Fall-ish Storm Thursday, Widespread [Light] Precipitation Expected

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Our relatively active September weather will take a break over the next few days, but the pattern will allow a large, deep Gulf of Alaskan upper-level low with a raging 150kt+ jet streak wrapping around the base will support the development of a new wave in the base of the trough along the strong jet. The presence of strong upper-level winds and favorable conditions atmospheric conditions will combine perfectly to form a strong surface cyclone off the Pacific Northwest coast during the day Monday and into Tuesday. GFS & ECMWF nearly indicates a bombogensis scenario between early Monday and early Tuesday (12z to 12z), with a 17mb pressure drop during cyclogenesis, but is 7mb away from the 24mb bombogensis classification.

18z GFS's forecast sea level pressure and precipitable water values, valid Tuesday afternoon at 5pm.

18z GFS’s forecast sea level pressure and precipitable water values, valid Tuesday afternoon at 5pm.

Nevertheless, this rapid intensification of a surface cyclone will bring in some gustier winds to northern California later Wednesday into Thursday, as well as large swells along the west coast – perhaps with some 8 – 10 footers Wednesday through Friday. Perhaps more importantly, this system will also likely mean some rain for a good chunk of northern California, with the cold front delivering some widespread rain as it slides through northern California Thursday morning. Models have fluxuated with timing and a bit with the strength of the low, which directly affects how much precipitation we may see given the faster the low weakens/fills, the faster the cold front decays with waning upper-level support.

18z GFS's forecast 500mb wind and heights for 5pm Tuesday, at the peak of the cyclone's intensity.

18z GFS’s forecast 500mb wind and heights for 5pm Tuesday, at the peak of the cyclone’s intensity.

Moisture wise, we don’t have much to worry about. While narrow, the cold front will have a band of great moisture, with precipitable water values in excess of 1.4 – 1.5″. Lift looks moderate, with 700mb vertical velocities around -20 to -30 ub/s banking up against parts of the mountains, and -10 to -15 ub/s elsewhere. Behind the front, more unstable air slides overhead, as with most cold-core Pacific storm systems in the cool season, which may bring some additional showers and possibly some thunderstorms later Thursday morning into the afternoon, before precipitation decreases in coverage Thursday night as moisture tappers and lift weakens as the surface low itself weakens. Currently looks like the Sacramento valley will see anywhere from a few hundredths to a couple tenths of an inch of rain, with a quarter to a half inch along the west slope, and up to an inch or so in orographically favored slopes, as well as along the northwest coast.

September 21st

Models diverge right then and there – by Thursday afternoon and beyond. The ECMWF indicates more of a positively tilted trough, with an upper-low pulling back and cutting off over California…while the GFS progressively ejects everything into the Great Basin and beyond. The ECMWF holds the now closed upper-low over California into the weekend… somewhat similar to what happened this weekend (except the low was sitting just offshore). The ECMWF then brings in a surge of monsoonal moisture from the southwest, wraps it around the low, and right into northern California by Saturday and Sunday, setting up what would be another, but much more widespread shower and thunderstorm event across the region.

I’ll be posting another update Tuesday or Wednesday night, hopefully by then the weekend is becoming more clear.

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