Winter Forecast for the Northwest

I have been under a lot of pressure to provide the winter forecast.  And the snow outlook is probably the biggest demand. 

OK, you will get what you want.

What some folks want

The skill of our forecasting models really fade after two weeks with the extended prediction models having virtually no skill months ahead of time.  The most useful tool we do have is the correlation of the temperatures of the tropical Pacific, quantified as El Nino (warm water), La  Nina (cool water) or neutral conditions (La Nada), and the weather over the Northwest.     The correlation is not perfect, but gives one an idea which way the atmospheric dice are weighted.

Let's start by look at the temperatures of the tropical Pacific, with the temperature of the Nino 3.4 area being the most closely monitored (see map).

The sea surface temperature (SST) anomaly (difference from normal) is shown below.   As you can see, the Nino 3.4 area  this spring went from colder than normal (La Nina) to weakly above normal.    Neutral or La Nada conditions area associated with the SST anomalies being between .5C and .5 C.    That is where we are now.

But it appears, we will get to at least to a weak El Nino (Nino3.4 temperature anomalies between +.5 and 1C).    One hint is that the temperature of the upper ocean is warming (see below)

And if we look at a collection of forecast models, they are pushing the temperature anomaly to about 1C above normal in the Nino 3.4 area (see below).  A weak El Nino.

 The official Climate Prediction Center prediction is for us to move from neutral to weak El Nino, with roughly a probability of 65%.

But we have an issue.  To get a significant El Nino signal, one that really changes the midlatitude weather pattern, one needs a moderate to strong El Nino.  In those circumstances, the Northwest is generally warmer than normal, the jet stream splits over the northeast Pacific and good portion of the jet stream heads south towards southern CA.

The El Nino this year appears to be a marginal one, with far more limited impacts.   Even in a weakened state, the El Nino tendency would be for warmer than normal conditions and less snow than normal.   And add to that the decadal trend to slowly increasing temperatures from global warming, and the fact that an important source of natural decadal variability, the Pacific Decadal Oscillation (PDO), recently moved into a warm phase. (see below)

So warmer than normal seems a good bet. 

Let's take a look at one of the best extended forecasting modeling systems (but still with marginal skill), the North American Multi-Model Ensemble (NMME).  I will show you the average anomalies (differences from normal) for precipitation and temperature for November through January.

Precipitation?  Drier than normal over much of the Northwest, but close to normal over western Washington.

 Temperatures?   Around .5C above normal.

The implication of all this would be a lower than normal snowpack in our mountains this winter.  Lower heating bills.  A slow fill for our regional reservoirs.   

One thing I do know...the next three days will represent the most perfect fall weather you can imagine.  Perfect day time temps, cool nights, leaves starting to turn.  My favorite season.

from Cliff Mass Weather and Climate Blog https://ift.tt/2xHIW9M

Super Convergence Zone Dumps Heavy Rain

One of the most striking peculiarities of western Washington weather is the Puget Sound convergence zone  (PSCZ), which can produce a heavy band of precipitation across northern Puget Sound and into the Cascades.

The PSCZ  results when low-level winds on the coast are westerly (form the west), move around the Olympics, and then converge together over Puget Sound, producing upward motion, clouds and precipitation.

This is exactly what occurred Saturday evening, as illustrated by the plot of surface observations at 7 PM (a large scale and close up view is shown below)

The air was relatively unstable Saturday evening and an upper level trough was producing additional uplift....so the convergence zone really revved up.    

Now let me impress you...here is a radar image around 7 PM Saturday.  Red indicates very heavy rain, and yellow is moderate to heavy. Intense convergence zone band.  Note the sharp southern edge of the rainfall, which is very typical of strong convergence zones.

There was substantial convection and thunderstorms in that heavy precipitation band, and lots of lightning was noted by the regional lightning sensors (see below-24h lightning ending 1 AM Sunday)

Seattle RainWatch combines radar and observations to provide a good estimate for rainfall.   Here is the RainWatch 24-h total ending 5 AM Sunday.  A substantial area got 1-2 inches, which even more in limited regions.  Pouring over Snohomish County but downtown Seattle was dry.   You got to love living around here.

Local rain gauges had 24-h rainfall totals that were consistent with RainWatch, with over 2 inches around Monroe.

I know someone that had a wedding celebration near Monroe...thankfully they had tents, but it was a soggy affair.

Now the really exciting part for me is how well this event was forecast....our high-resolution models was very skillful, days ahead of time.  To illustrate, here is the forecast from the UW WRF high-resolution (4/3 km grid spacing) model for the 24h precipitation ending 5 AM Sunday.  

Nice convergence zone.  It underplayed the total amounts a bit...but it was clear that Snohomish County was going to be drenched, which downtown Seattle would be dry.

from Cliff Mass Weather and Climate Blog https://ift.tt/2xC8jtn

Super Convergence Zone Dumps Heavy Rain

One of the most striking peculiarities of western Washington weather is the Puget Sound convergence zone  (PSCZ), which can produce a heavy band of precipitation across northern Puget Sound and into the Cascades.

The PSCZ  results when low-level winds on the coast are westerly (form the west), move around the Olympics, and then converge together over Puget Sound, producing upward motion, clouds and precipitation.

This is exactly what occurred Saturday evening, as illustrated by the plot of surface observations at 7 PM (a large scale and close up view is shown below)

The air was relatively unstable Saturday evening and an upper level trough was producing additional uplift....so the convergence zone really revved up.    

Now let me impress you...here is a radar image around 7 PM Saturday.  Red indicates very heavy rain, and yellow is moderate to heavy. Intense convergence zone band.  Note the sharp southern edge of the rainfall, which is very typical of strong convergence zones.

There was substantial convection and thunderstorms in that heavy precipitation band, and lots of lightning was noted by the regional lightning sensors (see below-24h lightning ending 1 AM Sunday)

Seattle RainWatch combines radar and observations to provide a good estimate for rainfall.   Here is the RainWatch 24-h total ending 5 AM Sunday.  A substantial area got 1-2 inches, which even more in limited regions.  Pouring over Snohomish County but downtown Seattle was dry.   You got to love living around here.

Local rain gauges had 24-h rainfall totals that were consistent with RainWatch, with over 2 inches around Monroe.

I know someone that had a wedding celebration near Monroe...thankfully they had tents, but it was a soggy affair.

Now the really exciting part for me is how well this event was forecast....our high-resolution models was very skillful, days ahead of time.  To illustrate, here is the forecast from the UW WRF high-resolution (4/3 km grid spacing) model for the 24h precipitation ending 5 AM Sunday.  

Nice convergence zone.  It underplayed the total amounts a bit...but it was clear that Snohomish County was going to be drenched, which downtown Seattle would be dry.

from Cliff Mass Weather and Climate Blog https://ift.tt/2xC8jtn

REX Will Dominate Northwest Weather This Week

During the past two weeks, the Northwest has enjoyed absolutely normal weather.   Typical temperatures (see plot below, purple and cyan are normal highs and lows) and nearly normal precipitation (see cumulative precipitation for the same period).

 Normal has been good for us, giving relief to our plants, restoring water to the surface soil layers, and radically reducing the water usage in urban areas such as Seattle (see below).

The occasional precipitation has been associated with transient upper-level troughs that have moved through the region, such as one moving across the Northwest this morning (see upper level, 500-hPa, ap for 5 AM this AM)

But this week something interesting is going to happen:  a very stable REX BLOCK will develop over the eastern Pacific, shutting off precipitation, leaving us with lots of sun, dry conditions, and moderate temperatures. 

It will be wonderful.

So what is a Rex Block?   It is a configuration of the atmosphere where the upper level circulation has a ridge of high pressure (or heights) north of a trough of lower pressure/heights.  Here is a schematic of such a situation.

A Rex Block is a very stable atmospheric configuration, with the ridge and trough reinforcing each other, keeping the flow pattern in place  (for reasons I talk about in my graduate synoptic/dynamics class, but won't go into here).

The ridge/high part produces descending motion and fine weather, particularly on its eastern side.  Storms are sent far northward--in the example shown, well into Alaska where they belong.

The Rex Block has nothing to do with kings or dinosaurs, but with a meteorologist named D. F. Rex, who wrote a seminal paper on this features way back in 1950 (see below).

Rex, D. F. (1950). "Blocking Action in the Middle Troposphere and its Effect upon Regional Climate". Tellus. 2 (4): 275–301.

Now that you are Rex-trained, lets look at the upper-level forecasts for this week.

11 PM on Sunday?  Rex is here!  Huge ridge of high pressure/heights over the northeast Pacific, with a low underneath.

 2 PM on Tuesday....classic Rex!

 5 AM on Thursday.   A Rex fiesta.

The REX block shifts westward on Friday (see below), allowing a trough to move southward over us late Friday night.   Some showers and cooling if it happens.

Will the REX block reestablish itself?   Stay tuned.

from Cliff Mass Weather and Climate Blog https://ift.tt/2znu8hL

REX Will Dominate Northwest Weather This Week

During the past two weeks, the Northwest has enjoyed absolutely normal weather.   Typical temperatures (see plot below, purple and cyan are normal highs and lows) and nearly normal precipitation (see cumulative precipitation for the same period).

 Normal has been good for us, giving relief to our plants, restoring water to the surface soil layers, and radically reducing the water usage in urban areas such as Seattle (see below).

The occasional precipitation has been associated with transient upper-level troughs that have moved through the region, such as one moving across the Northwest this morning (see upper level, 500-hPa, ap for 5 AM this AM)

But this week something interesting is going to happen:  a very stable REX BLOCK will develop over the eastern Pacific, shutting off precipitation, leaving us with lots of sun, dry conditions, and moderate temperatures. 

It will be wonderful.

So what is a Rex Block?   It is a configuration of the atmosphere where the upper level circulation has a ridge of high pressure (or heights) north of a trough of lower pressure/heights.  Here is a schematic of such a situation.

A Rex Block is a very stable atmospheric configuration, with the ridge and trough reinforcing each other, keeping the flow pattern in place  (for reasons I talk about in my graduate synoptic/dynamics class, but won't go into here).

The ridge/high part produces descending motion and fine weather, particularly on its eastern side.  Storms are sent far northward--in the example shown, well into Alaska where they belong.

The Rex Block has nothing to do with kings or dinosaurs, but with a meteorologist named D. F. Rex, who wrote a seminal paper on this features way back in 1950 (see below).

Rex, D. F. (1950). "Blocking Action in the Middle Troposphere and its Effect upon Regional Climate". Tellus. 2 (4): 275–301.

Now that you are Rex-trained, lets look at the upper-level forecasts for this week.

11 PM on Sunday?  Rex is here!  Huge ridge of high pressure/heights over the northeast Pacific, with a low underneath.

 2 PM on Tuesday....classic Rex!

 5 AM on Thursday.   A Rex fiesta.

The REX block shifts westward on Friday (see below), allowing a trough to move southward over us late Friday night.   Some showers and cooling if it happens.

Will the REX block reestablish itself?   Stay tuned.

from Cliff Mass Weather and Climate Blog https://ift.tt/2znu8hL

Time to Drop the Saffir-Simpson Hurricane Scale

One of the most familiar aspects of hurricane season is the constant talk about the strength of developing storms using the Saffir-Simpson scale, which ranges from Category 1 (a marginal hurricane) to Category 5 (a powerful monster).

Today's Saffir-Simpson scale is based only on the maximum SUSTAINED wind of a hurricane, the average wind over a few minutes and not the peak gusts.

As we shall discuss below, the S-S scale is an anachronism that poorly communicates the real threats accompanying hurricanes, and in fact can be quite deceptive, resulting in people being unaware of the real dangers that can threaten them.  It is also too hurricane centric, not highlighting major threats from "lesser" tropical storms and disturbances.

Hurricane Florence is a good example of the problem.  It reached the Carolina coast as a marginal category 1 storm, and few inland location experienced hurricane-force winds (74 mph or more).   But because the storm slowed to a crawl as it made landfall, very large rainfall amounts (as much as 30-40 inches over five days near the coast) occurred, producing very serious flooding.

So folks in vulnerable areas might have felt complaisant when they heard that "only"  a category 1 storm was approaching.    And even weaker tropical storms, not even considered a hurricane, can produce similar levels of precipitation, such as Tropical Storm Allison in 2001 (41 inches in Beaumont, Texas).

Hurricanes can produce serious damage in a number of ways:

1.  From the effects of the strong sustained winds and gusts damaging buildings and other structures.
2.   From storm surge, in which the hurricane winds push water up on to coastal regions and into rivers open to the ocean.  Storm surge is generally the most damaging aspects of hurricanes.
3.  Heavy rain, which can result in flooding, both near rivers and in low areas.

Storm surge is the greatest killer in hurricanes

The problem is that the Saffir-Simpson scale only quantifies the winds.

But the deficiencies of the S-S scale are far worse than that:

1.  It does not quantify the size of hurricanes, which hugely impacts the potential to do damage.  Hurricanes vary substantially in size, with the big ones obviously able to cause far more extensive damage.
2.  It does not quantify the amount of storm surge, the real killer.
3.  It does not quantify the amount of rainfall over any period.
4.  It does not quantify the level of flooding in the interior.

And it is worse than that.  

The effects of identical hurricanes can be very different depending on their speed of motion.  Slow moving storms like Harvey or Florence, did major damage because they moved very slowly, allowing big rainfall accumulations.  If they had moved more quickly, as for most storms, their effects would have been radically less.

And worse than that.

The effects of a storm can vary by terrain and coastal bathymetry (variations of water depth offshore), allowing identical storms to have differing impacts depending on where they hit.

Replacing the Saffir-Simpson Scale

The S-S scale made some sense when atmospheric and hydrological sciences were in a primitive state; when we lacked the capabilities to model and forecast the details of tropical storms and hurricanes and their impacts.

But times have changed.  Our ability to forecast hurricane tracks out nearly a week is now stunningly good.  And closer in time, we have good skill in predicting wind, rainfall, flooding, storm surge and the like, including the geographical distribution of the threats.     That is why the WeatherChannel folks and others were pushing the "catastrophic" rainfall and flooding threats during the day before landfall.

So why not drop the problematic and often confusing measure of hurricane strength (and one that neglects often-dangerous tropical storms) expressed in the S-S scale and simply warn folks of the specific threats, such as high-wind warnings, heavy precipitation warnings, storm surge warnings, flooding warnings.  And as we develop better probabilistic and uncertainty information, that can be communicated.

Forecasters can simply say a tropical storm or hurricane is approaching ("HURRICANE WARNING")  and describe the specific forecast threats and how they vary in time and space.

Here on the west coast of the U.S., we often experience the landfall of Pacific cyclones that are the equivalent in damage potential to minor or even major hurricanes, but we do well with providing the specific threats, without any categories.    Categories for tornadoes (the Fujita scale) are probably fine, since tornadoes are only associated with one type of threat (wind damage).

Probably OK.

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