Friday, September 25, 2015

The Affect of Western Pacific Typhoons on Weather in the West

Western Pacific typhoons are not always just a curious side note. In fact, they can affect the storm pattern over our region significantly, especially in the fall and winter. If a tropical system over the western Pacific gets absorbed into the storm flow at the right location, it can amplify/energize the pattern, potentially leading to stronger storms over North America.
Check out the video** below, which shows a typhoon (red symbol) just before absorption followed by the movement of the energy (shaded area) downstream after the typhoon is ingested into the flow. Notice how the flow (indicated by white lines) amplifies as the former typhoon energy moves across the Pacific and into Canada and the United States. Amplified flow can bring about a significant clash of air masses (cold Canadian and subtropical) and initiate the development of powerful winter storms.
Computer models often have trouble predicting how exactly tropical systems will be absorbed into the flow, which until the system is fully absorbed can make for a very uncertain forecast for the 4-7 day period. Watch for discussions about typhoons in our fall/winter forecast discussions, as that may indicate a highly uncertain forecast for the 4-7 day period!

**Video courtesy of a blogger at

Sunday September 27: Rare Supermoon Lunar Eclipse and the Sky Cover Forecast

On the evening of Sunday September 27th (mainly between 7 and 8:30 pm), we will be able to view an astronomical treat that hasn't happened in over 30 years! There will be a combination of a supermoon (perigree full moon) along with a total lunar eclipse! So what exactly is a supermoon? It's a new or full moon closely occurring with perigree - the moon's closest point to Earth in its orbit. This particular supermoon will be the closest supermoon of the year (other full moon supermoons this year: August 29th and October 27th) being only 221,754 miles away from Earth.
About three or four times a year (in the spring and the fall), the new or full moon coincides closely in time with the perigree of the moon - the point when the moon is closest to the Earth. Image Courtesy NOAA
Like I mentioned earlier, this September supermoon will be a treat because it will coincide with a total lunar eclipse! Check out this great animated video by NASA's Goddard Space Flight Center explaining the Supermoon Lunar Eclipse and its rarity. On the night of September 27th, 2015 this supermoon lunar eclipse will be viewable in the night sky for those living in North and South America. Those living in Europe and Asia can view it in the early morning hours of September 28th. The total eclipse will last one hour and 12 minutes. Earth's shadow will begin to dim the supermoon slightly beginning at 5:11 pm PDT. A noticeable shadow will begin to fall on the moon at 6:07 pm, and the total eclipse will start around 7:11 pm. Good thing that sunset will be around 6:48 pm here in Reno, so as the moon rises in the eastern sky the eclipse will already be in progress! NASA's Scientific Visualization Studio has a great website that will continue to update as September 27th-28th approaches. The next lunar eclipse that coincides with a supermoon won't occur until 2033 (and the next lunar eclipse viewable from the western U.S. won't occur until January 2018), so be sure to check this one out!

The Moon moves right to left, passing through the penumbra and umbra. Courtesy of NASA Scientific Visualization Studio 
Now that we have all the astronomical formalities out of the way, we will move on to an important aspect of this event...the meteorology! No, not meteors obviously. We are switching gears and need to see if clouds are going to limit the viewing of this rare event. 

The main concern for viewing will be the presence and thickness of cirrus clouds Sunday evening. Unfortunately, the latest forecast models are indicating a large area of cirrus clouds spreading across eastern California and western Nevada, due to southwest flow aloft ahead of a low pressure in the eastern Pacific. While this cloud cover could affect the viewing, all is not lost as the cirrus may be thin enough at times to allow for the eclipse to be visible. The presence of some thinner cirrus may even produce a more dramatic view (or photo) of this eclipse. At this time, there does not appear to be any specific nearby location where less cloud cover is expected, so planning a long drive to chase more favorable conditions is not recommended. 

The good news is that unlike a solar eclipse where the totality stage typically only lasts a few minutes, Sunday's event will last over an hour so there will be more opportunities to view at least a portion of this eclipse, even if some cloud cover is present. The evening timing is also more convenient for most people to make plans, without interrupting typical sleep time.   

Thursday, September 17, 2015

Meteorological vs Astronomical Fall and a quick Summer 2015 Overview

Let's take a look back at Summer 2015. Although astronomical fall hasn't begun, meteorological summer has ended. What? There is a bit of a difference between the two. Astronomical summer ends (or fall begins) when the Autumnal Equinox occurs, around September 22-23 (specifically September 23, 2015 at 8:20 UTC in the northern hemisphere) So why would meteorological summer end (or fall begin) at a different time? It's basically because the astronomical seasons are based on the Earth's position related to the Sun while the meteorological seasons are based on the temperature cycles through the year. For a more in depth discussion on what the difference is between the two, check out this great write-up by NOAA's NCEI (National Centers for Environmental Information).

 Anyways, back to the Summer summary for Reno. Looking back at the summer, we had a VERY active monsoon pattern which resulted in multiple days of thunderstorms and flooding for the Sierra and western Nevada. The most active period was during late June and early July where we received multiple reports of flash flooding, hail, and quite a bit of rain and lightning! We even had a waterspout at Lake Tahoe! Here are just a few of the pictures from some of the summer thunderstorms. 

This active period of weather kept temperatures relatively "cool" compared to normal for that late June and early July time frame. We typically will see some of our hottest temperatures during July but due to the cloud cover from thunderstorms we didn't see as many triple digit days. The chart below shows the amount of days that Reno airport hit 100 degrees or more during June, July, & August since 1996. One thing to keep in mind is that this chart only shows the temperature for the Reno airport since that is the official climate site that we use for the Reno area. You can really see that 2014/2015 didn't have nearly as many 100 degree or hotter days as 2013 did!

Let's take a look at July though. July historically is the hottest time of the year for the Sierra and western Nevada so let's compare previous July 100 degrees and hotter days. Here you can really see a difference. On average we will hit 100 degrees or hotter at the Reno airport around 4 times during July and we only did twice in 2015. Also notice how many more times 100 degrees or hotter was hit during 2014 and 2013! So 2015 did have less 100 degrees or hotter days than the previous few years.

Taking a look at the data just one more way.. here is table of the monthly mean maximum temperature for the Reno airport. Looking at July you can see that the monthly mean temperature for July was just below 90 degrees for 2015! Looking at this table this hadn't happened since 1997, so it really is reflective of just how much cloud cover and thunderstorms that we had in the local Reno area this past July. If you are interested in digging into more climate data be sure to check out the Climate page available on our website. There is a ton of neat data available there.

In other news, it really is starting to feel like fall out there! Did anyone see the snow on top of the mountains this morning? Although it does look like we will be warming up again going into the weekend. For all the forecast details please check out the discussion. Thanks!