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SPC Outlooks Explained

Posted on by MichaelV (Otsego)

I dusted off my Skywarn ‘Go’ bag yesterday which includes all the instruments I use should severe weather move into the area.  I would generally go south and west of of the Otsego/Plainwell area or head northwest on M-89 towards Allegan where there is flat land and fields where the visibility is unobstructed by trees and buildings.  I have a handheld scanner to keep track of weather, EMS, Ham and police reports, a lightning detector which can track lightning strength and movement, an anemometer to record wind speed, a high definition video camera and off course my cell phone which has Radarscope to track storm locations and to report to the NWS.  I won’t go out after dark or, if I do it won’t be far from home base, just far enough to get out of the valley I live in.  Bittersweet is a good location if the gates are open and I can get up the hill.

We haven’t had much in the way of severe weather the past couple years, just a few thunderstorms with heavy rain and a bit of small hail.

I will be at the Skywarn session in Shelbyville with my brother on Saturday.  Over the past month I have been talking to a lot of people at various area businesses in the Otsego/Plainwell area which have shown a lot of interest in attending.

Here are the latest SPC graphics for today and an explanation of what the colors mean.

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Storm Outlook

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Tornado Outlook

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Wind Outlook

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Hail Outlook

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Overview: Convective outlooks for days 1, 2, and 3 consist of categorical and probabilistic graphics that depict severe and general thunderstorm threats across the continental United States, along with a text narrative.

 

Graphical categorical risk: Using numbers, words, and colors, the categorical graphic depicts general thunderstorm areas (TSTM-light green) and up to five risk types (1-MRGL-dark green, 2-SLGT-yellow, 3-ENH-orange, 4-MDT-red, and 5-HIGH-magenta) based on the coverage and intensity of organized severe weather such as supercells, squall lines, and multicell thunderstorm complexes. Pulse type thunderstorms, consisting primarily of solitary brief severe updrafts (often found in weakly sheared environments), are not considered organized.

 

The TSTM area encloses where a 10% or higher probability of thunderstorms is forecast during the valid period. A 1-MRGL-dark green risk area includes severe storms of either limited organization and longevity, or very low coverage and marginal intensity. A 2-SLGT-yellow risk area implies organized severe thunderstorms are expected, but usually in low coverage with varying levels of intensity. A 3-ENH-orange risk area depicts a greater concentration of organized severe thunderstorms with varying levels of intensity. A 4-MDT-red risk indicates potential for widespread severe weather with several tornadoes and/or numerous severe thunderstorms, some of which may be intense. A 5-HIGH-magenta risk area suggests a severe weather outbreak is expected from either numerous intense and long-track tornadoes, or a long-lived derecho system with hurricane-force wind gusts producing widespread damage.

 

Graphical probabilistic risk: The probability of severe weather drives the outlook categories, with the tables below showing the conversion for each threshold. On day 1, the outlook contains individual severe probabilities for tornadoes, wind, and hail. With greater uncertainty about severe-storm type into the future, the outlooks on day 2 and 3 only forecast the combined probability of all three types of severe weather. For all outlooks, the probability values represent the chance of severe weather occurring within 25 miles of any point, which is about the size of a major metropolitan area.

 

As an example, if you have a 15% probability for tornadoes, this means you have a 15% chance of a tornado occurring within 25 miles of your location. This may seem like a low number, but a tornado is very uncommon at any one location. Normally, your chances of getting hit by a tornado or other severe weather are small, purely based on statistical average. Let’s say you have a 1% statistical (climatology) history of tornadoes within 25 miles on this day, which still is large. Having a 15% probability means 15 times the normal odds of a tornado nearby, meaning it should be taken seriously. The probabilities for severe thunderstorm wind and hail also have the same meaning as they do for tornadoes, but typically will be higher numbers than for tornadoes, since they are much more common.

 

Sometimes, a black hatched area will be overlaid with the severe probabilities. Black hatching means a 10% or higher probability for significant severe events within 25 miles of any point. “Significant” is defined as: tornadoes rated EF2 or greater, thunderstorm wind gusts of hurricane force (74 mph) or higher, or hail 2 inches or larger in diameter.

 

The outlooks on day 2 and 3 combine all forms (tornado, wind, and hail) into a single black hatched area for a 10% or higher significant-severe risk. In addition, for tropical cyclones (hurricanes, tropical storms and depressions), the outlooks on day 2 and 3 allow a 5% total severe probability to be a SLGT risk because they are specifically tornado-driven.

The U.S. map has been colorful the past couple days with a wide range of hazards.  There are multiple reports in eastern Colorado of wind speeds of over 80mph with snow up to 13 inches.  Kansas has also reported wind gusts over 80mph.  Storm reports can be found here.

We have flood watches for most of northern lower Michigan through tomorrow evening.  We will remain in an unsettle pattern until Monday when we will finally have a calm pattern setting up for a good portion of next week.

The threat for strong/severe storms this afternoon/evening looks lower than previously thought with areas south/east of Lower Michigan most at risk.  You can find the latest area forecast discussion here.

Below is the NAM forecast for today:

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Category: Michigan Weather Forecast

57 thoughts on “SPC Outlooks Explained”

  1. I was out chasing a little this afternoon these storms really pick up steam heading east we had a wonderful set up this early this evening not to often a sunny dry slot fires good storms if only Lk Michigan would of been a lot warmer storms would of been rocking hard in GR…Have a good night INDY!

  2. Some nice thunderstorms rolled thru here this afternoon. Little thunder & lightning. Heavy downpour a couple of times. Quiet now. I was just outside & temp feels cooler. This afternoon when the sun was out it was beautiful.

  4. Tornado warning for the cell that passed over us a half hour ago. It was ominous looking when it was over us.

    1. Yeah I’ve always wondere about that. They predict it’s coming, know it’s coming, but don’t put a watch out until after its hit. But they’ll throw up a snow advisory 2 days ahead of time for 1.3 inches of snow.

      1. NWS even mentioned us in the warning. Nowhere near severe level here. Very little wind and no hail.

  8. That 100mph storm looks almost unreal on the radar loop. When comparing it to all the other storms, it just rockets away from them like it’s in a drag race.

  9. Just got home. Sun has been shining for the past 45 minutes. Car thermometer read 62°

    2. The official high at Lansing was 62 at GRR it was 58 here at my house I reached 60
      Slim

  10. Ton of rain, some lightning, but little wind here. Suns out now with a brilliant rainbow to the NE! Actually a faint double rainbow!

  11. * Severe Thunderstorm Warning for…
    Southern Kalamazoo County in southwestern Michigan…

    * Until 545 PM EDT.

    * At 508 PM EDT, a severe thunderstorm was located near Schoolcraft,
    or 8 miles south of Portage, moving northeast at 45 mph.

    HAZARD…60 mph wind gusts.

    SOURCE…Radar indicated.

    IMPACT…Expect damage to roofs, siding, and trees.

    * Locations impacted include…
    Kalamazoo… Portage… Vicksburg…
    Galesburg… Schoolcraft… Climax…
    Fulton… Scotts…

  12. Per radio just now: a storm in Michigan is moving at 100mph. The fastest ever recorded in the state! I think it’s a storm on the east side. Wow

    1. STRONG THUNDERSTORM WILL AFFECT SOUTHERN LAPEER…NORTHERN
      OAKLAND AND SOUTHEASTERN GENESEE COUNTIES…

      At 513 PM EDT, a strong thunderstorm was located over Clarkston, or 9
      miles northwest of Pontiac, moving northeast at 100 mph.

      That’s incredible!

      1. Just WOW! Of course that also means that should it decide to drop a funnel, there’s going to be no warning it…..

  13. The National Weather Service in Grand Rapids has issued a

    * Severe Thunderstorm Warning for…
    East central Allegan County in southwestern Michigan…
    Northwestern Barry County in southwestern Michigan…

    * Until 530 PM EDT.

    * At 453 PM EDT, a severe thunderstorm was located near Martin, or 8
    miles north of Plainwell, moving northeast at 50 mph.

      1. Heavy down put and tiny hail….round 1? Seems to have stopped as quick as it came up.

  14. …SIGNIFICANT WEATHER ADVISORY…

    At 443 PM EDT, Doppler radar was tracking strong thunderstorms along
    a line extending from Port Sheldon to near Pullman. Movement was
    northeast at 60 mph.

    Dime size hail and wind gusts around 50 mph with downed tree limbs
    will be possible with these storms.

    Locations impacted include…
    Grand Rapids… Holland… Allegan…
    Lowell… Wyoming… Kentwood…
    Walker… East Grand Rapids… Hudsonville…
    Rockford… Zeeland… Coopersville…
    Sparta… Wayland… Otsego…
    Middleville… Caledonia… Fennville…
    Saugatuck… Freeport…

  15. The National Weather Service in Grand Rapids has issued a

    * Severe Thunderstorm Warning for…
    Northwestern Van Buren County in southwestern Michigan…

    * Until 500 PM EDT.

    * At 415 PM EDT, a severe thunderstorm was located near Benton
    Harbor, moving northeast at 50 mph.

    HAZARD…60 mph wind gusts and quarter size hail.

  16. * Severe Thunderstorm Warning for…
    Northwestern St. Joseph County in north central Indiana…
    Northeastern La Porte County in northwestern Indiana…
    Southeastern Berrien County in southwestern Michigan…

    * Until 430 PM EDT/330 PM CDT/.

    1. Yup, looking at more spring like temps next week.. The snow piles are all but gone here ..

      1. bring your truck over still a good amount of snow piles here yet. More like ice piles
        Slim

  18. …WIND ADVISORY IN EFFECT UNTIL 11 PM EDT THIS EVENING…

    The National Weather Service in Grand Rapids has issued a Wind
    Advisory…which is in effect until 11 PM EDT this evening.

    HAZARDOUS WEATHER…

    * Occasional wind gusts over 45 mph today into this evening.

    * Several rounds of showers and thunderstorms could easily
    produce very strong gusts. These gusts are possible even
    without any precipitation…especially along the lakeshore.

    * The first round of showers and storms was lifting northeastward
    through Indiana as of 930 am and should arrive in the Kalamazoo
    to Jackson area between 11 am and noon. Strong lakeshore winds
    also expected to develop by noon…especially from Muskegon
    north towards Ludington.

  21. Getting really dark to the SW. Radar loop shows pretty good thunderstorms heading directly at us.

  22. There are some heavier showers in southern Lake Michigan that had some lightning with it. That line is heading NE so will be in SW Michigan shortly the current temperature here at my house is 57 with some breaks in the clouds. The last reading at GRR was 56 with light rain.
    Slim

  23. 52* degrees now out at thee YARDofBRICKS NE GR with light rain and clouds I’m thinking out Severe weather and mid 60’s today like the NWS forcast is out the window still low 50’s feel wonderful..INDY!!

    1. 55° here at my house so it might not reach 60 today. BTY the last reading at GRR was 57 and that was you guess for the March maximum. We shall see
      Slim

  24. Here are the maximum March temperature guesses from last week Rocky 49.4, Indy 57, Mark 59, MV 62, Mookie 62, Andy 65, Sandy 69, SS 70, Barry 71, Sarah 72 and my guess is 64. The latest reading at GRR was 54.
    Slim

    1. Don’t forget my guess of 69. We are going to get close to some of these today.

      1. Sorry Sandy I will add your guess of 69. Rocky is already out and at this time Indy is right on it as the current temperature at GRR is now 57
        Slim

  25. Here at my house I received 0.36″ of rain yesterday. And while there is a lot of bare ground here there is still a lot of snow in the woods and in the shady areas and of course the snow piles will be with us for some time yet. With some sun the current temperature here is 55° Will have to see just how “warm” it gets today.
    Slim

  26. We went sledding yesterday…. snow was very icy & slushy in spots, but we were out enjoying the last day *fingers crossed*!!!

  27. “No snow on the ground”???…hard to imagine for Ada…maybe April 🙁 A line of severe storms?…we all know better. Try to enjoy the temporary thaw, as more snow is on the way.

  29. As suspected, GR is now reporting zero snow on the ground. Yesterday, was +10 degrees above average, and this morning felt very springy.

    2. While there may no a reported “zero” snow on the ground at the airport and much of the city of Grand Rapids, here in my nick of the woods there is still a lot of snow on the ground in the woods and in shady areas. So here in this part of Kent County the report would be a trace.
      Slim

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