Monday, March 30, 2015

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Classic April Roars In

Been enjoying those calm 70+ degree days with blue and sun. Been out cycling/running/something staring at your unused skis as you return. That's over for a few days, maybe longer. An early April Fools? NFW! Classic April roars in a day early tomorrow bringing a pattern change and Wind putting a damper on those plush warm days. The breakdown over the next couple days goes as follows:

  • Wind and slightly cooler overnight with cool fropa 
  • Wind re-intensifies with another cool front on Tuesday 
  • Another more impressive cold front on Wednesday associated with a strong shortwave
  • Some light snow and snow showers above 5000 ft on Wed/Thr can't be ruled out
  • And this is just setting the table for early next week after a brief rebound/hiatus Friday
  • Yes, a colder situation is looks promising beginning Sat. night onward, could be interesting
A rather dramatic pattern shift began this afternoon, although April is notorious for such events, with a series of shortwaves hitting the West coast. The first (#1) passing through tonight across the Pacific northwest tightening the pressure gradient and dragging cooler across Tahoe and western Nevada. Another shortwave (#2) moves across the southern Oregon on Tuesday prompting more wind for our area. And then a quite impressive wave (#3) on Wednesday night is something to watch bringing strong upper-level dynamics and much colder air (Figures 1 and 2). After a brief hiatus on Friday (Figure 3), another strong shortwave (#4) approaches Tahoe Saturday night into Sunday. In addition to strong dynamics, this time there is some moisture to work with (Figures 4 and 5). This is really something watch!

Figure 1. Illustrating shortwaves #1 and 2 via 500 mb heights/vorticity.
Figure 2. Illustrating shortwave #3 via 500 mb heights/vorticity.

Figure 3. Friday hiatus with temporary ridge, made of paper not steel, via 500 mb heights/vorticity.

Figure 4. Illlustrating impressive wave #4 for Saturday night via 500 mh heights/vorticity.

Figure 5. Illustrating some moisture associated with impressive wave #4 via precipitable water and lifted index.

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Sunday, March 22, 2015

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Too little too late

The first spring storm of the year will begin impacting the Sierra this evening with light snow accumulations below 7000 feet and up to a foot at the highest elevations along the crest.  Light and scattered snow showers will persist into Tuesday night before the ridge begins to rebuild to its former glory....

- Snow beginning this evening with snow levels dropping to lake level tomorrow morning.
- Light accumulations below 7000 feet with generally 3- 8 inches above and up to a foot along the highest peaks of the crest. 
- Snow amounts will drop off south of Tahoe and increase as you head north.
- This storm will do little to save the rapidly disappearing snow pack.

Current storm is slowly getting its act together and is spreading some showers into Northern California.  Not much in the way of moisture with this storm as there is no significant subtropical tap as can be seen in Figure 1. 

Figure 1: Forecasted precipitable water (the amount of precipitation that would occur if all of the moisture in the column was wrung out) for this evening at 1700 in centimeters.
The dynamics are also rather benign with a very weak trough passing to the north of Tahoe along the CA/OR border.  However, a respectable shot of cold air  (for this season...) will accompany this system (see Figure 2) and allow the snow level to fall to Lake level as 700 mb temps drop to around -6 C.

Figure 2: 700 mb (~10,000 ft) temperature (colored contours) in Celsius and relative humidity (color fill) for 5:00 am tomorrow morning.
As can be seen in Figure 2, the bulk of the high humidity air is behind the -5 degrees Celsius contour which for us back to back World War champs is a chilly 23 degrees Fahrenheit. This will allow the majority of the precipitation to fall as snow down to about 7000 feet providing a fresh new coating of white. However, due to the lack of moisture and dynamics, that coating will only be enough to cover small obstacles and fill in some bare spots.  Figure 3 shows the forecasted precipitation amounts from a high resolution model. The crest will be favored with precipitation amounts dropping off as you head south of Lake Tahoe.  Meanwhile, high elevations north of Lake Tahoe, like Mt. Shasta, look to do quite well with several feet of snow above 8,000 feet or so.

Figure 3: Forecasted accumulated precipitation (in inches) over the next 72 hours. 

Although this storm will help give everything a fresh coat, it is merely a drop in the bucket as the snow pack is disappearing at an astonishing rate.  Figure 4 shows just how early and scary the loss of snow is. 

Figure 4: Current snow water equivalent (bold blue line) and total precipitation (bold red line) from the start of the water year for various Snotel sites near Lake Tahoe.  The non-bold red and blue lines are the median (or "normal") precipitation and snow water equivalent.
The observations speak for themselves.  This winter, or lack there of, has been dismal except for at the highest elevations.  The "Spring" run off has started as much as two months early and is going to provide little relief to the already depleted water supplies across the region.

But to end on a more positive note, get out there and make some turns in the fresh snow before you either have to hike up Shasta or fly south, way south for the Summer.    

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Thursday, March 19, 2015

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Pleasant end of the week and a breezy weekend

  • Seasonable to slightly above average temperatures with a cooling trend into the weekend
  • Light winds today/tomorrow turning breezy out of the southwest
  • Possible precipitation in the northern region of CA, better chances for Tahoe on Monday

Keep those fingers crossed for some! any! Pacific moisture to make it into our parched region. A few small storms this spring could actually keep a little bit of skiing going into April or even longer if we get lucky. I recall an Amazing April where 110" of snow fell (~13" liquid); this would nearly double what we have accumulated during snow drought 2015. Fairly slim chances on this, but if we knew the answer, there would be no fun in playing the game!
 If this verifies, this weak trough might produce some of the lower snow levels experienced this winter. Yikes.

Should the Monday event be realized, we could see 6+ inches of snow fall above 7000ft. Not much help for the snowpack, but it will fill in a few skiable lines with a fresh coat of shreddability! The only place to really ski in CA, Mt. Shasta, will again be a big winner!

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Saturday, March 14, 2015

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Warm, Cloudy, and Possibly a Few Showers

Above normal temperatures will continue for much of the western U.S. with no real end in site. The plot below shows the temperature for the last 7 days at the Reno UNR weather station. We've been in the upper 60's and low 70's for the last week which is well above the normal for this time year, about 57-60 F.

You'll notice that we have had several nights with very warm lows, around 50 F. These warm nights coincide nicely with the incoming longwave radiation plot below. This is due to the abundant cloud cover, which absorbs shortwave radiation, and then emits longwave radiation towards the surface (throughout the night), keeping things much warmer than clear sky nights.

Today's clouds are due to an active, mostly meridional (south to north orientation), jet stream off the CA coast that is directed at the Pacific Northwest region. The storm shows up nicely on satellite, with a stream of moisture extending from the tropics.

Unfortunately, the main impacts for the Sierra will be strong winds tonight and tomorrow, with a few very light showers possible tomorrow. You can see that we are on the southern edge of a fairly strong 700 mb (~10,000 ft level) jet core by 18Z (10:00 AM) tomorrow morning. 
We will remain in this type of pattern for several days, with plenty of clouds, warm temps, and a better possibility of some showers (high snow level) by Tuesday or Wednesday, according to the GFS. Even if this does pan out, it won't do much for our snow pack. Get some turns while there is still snow, or do like many have already done, and make the transition to summer activities! No signs of a March Miracle at this point.

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Tuesday, March 10, 2015

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A Little Somethin'

The ridge of steel has been slowly pushing east across the Great Basin today and will take a temporary hiatus tomorrow as a mature closed low currently over the eastern Pacific pushes northeast into the WA/OR border overnight. As it does so, two small pieces of energy will break southeastward into the Sierra (Figure 1). The first piece will arrive early Wednesday morning and the second during the afternoon. Precipitation will be light on the west slope (0.45"of liquid +/-) and limited amounts east of the Crest (Figure 2). Conditions will cool down with time, but snow levels with remain above 7000 ft with light accumulation possible, particularly above 7500 ft. Over the next 24 hours, we can expected:

  • Mild and overcast this evening with increasing SW winds (breezy overnight)
  • Light rain (snow) over Tahoe below (above) 7500 ft by Wednesday morning
  • Precip will become isolated around mid day with another light pulse in the afternoon
  • Temperatures cooling through the day on Wednesday
  • 2-5 inches of snow possible above 7500 ft with a few wet snowflakes below 
  • Ridge builds back on Thursday with north winds keeping temps cooler
  • Rebounding temps on Friday as winds turn easterly
  • Ridging Friday-Saturday with another mild event possible by Sunday evening

Figure 1 illustrating the synoptic situation over the mid troposphere this afternoon. The departing ridge over the Great Basin with the closed low over the eastern Pacific and the 2 small pieces energy expected to arrive over the Sierra Wednesday.

Figure 2 illustrating the liquid precipitation expected over the Sierra through Thursday.

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Sunday, March 8, 2015

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Recapping a Bummer (Future?) Winter

Well it's March now and if you felt like we didn't really have a winter (December through February), you are somewhat correct. The Sierra and the Cascades had a warm winter as evident in Figure 1. The Cascades received approximately their average precipitation, but as you may have noticed, we did not (Figure 2).
 Figure 1. Average temperature departure from average from December 8 through March 7. 

Figure 2. Water year (since October 1) precipitation percentage of normal. 

Record to near record winter temperatures (Figure 3) significant hurt snowfall totals leading to much below average snow-water equivalent percentages (Figure 4) and record to near record low snowpack in the Sierra (Figure 5). All of this means less spring skiing and a small window for that desired corn cycle we enjoy in March and April each year. Low snowpack equates to quicker melting, which not only hurts our spring skiing, but has major implications for water especially in a historical drought.

Figure 3. Average temperature ranks for climate divisions across the United States for December through February.

Figure 4. Snow-water equivalent percentage of normal for March 8.

Figure 5. Snowpack percentage of average for March 3 and April 1. Also shows the minimum and maximum snowpack on record. 

Is this a sign of things to come for future winters? That is the million (trillion?) dollar question. Some climate projections under climate change forecast rising snow levels. Dr. John Abatzoglou addresses this question (and others) in a great blog post located here:

Is there any good news? Possibly. We do have a weaker storm moving through this week (Wednesday-Thursday). However, as of now, it's not anything to get too excited about. Additionally,  models forecast the Madden-Julian Oscillation (MJO) to amplify and move into phases favorable for moisture transport into the western United States. However, the GFS model forecasts a larger MJO amplification than other models (Figures 6-7).

Figure 6. MJO phase diagram with GFS ensemble forecast (green lines). Phases 6-8 can be favorable for western United States moisture transport. 

Figure 7. MJO phase diagram with statistical forecasts (green, pink, blue lines). Phases 6-8 can be favorable for western United States moisture transport. 

If this occurs, we can expect some atmospheric river events for the West Coast. Those events are wet and usually warm, especially during spring. Any precipitation is welcomed especially for drought stricken California. However, it probably won't do much for the snowpack and it could actually hurt it with any rain on snow.

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Wednesday, March 4, 2015

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The Unwelcomed Guest Returns

After a welcomed moderate snowfall this past weekend, our wintertime foe, aka Omegasaurus Rexbloxus, the Ridge of Steel, the Impenetrable Wall, or the Unwelcomed Guest, returns for another extended visit for the unforeseeable future. Figure 1 illustrates the flow blocking demon reestablishing a presence.
Figure 1. 300 mb wind/height illustrating the intensifying ridge off the California coast. Courtesy of Golden Gate Weather Services.
The day/night trend ahead, at least for the next week, will be a familiar one with the following expected:
  • Rebounding daytime temperatures with mainly sunny skies
  • Light north winds becoming easterly with time 
  • Cold mornings in the valleys with moderate inversions and slow mix out
  • Pleasant and milder at higher elevations with south-facing snow softening by mid-late morning
Much attention centers on the root cause of the persistent ridging we've been experiencing throughout the winter. In fact, the last three winters have been pretty exhausting for Sierra snow enthusiasts. An overwhelmingly positive Pacific North American (PNA) pattern with a dominant ridge off the west coast of California and a trough over the eastern two thirds remains a consistent theme. PNA behavior has been linked to all the prominent intraseasonal (within season) modes of variability such as the Arctic Oscillation, North American Oscillation, and not excluding the more slowly changing Madden Julian Oscillation. Interseasonal (between season) variability such as El Nino/La Nina modulates the behavior of the former intraseasonal indices as well. The full extent of the interaction amongst these modes remains unclear despite all the teleconnection studies that have been published. The near-neutral (macroscopic) signal for El Nino/La Nina over the last three winters is the only persistent observation to be gleaned. It sure seems like there's a larger component of this puzzle at work. The elusive nature cries out to lesser understood phenomenon such as a natural external forcing mechanism (e.g., the variation in solar activity), or an internal forcing mechanism as part of the oceans?

For now, put on the sunblock and shades and enjoy our "not too shabby" snowpack while we have it!

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