Rather active fall pattern in the Pacific Northwest

As one might expect during a significant La Nina year, the early-season storm track is currently more active than it would normally be this time of year. A very cold system (for this time of year, anyway) is currently dropping down from the Gulf of Alaska and is bringing open-cell convective precipitation to all of coastal WA/OR and eventually the northern half of CA. Southern California will miss out on the precipitation from this system, though it will see an increase in winds and cooler temperatures, as it often does with storms of this variety. Northern Californians can expect a dramatic cool-down in many areas with increasing westerly winds tomorrow, and clouds will increase late in the day. Overnight, enough cold air moves in aloft at the 850 and 500 mb levels to initiate some convective activity, especially over the ocean and over the immediate coast. Although precip will be very scattered, isolated thunderstorms are possible and these could produce a fair amount of rainfall and even some small hail as 850 mb temps approach 1 C. Over inland areas, including the Sacramento Valley, showers will probably develop later Thursday night and persist into at least the first half of Friday. Thunderstorms may be a bit more widespread here as surface temperatures further increase lapse rates and convergent flow on the lee side of the Coastal Range enhances serves to locally enhance dynamic instability. Small hail and downpours are likely in a few locations on Friday. By late Friday afternoon, any lingering showers should be confined to the Sierras, which will, incidentally, recieve the first locally significant snowfall of the season (locally up to 6 inches or so in heavy convective showers). The pattern clears and dries for the weekend, though not dramatically so. Next week’s weather is somewhat more in doubt; the models have been indicating for some time now a fairly active northern branch of the jet stream and this may allow some storms to begin to impact CA at more southerly latitudes in the next 1-2 weeks. Stay tuned…