HMT-West 2007: Forecast

There were some signs of a bit more potential to this system for late Wed into Thu, but signals are somewhat mixed. The 12z NAM is a little bit wetter than the 06z and 00z runs, with accumulated precipiation (which would be snow for the most part) at the higher elevations beginning a bit later than yesterday, probably 21z to 00z, but accumulating a little more. Most of the precipitation is predicted to fall in the HMT area between 00z/11 Jan (Wed evening) and 06z/12 Jan (Thursday night), with up to 0.75 inches max in the NAM, and between a third and 0.40 inches in the GFS (06z run for the GFS). The latest 12z GFS run is a little drier, however, more towards a third of an inch maximum precipitation. NCEP does two WRF 5km window runs for the western region initialized at 06z out to 48h. They are consistent with producing 0.25 to almost 0.5" in the 6-h period ending at 06z/11 Jan (the end of their run time but likely not quite the end of the precip), with timing similar to that noted above. My experience with these models in high terrain in Colorado is that they tend to be on the high side (they are run at 5 km resolution with no convective parameterization), but were quite good with some of our storms in locating the precip maxima. Our 06z run of the WRF-NMM at 3 km resolution produced total precipitation in the 0.5 to 1 inch category in a nw-se band, with the precip falling at elevations generally above 2500 feet and westward to about Truckee, basically in a pattern similar to the NAM. I noticed the forecast from Sacramento calling still for very light amounts of snow even in the mountains, but snow flurries possible to the valley floor by Thursday night. Poking around though at their interactive map finds that they are forecasting POPS up to 60 percent over the higher terrain with snowfall up to 4 inches total just west of Truckee. So certainly a cold system. Snow showery type conditions could persist into Friday as well before the upper low shifts eastward.

The amount of moisture with this system depends on how much is left of the moisture plume at the leading edge of the trough which extended all the way back towards an upper low northwest of Hawaii on Monday. This plume though continues to diminish with time, and the upper low is retreating westward to the nw of Hawaii. So the amount of moisture in the mountains will depend on how much is left in this band/frontal zone when it passes across the area and gets squeezed out initially with increasing westerly flow in the band and then for a period of time in strong cold air advection behind the 700 mb trough axis. The band of moisture begins to reach the HMT area with cooler temps at 700 mb and W to WSW flow of ~40 kts forecast at 00z/11 Jan on the NAM 12z run. The first surge of precip falls with this band, then more is squeezed out behind a strong 700 mb front which hits the HMT area about 06z/11 Jan with the winds shifting to NW on the coast but continuing more Wly over the HMT higher terrain. Most of the steady precip is over by 12z as 700 mb winds decrease considerably by then, but scattered snow continues with generally unstable conditions. In fact, the 12z NAM picks up the snow a bit near 00z/12 Jan, although the flow looks like it could go more to downslope by then but doesn't quite do so in the NAM. Finally, after the 500 mb upper level low/axis passes south and east of the HMT area by 06z/12 Jan, the precip comes to an end. In terms of the moisture plume at this time, the latest SSMI imagery at 13z shows only 1 to 1.3 g/cm2 in the band and the 00z GFS forecast by later Wed had maximum PW values around half an inch or so lurking off the coast in the band. I just noticed (but not at the time of the telcon just a while ago) a slight barclinic-looking development near 40 N/144 W in this band in the 18z water vapor imagery. Hopefully (since we are not doing anything) this isn't some kind of unanticipated enhancement that could lead to more moisture than the models are suggesting. I am always suspicious of how well the models handle these sort of things (including the one currently approaching Southern California that may bring overrunning moisture for our storm in Colorado...). At any rate, given there is a lot of time left in the experiment, probably prudent not to mess with this system.

Looking ahead, the upper level ridge holds firm just off the West Coast of North America through next week, with good agreement among the models and ensembles. However, a glimmer of hope presents itself with a potential trough, albeit that could be too weak to bring much (although this is not by any means certain), that drifts through the ridge and towards the California coast by late in the week or the weekend (~19-21 Jan). While a weak closed low is left behind from the current trough west of Hawaii, with another even weaker one to the east of Hawaii, with plenty of moisture probably plaguing the islands into next week, the system of possible interest for later next week peels off from the leading edge of the main jet moving across the Pacific (and now currently moving off of Asia) as the jet encounters the mean upper level ridge early next week (it may eventually tap into some of this moisture in place near Hawaii). The ECMWF and GFS runs are in decent agreement through Monday/15 Jan with this, then there is a difference in the solutions. The GFS drops a piece of the next wave that dives into the Intermountain West off the Southern California coast and creates a detached upper level low by late next week, effectively steering the approaching weak trough away from California, while the ECMWF is much weaker with this detached upper low, allowing for the approaching wave to maintain its structure and presumably make landfall with some possible precipitation in the HMT area next weekend (20-21 Jan). A few of the ensemble members, moreso with the Canadian ensemble, favor the ECMWF solution. The latest 12z GFS ensemble actually also has some members more in line with the ECMWF and showing the deep upper low off Southern California solution of the GFS to be somewhat of an outlier. So, maybe some hope, perhaps enhanced by the possibility of such a slow-moving system tapping into some of the moisture hanging around Hawaii.

Overall, however, it should be noted that the majority of ensemble members basically support an overall pattern of a mean upper level off the West Coast, cold weather into the CONUS, and probably not much chance of anything in the HMT area, through the 15 day period (taking us to 24 Jan), unless this system for late next week/weekend can amount to something.

ed szoke