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    Many things go into if there will be thunderstorms or how widespread they will be and how strong they will be. There are different types of thunderstorms. I won’t go over all that now but have covered that in past blogs and will no doubt do so at some point in the future. This is a simplified explanation of what goes into SEVERE THUNDERSTORM and tornado potential. A severe storm is defined as one that can cause damage through wind or hail. (lightning can and does do damage and kill, but all thunderstorms by definition have lightning even weak ones not just severe ones). Thunder is caused by lightning, an old time description of a thunderstorm was “an electrical storm”. This was both because of the electricity of the lightning but also because the electric power often went out during storms when lightning struck power poles or trees hit wires. This is less common in modern times thanks to the improved power grid and proactive pruning. We analyze the weather set-up using current observed and model forecast atmospheric soundings or Skew-T thermodynamic diagrams and Hodographs obtained from the twice a day release of weather balloons that sample the atmosphere. Here are a couple examples of what they look like:   Severe weather forecasting is ingredients based. Here is the basic 4 part recipe: MOISTURE/INSTABILITY We look for surface dew points of at least 55 with 65 or higher better, we look at the precipitable water values that show the total moisture available in a vertical column of air measured in inches, the more the heavier. There are many other measures of moisture we look at I wont go into here. We also look at CAPE. Convectively Available Potential Energy, a measure of buoyancy and instability or ability for air parcels to rise up, measured in Joules per kg, the more above 500 the more unstable the air mass. There are other measures of instability I am not covering here such as lapse rates. WIND SHEAR/Helicity (Kinematics): This is very complicated so I won’t cover every nuance. Wind shear is the change of speed and/or direction of the wind as you go vertical. (Vertical speed shear and vertical directional shear). There are hundreds of different variables we look analyze. The more the wind changes direction with increasing height from surface to the stratosphere the more likely storms are to rotate. So for example a Southeast wind at the surface and a Southwest flow at jet stream level is favorable for thunderstorm rotation. For reasons too complex to go into this helps storms generate larger hail and grow stronger in all facets. The SAME for increasing wind SPEED as you go up from the surface to the stratosphere. Speed shear of at least 25 knots is needed. The greater the bulk wind shear the greater the threat of rotating and thus severe storms. PVA or positive vorticity advection at jet stream level can aid this process. But we also want to know if there will be a tendency for wind updrafts and downdrafts from cloud base to earth to rotate. This is helicity and we look for values of 100 m2/s2 or higher in the 0-1km layer. There are dozens to hundreds of other parameter formulas for rotation and spin and updraft velocity that forecasters look at I wont cover here. JET STREAM STRUCTURES: There are low-level jets and the more familiar main jet stream aloft at various levels and locations. The presents and configuration of jet streams are related to wind shear and lift in the atmosphere, the patterns of diffidence and divergence that aid in air wanting to rise. Again, too complicated to go into all the details since this isn’t a college course. Suffice to say, that crossing jet streams that are strong at least 45 knots and the higher the more favorable for severe weather. SURFACE BOUNDARIES: A surface boundary such as a trough of low pressure, a warm front or a cold front and or low pressure center or buoyancy gradient can all help focus lift and all the other ingredients. Other types of boundaries such as small scale difference in temp or moisture or mesoscale outflow boundaries can all serve the same purpose of focusing the potential for thunderstorms into reality. Note how research finds a correlation between Helicity (SRH) and CAPE in tornado potential: There is often a trade-off between key ingredients for a severe thunderstorm so that the maximum positive ingredients are not in sync, they are out of phase with one another. So you might have plenty of one but not enough of the other making it a more complicated forecast, because severe storms are still possible but predicting them is a more difficult call. That happens a lot here in the Southeast if we have a strong system approaching but we are in one of our “wedge” (CAD) patterns. It also happens when a strong system to our West moves into our area but at a cooler less favorable time of day: late night or morning. Here’s an example from March 2018. See how two main ingredients (buoyancy and vertical shear/rotation) are not at a maximum at the same time at the same location: Sometimes there’s enough of one to overcome the lack of the other, that’s what we mean by the trade-off. It make the severe weather threat far more uncertain in such cases. In that case, a wedge helped kill storms as they tried to move into it (blue shade) while most of the tornadic storms formed on the edge of the greatest CAPE (buoyancy) instead of where it was at a maximum. The blue shade CINH or Convective Inhibition, is where conditions want to resist upward air motions. High CINH tends to inhibit the release of buoyant energy. The greatest threat of tornadoes and other forms of severe weather occurs where moist buoyant unstable air (high CAPE) and maximum wind shear and rotation (Helicity) overlap. We saw this happen again Sunday April 15th, much like the March 19th event, although this time there was no wedge in place, but the low pressure and front arrived in Atlanta in the morning when the air was cooler and more stable, hence the same effect. Where the storm arrived at the warmest part of the day to our West the storms were worse, when it arrived here weaker with the cool morning, then the storms fired up again to our East as the front arrived there in the warmest part of the day: The Atlanta area was again fortunate to be where key parameters were out of sync and not overlapping, we had a lot of spin but not enough of instability/buoyancy). But to our South and East where the front arrived later the ingredients again became more juxtaposed: Like many things in life, timing matters and the atmosphere is complex with many interactions. There are hundreds of charts a meteorologist must look at to make a forecast, not just some “Future radar” animation. That won’t cut it and neither will automated web pages or phone APPS, except on days the weather is simple. FOLLOW me on Twitter @MellishMeterWSB.  
  • The threat of severe weather has ended for Metro Atlanta and scattered showers will be ending, but temperatures will slowly fall this afternoon behind the cold front. A touch of snow may even mix with rain overnight in the higher elevations of the Northeast Georgia mountains. Clearing for Monday but temperatures will struggle to top 60 with lows in the upper 30s to low 40s the next couple nights. LATE SUNDAY SURFACE WEATHER MAP: LATE SUNDAY NIGHT: MONDAY SURFACE WEATHER CHART: MONDAY AM LOW TEMPERATURES: TUESDAY MORNING LOWS: However, a quick rebound to highs in the 70s is due Tuesday into next Saturday, Wednesday may reach 80. The next significant chance of rain and thunderstorms looks to be next Sunday: Follow me on Twitter @MellishMeterWSB.    
  • As is typical of Spring “winter” storm systems this one will run the gamut from blizzard and ice storm North to flooding rain and some tornadoes South, a pattern that started yesterday and will continue through the weekend. Most of us in Metro Atlanta stay dry during the daytime, but the chance of a shower or storm goes up before sunset, especially North and West suburbs. The greatest risk of severe weather comes after midnight through early Sunday morning. The risk level is currently a 1 on a scale of 1-5, (marginal) but COULD go up to level 2 in updates tonight. The level 2 risk is considered a standard risk. Hopefully the late arrival of the system will keep the risk of a tornado or damaging strait-line winds to a minimum. Highest thunderstorm chance area late afternoon to midnight shaded: SURFACE WEATHER CHART END OF DAY SATURDAY: RAINFALL ESTIMATE SATURDAY AFTERNOON: RAINFALL ESTIMATE SATURDAY EVENING: ESTIMATED RAINFALL AFTER MIDNIGHT: RAINFALL ESTIMATE FIRST HALF OF SUNDAY: ESTIMATED RAINFALL AFTER 2PM SUNDAY: SURFACE WEATHER MAP 8 AM SUNDAY: SURFACE WEATHER MAP 8 PM SUNDAY: Follow me on Twitter @MellishMeterWSB.
  • Warmer than normal temperatures into tomorrow (average is now 72) but noticeably cooler Sunday, and then Monday highs will struggle to reach 60 with lows in the upper 30s to low 40s Tuesday morning. We need to monitor heavy rainfall potential (looks like an average of 1 inch, with isolated higher totals) and possible some severe thunderstorms in the warm to chilly transition. SATURDAY AFTERNOON SURFACE WEATHER CHART: SUNDAY SURFACE WEATHER CHART: SURFACE WEATHER CHART MONDAY MORNING: ESTIMATED AVERAGE RAINFALL SATURDAY: ESTIMATED AVERAGE RAIN AMOUNTS SATURDAY NIGHT: ESTIMATED AVERAGE RAIN SUNDAY: SEVERE WEATHER OUTLOOK LATE SATURDAY AFTERNOON/SATURDAY NIGHT: SPC SUNDAY SEVERE WEATHER OUTLOOK: Here is the forecast discussion from the National Weather Service Atlanta office in Peachtree City: “There is some risk for isolated severe storms very late in the day for portions of N and W GA but that will be very dependent on the timing of the entire system which looks to have it`s most significant effect after Saturday. It appears the instability will be fairly limited for Georgia with this system. The best forcing along the front moves into our area early Sun morning…and with the lack of daytime heating this should help to suppress any stronger thunderstorms from developing. However, there is enough instability in the far western and northwestern portions of the state Saturday afternoon and early evening that may enable a few marginally severe storms to form ahead of the main system. While MU and SBCAPE looks meek, low level shear may be enough to drive a limited QLCS severe threat. Conditions will need to be monitored closely Saturday afternoon to ensure the area of higher instability remains across MS/AL”. The MU/SBCAPE stands for Most Unstable and Surface Based Convective Available Potential Energy (a measure of instability/buoyancy of the air mass measure in Joules. QLCS stands for Quasi-Linear Convective System, (a squall line). There is often a trade-off between sufficient instability and wind shear in deciding the degree of severe weather threat, especially here in the Southeast U.S. and especially with systems that enter a wedge pattern or that come into the area in the morning when the atmosphere is cooler and thus generally less unstable. So most of us stay dry Saturday but a few isolated strong storms can’t be ruled out, with more widespread rain and some storms likely during Sunday. Obviously the timing and threat level can change so stay tuned. I’ll be on the radio with updates through the weekend. Follow me on Twitter @MellishMeterWSB.      
  • A very PRELIMINARY outlook for the coming tropical cyclone season was issued last week by some of the pioneers in the field at Colorado State University. It was incorrectly characterized by many in the news business as a prediction for a “bad” season or “another busy season”. I would not describe the outlook that way number one, and secondly there are multiple sources that make forecasts not just one! There is no “Royal decree” on the matter. What is more, the track record of outlooks issued this early is not good. Hwever, the one’s that are issued in the coming months show useful accuracy. The April/May sea surface temperatures and ENSO trends have better correlations with better initial conditions to project the tropical season ahead, with the Summer outlooks better still. Past history has shown what we call the El Nino/La Nina “Spring predictability barrier”. This is because in the Spring the conditions often change rapidly. Performance testing shows model ENSO forecasts are more reliable after the Spring transition season. For example the April outlook issued last year foresaw a normal to below normal season: (Image: Capitol Weather) (Image: Tropical Storm Risk) But by the Summer the updated forecasts correctly saw an above-normal season coming. In fact, the CSU outlooks issued by June have shown considerable skill over the years. The 2017 Atlantic hurricane season had the highest ACE value (223) since 2005 and ranks in the top 5 years since 1900. ACE correlates with named storm frequency and intensity during a hurricane season. As of now, most of the numerical equation variants suggest we transition from the long La Nina we’ve had in the Pacific to a weak El Nino by the Autumn. The CSU group believes neutral conditions are the best call. As a rule El Nino is a sign of a below normal season and La Nina with a normal or above-normal season, although sea surface temperatures and wind shear in the Atlantic and Caribbean also play a significant role. As I said there are multiple hurricane season forecast groups. Some are based on rigorously tested systems and years of experience with peer-reviewed science, others are not. At this point some groups are predicting a slightly above normal season and others a  slightly below-normal season. So you can see with the uncertainty high in the Spring and with low skill in predicting something 5 months away it’s too soon to focus on hurricane season. The CSU group outlook: Their forecast details here. Track multiple outlooks from various groups compiled here. Follow me on Twitter @MellishMeterWSB.        
  • ATLANTA FORECAST Today: Increasing clouds. High: 73 Tonight: Showers likely. Low: 48 Tomorrow: Rain and isolated thunder. High: 62 » For a detailed forecast, visit The Atlanta Journal-Constitution weather page. Rain and cooler temperatures are coming to metro Atlanta. A cold front expected to move through Georgia will mean a 90 percent chance of rain Saturday for Atlanta, according to Channel 2 Action News.  And there is an 80 percent chance of rain during the third round of the Masters at Augusta National Golf Club. “So it really looks like the cold front that will be impacting us will also be impacting the Masters for tomorrow,” Channel 2 meteorologist Katie Walls said. “I do think some of the play will be impacted.” RELATED: How to watch the 2018 Masters Tournament MORE: After one round, is Tiger Woods done at Masters? Rain started in far North Georgia earlier this afternoon, and now showers are beginning to arrive in the metro area. They are expected to continue overnight and into Saturday, likely tapering off around noon. “If you do have outdoor plans this evening, grab that umbrella,” Walls said. “Grab that poncho.” It’s currently 68 degrees in Atlanta. But it’s about to get colder.  A freeze watch was issued for part of metro Atlanta over the weekend. It applies to several North Georgia counties, including Bartow, Carroll, Cherokee, Cobb, Douglas, Floyd and Paulding, according to the National Weather Service. It is expected to send temperatures into the low 30s Saturday night into Sunday morning. The high is expected to reach only 62 Saturday, 60 Sunday and 61 Monday, when there is a 20 percent chance of rain. “Don't plant your gardens this weekend,” Channel 2 meteorologist Karen Minton said. “I'll keep you posted when it's safe.” The pollen count is still in the high range. It increased from 1,254 particles per cubic meter of air Thursday to 1,495 Friday, according to Atlanta Allergy and Asthma. “Pollen counts this morning were just 5 particles short of ‘extreme,’” Walls said.  But even though the count is high, it’s still below the 4,667 total achieved Wednesday. » Download The Atlanta Journal-Constitution app for weather alerts on-the-go.
  • I don’t have a vegetable garden but I will hold off planting any new sensitive bushes or flowers until around or after April 20th just to be safe given the Topsy turvy pattern that looks to continue. Cold snaps and warm spells look to continue to trade back and forth. At least one more and possibly two more nights with lows in the 30s before this month is done, especially away from the urban heat island around in and around I-285. But then as often happens the rubber band snaps and suddenly there’s a big warm-up and winter is firmly in the rear-view mirror and much of the country lunges toward summer by May and June as the jet stream migrates or leaps north and upper level ridge takes hold in the south-land. The back and forth means and active storm track continues next couple weeks with precipitation every two or three days averaging out above-normal: Follow me on Twitter @MellishMeterWSB.
  • Beating a record is usually a good thing, but not this time. Wednesday’s pollen count soared to an extremely high 4,667 particles per cubic meter of air, which is bad news for allergy sufferers, according to Atlanta Allergy and Asthma. RELATED: Feeling yucky? It’s probably the high pollen count RELATED: Tips to allergy-proof your home RELATED: Atlanta's pollen: A bad wheezin' and sneezin' season RELATED: Metro Atlanta pollen count and allergy index That eclipses last year’s season high of 3,559 and the 2016 season high of 4,106. “Today marks the highest pollen count since April 9, 2015,” Atlanta Allergy and Asthma said in a statement to the The Atlanta Journal-Constitution. “On that day it was 6,152.” HIGHEST POLLEN COUNTS OVER THE PAST DECADE March 31, 2017: 3,559March 29, 2016: 4,106April 9, 2015: 6,152April 4, 2014: 5,230April 11, 2013: 8,023March 20, 2012: 9,369March 24, 2011: 3,939April 7, 2010: 5,733April 6, 2009: 3,584April 14, 2008: 3,332 After days of temps in the 80s, trees and other plants are finally releasing excessive pollen, Atlanta Allergy and Asthma Dr. Stanley Fineman told The AJC. “The windy conditions also contribute to higher pollen counts,” he said. “We are seeing patients with increasing allergy symptoms in light of these weather conditions.” The good news, however, for allergy sufferers is that temperatures are dropping. Bartow, Cherokee and Paulding counties are even facing a freeze warning early Thursday, according to Channel 2 Action News. MORE: Freeze warning issued as rain subsides » Download The Atlanta Journal-Constitution app for weather alerts on-the-go.
  • An overnight storm sent a pole onto a fire truck and a tree into three houses in one southwest Atlanta neighborhood. Rosie Gray, who has lived on White Oak Avenue for nine years, said Friday she saw a wire catch fire in the tree before her roof caved in.  “I had just laid down, and I heard something crumbling,” she told The Atlanta Journal-Constitution.  Gray called 911 and dispatchers sent a fire crew to the scene. About 15 minutes after firefighters arrived, a pole came down on the fire truck, Gray said.  “And the next thing I know, the whole tree was down and that roof is in my bedroom window,” she said. Although Gray escaped the wreckage, she was hit in the head by an unknown object sent flying from her bedroom dresser. No other injuries have been reported in the incident. 
  • ATLANTA FORECAST Today: Clearing and windy. High: 65 Tonight: Mostly clear. Low: 54 Tomorrow: Sunny. High: 70 » For a detailed forecast, visit The Atlanta Journal-Constitution weather page. Overnight rains ushered in cooler air Friday, but temperatures are expected to rise back to the 70s this weekend, according to Channel 2 Action News.  And expect more of that yellow stuff. “It’s going to be great this weekend,” Channel 2 Action News meteorologist Katie Walls said. “But if you’re an allergy sufferer, bundle up.” The pollen count soared from 1,573 particles per cubic meter of air Thursday to 2,928 Friday, according to Atlanta Allergy and Asthma. That is the highest total so far this season. And “it's expected to climb higher over the weekend with a sunny sky and warming temperatures,” Channel 2 meteorologist Karen Minton said. Last year’s season high was 3,559. The 2016 season high was 4,107. Atlanta could see highs of 70 degrees Saturday and 75 Sunday, according to the latest forecast. Although it’s failed to help out the pollen count, recent wet weather has helped in other areas. “Most of North Georgia is drought free,” Minton said. The exception is an area south of I-20 that is experiencing moderate to abnormally dry conditions. There’s no rain in the forecast until Tuesday, when there is a 20 percent chance, Channel 2 reported. Until then, expect a pleasant Easter weekend. Currently, it’s 56 degrees in Atlanta. The high reached 63. » Download The Atlanta Journal-Constitution app for weather alerts on-the-go.