HMT Forecast 1/31/2008 19:43 UTC
This morning we are seeing the start of the next system to impact the ARB with light precipitation to the north, BLU reporting 25F and S- and Truckee with 23F and cloudy. Most locations in the ARB are reporting cloudy conditions this morning.
Offshore we have two major circulation systems in the satellite water vapor, one at 50N 169W and the second at 48N 133W, the latter is responsible for circulating the current moderate to weak moisture plume to the ARB south of its location. Last night's GFS has this plume containing about 2.5 cm TPW sweeping by the ARB by 6UT 2/1. This morning's SSMI measurements at 1/31 11ut shows this to be a reasonable estimate of TPW with a TPW max offshore ranging up to about 3cm. The GFS has a second plume of about equal intensity as the one we are looking at this morning passing by the ARB at about 15UT 2/3. Each of these is about equal in intensity according to the model, but the second appears to sweep by the region at a faster clip reducing the precipitation.
The system that is upon us today begins with mostly southerly winds ranging from 10 kts increasing to a peak of 50 kts at 03UT 2/1. It appears that we have a trof passage about 09UT 2/1 and this coincides with a sharp decrease in the precipitation rate which has a peak of 0.63 inches in 3h at 06 UT 2/1. Total precipitation for this first event appears to be about 2.13 inches (NAM) 1.72 inches (GFS). The HMT high res ensemble precipitation amounts forecast lower amounts for this system with a regional total closer to the GFS of about 1.5 inches and more to the north of the ARB and less to the south. SREF probabilities have the maximum probability of precipitation at 03UT 2/1 (about 3h earlier than this morning's NAM) and it has the event ending at about the same time (18UT 2/1). Temperatures for this episode appear mostly cold with the freezing level starting at about 900 hPa (3400ft) and remaining there throughout the event.
The next precipitation event for the area is forecast to begin 00UT 2/3 (Saturday evening local time) about 27 hours after the current episode ends. It is shorter in duration ending about 21UT 2/3 late Sunday. Temperatures for this event appear a bit warmer overall, but with freezing level starting colder than the current event, at about 950 hPa (2300ft) and rising in altitude to about 850 hPa (5000ft) at 15UT 2/3. It appears that for the mountain portions of the ARB, both this and the prior system will be snow events. The event ends with trof passage about 21UT 2/3. SREF puts the maximum of the event at about 06 UT 2/3 with the NAM showing this later with two consecutive 3h periods of 0.5 inch accumulations (liq) at 12 and 15UT 2/3. The NAM indicates this system will bring a total of about 2 inches of precipitation to the region (NAM) (1.8 GFS) maybe with a heftier punch than the current episode now starting. This is borne out in the winds which start from the SW at 30 and increase to 55kts over a deeper extent of the atmosphere peaking about 12UT 2/3. Of the two events, I would say this second one, though bringing less overall precipitation, might be more interesting of the two to study even though it is of shorter duration. I say this because the winds are at a more favorable direction for orographics and the wind speeds are greater.
After this second episode ends at 21UT 2/3, another moisture plume possibly passes through the area around 06UT 2/6, but the GFS shows the ARB may just be grazed by this event, mainly passing to its north. A strong plume then is forecast to form out west of the coast at 11UT 2/11, but appears to remain offshore. If something were able to bring this into the region, it might prove to be a very good case, but as of this time, it remains offshore. For the most part the region appears to remain dry at least as far out as 384h (2/16) at which time the GFS ensemble hints at possibly some new precipitation for the area.
Dan Birkenheuer ESRL/GSD