Big Pattern Change Likely to Unfold This Weekend

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After countless weak to moderate-strength teaser-systems barely breaking through ridging either offshore or to our east over the last one to two months delivering relatively insignificant precipitation amounts, it appears that we may see a big shift in the pattern in time for this holiday/thanksgiving weekend, when travel is usually at it’s peak. One thing I’d like to point out is that while most of the storms we’ve seen so far have been for the most part modest, we’re off to a much better start than we were last season. It took until January in some cases for us to see the amount of precipitation we’ve seen already, but in reality it hasn’t affected (improved) our drought much. However, the potential storms looming this weekend into at least early next week could deliver significant amounts of precipitation across a good chunk of central and northern California (probably even some notable totals in southern California as well), with models remaining fairly consistent in the idea of a stormy/wet couple of storms, but the overall pattern varies by model a good bit, even though two out of three main global models (GFS & ECMWF… along with the lesser-known the Navy model). The GEM is an outlier at the moment with a dryer pattern. The NCEP Ensemble Forecast depicts a much wetter pattern during the same time frame, in correlation with a large upper-level (as well as a fairly deep surface low) somewhere off the west coast, which is in good agreement with the GFS and ECMWF.

November 24th

Currently appears the first system works its way into OR/WA Friday night into Saturday morning, bringing in the first front and widespread precipitation. Another system is poised to trail behind this first system, tap into it’s stream of moisture at some point Saturday, and strengthen offshore, continuing precipitation across northern California as a surface low deepens quickly off the California coast. The GFS and ECMWF have different timing and strength, with the ECMWF being much more bullish with the strength of the system, as well as indicating much slower progression, keeping the system around until Tuesday, while the GFS is weaker and kicks the low out by Monday morning, with quite a bit less precipitation, though still much more than we’ve seen from storms so far this season. The ECMWF has the low deepen into the higher 980s (in millibars), while the GFS keeps it in the low to mid-990s. The stronger (ECMWF) solutions seems to be supported by the ECMWF’s much more pronounced positive tilt of the upper-level trough, allowing the trough to dig a bit deeper south and keeps its positive tilt even as the low/trough begins to move inland, while the GFS begins to lessen the positive tilt into more of a neutral tilt, allowing the system to be kicked out faster.

12z GFS's forecast 6 hour precip and MSLP valid Sunday afternoon.

12z GFS’s forecast 6 hour precip and MSLP valid Sunday afternoon.

A positively tilted trough is for the most part usually considered less-favorable for big storm development, but not the case every time, and it looks like this won’t be one of those cases (in which the positive tilt hinders strengthening), at least at the moment. Currently, it appears the mid and upper-level jet will be SW to NE oriented, which is great for SW facing slopes, for instance a chunk of Butte, Plumas, Nevada, and parts of Shasta county foothills and mountains, but less favorable for the west slope of the sierra. That’s not at all to say they won’t get much precipitation if what is being modeled now is true (which is still iffy in my opinion), just not as much as they could get if the jet was more W to E oriented. In any case, current model solutions dump anywhere from 3 – 5″ in the mountains/foothills (north of Highway 50 mostly) t0 amounts in excess of 6 – 8″ in Butte/Plumas counties, if the current ECMWF were to be correct. The GFS and ECMWF both currently agree that most of the Sacramento valley would score 1 – 3″ over the course of the weekend into Monday or Tuesday, but again, if the wetter ECMWF were to be closer to correct, upwards of 4 – 5″ could fall in some parts of the east side of the valley north of Sacramento.

18z GFS's forecast QPF through next week. The ECMWF is 20 - 40% wetter depending on location.

18z GFS’s forecast QPF through next week. The ECMWF is 20 – 40% wetter depending on location.

Snow levels look pretty solid agreement wise with models, starting off pretty high Saturday and dropping off Sunday and beyond. Snow levels could lower well below pass level late Sunday into Monday, but amounts are extremely variable due to models having different amounts and coverage of precipitation at different times. Wind could also be impactful depending on placement and strength of the low – for instance a deeper low closer to the coast could bring through a powerful low-level jet – with the 12z GFS depicting a 60kt+ 850mb jet slide through with the main cold front Sunday night into Monday morning. The ECMWF was quite similar, maybe slightly weaker with the wind, but has much heavier/longer-lasting precipitation as mentioned.

I don’t want to say simply that this is going to happen, because models can have their issues and over or under-forecast certain things, and can have a tough time with big storms, for instance models have had a hard time with the Nor’easter taking shape for the eastern U.S. (they’ve got it down pretty well now, though). It’s hard to say something that hasn’t even formed yet and is just a computer simulation is going to happen, but a few indicators of potential for a more unsettled west coast are: 1). a negative PNA (Pacific/North American pattern), usually occurs when there’s troughing in the western U.S., 2). a positive AO, though a tricky one – sometimes a positive AO can lead to lots of ridging in the mid-latitudes, but can also lead to the opposite between ridges, and 3): we’re in a moderate El Nino, however this is another tricky one, because not every warm ENSO event leads to wetter weather in California (and the west in general).

This entire forecast is still up in the air, and models could most definitely significantly weaken these two systems. We’re closing in on the entire event duration (3 to 4 days) being within a week out, once it’s within that range I usually consider model forecasts more acceptable/useable, though even when you’re dealing with potent storms models can have issues up until it occurs.

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