Unit 4 - All Weather PDF

WEATHER Meteorology: Unit 4  Air Masses & Fronts Meteorology: Unit 4  Air Masses Air Mass - an extremely large body of air with similar temperature and moisture across the air mass at any given altitude Source region - the area where the air mass originates. ○ Most often polar...

WEATHER Meteorology: Unit 4  Air Masses & Fronts Meteorology: Unit 4  Air Masses Air Mass - an extremely large body of air with similar temperature and moisture across the air mass at any given altitude Source region - the area where the air mass originates. ○ Most often polar or tropical ○ Mid-latitudes is an area where air masses clash & produce a variety of Air Masses Classifications - according to temperature and humidity/moisture (influenced by the source region) Humidity level ○ Maritime (moist) ○ Continental (dry) Temperature ○ Polar (cold) ○ Tropical (warm) ○ Arctic (extremely cold) Air Masses of North America cP and cA - produce extreme cold winter weather as they move across N. America Lake Effect Snow - occurs when extremely cold air moves over aon heavy snow relatively the downwind shoreline warm body of water (e.g. Great Lakes) and creates Air Masses of North America mP air masses are cold & moist and are responsible for the cold, damp weather along NE coast of N. America AND for cold, rainy winter weather along the west coast of N. America  Air Masses of North America mT are warm & humid and are responsible for the hot, muggy weather found in the eastern United States in summer. cT are hot & dry and are responsible for heat waves of summer in the western half of the United States. Fronts Front - is a transition zone between 2 air masses of different densities Often associated with temperature differences and humidity differences 4 types Fronts How do we locate a front on a surface map? Sharp temp. change over relatively short distance Changes in air’s moisture content (shown by dew pt) Shifts in wind direction Pressure & pressure changes Clouds & precipitation patterns Fronts Cold Front: Represents a zone where cold stable air replaces warm unstable air Drawn with a solid blue line with blue triangles along the front showing the direction of movement Often have rain & strong thunderstorms along the leading edge of the front Air behind the front cools quickly Cold Front, cont. Tend to move toward the S, SE, or E ○ Occasionally special conditions can create a cold front that moves in FROM the east - known as a “back door” cold front ○ NC can also experience cold air damming - when a cold front moving westward stalls at the Appalachian Mts … Fronts Warm Front: Represents a zone where warm unstable air replaces a colder air mass Drawn with a solid red line with red half circles (semi-circles) along the front showing the direction of movement Moves much slower than cold fronts (~½ speed) Associated with light rain and more gradual changes than what occurs Fronts Stationary Front: Has essentially no movement Red semi-circles point toward colder air Blue triangles point toward warmer air The line marks the boundary where cold, dense air meets warm air Can transition to either a warm front or cold front, depending on which side retreats & which pushes Fronts Occluded Front: Forms when a cold front catches up to and overtakes a warm front Shown by a purple line with alternating cold front triangles and warm front half circles ○ On the same side of the line ○ Pointing in the direction the front is traveling Air behind the front is colder than air ahead of it Can have heavy precipitation at the point where the cold front overtakes  Middle Latitude Cyclones Meteorology: Unit 4 Mid-Latitude Cyclones The fluid (moving) boundary where warm tropical air masses meet cold polar air masses is known as the polar front. The ridges (∩) and troughs (∪) move and change (“stretchy”) with the different air masses that create this boundary. The meeting of contrasting air masses (polar and tropical) here leads to the development of mid-latitude Cyclogenesis The Norwegian Model (1910s- 1920s) stated that depressions form from waves that develop along a polar front. This was expanded further to today’s model of cyclogenesis. Cyclogenesis is the generation of depressions and the formation Remember that cyclone ≠ of cyclones. tornado. Stages of Cyclogenesis 1. A segment of the polar front is a stationary front High pressure is on both sides ○ Warm air below, blowing from the west ○ Cold air above, blowing from the east This create wind shear ○ Counterclockwise = cyclonic Stages of Cyclogenesis 2. A low pressure system develops at the point of rotation - which is the junction of the 2 fronts (known as “central pressure”) Stationary front starts to move ○ Warm front to the east ○ Cold front to the west Precipitation begins to form at this junction Stages of Cyclogenesis 3. Cold air along the cold front displaces the warm air upward This creates precipitation ahead of the warm front The “kink” deepens ○ Several isobars surround the central Precipitation pressure along (kink) the cold front is greater than along the warm front because faster motion = greater lift An area of warm air (warm sector) develops between the 2 fronts ○ Usually partially cloudy, could have rain/storms if Stages of Cyclogenesis 4. The cold front moves faster than the warm front Warm sector decreases in size Central pressure continues to drop It is now known as a mature cyclone Greatest likelihood for intense thunderstorms ○ Widespread rain Stages of Cyclogenesis 5. The cold and warm fronts merge at the point of central pressure (kink) and an occluded front forms at this junction. Referred to as the “triple point” The system begins to dissipate ○ The warm because sector cold air is away from the is farther now on both center sides of the of the storm, therefore no more energy occluded front from the rising moist air. Stages of Cyclogenesis The old storm dies out and gradually disappears. This entire cycle can take a few days to more than a week. It can easily cover a large amount of land (or water). Convergence & Divergence Convergence: piling up of air in a region (as it slows down) Divergence: spreading out of air in a region (as it speeds up) These both occur all along the polar front.  divergence convergenc convergence e divergenc e  Convergence & Divergence Both can occur from changes in either wind speed or wind direction. Convergence: Winds coming together (traffic merging) Winds slowing down Divergence: Winds spreading apart (traffic with new lane) Winds speeding up Conditions for Cyclogenesis What conditions lead to cyclogenesis along the polar front? The answer is complicated because there are many factors. Two main factors: 1. Ventilation/air flow 2. Sharp contrast between temperatures of opposing air masses Conditions for Cyclogenesis Ventilation of the rising/lifting column of air Convergent flow at the surface pushes air inward Divergent flow aloft pulls air outward ○ This comes from the jet stream because the speed Dips south changes at a trough andas slows down = it meanders convergence As flow leaves the trough, it speeds up = divergence  Conditions for Cyclogenesis Requires a sharp contrast between the temperatures of opposing air masses. There are several areas in the US where this is more likely to occur. Hatteras Low - warm gulf water creates warm moist air below the stationary front ○ Often creates Nor’easters which move up the Atlantic coast and affect the northeastern US Colorado Low - forms on the leeward side of the Rockies (known as a leeward low) ○ Provides mP air and then meets continental air (either P or T) Special Cyclonic Storms The severity of the storm can be measured by the rate at which the central pressure falls. When the central pressure falls by more than 24 mb in 24 hours (extremely steep pressure gradient), it becomes known as a March bomb cyclone. High 2022 winds, rapid cloud development and heavy rain March 26, 2014 had a bomb cyclone where the No cyclone has a life cycle that exactly follows this model. Most depressions do have a warm front & cold front that could eventually lead to occlusion BUT the strength of each varies Not all depressions provide the same amount of precipitation Some form quickly & fizzle out and some persist for more than a week AKA: meteorology/predicting the future is hard!  Weather Forecasting Meteorology: Unit 4 Weather forecasts are issued to save lives, property, & crops and to tell us what to expect in our atmospheric environment. vs Weather Observations We use observations to predict how Dopple r current atmospheric conditions will Radar Weath change into the future. er Statio Station data (temp, dew pt, n wind, pressure @ various heights) ○ Over 10,000 land-based stations ○ Hundreds of ship & buoy NWS office in Morehead City, NC stations ○ Airports gather info every hour ○ Weather balloons ○ Aircraft gather data during Weather Observations Satellite images Dopple ○ Visible and infrared images r Doppler radar data (there are Radar Weath er 155 doppler radar units in the Statio USA) n ○ Provides round-the-clock information on rain, snow, sleet, and hail NWS office in Morehead City, ○ Provides various storm NC warnings ○ Forecast This info isfunnel put into- a sequence of steps forecasters computer use to analyze atmospheric conditions as they move from large models scale to small scale forecasts Weather Forecasting Methods Numerical weather prediction - routine daily forecasting of weather by computers using mathematical equations Models are used because atmospheric conditions are always changing ○ Based on Weather Forecasting Methods Many different models exist: One standard NWS model produces weather depictions every hour out to 36 hours ahead Another comes out every 3 hours, but projects forward 84 hours (3.5 days) Another forecasts 16 days into the future or seasonally  Weather Forecasting Models The final forecast chart representing atmosphere at a specified future time is called a prognostic chart (aka “prog chart”) These require super-computers. 24 hr forecast for N. Hemisphere requires 100s of millions of mathematical calculations Many different computer models exist - each based on slightly different analyses - that are used in conjunction with each other to see a “full picture” of the atmosphere Forecast models can reasonably predict weather about 4-6 days into the future. Temperature is easier to predict than precipitation - which has far more variables involved. Interpretation of each atmospheric level on the following slide Forecasting Techniques Ensemble Forecasting uses several forecast models and looking for similarities (where they agree) They often have slightly different initial conditions to account for the degree of uncertainty Sometimes referred to as a spaghetti plot Forecasting Techniques Observational Forecasting: Some types of weather predictions can still be made by observing the sky Oldest methods of forecasting ○ Ring around the moon, rain or snow in the next three days ○ The higher the clouds, the better the weather ○ When clouds appear like towers, the Earth is refreshed Forecasting Techniques Persistence forecast - predicting that future weather will be the same as present weather Trend forecast - (aka steady state) surface weather systems tend to operate the same way they have been moving Pattern recognition forecast - when existing features strongly resemble features that produced certain weather conditions in the past (“I’ve seen this before …”)  Forecasting Techniques Probability forecast - based on prior 30 years of data (ex. probability of a white Christmas) Weather type forecast - where weather patterns are grouped into similar groups or types (ex. “when the jet stream dips down, storms tend to …”) Climatological forecast - typically made a month or more out and are based on known climates of an area, rather than current conditions What is a chance of rain? 1. The percent coverage of area that will see rain 2. The percent of time in the day it will be raining 3. How confident the forecaster is in it raining based on the atmosphere 4. A random number that a dog picks based on which food bowl it eats from What is a chance of rain? 1. The percent coverage of area that will see rain 2. The percent of time in the day it will be raining 3. How confident the forecaster is in it raining based on the atmosphere 4. A random number that a dog picks based on which food bowl it eats from A 40% chance of rain means that if you have the same atmosphere 10 times, a specific point will see measurable rain (>0.01”) 4 out of the 10 times. This is dependent on where the point is and how long the time range is (1-hour vs. 24- hours). Ideally, if you group together each day there is a 40% chance of rain, 40% of them will have measured at least 0.01” of rain for that location. Time Range of Forecasts Very short range (aka nowcast) - up to 6 hours Uses Doppler radar and steady- state forecasting Pattern recognition & prior experience play a significant role Watch vs. Warning ○ Watch - conditions favor hazardous weather during a given time period in a given area ○ Warning - the hazard is Time Range of Forecasts Short range - range from 12 - 72 hours (3 days) Uses Doppler radar and surface weather maps, as well as pattern recognition Medium range - extends from 3-8 days into the future Uses statistical forecasts and charts Long range - extends beyond 8 days (up to 16 days) Uses computer modeling Often referred to as “outlooks” ○ Not so much forecasts, but more of an overview of expected temperature and precipitation patterns Time Range of Forecasts Climate Prediction Center Creates seasonal forecasts Based on teleconnections (linkage between weather changes occurring in other regions of the world) Ex. El Nino and La Nina lead to predicted changes for rest of US Accuracy and Skill in Weather Forecasting In the US, there is a forecasting bias for fair weather. If your forecast was for fair weather EVERY day, you’d be right 50% of the time. Requires no skill Doesn’t prioritize accuracy Skilled forecasts should be better than one based solely on persistence (current weather) and climatology (normal patterns) for that region. Accuracy and Skill in Weather Forecasting Forecasting large-scale weather events several days in advance is more accurate than forecasting the precise movements and conditions of short-lived events like thunderstorms and tornadoes. With improved technologies, the accuracy of forecasts is improving … saving lives and property. This also depends on the abilities of the weather forecasters to effectively communicate forecasts to the public. Determining Movement of Weather Systems (Rules of Thumb) Mid-latitude cyclonic storms and fronts move in the same direction and speed as for the past 6 hours. Low-pressure areas move in a direction that parallels isobars of warm air. Lows move toward the greatest pressure drop. Highs move toward the greatest pressure rise. Surface pressure systems move in the same direction as the wind at 5,500 m. Forecasting: Current Conditions for 6 Cities Shown Forecasting: Projected Conditions for 6 Cities Shown - 12 & 24 hr Forecasting for Denver Current High pressure is projected to move southward (slightly east) Projected High pressure should indicate minimal cloud cover Colder air will move in with the high pressure system Winds should continue to come from the north-  Current Forecasting for Dallas Increasing cloud cover with good chance of showers and Projected thunderstorms Colder air will come in with the passing cold front Winds should become northwesterly Skies should be clear  Current Forecasting for Memphis Light rain should continue Projected as the warm front moves through Temperatures projected to rise with the passing warm front Later on the cold front will move in and bring ○ Colder temperatures ○ Stronger winds - from the south/southwest  Current Forecasting for Augusta Winds from the south should bring warmer air, causing Projected temperatures to rise Humidity should increase, leading to increased cloud cover Rain showers should be in the future (but not in the next 24 hours) Forecasting for Chicago Low pressure system projected to move east Temperatures should remain cold (below freezing) Predicted snowfall as system moves closer ○ Could be heavy due to lake effect snow Winds should be from the east  Current Forecasting for Washington, DC Clouds projected to increase as high pressure system pushes offshore and low pressure system moves Projecte in Temperatures should remain cold (around freezing) Precipitation approaches and may start as snow if the cold front arrives first