HMT discussion for Sunday 18-March-07
Our system remains marginal for an IOP with a trend generally in the models towards less precipitation but still possibilities of around an inch in under 24 h for the higher terrain in the HMT area. Timing is about as discussed yesterday with onset of showers possibly as early as 03z/Tue but more likely after 06z, and then most of the precipitation ending by 21z/Tue to 00z/Wed, perhaps some cold advection showers continuing until about 06z/Wed. The main trend in the models today is towards more energy in the southern portion of the trough which is reflected in downstream digging much farther south, but too far to the south to benefit the HMT area. Additionally, the two plumes of moisture we were watching in the Pacific continue to diminish, with the SSMI imagery indicating barely an inch of precipitable water in the lead plume and less with the main trough, the center of which is now near 45N/145W.
As for details of the precipitation forecasts, the 12z GFS has barely 0.50 inches in the HMT area but a little bigger 0.25-.5 inch area than in the 00z run. More in the NAM in general, although right over the HMT about 0.5 inches, there is a spot with an inch to the north. The 09z ensembles (SREF) have a general area of .25-0.5 with most members in this range except a few Eta members around an inch as outliers. The 12z Canadian run is in the .5-.6 inch range. The latest GFE grids from SAC WFO have precipitation beginning around 06z/Tue with near 0.7 inches at Blue Canyon by 00z/Wed. The HPC forecast was more optimistic with almost a one inch max at the higher terrain. They noted the strong orographic flow which the models do have at 700 mb for a short period (maybe 12 h) of around 30-40 kts out of the sw through 18z/Tue then a trough passage 21-00z. Not such great flow at 850 mb, however. The potential for the orgraphic enhancement and the fact that there at least is an extensive, albeit diminishing, plume approaching (as the two plumes combine) with possible convective activity gives a little reason for optimism that could lead to local amounts over an inch. But the overall trend in the model forecasts goes the other way, so in general this remains marginal but perhaps interesting for a short-duration event.
In the longer range the very strong Pacific jet will be the main feature this week with zonal flow over 180 kts in the jet max. The lead edge of this jet reaches the west coast next weekend with a potential system probably in the Sunday/25 March timeframe. However, most of the ensemble members take the bulk of the precipitation to the north with this as it then dives into the intermountain west. This suggests another fast-moving event if it does materialize. A nice tap to the western Pacific tropical moisture early in the week with this system but then the moisture plume is stretched out tremendously by the time it reaches the west coast, so no great tropical connection is likely. Still, with such a strong jet could be potential for a decent event. Beyond this period the flow in the Pacific buckles again into an active pattern of progressive waves which tease the HMT area into April. Nothing definitive but potential, and not a strong ridge over California to say nothing, so lots to sort through tomorrow.
ed szoke NOAA/GSD