The Jet Stream … and its associated upper-air features Polar jet Subtropical Jet By: Michael Vuotto and Jake Mulholland

What is a Jet stream? Jet streams are “swiftly flowing air currents thousands of kilometers long, a few kilometers wide, and only a few kilometers thick.” (Meteorology Today: Ahrens) Usually found between 6mi and 9mi above sea level (Or between 500250mb level) What is the underlying cause of the Jet Stream … ???? (Answer: North/South Temp. Gradient)

How they were discovered? First evidence of the jet stream was found during WWII Fighter aircraft encountered high altitude “currents” of high velocity winds traveling from west to east during their missions

How jet streams form? Differences in air temperature cause differences in air pressure  higher wind speeds  the jet stream The greater this difference (Greater delta-T’s), the greater the wind speeds within the jet When is this difference greatest? (i.e. – when is the jet stream the strongest?) weakest? Usually this difference is greatest in winter and weakest in summer

Names for Common Jet Streams “Polar Jet Stream” – Occurs within the higher latitudes, where cold Arctic air clashes with the warmer southern air “Sub-Tropical Jet Stream” – Occurs ~ 20˚ lat. north and south of the equator “Eastern Asian Jet Stream” – Eastern Asia  duhh! Why aren’t there any jet streams typically in the tropics … ????

Effects of the jet stream (3 main ones …) 1) Transfers heat from the equator pole ward and cold air equator ward (mixes out temperature gradients) 2) Helps in formation of mid-latitude cyclones (i.e. – upper-level divergence!) 3) Steering mechanism for mid-latitude cyclones

The low-level jet A stream of fast flowing air that typically occurs at night (after boundary layer decoupling-above the stable nighttime layer) Also known as the “nocturnal jet stream” or “nocturnal low-level wind maximum” Helps in the formation of thunderstorms at night over the Central Plains (ex - Moisture Transport) Also adds shear to atmosphere  good for severe t-storm development Usually we look at the 850mb level for the presence of one

Low level jet at the 850mb level

Severe Weather and the LLJ … 00z – 7pm night of the El Reno tornado … widest tornado in history … *Anyone know the date of this map … ???? 00z – 850 map

Sounding Analysis – May 19th 2013

Jet stream’s role in mid-latitude cyclone formation For a surface storm to intensify, diverging air above the storm must be present Jet streams help by removing air from above a developing surface cyclone and adding air to the surface of a surface anti-cyclone A special feature of the jet stream helps aid in this development …. Any guesses … ???? Answer … see next slide …

The Jet Streak A jet streak is the maximum wind speed area of a jet stream They have winds of at least 50 kts Also help with formation of mid-latitude cyclone formation because they naturally have areas of convergence/divergence and associated rising and sinking motions See next page …

Cont. jet streak

Another image …

In Correlation with Severe Wx. Jet streaks, with their divergent areas, help in the formation of severe thunderstorms, including supercells Divergent air aloft surface convergence positive bouyancy of air parcels formation of clouds showers, storms, supercells, etc.

Other notes The jet stream typically steers mid-latitude cyclones (i.e. - 500mb flow) When discussing this topic we like to ask the question: “Does the storm have upper-air support?” (aka- jet stream present???!!!) Typically we look at the _________ mb level for jet streams and jet streaks Wind speeds may exceed 275mph within a jet streak!!!

Other Noteworthy Upper-Air Features Rossby Waves (i.e. – long waves) Short Waves Blocking Patterns (Helps “back” up the atmosphere and prevents systems from continual propagation) See examples on next few slides … Also, upper-level fronts (not included)

Rossby Waves (Long Waves)

Short Waves

Omega Blocks

300mb chart from GFS showing Omega Block Any guesses what this is … ????

Ex – Greenland Block

Rex Blocks

500mb chart from GFS showing Rex Block

Useful websites Jet stream analysis and forecasts at 300mb http://squall.sfsu.edu/crws/jetstream.html USA Wind Map http://hint.fm/wind/ Earth Wind Map http://earth.nullschool.net/ Current and forecast Jet stream positions http://www.weatherimages.org/data/imag192.html http://www.stormsurfing.com/cgi/display_alt.cgi?a=glob_250

*SHOW THEM HOW TO GO ONTO GARP TO LOOK AT MODEL DATA FOR 500 MB LEVEL, 850 LEVEL, 300MB LEVEL, ETC.* Let’s now use what we learned about the Jet Stream to forecast this winter storm for CNY and Oswego! (aka – look for the Jet Streak over our region! )