Wet Pattern Through the Weekend, Wettest Systems in a Year

· FEBRUARY 5, 2014

Alas, a new post! I didn't want to wait until the last minute when we have a storm knocking on the door, but models have been dreadful the last few days, and still have some issues but aren't as significant as before, and I’ll work through those issues in this blog.

As you probably know, a weak to moderate strength system is taking shape offshore in the form of a developing surface low on the nose of a plume of sub-tropical moisture, allowing for precipitation enhancement along the systems front given the available lift, divergence aloft, and low/mid-level moisture… similar to that of a baroclinic leaf, minus a very strong thermal gradient and deep low.

VIS satellite imagery of a precipitation band associated with a weak offshore low.

Right now, surface low itself is contained to one large system – however a figment separates with the moisture plume tonight, allowing for the system to enhance a bit more through its impact tonight through Thursday morning or perhaps early afternoon depending on exact track. It appears the splitting low will ever so slightly deepen to around 1010 – 1012mb, compared to the broad area of 1014 – 1016mb currently present off the OR/WA/N. CA coast. The small low associated with the precipitation band is generally expected to impact the somewhere around San Francisco. Which puts the best precipitation from the head of the precipitation band somewhere in the north or central Sacramento valley south into the central San Joaquin valley, with the heaviest/steadiest rain from around San Francisco into the San Joaquin valley and adjacent mountains.

Snow levels with this system may start out around 2500 – 3000ft along the west slope, and as low as 1000 – 2000ft in the northern mountains, rising into the 3000 – 3500ft range by Thursday morning, and possibly remaining trapped between 2000- 3000ft in the northern mountains.

4km NAM’s forecast precipitation forecast valid for early Thursday morning.

This system could drop somewhere around .50” of rain in the northern San Joaquin valley, with .50 – 1” along the coast from Santa Rosa south, however 1 – 2” is possible along the northwest coast due to enhancement by the broad surface low offshore spiraling in waves of additional energy/precipitation/convection. .5 – 1” is possible in the northern mountains, with pockets of near the same in the west slope of the sierra, especially south of I-80. In the Sacramento valley, .10 – .25” seems to be a good bet for many locations, perhaps locally more near the delta.

These QPF amounts would bring anywhere from 2 – 6” of snow in the sierra, with locally 8 – 10” above 5000 – 6000ft south of Highway 50 in the central and southern sierra.

Moist westerly flow may keep showers going into Thursday afternoon or early evening before moisture cuts off ahead of a stronger system in the eastern Pacific.

This stronger system consists of two deep lows in the east-central Pacific: one in the Gulf of Alaska, and another north of Hawaii by around a thousand miles. As the lowermost low strengthens, westerlies in the western Pacific cut underneath the low, tapping into a deep sub-tropical moisture plume streaming northwest from the Hawaiian Islands.

GFS’s 300mb winds and MSLP valid for Friday afternoon, indicating a deeper low north of Hawaii, with a weaker low splitting off along the strong jet streak towards Oregon.

Overnight Thursday, the westerlies stretch out the southern low, allowing a piece to split off and eject east along the fierce 120 – 130kt jet streak towards the U.S. west coast. As the shortwave nears the coast Friday, a shield of associated precipitation slams into northern California, bringing widespread rain and potentially strong winds as a piece of the low slams into Oregon. There are still some timing and strength differences among models, but generally it looks like precipitation begins to flood inland by late morning and afternoon, possibly a bit earlier if warm air advection is strong enough to support orographically/upslope precipitation development ahead of the main band.

Precipitation from this first system along an atmospheric river of moisture continues into Saturday, though the initial moisture band and cold front may sag southward.

Friday night into Saturday, the two separate lows in the eastern Pacific merge, creating one large and deep low south of the Allusion Islands, reloading one to two more shortwaves through Sunday night, each bringing a surge of sub-tropical moisture northward in combination with strong lift along each systems’ front.

GFS’s 300mb wind and MSLP valid for Saturday evening, indicating one large, deep low south of the Allusion Islands, buckling the jet downstream with a shortwave.

Some locations may receive short breaks between systems, but quite a few locations may not see a break at all in precipitation from Friday afternoon through Sunday night or even Monday morning, until the deep low between Hawaii and Alaska starts to weaken and amplify our ridge off the California coast, which moves overhead through the day Monday, drying us out.

Snow levels will begin around 3000 – 4000ft Friday, rising to around 5000 – 6000ft Saturday, and somewhere around 7000 – 8000ft by Saturday night and Sunday as sub-tropical moisture slowly modifies leftover cold air in the mountains/sierra.

Precipitation amounts between Friday and Monday are looking significant, perhaps the most we’ve seen from a series of systems since November/December of 2012. Currently looks like the Sacramento and northern San Joaquin valley could see anywhere from one to four inches of rain (heaviest along the east side of the valley), with 3 – 6” along portions of the north-central coast between San Francisco and Eureka, along with in the foothills and mountains of interior northern California. Orographically favored slopes could see 5 – 8” of liquid precipitation, possibly more depending on the placement of each system’s precipitation band.

700mb vertical velocities plotted through the 12km NAM valid for Friday evening, indicating very strong orographic/upsloping against the east side of the valley up the western slope of the sierra, as well as against the west side of the coastal mountains. Similarly strong upward motions are forecast for each successive system impact over the weekend.

1 – 2ft of snow could fall in the sierra above 5000 – 6000ft Friday into early Saturday before snow levels rise into the 6000ft range through the weekend, which is quite a bit compared to what we’ve seen from recent storms. Over the weekend, another 1 – 2ft of wet snow is possible over the highest elevations of the sierra.

While we are indeed in a historic drought and would need several more of these types of systems to effectively put a dent in this drought, this amount of precipitation could still cause some short term minor to moderate flood and debris flow issues. The number one threat from this much precipitation would likely be in the form of mud and debris flows from this past summer’s wildfire burn scars, including that of the Rim and American fires. In addition, some street drainage systems may still have quite a bit of leaf litter from fall that hasn’t been fully washed away… this could cause some localized urban flooding. Apart from that and maybe a few creeks running high, I wouldn’t count on any issues on major streams.

Wind wise, it appears the first system, Friday’s, system could come with a band of 40 – 50kt 850mb winds, which combined with moderate to heavy rain in the precipitation shield, some 35 – 45mph gusts could be brought down to the surface between Friday afternoon into the overnight hours, remaining breezy into Saturday. Another wave on Sunday could also be accompanied by some 25 – 35mph winds depending on the strength of the shortwave. These periods of gusty winds combined with moderate to heavy rainfall could end up bringing down some trees, which could knock out power to some regions and cause damage elsewhere.

NAM’s forecast 850mb winds (and MSLP) for Friday evening, indicating a swath of 40 – 55kt winds swiping across northern California.

Currently looks like our dreaded ridge returns by next week, shifting storms into the Pacific Northwest. If things don’t get windy behind our weekend systems, valley fog could become a notable issue due to immense ground saturation. That’s something we can look into more once we are closer to next week… but first, let’s enjoy these storms we have coming.

I’ll have another update out probably later Thursday or Friday regarding the weekend train of systems with some revised timing and a QPF forecast map.