HMT wx forecast discussion: 7 February 2008 at 1945 UTC
All is quiet on the western front. The latest SSM/I and AMSU satellite imagery of integrated water vapor shows very low values (<1.5 cm) across the entire eastern Pacific basin, save a dying band of slightly enhanced PW (1.5 to near 2.0 cm) impacting the PacNW associated with the shortwave trough embedded in the racetrack northwesterly flow aloft. Companion IR and upper tropospheric vapor-channel loops have a firm lock on this shortwave and its somewhat enhanced cloudiness in WA, OR, and northernmost CA. Some light shower activity with this S/W has crossed the Oregon border into extreme N CA, but the precip will remain well N of the ARB as the S/W propagates SEward into the Great Basin today.
Ridge building aloft will commence just offshore of CA tonight and tomorrow, thus keeping the ARB in dry, subsident, NWerly flow aloft. The ridge flattens and moves inland on Saturday into Sunday, as weak S/W energy begins coming ashore into the PacNW. The 12Z NAM is farther S and a bit stronger than its 12Z GFS counterpart with the landfall of the next S/W later Sunday into Monday centered near 00Z 11. In fact, the 12Z NAM shows a 500 mb vort. max approaching the OR/CA coast at 00Z 11. Nevertheless, even with this southern-track scenario, moisture should be limited with this system… especially on its southern flank across the ARB, and the lower-level W to NW flow will be unfavorable for orographic forcing. In addition, as with the majority of our other storms this winter, temperatures will be relatively cool with this transient wave. Hence, do not expect much in the way of meaningful precip.
As this next wave drops into the Great Basin early next week, midtrop heights will once again rebound in response to ridge building in the eastern Pacific. Based on the extended model forecasts (both the control and ensemble solutions), the ridge will flatten every couple days thereafter during the landfall of transient S/W troughs along the West Coast. The GFS keeps these waves mostly N of the ARB with no significant precip., whereas the new 12Z EC model tends to dig these waves in the NWerly flow aloft (downstream of the EPAC ridge) along the West Coast, with the possibility of precip and low snow levels in the ARB. Given the history of this winter to date, I cannot discount this latter scenario from playing out. Time will tell.