Saturday, September 22, 2018

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.

Wednesday, September 19, 2018

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 complacent 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.
If we do drop the Saffir-Simpson, there is one group that will probably complain: