NASA MODIS imagery of SoCal wildfire on 5/3/2013

Over the coming week or so, however, it does appear that a very unusual large-scale pattern will allow for some more unsettled conditions than we’ve seen in quite some time here in CA. By tomorrow, a cutoff low will retrograde over NorCal from Nevada under a rather strong Rex block. Cutoff lows are not uncommon near CA–in fact, our region is well-known for its propensity to produce these slow-moving features–but it is far from typical for a cutoff to form over the Great Basin and make it all the way off the coast of San Francisco, as is expected to happen over the next couple of days.

NAM depiction of precip early next week

As it does so, this initially weak low will deepen and entrain some moisture over the ocean, slowly increasing lift and instability over a broad region of the state. Scattered showers and probably some thunderstorms should develop over the higher terrain by Sunday afternoon, slowly becoming more widespread Monday and Tuesday. Just about every part of the state stands the chance of an isolated thunderstorm at some point this week, though most places will see very little precip and lightning is most likely in the mountains. It is possible, as sometimes happens with spring/fall cutoff lows in CA, that some mid-level instability develops and produces more widespread thunderstorm activity than currently expected. If that happens, I anticipate the possibility of significant fire weather concerns, as such storms tend to be rather dry and we’ve seen how dry conditions already are throughout the week by the fires that have occurred. Could be an interesting week ahead!

© 2013 WEATHER WEST

Well, it does appear than  a more unsettled pattern is headed toward California over the coming week–with a higher potential for meaningful rainfall in most areas than has been experienced in quite some time. A cutoff low will approach from the north and west by next weekend, and although there’s always a lot of uncertainty associated with features removed from the mean flow it does appear that this system will have sufficient moisture and potential instability to get some decent precip going over at least some parts of the state.

GFS depiction of slow-moving cutoff low near California next Saturday.

Depending on the particulars of where the low sets up and the degree of synoptic-scale forcing available, it’s also possible that there could be a fairly widespread thunderstorm outbreak. We’ll have to keep an eye on that as we get closer to the main event. Stay tuned!

© 2013 WEATHER WEST

18z GFS depiction of incoming cold low.

A small but very cold low is currently dropping  southward off the U.S. Pacific coast and will arrive in NorCal by Tuesday morning. This system is fairly moisture-starved but does have over-water trajectory so it appears there is sufficient moisture in this case to allow for significant convective activity in the presence of very steep lapse rates and some dynamic lift as the primary frontal band moves through. All the models have recently trended somewhat colder with what was already expected to be a pretty cold system, with 850 mb temps now progged to get down below -5 C over parts of NorCal and probably as low as -3 C in SoCal. With 500 mb temps in the -30s C, snow levels could actually be quite low, especially given the potential for some stronger convective downdrafts. Also worth noting is that the cold air actually precedes the onset of precipitation, which is pretty unusual for a system of this type. As a result, a larger fraction of total precipitation than usual will probably fall as snow at low elevations. I would expect snow to fall as low as 1500 feet potentially as far south as the Los Angeles area, and convective showers could bring some flakes to even lower elevations (especially in NorCal). I also anticipate the possibility of some pretty significant snow accumulations between 2000-3000 feet in many areas given the unusual cold air/precip phasing.

NAM depiction of incoming low. Note the unusual phasing of precip and cold air.

Convective activity could be fairly impressive with this system, as well. Very cold mid-level temperatures will be highly conducive to small hail (and possibly accumulations), and low-topped cells are likely to contain lightning. Severe weather potential seems pretty minimal, but I would not be surprised to see reports of remarkable local hail accumulations in a few spots.

Unfortunately, this system appears to mark only a very brief break in the meteorological monotony as a dry and calm pattern appears likely to reestablish itself by week’s end across the entire state outside of a few North Coast showers. We do need the rain now…and we’re not getting it any time soon.

Please share any photos or videos from the upcoming cold storm on the Weather West Facebook page! Share with your family and friends!

© 2013 WEATHER WEST

Water vapor imagery depicting a large area of cold air cumulus over the Eastern Pacific Ocean. (NOAA)

The primary result of this modified Arctic airmass will be to bring a dramatic cooling trend, with cold temperatures and strong winds expected  for the next several days at least. Well below freezing temperatures are expected inland, and even near-coastal regions could approach the freezing mark during the overnight hours. Moisture associated with this long wave trough is very limited, but there is enough overwater trajectory coupled with extremely steep lapse rates to generate convective showers, mainly near the coast. It’s not clear exactly how widespread convective showers may be, since there is a very large cold air cumulus field in the East Pacific headed for CA, but numerical models project only light precipitation and generally confine it to the immediate coast. Very low snow levels are already in place, however, and will continue to fall through tomorrow. Snow will fall around 500 feet along the North Coast (locally lower), around 1000 feet in the Bay Area, and possibly below 2000 feet as far south as the Mexican border. While precipitation will mostly be very light or nonexistent, it’s possible that a few locations that rarely see snow could see a dusting or maybe an inch or two in some of the higher coastal hills. This system is also poised to bring snow at sea level to both Seattle and Portland, which gives some indication of how cold this airmass is.

The deep trough will shift inland slightly over the weekend, shifting the axis of coldest air to the Great Basin region. However, a few weak disturbances will slide down the backside of the trough and could possibly produce snow at very low elevations where trapped cold air remains, which is a significant possibility given the multiple days of sub-freezing overnight lows expected. It’s still too early to tell which regions might be affected, but it’s certainly worth keeping an eye on over the next couple of days. The other effect of this deep trough will be to induce very steep pressure gradients between SoCal and the Great Basin region, which could produce some very strong offshore winds this weekend. Again…it’s still a bit early to tell, but this could end up being a pretty significant cold wind event. Stay tuned!

© 2013 WEATHER WEST

Winter has gotten off to a pretty active start across much of California. A series of major storms–including an impressive atmospheric river event–brought some flooding to Norcal at the beginning of December, but fortunately a longwave pattern shift brought an end to the persistent heavy rains and prevented some of the more serious hydrological problems that had been feared. This series of storms have pushed Water Year-to-date precipitation totals well above normal for some parts of Norcal, while Socal remains modestly below average for this time of year. The flow over the far Eastern Pacific and California has once again become very active and is expected to stay that way for the foreseeable future.

Water-Year-To-Date precipitation across California. Most of Norcal is above average; most of Socal is below average.

Short Term

An ongoing series of cold storms has been impacting mainly the northern portions of the state for the past several days. Cold air trapped in the Central Valley behind one of these systems brought accumulating snow to the Sacramento Valley floor near Redding on Saturday and as far south as Red Bluff on Sunday morning, which is a pretty unusual occurrence. Additional snow could fall locally between 500-1000 feet with the midweek storm, and snow levels elsewhere will also drop to fairly low levels (perhaps a bit below 2000 feet as far south as the Bay Area). However, precipitation is expected to be limited to widely scattered convective showers by the time the coldest air arrives, so I doubt there will be more than an inch or two of snow in some isolated lower elevation locations. Elsewhere in Norcal, periods of cold rain will continue for much of the upcoming week, and rain totals will start to add by by Thursday and Friday. Socal will see some precipitation, but it will be considerably lighter than in the north.

Long Term

The numerical models have been struggling tremendously with the current and near-future pattern. The prognosis beyond day 5 has been changing pretty dramatically from day to day, but the screaming message is this: an already active pattern is likely to become even more active by next weekend and to stay that way through the end of the month. The details are very fuzzy at this point, but today’s GFS and ECMWF runs are keying in on potentially strong storm systems on day 7/8 and again on days 11/12. It remains to be seen if these features still exist in tomorrow’s runs, but regardless I expect a very wet pattern to continue for the foreseeable future.

Animation of GFS jet-level winds days 5-16. Note the persistence of the C. Pac. block.

It appears that a strong high pressure region is trying to set up shop in the Central Pacific, far enough to the west of California that the persistent equatorward flow in the downstream trough will be offshore. This is a very favorable pattern for cold and moist systems from the Gulf of Alaska to impact California. While such a setup makes moist subtropical connections less likely, it increases the potential for rapidly deepening surface lows to impact the CA coast directly (bringing occasional strong winds and intense rains) and also opens the door to the possibility of low snow level events if retrogressive flow over western Canada sets up. The 00Z GFS depicts a remarkably stable and increasingly high-amplitude block straight through the end of the calendar year. Stay tuned!

© 2012 WEATHER WEST

November in California is certainly going to close on a very active note! A very moist and persistent atmospheric river, coupled with two powerful storm systems over the coming weekend, will produce extremely heavy precipitation over a wide swath of Northern California from the Bay Area all the way up to the Oregon border. Flooding is likely in affected areas, initially along small streams tomorrow and then later on the larger rivers by Sunday. Strong winds and thunderstorms will also be possible at times through Sunday afternoon. While rainfall will most likely decrease in intensity next week, a plume of enhanced moisture will continue to stream over the state and produce additional precipitation.

Short Term Discussion

The much-advertised series of increasingly powerful storm systems continues over California at this time. A strong cold front is currently situated across the North Coast region, and is bringing heavy rain, strong winds, and more recently strong thunderstorms to a broad swath of Norcal this evening. Rain from this storm has just reached the Bay Area and will continue to intensify overnight into tomorrow morning. With soils saturated from the quick-hitting but intense rainfall earlier this week, flooding problems will start to crop up overnight on the faster-responding tributaries and in urban areas since the cold front and associated atmospheric river will be slow to move away to the southeast. Winds will not be extremely strong with this storm, but they will potentially be strong enough to start causing minor damage now that soils are completely saturated. Given the recent outbreak of strong thunderstorms along the North Coast and favorable thermodynamic profiles that will persist for much of tomorrow around the Bay Area, I think strong convection is a possibility along and behind the front tomorrow.

Satellite-assimilated precipitable water imagery showing the developing atmospheric river.

The main flooding concern, however, is focused on the Saturday/Sunday system. The models have been very consistent in bringing a very strong and extremely moist storm somewhere into Norcal this weekend and have been generating phenomenal rainfall totals over a broad area (at various points the coarse-resolution GFS has been spitting out aerial averages of 15-20 inches through Sunday). The question has been precisely where the 200-300 mile wide axis of extreme precipitation will set up and cause serious flooding problems. For the past several days, the GFS was targeting the Bay Area with the Saturday/Sunday system; recent runs have shifted the highest precip values slightly north by 50-100 miles into Mendocino County. It’s possible the actual precip maximum could shift another 100 miles in either direction by the time all is said and done; regardless, much of Northern California is slated to receive an amount of rainfall that will certainly cause creeks and streams to overflow and very possibly some of the mainstem rivers as well. I think that event-total precipitation through Sunday afternoon will probably top 20 inches in orographically-favored areas of the Coastal Mountains in Mendocino County and also in the Shasta Drainage region. Snow levels will be quite high through Sunday–generally above 7000 feet–so nearly all of this precipitation will fall as rain and this will contribute to even higher runoff rates than might otherwise occur. And, as with the rest of the systems this week, gusty winds and thunderstorms will once again be possible with the Sunday storm.

Visualization of the evolution of the persistent atmospheric river (NOAA/ESRL).

Long Term Discussion

There is presently some substantial uncertainty regarding the prognosis for next week. The Sunday storm was initially slated to be the last in the series, with a pronounced drying trend helping the state to dry out during the first week in December. The ECMWF model was the first to suggest that the active pattern might continue into next week, and today the GFS started to trend towards the recent ECMWF forecasts. The 18Z GFS actually brought  heavy precipitation back to Norcal by Tuesday, while the most recent 00Z is slightly less bullish. Interestingly, what all the models appear to be keying in on is the continued persistence of our friendly atmospheric river, which by early next week will stretch from just west of the Hawaiian Islands to the coast of Northern California. It’s important to keep in mind that the presence of an atmospheric river does not necessarily mean that extreme precipitation will occur: while the extremely moist atmosphere that characterizes an atmospheric river provides one ingredient necessary for heavy precipitation, a strong synoptic-scale storm system is usually required to generate enough lift to cause significant precipitation (as will occur this weekend). The GFS is not currently indicating the there will be any large-scale storm features to act upon the very moist plume next week, so precipitation forecasts at this time are relatively modest. This is a situation that bears watching, though, since it wouldn’t take much to produce some additional heavy rainfall in the presence of an ongoing atmospheric river event. Stay tuned!

© 2012 WEATHER WEST

A dramatic change to an extremely active weather pattern is currently unfolding over the Eastern Pacific. The first in a series of powerful storm systems will approach the California coast tonight, bringing a brief period of heavy rain, high winds, and possibly thunderstorms Wednesday morning. Additional storms will continue to slam into the coast through the upcoming weekend, bringing additional periods of very strong winds and heavy rainfall. An atmospheric river event could bring extremely heavy precipitation to a relatively small area by this weekend, possibly resulting in significant flooding.

Short Term Discussion

A quick glance at satellite imagery appears to verify what the numerical models are currently depicting: a rapidly strengthening storm system 500-800 miles off the coast of California. The developing surface low is expected to move in a more northeastward fashion overnight as the associated cold front moves almost due east and approaches the coastline by dawn tomorrow. The models are projecting that this front will be quite intense as it moves ashore, accompanied by a burst of very heavy rain and a period of strong to very strong winds. There is already very strong dynamic lift along and just ahead of the front itself, and this is only expected to increase over the next 12 hours. Strong upper-level divergence, coincident with some rather impressive positive vorticity advection, support a fairly dramatic cold frontal passage tomorrow morning that may be accompanied by thunderstorms containing torrential rainfall and winds in excess of 50-60 mph.  850 mb wind fields are already quite strong–in the 50-70 kt range–so I would not be at all surprised to see some potentially damaging gusts over 60 mph gusts mix down to the surface in convection. Flooding is not a big concern with this first system since it will be very fast-moving, though that will change with subsequent storms later this week. Most of Thursday will be a “break” day except over the far north, though rain will return in earnest by evening.

Long Term Discussion

Even stronger and more impressive storm systems are slated to arrive by Friday and continue through at least Sunday. Friday’s storm will, of course, contain heavy rain and strong winds once again, but there indications that the synoptic scale feature may act on an existing plume of subtropical moisture (atmospheric river) to produce excessive rainfall in some part of Norcal on top of the intense cold frontal rains. The models have been struggling with the placement (and even existence) of this particularly enhanced band of precipitation, so there remains uncertainty regarding where it might fall and how much rain it might produce. Regardless…rainfall totals will really start to add up by Friday evening (and will probably be approaching double digits already in orographically-favored areas).

The Saturday-Sunday storm system, though, is the most concerning and potentially the strongest of the bunch. The orientation of the deep longwave trough off the coast will begin to shift by the weekend, directing parcels of positive vorticity right at NorCal. One of these disturbances will be in a very favorable position to rapidly develop a surface low as it approaches the coast on Saturday, and given the extremely moist subtropical atmosphere already expected to be in place at that point some very heavy precipitation could result. The persistent atmospheric river will be directed over Central California at this time, and may even shift back northward temporarily as the developing synoptic-scale storm system offshore matures. This could mean that the heavy precipitation band actually passes over a single location three times–first on its slow southward trek, then again as it lifts north ahead of the main front, then one final time as the front finally makes progress inland. Rainfall totals where this occurs are going to be excessive, and right now the focus of this final system is squarely on the Bay Area. Significant flooding will probably result if this scenario pans out as currently depicted. It’s also worth noting that the 12Z operational run of the GFS brings very strong winds with this final system as well, possibly stronger than the Wednesday storm.

It’s worth noting that Southern California isn’t likely to see a great deal of precipitation or wind out of any of these systems. While a soaking rain may occur as far south as Los Angeles, in general the rain/no rain (or probably more accurately the heavy rain/light rain) line will be rather sharp, as is often the case with atmospheric river-related heavy precipitation events. In any case, the best chance for some interesting weather in Socal out of this storm series probably will come with the Sunday system, as that one will dig significantly farther south along the coast than either the Wednesday or Friday storms in Norcal.

Even further out, the prognosis is pretty fuzzy. The models do generally depict a drying trend by Monday as the deep trough loses amplitude and the jet shifts northward. However, there are some indications that this break may be short-lived, with significant precipitation potentially returning by midweek (at least to northern portions of the state). Let’s get through the next week first…

© 2012 WEATHER WEST

After 1-2 year lull in powerful winter storm activity across California, a series of very strong storm systems will come ashore this week and bring widespread heavy rainfall and strong winds to the northern half of the state. There remains a considerable amount of uncertainty regarding the exact location and magnitude of rain and wind impacts, but right now it does appear that winds could be locally damaging at times over the next week or so and rainfall totals by this time next week could be extremely impressive.

Short Term Discussion

A deep longwave trough is expected to develop over the Eastern Pacific early this week, with its axis remaining around 1000-1500 miles off the coast of California. By Wednesday, a strong disturbance is expected to round the base of the deep trough and develop a surface low as it approaches the far Norcal coast. This initial system will be quite strong, bringing a period of widespread heavy rain to Norcal and probably a period of strong to very strong winds as well. The GFS is currently indicating a sub-990 mb low near the OR/CA border in relatively close proximity to a modestly strong surface high pressure area over the Desert Southwest, and a pressure gradient of this magnitude could easily produce the strongest southerly winds we’ve seen in at least a couple of years here in NorCal. A note regarding the 11/25 18Z operational GFS: I’m seeing 850 mb wind speeds of 65 kts, which is very impressive. While sustained winds of this magnitude will not be observed at the surface, gusts near or over 65 mph are certainly possible.

GFS depiction of a strong cold front approaching California on Wednesday (NCEP).

This system will exit the region quickly by Thursday, and it’s possible there may even be a brief dry period on that day. Soon thereafter, though, warm advection rains from the next storm will begin in earnest over the North Coast and spread quickly southward. By Friday, a long zonal fetch of very high PWs will be riding a strong low-level jet into NorCal, producing a 200-400 mile wide band of heavy to very heavy rain. This is the classic setup for a very moist “atmospheric river” event, which can produce tremendous amounts of rainfall if it remains stationary for any length of time. It’s not entirely clear where this axis of potentially excessive rainfall will set up, and there could be pretty sharp rainfall gradients north and south of the main band. I will say this, though: wherever it does set up, favored areas are almost certainly looking at double-digit rainfall totals.

Long Term Discussion

And…the parade of storms does not end there. The GFS and ECMWF are both depicting at least one more big storm capable of producing heavy rainfall and potentially strong winds next weekend, with the potential for an additional 1-2 major storms next week. Rainfall totals by the middle of next week are likely to be very impressive in Norcal, and if currently progged conditions actually come to pass there will be flooding problems at least in creeks and streams and probably in the faster-responding river basins, as well. As for Socal, there will almost certainly be rain at times over the next week or so, but it does appear that the heaviest stuff will by far be focused on the northern and central parts of the state. Towards the end of the storm series, I think there’s a good chance that one of the stronger storms will finally dig southward enough to give Los Angeles and San Diego some substantial rain and wind. That’s quite a ways out, though. In any case, it does appear that we’ll be erasing some rainfall deficits in short order…

Event-total precipitation from the 18Z GFS. Most of Norcal receives over 10 inches of rainfall in this scenario, and some regions receive over 20 inches.

© 2012 WEATHER WEST

Climate Summary

Fall thus far has been pretty typical for much of the state. Weak systems started bringing precipitation to the North Coast and as far south as the Bay Area by mid-October, and significant precipitation has now fallen as far south as the Santa Barbara area. Several convective events brought notable impacts to small regions within the state, including widespread lightning across much of Southern California on two occasions and even a tornado outbreak in the Central Sacramento Valley in early November. Water-year precipitation thus far have been below average for most of the state outside of a small region in the north-central Sierra Nevada Mountains, but deficits are running much lower than this time last year. And, as I’ll discuss shortly, any existing short-term shortfalls should be erased within the next 10 days.

ENSO Update

What had over the summer appeared to be promising indicators of an El Nino event for Winter 2012-2013 have since dwindled. Sea surface temperature anomalies are currently minimal in the main regions of interest in the tropical East Pacific, and the medium-term trend has actually been negative. It is unclear exactly why El Nino appears to have fizzled this year, and in the historical record it’s difficult to find other instances of such a dramatic reversal of what at one point appeared to be a strong signal for a modest-strength El Nino event. Dynamical and statistical models, which at one point were pointing to the imminent development of a weak to moderate El Nino event, are now depicting a return to ENSO-neutral conditions for the rest of the winter season. I’m not entirely convinced that this will actually come to fruition, since I was surprised to find significant positive subsurface temperature anomalies persisting across almost the entire Pacific basin in recent weeks. Whether these subsurface anomalies eventually propagate to the surface and initiate a coupled atmospheric response is anyone’s guess right now, though I certainly wouldn’t count on it. In short: a best guess for this winter is an ENSO-neutral environment, which means there are no meaningful signals with which to make a prediction regarding precipitation this winter in California.

Short Term Discussion

It does appear that California is about to enter a period of active weather over the next 5-10 days and possibly lasting for much of the rest of November. Present model forecasts depict a rather strong but very slow-moving low off the coast by mid-week, bolstered by strong upper-level divergence and a strong jet streak on the east side of the system. However, southwesterly flow ahead of the low is expected to be quite dry over California, so even as a 160 kt+ jet streams overhead we can expect mostly  dry conditions until the low approaches the coast by Thursday or Friday. By this time, the system will have lost much of its upper-level support and should be quite unimpressive, probably bringing some modest showers to Northern and Central California.

The real story is what may be headed our way next weekend, however. The operational GFS and its ensemble members currently agree on the potential for a strong to very strong storm about seven days from now that would likely impact the entire state. Model solutions over the past several days have been consistently depicting a deep and rather sharp trough off the California coast and appear to be keying into a particular piece of energy that rides down the backside of the trough offshore and morphs into a pretty impressive storm system as it rounds the base on its final approach. Right now, there appears to be the potential for some impressively heavy rains over much of the state next weekend, and depending whether a surface low can form near the base of the trough we may be looking at strong winds as well. Very early indications are that this system could be more convectively unstable than is typical for systems at this time of year, especially given some of the very impressive vertical motion profiles currently depicted. In short: looks like our first real storm of the year is probably on the horizon. And, for what it’s worth, the current ensemble forecasts favor a trough and fairly low-latitude jet persisting across the west coast in the day 10+ period. Stay tuned!

 © 2012 WEATHER WEST

Source: NWS/NOAA

At the present time, a fairly complex and increasingly active pattern is evolving over the far Eastern Pacific. A cutoff low is currently circulating several hundred miles off of the California coast, which triggered mountain thunderstorms this evening in the moist, warm, and slightly divergent southerly flow on its eastern side. As this low begins to approach the SoCal coast over the next 48 hours, I expect the coverage of showers and thunderstorms to increase. Tomorrow afternoon, a weak perturbation and associated vorticity maximum will lift north over NorCal, likely triggering scattered convection in many places (especially over the mountains). Similar activity could occur on Tuesday, though there doesn’t appear to be a specific focusing mechanism on that day so shower/thunderstorm coverage could be somewhat more limited.

On Wednesday and Thursday, the cutoff low will move inland near the Los Angeles area, bringing a fair bit of rain and possibly thunderstorms to a wide swath of the state. At the same time, a modestly stronger and colder low will begin to approach the state from the northwest. The confluence of these two systems will bring unsettled and potentially active weather over most or all of the state into late Thursday.

Source: NCEP.

Thunderstorm potential is difficult to assess at this point given the complexity of the developing pattern, but there may be lightning observed somewhere in the state each day between now and Friday. Right now, the mountains, the Central Valley, and the coast from the L.A. Basin sourthward appear to have the highest thunderstorm potential this week, but this may change. Stay tuned!

© 2011 WEATHER WEST