2005-12-21 Daily Wx Forecast Discussion (1955UTC)
In the last 24 h, the coast ranges NW of SFO received more than 1.5” of rain. Blue Cyn in the ARB received over an inch, although almost all of that fell since 12Z this morning when an organized cloud band moved Eward across the region. Snow level observations from nearby wind profilers and vertically pointing S-band radars have been rather high, ranging from 8.5-9.5kft.
The satellite animation shows the next in a series of shortwaves making landfall in NWern CA. Its comma cloud tail extends SWward off the coast to a region of cooling cloud tops approaching CA. SSM/I observations in this region portray an enhanced SW-NE oriented PW filament making landfall near Pt Arena and about ready to merge with the stalled atmospheric river intersecting the Bay area. Not surprisingly, local PWs remain high, with Bodega Bay hovering in the 2.5-3cm range. The SSM/I also shows a coherent region of rain (rates exceeding 4 mm/h locally) closing in on 130W and, ultimately, on CA. This landfalling shortwave holds promise for producing significant precip in the ARB for the HMT06 radar crews before they need to wrap up later tonight or very early Thursday (see forecast below).
The 12Z NAM shows the aforementioned shortwave impacting the ARB this afternoon and overnight, with modest dynamics, enhanced 850-700 mb warm advection, enhanced upslope SWerly flow, and an impressive moisture feed that taps the subtropics. Precip totals will likely range between 1.5-2.5”. The midtrop flow veers with time from SWerly today/tonight to more zonal on Thursday. Weak vort spokes will be embedded in the zonal flow, so expect precip to persist tomorrow, and end by tomorrow night or Friday morning as the midtrop ridge pops.
The medium to long range outlook is confused at best, depending on which model, ensemble member, and initialization time is considered. There is a full suite of solutions, ranging from mostly ridge conditions from this weekend through next week (i.e., our HMT06 start-up period is next Thursday evening the 29th) to shortwaves beating down the ridge by early next week with transitory precip events possible through next week. I will address this issue during the resumption of HMT06 forecasts next Tuesday.
Paul Neiman