Storm to Bring Widespread Rain Late Friday into Saturday

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It’s been a cool, showery last couple of days across northern California as a duo of systems have traversed across the region. A third, and much more moist system with a substantial subtropical moisture plume being fed into a potent westerly jet stream will slide through northern California late Friday into Saturday (morning), bringing much more widespread rain, but higher snow levels given the origin of the moisture. Some locations were lucky to pick up a few hundredths of an inch from these couple systems, for example here in Rocklin both storms totaled out to .15″, while some places west and north of Sacramento scored a quarter to half an inch. This third (and final) storm will bring more uniform/predictable precipitation, instead of the scattered and hard-to-forecast showery nature of the last two systems.

An expansive shield of moisture and clouds can already be noted on satellite imagery just about to nose into the Pacific Northwest. This train of moisture is associated with a cold front and long and fairly strong jet raging across central and eastern Pacific, which itself is associated with a large upper-level low spiraling around in the Gulf of Alaska, driving the fierce westerly jet eastward toward the western United States. Precipitable water values are forecast to be in the 1.4 to 1.6″ range along the plume of moisture, which is quite decent. Precipitation looks to begin to fill in after midnight Friday into early Saturday, with the bulk of the precipitation occurring for most areas between Redding and Sacramento between 5am and 10am, with things drying out after about 12pm from north to south.

The very strong westerly jet in excess of 150 knots over the eastern Pacific and >100kts across much of northern California will support fairly decent orographics, with a good bit of precip banking against the east side of the valley on up into the foothills and sierra (as well as much of the other higher elevations including the coastal mountains/northern mountains), whereas parts of the west side of the valley could see some rain-shadowing effects and receive less precipitation than locations further east/closer to the foothills and west slope of the sierra.


0z GFS's forecast 300mb wind and heights (across the Pacific), valid early Saturday morning.

0z GFS’s forecast 300mb wind and heights (across the Pacific), valid early Saturday morning.

Currently looks like a good bet a good chunk of the Sacramento valley from the median eastward will receive a a tenth to four-tenths of an inch of rain, with the foothills from around I-80 northward picking up about a half inch to an inch in Butte county where orographics are generally more noticeable, with one to two and a half inches of rain in the favored terrain of the northern sierra from higher Nevada county northward through Shasta county. For coastal areas: a tenth to quarter inch in the Bay Area is currently most likely, with an inch to two inches along the north coast, and upwards of three to four inches for the northwestern coastal mountains.


0z 4km NAM's forecast QPF through Saturday.

0z 4km NAM’s forecast QPF through Saturday.

Strong ridging builds in through early next week as troughing re-intensifies in the middle/eastern half of the U.S., and may continue to dominate our pattern for quite a while it currently seems, with the ECMWF keeping us dry under the ridge through the rest of the month after this last system, with the GFS also stretching to try and keep us dry. An interesting note: the CPC updated their 3-month outlook (which covers most of winter), and they’ve decided to go with a chance of above normal precipitation from the Bay Area southward, with equal chances northward, with a chance of above-normal temperature for the western third of the country. We’ll have to wait and see, and hope that their glimmer of hope for some wetter weather through the next three months or so is correct, because at this rate, though still quite early in the season, the general pattern of flimsy/weak storms aren’t cutting it.

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