Wet Weather by Mid-Week, Though no Drought Buster

Wet Weather by Mid-Week, Though no Drought Buster 

After a very long period of dry weather with weak storms here and there, models have locked onto a system developing in the east-central Pacific this week, kicking east off the western U.S. coast by mid-week and offshore by late-week, which should bring some more widespread precipitation to at least the northern half of the state between mid and late-week.

Currently, a cutoff low is spiraling around off the southern California coast, one of a few that have either come up from the southeastern Pacific, or east from the Great Basin. To the west of that, a large deep low about 1000 miles off the WA/OR coast is forecast to ride over the ridge sitting over the western U.S. coast, sending it into Alaska Monday.

It appears as though the tail end of the large system off the western U.S. coast may bring northern California some light precipitation late Monday into early Tuesday, and while it wouldn’t be significant, this system will open door for the next.

A piece of energy rotating around this strong, deep low in the Gulf of Alaska will dive north of Hawaii Tuesday, tapping into a plume of sub-tropical moisture. When this occurs, a tighter thermal gradient develops, and allows the system to strengthen into a more large scale trough/upper low, pushing eastward Wednesday.

12z GFS’s 300mb wind and heights valid for Wednesday morning, indicating the base of the trough off the western U.S. coast by about 1000 miles, directing strong jet stream level winds into California, Oregon, and Washington. You can also point out how far south the jet extends, where it;s likely tapping into some sub-tropical convection east of Hawaii.

It appears this sub-tropical moisture plume and band of frontal lift will begin to nose into northern California as early as Wednesday, increasing clouds and beginning the onset of precipitation across the region through the day. Precipitation is currently forecast to continue into at least Thursday before the base of the trough moves its way inland, sending the back edge of the cold front through and cutting off lift, and precipitation with that. Colder air aloft within the trough may promote some instability during the day Friday behind the front, so some convection isn’t out of the question, depending on placement of the upper-level low.

Precipitation is currently modeled to be more or less light to moderate through most of the event, however given 1.1 – 1.3″ precipitable water values it doesn’t take much variation in the strength of the upper low to affect precipitation intensity and amounts. I should know more by the next update I post on this system Tuesday.

Snow levels with this system are forecast to begin rather high due to the tropical origin of the moisture, likely around 6000 – 7000ft, but should lower to around 4000ft by Thursday as colder air behind the cold front works its way overhead.

Current model guidance indicates .25 – .75″ of rain in the valley, however models have had issues with the strength of the system and how long it sticks around, with some model runs and ensemble members dumping over an inch of rain in the valley, which does seem plausible given the significance of the sub-tropical moisture plume… but for now we can wait a couple more days for more precise/higher resolution model guidance and run to run confidence. 1 – 3″ is the mean of model indications in the foothills and mountains, with locally upwards of 4 to 5″ in orographically enhanced areas, but there could be even more if wetter solutions were to vertify.

Several GFS ensemble members’ (not all are shown) 72 hour precipitation for the Wednesday – Friday trough/precipitation. event. The members that produce the most precipitation indicate more purple, which indicates 1.5″+.

Right now, it appears we’ll remain under the cold trough into the weekend, but unsure if any additional shortwaves within the trough will bring us any additional precipitation at this time. For now, main concentration is on the Wednesday – Friday system, and I plan to have another update by Tuesday as aforementioned, when high-res models are in range, and more is known about what the weekend, and possibly even what early next week has in store.

Even if the wettest models were to be correct, it certainly won’t be enough to produce any significant drought relieve, but of course any bit we can get will help at least somewhat, but we need an exceptional amount of precipitation from here on out through our known wet season (which continues into April, and some years lingers into May/early June) to get us out of these severe drought conditions.

Precipitation required to end a drought based on a -2 PHDI through the next 3 months. This was based on conditions through December, though, and given we’re in later January and the same pattern has stuck, we may be in a more widespread -3 PHDI now, requiring even more precipitation.

Stay tuned for the next update on this system.