Thursday, October 23, 2014

A warm weekend in store; rain chances hold off until next week

A rather cool afternoon is underway with temps in the mid to lower 60's as a partial solar eclipse will hopefully be visible in the Mid-South. The only issue at this time is some high clouds over the area, but these are showing some signs of breaking up and we're hoping we'll be lucky enough to catch a break for the eclipse beginning at 4:52 PM (more info in our previous blog post). (If we aren't able to view it locally, you can always tune into the Slooh Observatory live broadcast.) However, our main story concerns the fairly drastic change in temperatures for this weekend and into next week.

GFS model valid Friday night showing temperatures (in °C) about 5000' feet above the surface
Warmer temperatures will begin streaming into the Mid-South starting tomorrow mainly due to a shift in the upper-level atmospheric pattern. A ridge (area of higher pressure) to our west will bring northwesterly flow aloft which will advect warmer air into the Mid-South from the lower plains. This may seem like a bit of an unusual pattern, but warmer air is often brought into the plains earlier then the south after several rounds of cold fronts. The ridge to our west and trough to our east promote this atmospheric pattern.

GFS model Friday night showing temperatures (in °F) at the surface
At the surface the pattern is more of what you would expect, with winds shifting towards the south with a weakening surface high over the area (which was responsible for the cooler air). Expect temps to begin increasing tomorrow with highs in the mid 70's. Warming continues into Saturday and Sunday with temps getting all the way into the lower 80's. It will feel quite warm during the day, but by Saturday night temps will only get into the 60's at night, making it relatively mild even in the mornings to end the weekend.

GFS model Tuesday afternoon showing Precipitable Water Values
Rain chances will return early next week as the next cold front moves through. Models are indicating a strengthening low pressure system to our north with a developed cold front sweeping through the Mid-South. The band of high precipitable water values along the cold front represent deep moisture which could bring thunderstorms along with it. We'll keep an eye on this threat and will let you know if we expect anything significant. As always you can check out the full MWN Forecast here.

William Churchill
MWN Social Media Intern

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Wednesday, October 22, 2014

Partial solar eclipse visible to Mid-Southerners on Thursday

In an event that won't be seen again for nearly 3 years, a partial solar eclipse will grace the sky over North America on Thursday, October 23. Partial solar eclipses happen when a new moon comes between the sun and the Earth, but they don't align in a perfectly straight line. Therefore, the moon only partially covers the sun's disc. 

Diagram of how a solar eclipse occurs, as a new moon passes between viewers on Earth and the sun.
Graphic credit: EarthSky.org.
In the Memphis metro, the partial eclipse will begin at 4:52pm Thursday afternoon as a small shadow on the right side of the sun. As the shadow moves across the top of the sun, a maximum eclipse (shown below) will occur at 5:53pm low on the western horizon. The sun will slip below the horizon at 6:14pm, thus ending the viewing opportunity prior to the end of the eclipse.

How the sun will appear at maximum eclipse (5:53pm Thursday) just prior to setting. For an animation of the complete eclipse cycle, see TimeandDate.com.

The next solar eclipse opportunity won't be until August 21, 2017 when a total eclipse takes place. While we look forward to that opportunity, don't miss the chance to see this partial eclipse or you'll have to wait another 3 years to see it again!

As to viewing an eclipse, remember the #1 rule is to NEVER look directly at the sun, eclipsed or otherwise, without protective eyewear! The sun’s UV radiation can burn the retinas in the eyes leading to permanent damage or even blindness. The only way to safely see a solar eclipse is to wear protective eclipse glasses, look through welder's goggles with a rating of 14 or higher, or to project an image of the eclipsed sun using a pinhole camera. Here's info on making a simple camera from Mr Eclipse.com:
One safe way of enjoying the Sun during a partial eclipse--or anytime--is a "pinhole camera," which allows you to view a projected image of the Sun. There are fancy pinhole cameras you can make out of cardboard boxes, but a perfectly adequate (and portable) version can be made out of two thin but stiff pieces of white cardboard. Punch a small clean pinhole in one piece of cardboard and let the sunlight fall through that hole onto the second piece of cardboard, which serves as a screen, held below it. An inverted image of the Sun is formed. To make the image larger, move the screen farther from the pinhole. To make the image brighter, move the screen closer to the pinhole. Do not make the pinhole wide or you will only have a shaft of sunlight rather than an image of the crescent Sun. Remember, this instrument is used with your back to the Sun. The sunlight passes over your shoulder, through the pinhole, and forms an image on the cardboard screen beneath it. Do not look through the pinhole at the Sun.
Be safe and have fun watching the eclipse on Thursday!

Erik Proseus
MWN Meteorologist

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New severe weather outlook categories roll out today

Beginning up to eight days ahead of potential severe weather, the Storm Prediction Center (SPC), a part of NOAA/NWS and the worldwide leaders in severe weather prediction, begins issuing convective outlooks (a.k.a. "risk areas") for the possibility of severe storms.  For many years, these outlooks have included a "general thunderstorm" area, as well as Slight, Moderate, and High Risk areas.

Background

In recent years, especially with the widespread use of social media and availability of information around the clock and at our fingertips, the general public has become familiar with these outlooks and the terms used.  There has also been a call to update the outlooks to provide better delineation of the risk that the public faces from severe storms. In particular, "Slight Risk days" seem to cover a wide gamut of severe weather possibilities, from a small threat for a damaging wind gust or large hail to something just short of a tornado outbreak.

Responding to the call to better define the risk, take into consideration the research and advice of social scientists who specialize in communicating risk or threat to the general public, and to provide better consistency with other NWS products, SPC is using a new classification system for their severe weather outlooks starting today (October 22, 2014).  The new system can be found in the tables below (pay particular attention to the first table).  The probability of tornadoes, 1"+ hail, and/or 58+ mph wind in a particular area defines the risk outlook category issued by SPC. These probability-to-outlook category conversion tables are shown below the new outlook classification system.  NOTE: There will be NO changes to the watches or warnings issued as a result of this modification.


SPC outlook categories prior to October 22 ("Old") and starting October 22 ("New"), as well as the numeric scale that will accompany the categories in SPC outlooks and MWN postings.


Probability matrix SPC uses for determining outlook category on the Day 1 outlooks. For instance, a 10% risk of a tornado or 30% risk of severe wind or hail warrants an Enhanced risk. "Significant Severe" means EF-2 tornadoes, 74 mph wind, or 2 inch hail.

Probability matrix SPC uses for determining outlook category on the Day 2 outlooks

Probability matrix SPC uses for determining outlook category on the Day 3 outlooks. High Risks are not issued on Day 3.

Details

The biggest change users will see will be the addition of two new risk areas - "Marginal" and "Enhanced."  The Marginal Risk indicates that the chance of severe weather is very low but not non-existent (or marginal). The Marginal Risk would replace the current "See Text" areas in the outlooks. According to first table above, areas under a marginal risk of severe weather have a low chance of severe storms - less than the slight risk of previous years.  Enhanced Risk indicates a more significant chance of severe storms and it will be used for "high-end" slight risk areas.  In other words, the current slight risk category will be split between "slight" and "enhanced."  Enhanced risk indicates a higher chance of severe weather than slight risk, but not quite up to a moderate risk.  There will be no changes to the moderate or high risk areas.

In addition, each Severe Weather Outlook text bulletin that accompanies the maps will contain a "public discussion" section that describes the weather risks for that day in non-meteorological jargon, so that the general public can understand the threat. An example of a day which had a high risk of severe weather (May 24, 2011) is shown below, followed by what the outlook areas would look like under the new classification system (click each for a larger image).

Example showing the classification system for severe weather outlooks as used on May 24, 2011

How the convective outlook would look for May 24, 2011 using the new classification system

How MWN will handle the change

We understand that this change seems to make things more complicated. While the changes will be fairly well understood by meteorologists and NWS partners, education will be necessary for those who do not look at these categories on a daily basis. We will employ a supplemental numerical scale (1 to 5), in addition to the new categories, to help better define the risk. These numerical categories are similar to the Saffir-Simpson Hurricane Wind Scale and the Enhanced Fujita Scale for tornadoes with a higher number indicating a greater risk of severe storms. (We have learned since deciding to use the numerical scale that SPC actually intends to do the same thing. They must've thought we were on to something there! :-)  The scales will be identical.)

More important than the number or name of an outlooked area are the threats posed. As a Weather-Ready Nation Ambassador that promotes information and education, we'll be sure that potential impacts are the key ideas in blog and social media posts, just as we have always done!  If you have any comments or questions, feel free to send them to us via our social media feeds or as a comment on this blog.

Erik Proseus
MWN Meteorologist

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Wednesday, October 15, 2014

Join us for a "Severe Weather Roundtable" Google+ video Hangout #wxchat

Secondary severe weather season is quickly approaching and, in fact, the southeast U.S. (where secondary season is centered) has just experienced a regional severe weather event. It is a good time to bring severe weather preparedness and education back into the forefront.

To that end, on Thursday night, October 16, at 9pm MemphisWeather.net will host a "Severe Weather Roundtable" discussion via live video-based Google+ Hangout. Joining MWN meteorologist Erik Proseus and other members of the MWN team on the panel will be local weather enthusiast and veteran of MWN hangouts, John Maddox, and special guest Rick Smith. Rick is the Warning Coordination Meteorologist at the National Weather Service office in Norman (Oklahoma City), OK and has roots in the Mid-South. He knows Mid-South weather.

For one hour, panelists will discuss topics related to severe weather policy, procedure, and practice, answering some of your questions, such as:

  • "Why does a watch extend for so many hours when the severe weather will be occurring very soon?"
  • "Why do warnings not cover entire counties, and if they don't, why do I still hear sirens across my entire county?"
  • "I hear that the traditional Slight, Moderate, and High risk outlooks are changing. What does that mean, how does it affect me, and how will I know how bad the weather is going to be?"
  • "I always hear people say to have multiple ways of receiving warnings. What are the best ways to ensure that my family remains safe?"
  • And finally, "Why do some schools dismiss early on severe weather days and is that really a good idea?"

You will be able to watch the broadcast live, and ask your questions online, via our MWN Hangout page and our Google+ page. In addition, we'll be live-tweeting using the #wxchat hashtag on Twitter. You may also ask your questions via Twitter using the #wxchat tag. If you can't watch the broadcast live, a recording will be available on YouTube via the links above after the broadcast ends.

This discussion promises to be educational, informative, and perhaps even a bit controversial. We hope you'll be able to join us Thursday night at 9pm!

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Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!