Modest Pattern Change This Week

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It’s been a windy, dry, and warm day across a good chunk of northern California here this Sunday, as a deep trough digging into the central U.S. significantly tightens pressure gradients. Winds are subsiding this evening, and will continue to do so through the night for the lower elevations. Monday will be dry and warm, perhaps a bit warmer than it was Sunday, as the ridge begins to get shoved eastward – directly over northern California as a large upper low in the Gulf of Alaska begins to strengthen and extend it self southward, forcing the offshore ridge inland to make room for its expansion/strengthening.

Tuesday a fairly strong onshore breeze develops as the upper low slides southward, placing a moderately deep (~990mb) surface low off the southern British Columbia coast, with additional surface low cores further north in the Gulf of Alaska. The strong onshore flow will bring temperatures down quite a bit Tuesday – with valley highs in the mid 70s to low 80s (warmer the further south you go). A rapidly weakening front associated with the low(s) in the Gulf of Alaska slides through northern California through the (overnight Tuesday into Wednesday morning, bringing some (light) precipitation to some northern California locales. Wednesday’s temperatures will fall off another few to several degrees as colder post-frontal air drapes overhead, keeping valley temperatures confined to the upper 60s to low 70s. The cooler air aloft combined with sunshine during the afternoon Wednesday could lead to some cumulus buildups and scattered convective showers, mainly along the coastal mountains/hills, with little threat of anything significant thunderstorm wise.

GFS's forecast precipitation amounts through Thursday night. As you can see, rainfall amounts are pretty miniscule, if any for a lot of locations.

GFS’s forecast precipitation amounts through Thursday night. As you can see, rainfall amounts are pretty miniscule, if any for a lot of locations.

Several days ago this system was looking quite a bit more significant – with widespread moderate rain. Now, it appears much of northern California south of Sacramento may not see any precipitation, with spotty trace to several hundredths possible north of the Bay area and Sacramento inland. The west slope of the sierra could pick up perhaps a tenth or two, and further north in the northern mountains & northwestern portion of the state perhaps half to three quarters of an inch of rain – more notable amounts due to the front holding together and a better shot at post-frontal showers.

Days ago, models were trying to indicate potential for another system later in the week, but it now appears this system, another decent-strength mid-latitude cyclone, will rapidly weaken as it runs into our re-developed ridge overhead, likely keeping us dry.

The remains of what once was Super Typhoon Vongfong get entrained into the Gulf of Alaskan trough late in the week and re-intensify it a bit for the next week (week of the 20th). ECMWF/GFS want to rotate the upper low and associated surface lows around and southward a bit once-more and give us another shot at precipitation at some point in the week, but we have some time to wait and see what actually occurs with the large-scale pattern.

18z GEFS's forecast MSLP anomalies valid on Friday evening, at peak of re-intensification of the Gulf of Alaskan low due to the absorption of Vongfong's remnants.

18z GEFS’s forecast MSLP anomalies valid on Friday evening, at peak of re-intensification of the Gulf of Alaskan low due to the absorption of Vongfong’s remnants.

Something else I’ve noticed in the long-range GFS is a large amount of tropical cyclone activity in the central and western Pacific by mid to late-month. October and early November can be notorious for massive post-TC systems in the Pacific, and can bring quite a lashing to the west coast if currently-existent ridging can be suppressed. The ECMWF has much less activity, this is another thing we’ll just have to wait and see for.

I’ll be sure to write a much more enthusiastic and graphic-filled blog once a much stronger and more impactful storm looms for northern California. This weeks setup just isn’t very notable.

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