2006-12-14 Daily Wx Discussion & Forecast
Another 1/4 to 3/4 inches of rain fell in the vicinity of the ARB during the last 24 h, ending near local midnight. The precip likely fell in response to the passage of weak vorticity spokes and warm advection embedded in modest but moist zonal flow. The SSM/I imagery showed a narrow PW plume (~2.5 cm) sagging as far S as the ARB last night. Melting level heights could not be tagged with the nearby wind profilers or S-band radars because only shallow, nonbrightband rain fell. The Chico profiler has thusfar shown no significant signature of a Sierra barrier jet… not surprising, given that the main storm system and trailing frontal zone is only now slamming into the Pacific Northwest.
This storm is captured well in the IR satellite imagery. The circulation center is closing in on 130W to the W of WA, and major warm-advection IR enhancement covers most of WA/OR ahead of the approaching cold front and 500-mb shortwave trough. A trailing cold-frontal comma-cloud tail extends WSWward from near OR to well out over the Pacific, and it is not yet showing signs of a Sward advance. The SSM/I imagery shows the PW plume migrating Nward toward OR as the deep-tropospheric flow backs with time ahead of the approaching storm system.
So… what is going to happen in the ARB in the next couple of days? Although the region is under the influence of weak warm advection and onshore flow today, the PW plume has migrated Nward into OR, and there is little evidence of cyclonic vorticity advection today into tonight. Hence, expect little in the way of precip (<0.1-0.2”) between now and near midnight.
Overall, today’s model solutions are similar to their predecessors regarding the storm affecting the ARB late tonight/tomorrow, if a tad slower. The cold front makes its Sward advance into N CA late tonight and across the ARB by midmorning tomorrow. Orographics will become quite favorable in the vicinity of fropa, with 50+knot WSW flow at 700 mb progged to slam into the Sierras. However, the primary band of cyclonic vorticity advection at 500 mb doesn’t impact the ARB until Friday afternoon and evening. Hence, the main frontal forcing/orographics should be decoupled from the dynamics aloft tomorrow. In addition, the primary region of 850-mb moisture flux anomaly is progged to remain north of 40N during the passage of the storm. The bottom line: this storm will likely produce moderate precip for a ~24h period starting late tonight (perhaps in the 0.75-1.25” range), with one period of enhancement during fropa tomorrow morning and a second period of enhancement as the cold air and dynamics move in aloft tomorrow afternoon and evening. Freezing levels should drop markedly from about 9+kft at the start of the storm to ~4kft at the end. One potential fly in the ointment is the possibility of the cold front stalling across the ARB tomorrow, thus allowing for phasing with the dynamics aloft…. resulting in significantly more precip than the QPF fields would indicate However, if the front stalls, it would likely be S of our domain. And none of the control solutions are showing this scenario unfolding.
The trough axis and cold air quickly moves E as an open wave across CA late Friday night and Saturday, so the postfrontal orographics should abate by ~ 12Z Sat. For the remainder of the weekend, the ARB will be under the influence of drying NW flow aloft, as ridge building commences over the eastern Pacific. Dry conditions will likely prevail through at least next Monday. Thereafter, it is unclear whether the eastern Pacific ridge will remain intact or begin to erode due to the approach of shortwave energy.