Big storms on the way for Norcal!

Filed in Uncategorized by on November 25, 2012 33 Comments

Overview

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

 

 

  • David

    sacmento is going too turn in too a big lake if this is right

    this is showing 18″ of rain

    http://raleighwx.americanwx.com/models/gfs/18zgfsp72180.gif

  • sc100

    I’m definitely more concerned after seeing today’s runs. This is the most dangerous type of pattern for California, especially since there’s virtually no break from Thursday until possibly Monday. This is looking more and more like the pattern during the 1997 floods. I’m particularly watching the storm for next Sunday, which I think will get stronger over the coming days and could be a real monster. When you add the wind into all this, it’s going to be one memorable week.

    This is as classic an atmospheric river pattern as you get. If you look at the full Pacific view you can see a very well-defined moisture tap stretching all the way back towards Japan. I’m going to be saving a lot of the data from this series of storms for future reference on atmospheric rivers.

    • This setup definitely has some major potential, though I’m not yet ready to compare it to 1997. A significant mitigating factor is that we’re heading into this from a pretty dry spell for almost the entire state except the far North Coast. Any time the GFS starts spitting out 15-20 inch rainfall totals, though, it’s time to take notice.

      • sc100

        You make a good point about the antecedent conditions. There were several big storms before the major ’96/’97 storms including a huge snow storm the weekend before, and a lot of snow was melted by the flooding storms. One of the things that stands out to me about the current setup is the consistent very high snow levels, which I don’t really remember seeing since ’97. We’ve had many extended stormy periods, but not with snow levels consistently above 6,500/7,000 feet. Norcal had a moderate flood event at the end of 2005 but I don’t think the snow levels then were as high as the current setup.

        Another thing that stands out is that the trend over the last several days has been strengthening of the pattern, whereas a lot of times the models start out overly wet and the precip forecasts end up being reigned in as the storms get closer. So I’m thinking there’s a good chance the pattern will strengthen even more over the next several days.

        So, all of this leads me to think there’s a reasonable chance this could be as bad as 1997, or close to it, but you’re right, we’re not there yet.

        • Dan the Weatherman

          At the same time it is not getting any more certain with the models whether Socal will get much out of this pattern or not as they seem to be in disagreement how far south the rain will be coming.

  • Dan the Weatherman

    Whatever actually happens later this week, I just hope that Socal can get some much-needed rain from this series of storms. I would hate for Norcal to get flooding rains, while Socal is left in the dust with nothing.

  • Nicholas

    The Euro still shows a decent compilation prize for So Cal but the GFS is not looking great and that has been the better model as of late If the Euro was right it would still be a lot better than most of the systems last winter.

    • Dan the Weatherman

      I hope the Euro remains persistent in bringing rain to the area and that the GFS comes into line with the Euro as the event draws closer.

      • I haven’t been watching the models’ handling of West Coast storms that closely lately but at least out East, the Euro has been far superior this fall.

  • GFS shifted a little more north, but still very wet for northern portions of the state. The ensembles also came in a little wetter than the last couple ensemble runs.

    • The 00z ECMWF has trended more towards that of the GFS it seems.

  • Tom in San Diego

    I’m with you Dan. Watched the gfs loop several times, but couldn’t get too excited about any prospects for this end. Anything substantial for us seems a bit away at the moment. However, it is important for all of us, especially ranchers, we get a good snow pack so im looking forward to the next week or two.

  • sc100

    AREA FORECAST DISCUSSION
    NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
    404 AM PST MON NOV 26 2012

    THE EXTENDED FORECAST WILL BECOME NOTEWORTHY AS WE MARCH FORWARD IN TIME. HIGH CONFIDENCE FOR VERY SIGNIFICANT RAINFALL FROM THURSDAY AFTERNOON THROUGH EARLY SUNDAY BUT NOT ABLE TO PINPOINT THE TIMING OF HEAVY RAIN WINDOWS AT THIS TIME DUE TO MODEL DIFFERENCES AND EVOLUTION ON HOW FRONTAL BOUNDARIES WILL PASS OVER THE REGION. THE GFS HAS SPIT OUT SOME PHENOMENAL RAINFALL TOTALS OVER THE NORTH BAY BY SUNDAY…IN EXCESS OF 15 INCHES. HOWEVER THAT REPRESENTS THE EXTREME FOR OUR DISTRICT. THE ECMWF AND GFS ON AVERAGE HAVE CONSISTENTLY BEEN SHOWING THE POTENTIAL FOR
    WIDESPREAD 4-8 INCH RAINFALL TOTALS OVER THE HIGHER TERRAIN OF THE NORTH BAY AND THE RUSSIAN RIVER BASIN. POTENTIAL IS THERE FOR BIG IMPACTS OVER OUR DISTRICT BUT GIVEN HOW MANY DAYS OUT WE ARE DON`T WANT TO GET TOO EXCITED. NEED TO SAY THAT AREAS TO OUR NORTH IN MENDOCINO COUNTY AS WELL AS THE NORTHERN SIERRA FOOTHILLS APPEAR THE MORE LIKELY LOCATION WHERE ATMOSPHERIC RIVER AND ASSOCIATED HEAVY RAINS WILL LINE UP IN THE EXTENDED RANGE.

    AT THE VERY LEAST THE FLASHY STREAMS OF THE NORTH BAY WILL LIKELY SEE IMPRESSIVE RESPONSES THIS WEEKEND BUT THE MAIN STEM SHOULD BE ABLE TO HANDLE THIS EVENT GIVEN THE CURRENT MODEL QPF AND TIMING. LATEST SOLUTIONS ARE ALSO INDICATING A BREAK IN THE WEATHER ABOUT ONE WEEK FROM NOW…WHICH WILL BE IMPORTANT TO WATCH AS THERE WILL BE SOME CONCERN FOR SETTING UP A PROLONGED WET PERIOD.

    SO FEELING HERE IS THE WEDS SYSTEM IS FAIRLY HIGH CONFIDENCE ON
    THE DETAILS WITH NO MAJOR HYDRO IMPACTS. ANOMALY DATA SUGGESTS THE WEEKEND TROUGH IS ABOUT 3.5 UNITS WHICH EQUATES TO A RETURN PERIOD OF THE TYPE OF EVENT WE EXPECT ABOUT ONCE A MONTH DURING THE WINTER. WILD CARD WILL BE IF THE BOUNDARY STALLS IN ANY ONE PLACE FOR A PROLONGED PERIOD OF TIME. LOTS MORE TO COME ON THIS. WILL GET THROUGH THE WEDS SYSTEM FIRST AND THEN HONE IN ON THE LATE WEEK/WEEKEND SYSTEMS.

  • URGENT – IMMEDIATE BROADCAST REQUESTED
    FLOOD WATCH
    NATIONAL WEATHER SERVICE SACRAMENTO CA
    1100 AM PST MON NOV 26 2012

    …PROLONGED WET WEATHER AHEAD…

    .A MAJOR SHIFT IN THE WEATHER PATTERN WILL BRING WET AND WINDY
    WEATHER TO NORTHERN CALIFORNIA BEGINNING MIDWEEK AND LASTING
    THROUGH THE WEEKEND. HEAVY PRECIPITATION IS EXPECTED WITH HIGH
    RAINFALL RATES FIRST ON WEDNESDAY THEN AGAIN ON FRIDAY. DEBRIS
    FLOWS ARE POSSIBLE…ESPECIALLY NEAR AREAS BURNED IN RECENT
    FOREST FIRES. URBAN AND SMALL STREAM FLOODING IS LIKELY…WITH
    WEIR OVERFLOW AND RUNOFF THROUGH BYPASSES POSSIBLE. MAIN STEM
    RIVERS WILL RISE…THOUGH MAJOR FLOODING OF LARGER RIVERS IS NOT
    AS LIKELY DUE TO LOW LEVELS AT RESERVOIRS.

    CAZ013>019-063-064-066>069-271300-
    /O.NEW.KSTO.FA.A.0001.121128T1800Z-121202T1200Z/
    /00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
    SHASTA LAKE AREA / NORTHERN SHASTA COUNTY-
    BURNEY BASIN / EASTERN SHASTA COUNTY-NORTHERN SACRAMENTO VALLEY-
    CENTRAL SACRAMENTO VALLEY-SOUTHERN SACRAMENTO VALLEY-
    CARQUINEZ STRAIT AND DELTA-NORTHERN SAN JOAQUIN VALLEY-
    MOUNTAINS SOUTHWESTERN SHASTA COUNTY TO NORTHERN LAKE COUNTY-
    CLEAR LAKE/SOUTHERN LAKE COUNTY-
    NORTHEAST FOOTHILLS/SACRAMENTO VALLEY-MOTHERLODE-
    WESTERN PLUMAS COUNTY/LASSEN PARK-
    WEST SLOPE NORTHERN SIERRA NEVADA-
    1100 AM PST MON NOV 26 2012

    …FLOOD WATCH IN EFFECT FROM WEDNESDAY MORNING THROUGH LATE
    SATURDAY NIGHT…

    THE NATIONAL WEATHER SERVICE IN SACRAMENTO HAS ISSUED A

    * FLOOD WATCH FOR PORTIONS OF NORTHERN CALIFORNIA INCLUDING THE
    FOLLOWING AREAS…SACRAMENTO VALLEY…WEST SLOPE NORTHERN SIERRA
    NEVADA…NORTHERN SIERRA FOOTHILLS…CARQUINEZ STRAIT AND
    DELTA…CLEAR LAKE AREA…SHASTA LAKE AREA…LASSEN PARK.

    * FROM WEDNESDAY MORNING THROUGH LATE SATURDAY NIGHT

    * CONFIDENCE IS HIGH IN A PROLONGED PERIOD OF PRECIPITATION…WITH
    THE MOST CERTAINTY OF HEAVY PRECIPITATION OVER THE SHASTA AND
    FEATHER RIVER DRAINAGES.

    * DEBRIS FLOWS POSSIBLE…ESPECIALLY NEAR RECENT BURN SCARS IN
    SHASTA…BUTTE…AND PLUMAS COUNTIES.

    * FLOODING EXPECTED ON SMALLER TRIBUTARY CREEKS AND STREAMS.

    * WEIR OVERFLOW AND RUNOFF IN BYPASSES POSSIBLE.

    PRECAUTIONARY/PREPAREDNESS ACTIONS…

    A FLOOD WATCH MEANS THERE IS A POTENTIAL FOR FLOODING BASED ON
    CURRENT FORECASTS.

    YOU SHOULD MONITOR LATER FORECASTS AND BE ALERT FOR POSSIBLE
    FLOOD WARNINGS. THOSE LIVING IN AREAS PRONE TO FLOODING SHOULD BE
    PREPARED TO TAKE ACTION SHOULD FLOODING DEVELOP.

  • Nicholas

    I think this picture is summing up how winter is looking for So Cal this season (I posted this els where as well)

    http://i46.tinypic.com/o9h2rn.jpg

    • Shady Blues

      I am starting to get worried too Nick. Its almost December and we have not gotten anything noteworthy. Our weather for the most part has been dead….lifeless….down here. Its just too quiet…

      • Dan the Weatherman

        I’m getting worried due to the overall inactive nature of the weather pattern since October, as we have had only one noteworthy Santa Ana wind and only .24″ has fallen here at my place in Orange since then. I don’t think having weather this calm going this late into the season is a good sign of the overall pattern as we go into winter.

        If the weather pattern does change and we start to get more rain and Santa Ana winds in the next few weeks, then I will feel somewhat better about the overall pattern.

        • Dan the Weatherman

          The Thanksgiving to New Year’s period is notorious for having series of Santa Ana events in Socal, and that happens during both wetter and drier years. Sometimes we get into a stormy weather pattern during this time and not have too many Santa Anas, like what happened in 2010. However, if this period is calm without much Santa Ana wind activity and very little rain, then I think that spells trouble ahead.

          Remember, we have one of the most unfavorable set of factors in play this season IMO as far as getting significant rain down here: +AMO, -PDO, and warm ENSO neutral together. If just one of these would change, I would have higher confidence for a better year.

  • Ken K.

    Dan, as time goes on it is looking less favorable for us to get much if any rainfall out of the systems coming through to the north. Sure does not look good for us here in the southland. The pattern seems to like the jet stream staying north this season. Looks like dry with periods of fog are the way to go. At least the centeral and northern part of the state will get some action. They can use it.

  • sc100

    It’s interesting to see that “hose” snake around from model to model. On the 18z GFS it’s now more centered over my neck of the woods. These hoses can take on a life of their own when they start impacting land so regardless of what the models are saying at the time all areas need to be on alert as these things can move around and stall out all of the sudden. And with a hose this wet things can get bad pretty quickly.

  • Dan the Weatherman

    The good news is that there is a 50% chance of rain on Wednesday night and a chance of showers on Thursday (not sure of POP% here as it is too far out) in Orange County and that has been increased from previous forecasts. Something is better than nothing.

  • Wow. The 00Z operational GFS is suggesting a potentially serious scenario for the Bay Area Friday-Sunday. The third storm is now expected to come in further south and the associated atmospheric river may stall out over Central California. Based on current progs 48 hour rainfall totals Friday PM-Sunday PM could be in the double digits, and that’s on top of 2-4 inches earlier in the week. While this amount of rainfall is very impressive anywhere in the state, Central Coast watersheds don’t have quite the capacity of the North Coast or Central Valley rivers, and after a quick check soils are actually more saturated than I expected based on antecedent conditions.

    More immediately, Wednesday’s storm looks quite potent if brief. I will try to have another full update tomorrow.

    • sc100

      I had the same reaction. Never seen anything like it for my area. Literally 2 1/2 straight days of heavy rain without a break. This setup has the potential to be very dangerous and I’m glad my local media stations are already telling people about it. It’s pretty obvious at this point that some area of the state is going to get it really bad. We’ll just have to see where.

      • In addition…the GFS is remarkably active all the way through day 16. Current progs have a truly monstrous storm hitting BC/Washington state on day 8…let’s hope that one stays well to the north.

        • sc100

          Yeah, that storm looks incredibly huge. And it’s amazing to see the tropical plume go all the way up to southeast Alaska! It’s a really incredible pattern that’s setting up. And also really rare that this is all setting up in late November. Norcal had a big flood event in November 1950 but besides that I can’t think of another one.

  • 00z GFS came in almost as bullish as yesterday’s 18z run…
    http://www.norcalweather.net/Model%20Run%20Images/11-27-12%2000z%20GFS.png

  • A great resource to follow atmospheric rivers in action:

    http://tropic.ssec.wisc.edu/real-time/mimic-tpw/epac/main.html

  • sc100

    I just saw that the next weather-hydrology briefing from the State-Federal Flood Operations Center was moved from Thursday to tomorrow morning. So they’re obviously gearing up for this week in earnest. You have to request to get these briefings but they’ll probably have a news conference at some point, which they usually do in situations like this.

    http://www.water.ca.gov/floodmgmt/hafoo/fob/focs/weatherbriefing.cfm

  • Ken K.

    Well looks like, I will have to travel north to see any rain, back down to a 20% chance Wednesday evening and most likely no rain at all this far south. I think that Mexican high is hanging tough here.

  • AREA FORECAST DISCUSSION
    NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
    501 AM PST TUE NOV 27 2012

    IN ADDITION HAVE ADDED THUNDERSTORM CHANCES FROM
    12-18Z WEDS WITH THE FRONTAL PASSAGE. LIFTED INDICES ONLY AROUND
    ZERO BUT BIGGER CONCERN IS STRONG SYNOPTIC LIFT WITH THE SURFACE
    FRONT ITSELF VERSUS VERY COLD AIR ALOFT. WENT AHEAD AND ADDED
    POSSIBLE SEVERE WORDING BASED OFF OF LOW LEVEL DIRECTIONAL AND
    SPEED SHEAR WHICH WILL ONLY BE ENHANCED BY LOCAL TOPOGRAPHY.
    FORECASTING SEVERE WEATHER AROUND HERE IS NEVER EASY GIVEN HOW
    RARE IT IS BUT THIS SYSTEM WILL BE DYNAMIC ENOUGH TO SPIN UP SOME
    MINI-SUPERCELLS. OUR IN-HOUSE LIGHTNING DATA HAS SAMPLED NUMEROUS
    LIGHTNING STRIKES WELL OFFSHORE WITH THE FRONT…CURRENTLY
    APPROACHING 130W. IR AND WATER VAPOR SHOW WELL DEFINED SHORTWAVE THAT IS CARVING AND ROUNDING THE BASE OF THE TROUGH. THIS FEATURE WILL BEGIN TO CATCH UP TO SURFACE BOUNDARY AS IT MOVES ONSHORE AND INCREASE THE CHANCES OF CONVECTION.

  • Nicholas

    After looking at the new model runs for So Cal I would rather just go back to bed till next Winter.

  • redlands

    Nicholas — Where do u get these model runs ???? Are u saying he coming rain season is gonna be a flop ???

  • sc100

    The 12z GFS looks a little less crazy but the overall trend remains in place with a bullseye right over my area. Trying not to get too hung up on the exact details as these will change from model to model but instead focusing on the trend. The last storm in these sort of sequences is almost always the wildest so I’m expecting strengthening of the storm on Sunday and it could get quite a bit larger. Also, people are now talking about snow levels OVER 8,000 feet, which is a big deal as that’s a lot more water coming into local rivers.

    Each run is moving next week’s Tue/Wed storm further south and the GFS now has it over Norcal. Looks very wet too. So this bears a lot of watching.