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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Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

Monday, October 13, 2014

Monday morning update on today's severe weather threat

UPDATE -- 9:00am Monday

As suspected last evening, the Mid-South has been upgraded to a Moderate Risk for severe storms today. Probability of damaging wind within 25 miles of your location is now at 45% with a 10% chance of wind over 75 mph. The tornado risk is also at 10%.

Current SPC severe weather outlook for Monday - a Moderate Risk is in effect for the metro.

Current SPC probabilities for damaging wind. Pink is 45% and the black hatching indicates a 10% or greater risk of wind over 75 mph.

Current SPC probabilities for tornadoes. Yellow is 10% and any black hatching indicates a 10% or greater risk of strong (EF-2+) tornadoes.

As far as timing, supercells capable of high wind and a few tornadoes will be possible by late this morning, while the squall line itself is on track for an afternoon (1-4pm) arrival. Everyone in the metro should prepare for the possibility of severe storms from late morning through early evening.

The remainder of the blog, posted last night, is still applicable, particularly the severe weather preparations discussed near the bottom.

--Erik Proseus,, MWN Meteorologist


ORIGINAL POST -- 8:25pm Sunday

As we blogged about yesterday, severe weather looks likely on Monday for a large portion of the middle and lower Mississippi Valley. In fact, the Storm Prediction Center has indicated as many as 42 million people from the Midwest to the Gulf Coast are under a risk of severe weather.

The greatest likelihood, where there is currently a 30% risk of severe storms within 25 miles, exists from St. Louis across the Mid-South to northern Louisiana. This same area also is under a threat of "significant severe" weather, which includes EF-2 or stronger tornadoes, 75+ mph wind, or 2"+ hail. The Memphis area is in both the 30% area, as well as 10% risk of significant severe storms. SPC has indicated that there is a decent chance parts of the 30% area could be upgraded to a Moderate Risk tomorrow, the first for our area since June 5 when a derecho caused widespread damage.


The Mid-South is currently forecast for at least a 30% chance of severe weather within 25 miles, as well as a 10% risk of "significant severe" weather. These probabilities could go up tomorrow morning.  
The area is currently sitting under a warm and moisture-laden southerly flow of air from the Gulf of Mexico, evident by dewpoints in the upper 60s this evening and south wind. Overnight, wind will begin increasing from the south and southwest aloft, further priming the pump for severe weather as moisture and wind shear increase. This will bring about a chance of a few thunderstorms tonight, though no severe weather is expected.

Forecast dewpoints as of 7pm Monday evening are still near 70 degrees. This amount of low level moisture is more than sufficient for strong to severe storms and is very muggy for October when dewpoints typically are in the 40s and 50s.
By tomorrow, low-level wind will really start to crank up, and in fact, wind at the surface ahead of the potent cold front will increase to 25-35 mph during late morning and afternoon, prompting the issuance of a Wind Advisory for the area from 10am - 7pm. If you need to, tie down any loose objects outdoors tonight or before leaving for work or school in the morning. Wind in the lowest 5000' of the atmosphere will also pick up to more than 50 mph during the afternoon, setting up a low-level wind shear scenario that is favorable for rotating storms and possibly tornadoes.

Forecast wind at about 3,000' early Monday afternoon. Values are in knots, which can be converted to mph by adding 15%. Eastern AR has values approaching 60 mph, while they are near 50 mph over Memphis. Strong storms can push this wind to the surface, creating a damaging wind scenario. The wind, coupled with changes in direction or speed above and below this layer, can create a wind shear scenario, which is necessary for severe storms.

Forecast wind shear values as of 4pm Monday afternoon. Values near 50 knots (present over the metro) are more than sufficient to allow storms to rotate, which is a precondition for supercells.
With an abundance of warm air and a period of dry weather expected during the day tomorrow, temperatures will warm to at least 80, more likely the lower 80s, by mid-afternoon. This heating will add fuel to the fire in the form of instability, another necessary ingredient for strong to severe storms. However, even if temperatures don't quite reach 80, there are plenty of other factors in play that could allow for storms to become severe.

Forecast CAPE values at 1pm Monday are near 2000 J/kg, which is sufficient for strong storms, given other factors which are also forecast to be present.
Given all of the above parameters - instability, moisture, shear - the final ingredient, lifting of the air into the primed airmass, will be provided by the cold front, or perhaps more likely, a trough out ahead of the front. Storms will move into western AR early Monday and then travel across the state during the day ahead of the front. A squall line of storms (or using meteorological jargon, a QLCS) will sweep across the metro most likely during the late afternoon to early evening hours. As of now, we're expecting this line between 3pm-8pm, but are favoring a rush hour impact in the metro. In fact, one of the high-resolution models (below) agrees.

Simulated radar (not a forecast), shows a squall line pushing into the city at 5pm. This type of forecast product is used for TRENDS. Radar is NOT expected to look exactly like this at that time, but gives us a good idea what to expect.

The main threat will be damaging straight-line thunderstorm wind of up to 60-75 mph with the squall line. However, as we saw with the last big squall line, "loops" or notches in the line are favored spots for rotation and a few tornadoes can NOT be ruled out.

The better chance for any rotating severe storms capable of tornadoes, however, will be in any renegade cells that form ahead of the line during the afternoon in a sheared environment. If these storms form, and can tap into the shear and become supercells, tornadoes would be possible, as well as some large hail. These threats are CONDITIONAL though, meaning we're not positive that this scenario plays out. If it does, the threat becomes more likely.

Once the line moves through, expect a quick drop in temperatures and a few hours of rain, perhaps heavy. The heavy rain along and behind the squall line could be capable of producing some flash flooding, so be aware of that threat during rush hour and into the evening as well.

Note a quick drop in temps behind the line, as shown by the high-res NAM model valid at 5pm. Temps in the 80s fall quickly into the 50s/60s behind the line.

Bottom line: prepare for the likelihood of severe storms tomorrow afternoon and evening, possibly first as supercells, then as a squall line around rush hour. Damaging wind is the main threat and appears to be a good possibility. Tornadoes and large hail are a secondary risk. A Tornado Watch is expected during the afternoon tomorrow. Know your plan for wherever you will be during the afternoon and evening should a warning be issued. Avoid windows even as a line of storms moves through due to the threat of high wind and lofted objects.

We'll bring you the latest all day tomorrow, including nowcasting of the events as they unfold, on our social feeds below. We highly recommend having a severe weather app to alert you to severe weather conditions. We prefer ours (links to info and download below), but honestly don't care as long as you have a reliable way to get warning information.

Also, remember that outdoor tornado warning sirens are for OUTDOOR use and the policies vary by municipality and county. If you hear one and aren't sure of the policy, take cover. In general (in Shelby County), Bartlett, Collierville, and Germantown only sound their sirens when the respective municipalities are under a threat. Memphis and Shelby County sound their sirens for any threat in Shelby County.

Click here for the latest forecast, or get it on our apps.  Stay safe!


Erik Proseus,
MWN Meteorologist

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Follow MWN on Facebook, Twitter, and Google+
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!