Microburst Vs Tornado

Microburst Vs Tornadoes

A microburst is a strong, localized column of air that sinks within a thunderstorm with a diameter of 2.5 miles or less. A microburst can occur in two different ways:

  • Wet Microbursts are accompanied by heavy precipitation.
  • Dry Microbursts do not contain precipitation as most of it evaporates before reaching the ground.

Microbursts form from strong thunderstorms that suspend rain/ice high up in the cloud from the strong updraft. As dry air works in from the mid levels, it causes evaporational cooling making the air “heavier”. The “heavier” air becomes more dense than the surrounding air and sinks to the surface. Other factors outside of evaporation include, melting of hail in the cloud and downward transport of higher momentum from the drag of the precipitation.

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Figure 1: A picture of a microburst with the colder air in the mid levels sinking to the surface.

Tornadoes

 The stronger more significant tornadoes form from supercell thunderstorms. Supercell thunderstorms have a rotating updraft that allows the thunderstorm to sustain its intensity for long periods of time.

Supercells are favored when the vertical shear is strong (greater than 30Kts) over the lower 6km of the atmosphere. This produces a separation from the warm inflow air and the cooler downdraft air. This process leads to strong upward motion in the updraft if the instability is strong enough.

Supercells gain rotation from the environmental winds in the lower levels increasing and changing direction with height. Directional shear refers to the winds turning with height, ie. from the south at the surface and westerly aloft. Vertical shear refers to the winds increasing speeds with height. The combination of these two variables produces the rotating updraft.

Not all supercells produce tornadoes, the number of supercells that spawn tornadoes is estimated at 20%. From case to case, it is very difficult to distinguish why some produce tornadoes and others do not. The latest research from Paul Markowski states that the temperature of the rain cooled air from the downdraft may be a key player. The intersection of the warm inflow and the downdraft on the forward flank produces baroclinic vorticity near the surface as the warm air overrides the cooler air. This vorticity generated parallel to the storm inflow is then wrapped around the supercell and stretched by the strong instability and vertical pressure gradients in the supercell. IF the outflow is too cold it becomes much less likely to be stretched as the air is not as buoyant. If it is cool but not cold it can be rapidly stretched despite being less buoyant. Each supercell is different in terms of how strong the “suction” is and how cool/cold the downdraft air is.

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Figure 2 Paul Markowski displaying tornadogensis from a supercell thunderstorm.

Tornadoes can also occur in ordinary thunderstorms through stretching of pre existing vertical vorticity. This is known as a “landspout”.

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Figure 3: Formation of a landspout.

 

Distinguishing between microburst vs Tornado:

microvstor

 

References:

http://www.nssl.noaa.gov/education/svrwx101/tornadoes/types/

 

https://sites.psu.edu/pmarkowski/how-tornadoes-form/

 

https://www.usna.edu/Users/oceano/barrett/SO441/markowski_richardson_2009.pdf

 

https://www.weather.gov/bmx/outreach_microb

Severe Weather Outlook April 20, 2017

Synopsis: A cold front will push through the region during the afternoon hours with a broken line of strong to severe thunderstorms. The thunderstorms will initiate along the cold front around 2pm CDT over Southern Illinois, Indiana, and into Southern Michigan. The thunderstorm activity will form into small line segments that will move eastward through the late afternoon/early evening hours. The main threat from the thunderstorm activity will be damaging wind gusts with hail upwards of 1-1.5″ in diameter in the stronger thunderstorms. The tornado threat will exist but will be on the low end, given the overall uni-directional wind shear profile. This will favor the development of a few bowing segments that will enhance the wind potential.

2-6pm CDT:

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The area outlined in black is where we expect the biggest threat for thunderstorm activity capable of strong wind gust. At the initiation stages, the wind shear will favor storms getting off the boundary. This means that they will be able to maintain discrete/semi-discrete structures resulting in the best chance for an isolated tornado/large hail report. As more storm develop, line segments will begin to form making the main concern damaging wind gust.

 

6-10pm CDT:

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Thunderstorms will be in several different line segments with the possibility of a few bowing segments (enhanced damaging wind potential). As we head towards 9-10pm CDT the instability will begin to weaken resulting in the weakening of the thunderstorms as they push into Central Ohio and Western Pennsylvania. From 6-9pm CDT the main threat will be for damaging wind gust from Southern Illinois through Indiana into Ohio.

The hail forecast from TrueWx shows 1-1.5″ diameter. a secondary area is seen over Western PA but this is dependent on if strong thunderstorms can develop well ahead of the stronger forcing. The lightning projection is showing frequent lightning with these thunderstorms.

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Severe Weather Outlook Thursday April 6, 2017

A cold front will push through the region in the morning hours on Thursday resulting in a rather unusual set up for severe weather starting in the morning hours and coming to and end around noon. The greatest severe threat is expected across North Carolina, Virginia, into Southern MD and Southern DE. To the north the threat for strong to severe storms will be possible, but the main threat will be from hail. The warm front failing to move through will keep thunderstorms “elevated”. This means that the threat for damaging winds and tornadoes will not occur in the regions north of the warm front.

8am

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At 8am, surface based thunderstorms will be ongoing across Virginia. This threat will continue to develop and expand eastward. Forecast soundings off the latest 3km NAM show an elevated mixed layer which supports the discrete supercell solution being portrayed in the image above. This will allow for the supercell(s) to take full advantage of the enhanced shear environment for the potential for a strong tornado. Our TrueWx tornado index is highlighting this potential with the morning supercell activity across VA as seen below.

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11AM:

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The supercells over Virginia begin to congeal into a line segment. This is because of the 0-6km vertical shear being largely parallel to the forcing which allows the line segment to form. However, tornadoes within the line and any semi-discrete supercells will be capable of producing a strong tornado. But with the congealing into a line segment this does bring down the threat slightly for a strong tornado, the winds will become the bigger threat in terms of coverage. To the north, we see soundings show over 1500J/Kg of CAPE with temperatures in the lower 40s! A very unusual threat but the CAPE through the hail growth layer will be capable of producing marginally severe hail of 1″ diameter. Areas on the border of the red/black outline need to pay close attention as a slight shift in the warm front could lead to a north or south adjustment.

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The top sounding is the elevated storms outlined in black and the bottom sounding is the area outlined in red across eastern VA. The bottom sounding shows a strong shear environment with strong tornado possibilities. As previously mentioned, the strong tornado threat will be present across the red outlined area but the congealing into line segments does diminish it slightly.

2PM

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The majority of the line moves well offshore by 2pm with storms lingering over NJ/PA into Southern New England. The main threat will be for small hail across the north with areas in the southern portion of the black outline receiving marginally severe hail. Areas in Southern NJ will see enough warming at the surface for winds to mix down to the surface, but these winds may be below severe limits.

 

Lightning:

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Lightning will be impressive from PA down into NC during the late morning hours Thursday.

Severe Weather Outlook Wednesday April 5, 2017

Synopsis:

We are watching two main areas for severe weather development on Wednesday. The first area is associated with an area of low pressure across Illinois which will extend a warm front over portions of Indiana and a cold front to the south. The second area will be a combination of a morning frontal boundary that lingers over the Southeastern US and the aforementioned cold front that swings through in the afternoon/evening hours.

Indiana/Kentucky:

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Active: We expect the strongest thunderstorms to develop around 3pm CDT and continue through 8pm CDT.

Latest 3km NAM shows discrete-semi discrete supercell formations through Kentucky and into Indiana where the warm front will be located. Any storm interaction with the warm front will enhance the potential for a tornado. Forecast soundings out of Kentucky are showing MLCAPE values around 2000J/Kg (with impressive CAPE found in the lower 3km) and veering winds in the lower levels of the atmosphere. This would support the threat for the potential for a strong tornado. The hail index for near surface based development also hints at large hail upwards of 1.75-2″ in diameter in the stronger thunderstorms that develop.

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Southeastern US

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Convection will be ongoing in the morning hours, which will add a degree of uncertainty to destabilization throughout the late morning and afternoon hours. This is why we are cautious on extending threat into Northern GA to start the day. Most of the activity will be elevated and pose a low risk for severe weather in the mid morning hours, but as we head towards the late morning and early afternoon activity will become rooted at/near the surface which will increase the potential for large hail/tornadoes (possibly strong) across the area highlighted. The second part of the day will feature a line of strong thunderstorms ahead of the cold front. The greatest threat will be for winds/hail across AL with the higher tornado threat occurring over TN/KY.  Images below valid for 1pm CDT show the potential for strong tornadoes and large hail greater than 2″ in diameter.

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Precipitation Type Forecasting

In most winter weather events, we see several different precipitation types ranging from snow, sleet, freezing rain, and plain rain. The temperature profile through the middle and lower levels of our atmosphere determine what precipitation type will fall.

Freezing Rain:

When freezing rain occurs it can occur from two different processes. The first one involves a snowflake aloft that enters a warm layer. This warm layer melts the entire snowflake/ice crystal into a water droplet. The water droplet then falls to the ground, where the surface is at or below freezing (32F). The temperature could be warmer than 32F, but as long as the ground temp is at or below freezing the rain will freeze. This is displayed on a Skew-T/Log-P diagram with the temperature being displayed diagonal in Celsius. The solid black line is the temperature with the hatched line being the dew point:

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Freezing rain sounding from the NWS Louisville http://www.weather.gov/lmk/

The image taken from the National Weather Service in Louisville shows the temperature profile of a freezing rain event. The temperature could go back to below freezing after the snowflake melted in the cloud, but for ice crystallization to occur we typically need to see the droplet reside in temperatures at or below -10C. This leads us to our second way of freezing rain occurring…

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Freezing rain sounding from the NWS Louisville http://www.weather.gov/lmk/

The sounding shows temperatures below freezing throughout the column, but the precipitation type will still be freezing rain. The lack of saturation in the cloud above 700mb only allows for temperatures of -5C in the cloud. This is not cold enough for ice crystallization to occur resulting in water droplets remaining super cooled. As a result, the shallow layer of moisture ends up producing light freezing rain that freezes on the cold surface below freezing.

 

Sleet:

Sleet has a warm layer involved in the cloud, but it is typically shallow or not as warm. Instead of the snowflake completely melting, as mentioned in the freezing rain case, the snowflake partially melts. This keeps the ice nucleus intact allowing for refreezing once the temperature drops below freezing.

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Sleet sounding from the NWS Louisville, http://www.weather.gov/lmk/

Snow:

 

The whole column is below freezing with ample moisture at or below -10C. This allows for enough ice crystallization for the snow to fall to the ground.

 

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Snow sounding from the NWS Louisville, http://www.weather.gov/lmk/

El Nino and La Nina

Sea Surface Temperature Anomalies (SSTA) in the equatorial Pacific alternate between warm and cool periods. The cooling of the equatorial Pacific waters is known as La Nina. The warming of the equatorial pacific waters is known as El Nino.

The SSTA are measured in 4 locations 1+2, 3, 3.4, and 4:

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ENSO Regions from the Climate Prediction Center. www.cpc.ncep.noaa.gov

Region 3.4 is used as the primary region of measuring for El Nino and La Nina. When the 3 month average of region 3.4 reaches -0.5C (or lower) or 0.5C (higher) for 5 consecutive monthly readings it is considered an El Nino or La Nina.

During La Nina, the easterly trade winds are stronger across the central-eastern basins of the equatorial Pacific. This creates upwelling and enhanced evaporation along the ocean’s surface cooling the waters. The warmer waters are pushed towards the Maritime Continent. The strength and orientation of the La Nina plays a factor in the pattern that follows for the United States.

The cooler waters having lower height anomalies over the equatorial Pacific create easterly momentum over the Tropics. This results in a lack of a southerly jet with the northerly jet being the big player. The ridging over the Northeast Pacific allows a dip in the Jet Stream over the Midwest and Northeast. Warmer than normal conditions is shown in the Southern half of the US extending into the Southeast.

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Typical pattern associated with La Nina from the Climate Prediction Center. www.cpc.ncep.noaa.gov

Typically a more basin-wide/stronger La Nina will focus the cold further west allowing for a warmer Eastern US. Another caveat is the strength of the polar vortex, as a strong polar vortex could allow for a much flatter ridge in the Northeast Pacific with a strong pacific jet. This would bring relatively milder air to much of the CONUS.

During the El Nino winter seasons, the tradewinds are much weaker over the central and eastern equatorial pacific basins. This allows the warmer waters over the Maritime Continent to push westward. The orientation of the warmer waters in the equatorial Pacific plays a crucial role in the winter weather pattern. If the warmer waters focus in the eastern basins this would result in a much stronger low in the Gulf of Alaska. This would flood the CONUS with milder Pacific air. A central-west based El Nino opens the door to a much more favorable pattern for cold and snow across the Eastern US with the strong low centered more towards the Aleutian Islands. This acts to pump up a strong ridge in the Western US that allows cold air to dive southward.

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Typical pattern associated with an El Nino winter from the Climate Prediction Center. www.cpc.ncep.noaa.gov.

The higher height anomalies from the warmer water and latent heating create a strong pressure gradient at a lower latitude than La Nina. This produces a strong southerly jet stream that brings in moisture across the Southern US with below average temperatures.

Arctic Oscillation and Polar Vortex

The polar vortex is a pool of cold air over the Arctic. This polar vortex develops from the difference in temperatures between the cold arctic air and the warmer air across the southern latitudes. When the polar vortex is strong it remains in place over the arctic with the cold air being confined to the northern latitudes with a fast polar jet. When the polar vortex is weaker it is more susceptible to warming, which allows the polar vortex to be split or displaced. This displacement allows for big dips in the jet stream that bring cold air south in the Middle Latitudes.

A measure of the polar vortex strength can be figured out through the use of the Arctic Oscillation. During the positive phase of the Arctic Oscillation, the upper-level height anomalies over the Arctic are below average. This signifies a strong polar vortex with cold air being wrapped up.

During the negative phase of the Arctic Oscillation, the upper-level height anomalies are positive over the Arctic. This shows a much weaker and displaced polar vortex with cold air diving southward.

The polar vortex is a cold core anomaly with very high static stability from the stratosphere. To weaken this vortex you need to warm up it up and disrupt the strong flow around it.

 

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Negative and Positive phase of the arctic oscillation from the NCDC http://www1.ncdc.noaa.gov/pub/data/cmb/bams-sotc/2010/bams-sotc-2010-brochure-hi-rez.pdf

The winter of 1988-89 featured a strong polar vortex with the cold air being locked up in the northern latitudes. The 500mb anomaly of that winter is pictured below with higher heights and warming in the Middle Latitudes:

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The winter of 1988-89 featured a very positive phase of the Arctic Oscillation. Reanalysis data from NCEP/UCAR. http://www.esrl.noaa.gov/psd/map/

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The winter of 2009-10 featured a very negative phase of the Arctic Oscillation. Reanalysis data from NCEP/UCAR. http://www.esrl.noaa.gov/psd/map/

Higher height anomalies were seen across the northern latitudes and lower height anomalies and cold air were displaced to the south across the Middle Latitudes.

Predicting the strength of the polar vortex is very difficult in long range forecasting, but we look into clues from the ENSO forcing, solar activity, and winds in the stratosphere. The quasi-biennial oscillation (QBO) is a measurement of the winds above the tropical stratosphere. During the westerly phase, we see a strong thermal gradient from the tropical regions (very warm) to the polar regions (cold). The easterly phase reduces the thermal gradient by cooling of the tropical regions and warming over the polar region.

Other factors can further influence the prediction of the strength of the polar vortex, such as the ENSO and solar activity. During a La Nina, we tend to see momentum added to the northerly jet, which could contribute to a stronger polar vortex. An El Nino reduces the momentum in the northerly jet with the strong southerly jet stream. This increases the chance of a weaker/disturbed vortex.

During a solar minimum, this could lead to a reduced thermal gradient between the pole/equator reducing the westerlies. This is what we are currently seeing October 2016 with low solar and a westerly QBO.

North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is the difference in sea-level pressure between the Icelandic Low and the Azores High. Fluctuations in the pressure differences alter the pattern across the North Atlantic, which has impacts that affect us in the Eastern United States.

During the positive phase, lower height anomalies can be seen over Greenland with higher height anomalies to the south over much of the Atlantic. The gradient between the higher Atlantic height anomalies and the lower height anomalies over Greenland produce a very strong jet stream traveling west to east. This prevents cold air from dropping southward and allows higher height anomalies (warmer air) to occur over Eastern US during the winter.

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A depiction of a positive NAO from the Climate Prediction Center. http://www.cpc.ncep.noaa.gov/data/teledoc/nao_map.shtml

This leads to the cold air to moving out before storm systems move towards the Northeast/Middle Atlantic.

 

During the negative phase of the NAO, the opposite occurs. The lower height anomalies over Greenland are replaced with higher height anomalies. The higher height anomalies in the Atlantic are replaced with lower height anomalies. This creates a weaker jet stream more susceptible to bending creating a meridional flow. This locks in cold high-pressure systems that are a key component to big snow events in the Northeast and Middle Atlantic.

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Negative phase of the NAO by the University of New Hampshire http://airmap.unh.edu/background/nao.html

The January 2016 Blizzard occurred during a negative period of the NAO. This allowed a strong high-pressure system to build across Eastern Canada and lock cold air in place:

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Analysis of the surface map from the Blizzard of 2016. Archived mesoanalysis is from spc.noaa.gov. Drawn in “H” and “L” to display the blocking.

Predicting the state of the NAO can be very challenging in 10-15 day forecast, but even more so in seasonal forecasting. The Sea Surface Temperature Anomalies (SSTA) make it possible for a seasonal forecast to gauge a prediction of the dominant state of the NAO.

The horseshoe SSTA pattern in the Atlantic can offer a lot of information to the probability of a negative or positive NAO:

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Sea surface temperature and NAO correlation from the University of New Hampshire http://airmap.unh.edu/background/nao.html

During a positive phase of the NAO, the SSTA pattern shows a cold horseshoe pattern extending from the equatorial Atlantic into Greenland. The cooler waters contribute to the lower height anomalies making the Icelandic low much stronger. The higher heights stay confined to the Central Atlantic creating a strong gradient accelerating the westerlies. The stronger westerlies are less susceptible to bending resulting in a lack of blocking.

A negative phase is shown by a warm horseshoe pattern across the equatorial Atlantic into Greenland. The reduced pressure gradient over the Northern Atlantic allows for a weaker jet more likely to bend and create a block.