Rain? It’s in the Forecast Thursday & Friday!

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Yes, you read the title right. Models are closing in on a potentially good chance of rain come Thursday as a trough sides into the west coast sending a cold front into the region.

Overall dynamics with this trough are fairly weak; however as the wave pushes downstream along a 70 – 80kt upper level jet wrapping into northern and central California, being enhanced by a 100 – 120kt jet streak along the back side of the trough, it is forecast to tap into a fairly significant sub-tropical moisture plume, with precipitable water values as high as 1 – 1.3″ pushing inland and dew points in the mid to upper 50s.

Overall, model agreement is good with the track of the trough… however the strength and accordingly, precipitation. At the time of typing out this blog, the 00z ECMWF was not available, thus I’ll only rely on it with limited confidence. Other models, including the NAM, GFS, and GEM indicate cold front passage between 10am and 3pm for most locations… with widespread showers or possibly a band of precipitation developing along/ahead of the front.

With temperatures getting into the low 70s in the valley, beneath a incoming -15 to -18c 500mb cold pool during the day Thursday, some convective development is possible both along and behind the front, though at the moment, instability looks fairly meager.

A weak mid/upper level low on the tail end of the system may pass overhead Thursday night and bring another shot at some showers, possibly even some overnight convection/thunderstorms depending where it moves overhead. High resolution NAM takes it into the central Sacramento valley with the best shot at overnight showers and thunderstorms generally north of Sacramento, and best in the north valley and northern mountains (mainly Shasta county).

Valid graphic depicting the setup for Thursday/Friday.

Valid graphic depicting the setup for Thursday/Friday.

As the cold pool slides deeper into California Friday, some more showers/convection could be possible as temperatures climb into the mid 70s or so.

Current models, mainly North American models (NAM, GFS, GEM), generate at least some light QPF across most regions from Modesto northward. A mean of these models would drop somewhere between a few hundredths to upwards of three tenths of an inch in the lower elevations (valley, delta)… with .25 – .40″ in the foothills, and locally up to half to three quarters of an inch of rain in the western slope of the sierra. In Lassen, northern Plumas, eastern Shasta, and northeast Butte counties, where orographics could be utilized, .75 – 1.5″ of rain is possible. Locally higher amounts are possibly in areas that receive thunderstorms.

00z GFS's forecast precipitation for the Thursday - Friday timeframe.

00z GFS’s forecast precipitation for the Thursday – Friday timeframe.

Things warm up and dry out over the weekend as a ridge offshore rebuilds into California. Temperatures may reach the 90s once again by Sunday. Another trough digging into the Great Basin/Intermountain West may bring some gusty winds in the Saturday – Monday time frame, but models are having some timing and track issues.

Forecast created this evening.

Forecast created this evening.

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  • micro job…

    Im obliged for your blog post.Much thanks once more. Much obliged….

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