Wednesday, February 27, 2013

Meteorological Winter vs. Astronomical Winter

Tomorrow is the last day of "Meteorological Winter" for the Northern Hemisphere.  Lets take a look at the differences in the customary "Astronomical Winter," which ends on March 20th.

First of all we use Meteorological Winter because temperatures and weather are more similar during this period and slightly more evenly distributed.  

 Astronomical Winter occurs usually right around December 20th, plus or minus a day depending on the year.  The Winter Solstice for the Northern Hemisphere is when the sun angle above the horizon is at its lowest point of the year.  It is also the exact moment the direct sun angle passes over the Tropic of Capricorn in the Southern Hemisphere(their summer)

The Spring Equinox, which typically occurs around March 20th, again plus or minus a day.  The equinox occurs when the direct sun angle passes directly over the Equator. At this point in time Earth's axis points neither towards nor away from the direct rays of the sun.

Now let's take a look at some factors that differ during these two different classifications for winter.

You'll notice that from the Winter Solstice to another 3 weeks our AVERAGE temperature (based from Climate records) in Redding bottoms out to about 45 degrees.  So that Means the coldest 3 weeks of winter, on average occurs during the first 3 weeks of official winter.  You'll notice that Meteorological Winter is more even distributed.
Same thing goes for the sun angle above the horizon at noon.  The difference between the Solstice and the Equinox is 23.5 degrees.  And Meteorological Winter is 13.5 degrees, more even distributed.

For length of Daylight, the difference is nearly 3 hours between the Solstice and the Equinox.

In conclusion if we wanted a fully evenly distributed winter, it would make more since to start winter somewhere in mid to late November.

data source:

Wednesday, February 20, 2013


First of all I want to say that there is no difference in the winds or damage potential between an EFO tornado or a landspout, so I'm not downplaying it.  Either one can be very dangerous and provide winds up to ~85 mph. There are actual cases of some landspouts reaching up to EF3 category or 136 mph wind gusts!

The American Meteorological Society defines a "Landspout" as " tornadoes occurring with a parent cloud in its growth stage and with its vorticity originating in the boundary layer.The parent cloud does not contain a preexisting midlevel mesocyclone"

Basically this means a strengthening storm picks up spin or shearing in the lower 1000m of the atmosphere and then spins up a quick landspout.   This differs from a tornado because the parent storm or supercell of a tornado organizes the shearing and helicity in the atmosphere and builds a mesocyclone.  The stretching of the mesocyclone then builds its own vortcity and pulls it to the surface.

Yesterday's "Tornado" in Gerber

Similar Landspouts in the Southern Plains of the US

Notice how all of these appear as a regular tornado but were eventually classified as landspouts.  All very cylindrical and  fattening at the base.  A lot of landspouts actually break away from the parent cloud and continue to spin until dying out.  Tornadoes do the opposite   Tornadoes weaken from the bottom up, rising back into the parent wall cloud.

here's a video of yesterday's Gerber "tornado" fast-forward to around 2:06 in the video and you can see how the landspout detaches from the parent cloud, typical of a landspout.

Let's take a look at some of the dynamics in place yesterday around 1:00 pm

CAPE values were running right around 100 joules/kg barely enough to form a thunderstorm let alone a tornado.  Big tornado out breaks in the Southern Plains typically work with around 2-3,000 joules/kg.  However we can form tornadoes here in Northern California with only 500 joules/kg.

0-1km helicity values were 0 meters squared per second squared.  So there would have been no spin for a mesocyclone to form.

0-6km shear values were near 30 knots.  We usually look for around 50-60 knots to form a supercell to eventually form a mesocyclone.

The "Signifcant Tornado Paremeter" at the time of the Gerber "tornado" was 0.  This composite brings all the ingredients for a tornado and normalizes them.  Anything above 1 and you could possible form a weak tornado. 

So where did the spin come from?  The convergence zone in the boundary layer.
at 12:54 you can see the convergence zone front has passed Red Bluff.  With winds now out of the NE.

Where at around the same time the convergence zone is still north of Chico.  Winds are still out of the SE.

So the convergence zone front is in between Red Bluff and Chico, right at Gerber by the time of the "Tornado."

So right at that front you are getting a tight counter-clockwise flow that gets stronger as you get above the surface (less friction)
This strong spin was ingested by the storm to form a Landspout.

A look back at the May 25, 2011 tornado outbreak in the Northstate, where we actually had a EF2 tornado in Butte county.  In that day we had CAPE values over 500 joules/kg (5 times as much as yesterday) and Shearing values approaching 70 knots.  That's why those were tornadoes and not landspouts.

Thursday, February 14, 2013

A Look Back at Valentine's Day 1977 "Heat" Wave

Today and tomorrow will feel like record highs in the valley floor. However, looking back to 1977 you can see just how warm we got up to on Valentine's Day.

(KRDD is the Redding Airport, records only go back
to 1986)

Here's our current set up over the next 24 hours.  Giant ridge of high pressure over the West Coast.  500 mb height levels will approach 576 dam)  Which is pretty high for this time of the year.

Now look back to 1977.   This image above is reanalysis form NOAA for Feb. 14th 1977.  We also saw a large West Coast ridge.  500 mb height levels were likely closer to 578-580 dam!

Today's 850 mb temperatures (about 4,000ft in elevation) are running near 9 degrees Celsius.  Bringing that air mass to the surface of the valley floor with condensing and warming, we are likely going to see some upper 70s today.  

Wednesday, February 13, 2013

Anomalous Warmth Followed by A Wet & Cold System Tuesday

The pattern the first 6 weeks of 2013 has been large blocking highs over the Northstate with weak systems every 10 days or so.  The latest ridge of high pressure continues to sit and amplify through the end of this work week.

(A look at Friday's anomaly afternoon highs.
You can see 10-20 degrees above normal highs)

The last 40 days the MJO has moved from phase 4 to 5 to 6 to 7 to 8 to 1 and now situated in Phase 2.  In the next few days the MJO index will move into phase 3 which is our wettest phase of the 8.  The MJO is one the biggest players in West Coast weather.

Here's a look at the all 8 phases compared to climate averages.  And you can clearly see how Phase 3 easily outweighs the other 7.

Our next system (pictured above) will rotate down from out of the North, Northwest Monday night and into Tuesday morning.  That trajectory is not the wettest route but this system will tap into a .80" PWAT plume (maybe under modeled at this time)

You can see the moisture tap out of the southwest with PWAT values (again maybe under represented) approaching .80" values.

GFS modeled precipitation by 10:00am next Tuesday morning.  850 mb temperatures would promote snow levels around 3,000ft and dropping.

GFS modeled precipitation by 4:00 pm on Tuesday, most of the Northstate under mountain snow and valley rain.

Accumulated snow with Tuesday's system.   This is extremely low-res modeling since we are still 6 days out. Look for better resolution and accuracy once we get within the range of the higher resolution forecast models.

Again extremely low-res modeling but you get the idea with total precipitation too.  Of course this image above is if everything fell in the form of rain.  I would expect maybe around a .50"of valley rain, unless we see the development of thunderstorms which would bump these totals.

Monday, January 21, 2013

Quiet Pattern Shifts To Slightly Active

For the last three weeks, thanks to a dampening MJO pattern and "Sudden Stratospheric Warming Event", the West has for the most part been situated under either amplified ridging or dry troughing.

The MJO (past and forecasted above) is now entering a somewhat amplified Phase 7.  For the last 3-4 weeks we went from no MJO to the very dry Phase 5 and 6 and now into Phase 7.

Take a look at the anomalies above for all 8 phases of the MJO.  Though not a soaking Phase, Phase 7 is a wetter pattern than the very dry phase 6-7, historically.

Something interesting that has been ingested in the models in the last 48 hours, is Air Force Flight Recon. data.  Flights went out in the Pacific to sample the southern stream of the jet stream.
From HPC......

This extra and more accurate data has changed model output drastically for both Wednesday's system and this coming Weekend's system.

Though not strikingly wet, Wednesday's system is now trending much wetter than a nearly dry system seen just 36 hours ago.

The image above is the Euro forecast model's output for Saturday's system.  You can see a positively tilted trough about to slam in to Northern California.  This should supply at least enough energy for scattered showers and storms, if not some sort frontal precipitation shield.  Something to watch the next 5 days.

Tuesday, January 8, 2013

Wednesday's System & The Warm Weather Next Week

After the wet and wild November and December, January for the most part continues to be relatively quiet.  Yes every few days or so we see a weak system but rain and snow totals for the most part remain marginal.

Tomorrow's system (Wednesday), pictured above, is a positively tilted trough diving in from out of the north, northwest.  I drew the center of the axis to show you how it is tipping over (positively tilted).  Typically with these types of troughs, the moisture tap is weak and therefore precipitation amounts are low.  We look for neutral to negatively tiled troughs for the bigger and wetter storms.  This will however bring in some much colder air though.

Besides the typical orographic aided areas, the Northstate will only see from a .10" of rain for the valley and .25-.50" of precipitation for the mountains.  Snow levels will be around 2000', dropping as the system progresses with cold air filtering in behind the initial front. 

At 12:30 tomorrow afternoon you can see the initial band of precip working through the Northstate.  

By 4:30 tomorrow afternoon all of the precipitation has already eroded out of the valley and remains only in the foothills and mountains.

The image above is the accumulated snowfall by tomorrow night.  A lot of locations see snowfall but accumulations for the most part, very light.

Now lets take a look at the big picture.

What once was looking like an El Nino winter (last summer) has quickly faded and now transitioning into a possible weak La Nina.  But even the global climate models erode it away by this spring.

Something very interesting has happened in the last 1-2 weeks.  It is called a "Stratospheric Warming Event" it is a very complicated process that I'm and still scratching my head at but basically it means a shift in the polar vortex.  What is happening is the polar vortex is splitting and shift to another part of the Northern Hemisphere   This will likely bring much colder air to the lower 48 in the next couple of weeks.

This could possibly create a double "Rex Block."  1 block off the coast of the western North America and the other over Greenland.  Take a look at the image above of one operational forecast for February.  The big globs of red are prolonged ridging of high pressure.  This will then force all the cold air to pool and slide down into the lower 48 of the United States.  The question is how close is the block to the Northstate?  Too close and we'll be mild and dry, while the rest of the US freezes.

Lets take a look at a previous "Stratospheric Warming Event."  In December of 1984 prior to the "Event" you can see the west was cold and the east was warm.  

After the "Event" you'll notice how the Lower 48 was well below normal for the month of January in 1985.

Back to our weather.  Next week we'll see the ridge developing off the coast and moving towards the Northstate.  You can see the cold air starting to slide down from out of the Arctic, Canada and approaching the lower 48.

By next Friday the ridge is still over the Northstate and Rockies and east enter a deep freeze.

The MJO wave continue to strengthen this week.  However, it is entering a dry, cooler phase for Northern California.  You'll notice the forecast (yellow/green line)  is moving into a Phase 5 and 6 in the near future.

Comparing the winter anamolies for a phase 5 and 6 shows a dry phase.  The image above has all 8 phases. Look for phase 5 and 6 and map and you'll notice the brown over Northern California, which is drier than average.

Phase 5 and 6 are actually cooler phases for Northern California so we'll see how it interacts with the "Stratospheric Warming Event."