HMT Forecast

Some fog is being reported along the windward slopes of the Sierras and in the Central Valley this morning, while IR imagery shows high/middle clouds starting to stream across the N half of CA with the onset of warm advection aloft ahead of the next approaching storm system. In fact, the KDAX, KBHX and KMAX 88-Ds are already showing some echoes moving eastward… probably associated mostly with virga for the time being. The Cazadero vertically pointing S-band radar is showing clouds moving in between 4-7.5 km MSL. Meanwhile, the latest SSM/I IWV imagery reveals a developing potent atmospheric river (core values of 3.0-4.3 cm) associated with the approaching southern-branch wave extending eastward to beyond 135W. This feature should impact CA tonight and tomorrow.

Overall, the various models are still on track for the landfalling storm to impact the ARB tonight through tomorrow…. although today’s model runs seem a little faster with both the onset of precip and its cessation. On the front end, the 12Z NAM is showing precip beginning in the ARB near or slightly before 00Z this evening, i.e., before dark. The vigorous mid-trop vortex approaching the Pacific NW tonight and Thursday will phase with the southern branch wave (this is seen nicely in the 12Z NAM with the consolidating northern/southern branch jet streaks over CA on Thursday). The northern branch wave is still progged to yield substantial large-scale dynamics while the southern branch system will provide the tropical moisture. The heaviest precip will likely occur between 12Z Thursday and 00Z Friday with the strongest jet dynamics, warm advection, onshore flow, and incoming tropical moisture transport within the atmospheric river, and the weakest stratification. During this period, there will likely be strong gusty surface winds >30-40kts. The newest models now show cold fropa across the ARB as early as 06-09Z Friday (i.e., before or near local midnight), with postfrontal showers persisting until perhaps 12-15Z Friday. We are still on track for a 3-5” precip event in the ARB, with perhaps 2/3 to ¾ of that falling between 12Z Thurs to 00Z Fri. Snow levels at the onset of precip will likely be in the 6-7kft range (or greater) this evening, rise to ~8kft later tonight into tomorrow, then fall dramatically with cold fropa tomorrow night… perhaps to as low as ~4-5kft by Friday morning. A trailing vort max Saturday may generate some light precip in the area, but most of that wave energy should remain N of the region.

Medium-range and longer models are now more consistent building midtrop heights offshore of the West Coast later this weekend and well into next week. The current thinking is that some wave energy may sag far enough S early/mid next week to bring some scattered light precip to the ARB, but once again, it is likely that most of the precip will remain to our N.

Paul Neiman