Pacific at this time but the moisture is limited, however orographic flow should be pretty favorable. The 00z runs were variable in their moisture amount with this first wave, with the Canadian Global model appearing to be the wettest. The 00z ECMWF run is faster with the first wave but also rather moist, producing up to 0.65" in the ARB by 12z/Mon, with not a lot more by 12z/Tue. The 12z Canadian Global model has quite a bit of precip by 12z Mon with this first wave. In the 12z GFS ensembles there is good consensus of a pretty decent amount of precip with this first wave. Timing has it beginning before 12z/Mon/17 Dec, with no real break before the next system sets in. Most of the members have ~0.5" of precip by 12z/Tue, with max amounts up to about an inch. The latest 12z ECMWF just in supports the earlier onset of precip shown in the Canadian and some of the ensemble members of the GFS. Precip in the ECMWF begins about 00z/Mon, with a max of 0.82" in the ARB by 12z/Mon, then precip diminishing by ~18z/Mon. Although a fast-moving wave, there is rather strong sw flow with it of 30-40 kts at 700 mb. Snow levels look to be 4500 feet or so with this wave.

The latest 12z GFS is stronger with the second wave, and stronger than the 00z GFS, trending towards the 00z ECMWF and a good number of the 00z GFS ensemble members, although then actually dropping farther south than all other models. There is no real break in the precip in the 12z GFS, with precip picking up Tue afternoon and about another inch at BLU by 12z/Wed. More to the south as the GFS takes a more southerly track with this stronger 2nd wave, as noted more south than any of the other models. A look at the U of Hawaii site indicates a better tap into the sw Pacific moisture with this system. In the 00z GFS precip also starts up again on Tue, with another ~2" by 12z/Wed. This is more of an open wave than the latest GFS. The latest NOGAPS has similar timing to what is noted above, but the system is farther north than the GFS. The 12z Canadian Global model is also an open wave, and considerably farther north than the GFS but with overall similar timing. The latest UKMET run is also an open wave farther to the north, and a little faster than most of the other models. The 12z GFS ensembles just in support more of an open wave that does not dive as far south as the operational GFS run. By 12z/Wed as the overall event winds down the accumulated precip for both waves ranges from over 2" to closer to a half an inch in the ensembles, with most members exceeding an inch. The 12z ECMWF comes in also as an open wave but very moist, and with sw to wsw flow at 700 mb reaching 40 kts by 12z/Tue produces a good amount of precip. Snow levels probably again in the 4500 foot range, perhaps rising a tad as the wave approaches then falling by early Wed. The ECMWF 12z run is faster than the GFS is picking up the precip in earnest with the second wave, with a half-inch new in the ARB in the 12 h ending 12z/Tue. By 00z/Wed the second wave has produced up to ~1.8" max in the ARB in the latest ECMWF, with a trough passage near 06z/Wed. Almost 3" in the EC for the combined waves by the time we reach 12z/Wed.

A third but weaker wave in all the models arrives Thu-Fri/20-21 Dec. In the 12z GFS this wave does not amplify and only drops another third of an inch or so in the ARB. In the 00z ECMWF this third wave has similar timing to the latest GFS but more moisture, with about 0.75" at BLU. The latest ECMWF run is more moist as well, with ~1.4" max in the ARB between 15z/Thu and 12z/Fri, with the trough axis in central Nevada by 12z/Fri. A few of the ensemble members in the 12z GFS ensemble are a little deeper and slower with this third wave, with a trough passage near 12z/Fri/21 Dec, but in all the members it is a weaker system than the second wave. We'll have time to watch this but currently there would be a break in the precip later Wed into Thu. Spread in the longer term but overall consensus favors ridging on the West Coast after the third wave passes into Christmas week.

ed szoke NOAA/CIRA/GSD