Very Heavy Rainfall Tonight into Sunday; Convection Also Possible...

Very Heavy Rainfall Tonight into Sunday; Convection Also Possible

FEBRUARY 8, 2014

It’s been a very wet passed 24 – 36 hours across northern California as an atmospheric river remains directed at northern California, with shortwaves traveling along the fierce 130 – 140kt jet streak blasting the west coast, and we’re not close to done.

The last, and perhaps the most dynamically strong system will slam into northern California tonight into Sunday, as the strong jet rides directly over the deep 1.3 – 1.5″ sub-tropical precipitable water plume. As the jet began to stream over top the stream of moisture through the day Saturday, the strong mid to upper-level winds combined with rich low/mid-level moisture and strong large-scale accent from a shortwave/weak surface low have supported strong cloud-top enhancement with the system’s precipitation shield. GOES-west IR satellite imagery indicated cloud top temperatures below -60c, which is quite notable for west coast storms.

The system itself consists of a shortwave trough/weak surface low, which itself isn’t expected to make landfall until Sunday afternoon/evening. Ahead of the low/vorticity max, the aforementioned intense precipitation shield will begin overspreading precipitation across northern California by this evening, and will continue to stream across the region through the day Sunday until the vorticity max behind the system moves inland.

Overnight, the precipitation band may become stationary over the central/southern Sacramento valley and adjacent foothills and mountains, potentially dumping quite a bit of rainfall in a several hour period overnight into early Sunday morning. Gusty southerly winds associated with the precipitation band will promote decent orographics/upsloping in the foothills and west slope of the sierra, even extending into the east side of the valley. From the median of the Sacramento valley east, 1 – 2.5″ of rain could fall through Sunday on top of what’s already fallen between Friday and now. In the foothills and mountains from Sonora to Butte county, 3 – 6″ could fall, with local amounts in excess of an additional 8 – 9″ being modeled by high resolution models along the west slope of the sierra.

0z 4km NAM’s forecast QPF through Sunday night.

Satellite derived estimated rainfall rates suggest hourly rates of .10 – .25″ will be possible tonight into Sunday given the very rich moisture and strong lift associated with the low, which could on top of what’s already fallen cause some urban flooding issues in poorly drained areas and in some creeks and streams.

4km NAM’s 3 hour precipitation totals between 10pm and 1am, indicating amounts of .5 – 3/4″ from Chico to Sacramento along the east side of the valley, with 1 – 1.5″ totals during the 3 hour period in the foothills and mountains adjacent to the Sacramento valley.

Sunday afternoon and evening, as the vort max comes inland, somewhat cooler air aloft (-20 to -24c at 500mb) will promote some weak instability, which combined with moderate to strong lift/orographics and decent low/mid-level moisture could lead to some convective development between noon and 9pm as the back edge of the moisture plume and strong jet slide south into central California through the overnight hours of Sunday.

Decent shear will exist from around Butte county southward beneath the strong 120 – 130kt upper-level jet, including 0 – 1km bulk shear values of 20 – 30kts and 100 – 200 M2/S1 0 – 1km SR helicity. However, given generally widespread cloud cover, believe buoyancy will be too weak to get stronger updrafts that can tap into this shear to use it at their advantage. This doesn’t mean some isolated to scattered thunderstorms can’t be ruled out though, just that the severe threat looks quite minimal at the moment. Any thunderstorms, or rather any convection that develops will be capable of immense rainfall rates and perhaps some small hail, and if a storm were to get strong enough some lightning.

I’ll monitor convective parameters through the day Sunday, and if anything changes allowing instability to become a bit stronger than currently indicated (50 – 150 j/kg SBCAPE), I’ll update the post.

Moist westerly flow continues into Sunday night, continuing showers across northern California even though main shortwave slides south. Through the early morning hours Monday, lift slowly subsides and things dry out beginning in the lower elevations, lastly in the higher elevations… however if there’s little to no wind after frontal passage Sunday night, valley fog could be an issue.

Another atmospheric river takes aim at northern California by mid to late week, but right now most of the precipitation is currently modeled to be north of SFO, though some differences among models do exist. I’ll have a post regarding this event by early in the week.