Extreme Weather -- Exam 1

Weather

Everyday atmospheric conditions of a particular place over a short period of time

Climate

Aggregate of weather (averages and extremes) at a place over an extended period of time

Why do we have weather?

Sun causes weather, specifically weather is due to the differential heating received on Earth. Warm air and moisture @ equator moves towards poles, cold air and moisture @ poles move to equatorial. Atmospheric struggle for equilibrium creates changing weather

Weather Severity Continuum

Benign Weather —> gentle, mild pleasant

Nuisance Weather —> Inconvenient, no significant issues

Severe Weather —> Potential to cause serious damage or loss of life 

Extreme Weather —> Unexpected, unusual, unpredictable, or never-before seen weather events

Normal Value

The average and extreme values of weather for a place over a period of 30 years

Helps in describing the climate and is used as a base to which current conditions can be compared

“New” normal calculated every 10 years

NWS Mission

Provide weather, hydrologic, and climate forecasts and warnings for the US , its territories and ocean areas for protection of life and property

Outlook

Used to indicate that a hazardous weather event may develop, gives time to prepare

Weather Statement

Special weather statement that may be issued by the NWS for hazards that have not yet reached warning or advisory status

Advisory

Weather conditions are expected to cause a significant inconvenience, not serious enough to warrant a warning

Watch

Used when the risk of a hazardous weather event has increased significantly, but its occurrence, location, and timing is still uncertain

Issued when there is potential in 48 hours

Warning

Issued when a hazardous weather event is occurring, is imminent, or has a very high probability of occurring

Used for conditions posing a threat to life and property, 12-18 hours

“Ready”, “Set”, “Go” Concept

Three-tiered concept to convey severity

ex) Hydrologic Outlook (R) —> Flood Watch (S)  —> Flash Flood Warning (G)

How do civil emergencies differ from NWS alerts?

Issued by government entities, may relate to NWS emergencies but differ in their scope

Local, state, and federal levels

Empowered to perform actions that would not normally be permitted, spend money that isn’t budgeted, enact emergency plans

Wireless Emergency Alerts (WEA)

Potentially life-saving emergency messages, usually less than 90 characters, that are sent directly to your phone by authorized government alerting authorities through your mobile carrier

"Ordinary“ Warnings do not activate WEA, only those designated as catastrophic with threat tags like “emergency” are broadcasted

Heat Dome

Weather phenomenon consisting of extreme heat, caused when a strong high-pressure system traps hot air over a large area, and preventing it from escaping

Occurs over days or weeks

Forms when jet stream slows or stalls, leading to sinking air that compresses and heats the air below

Jet Stream

Fastest-moving part of the Rossby Wave, flows West to East, located at the top of the troposphere

Controls the location, strength, and direction of surface pressure systems

Zonal Jet Stream

Strong temperature gradient creates strong pressure gradient, which produces faster winds and zonal flow

Meridional Jet Stream

Weak temperature gradient, weak pressure gradient

Produces slower winds and meridional flow

Can cause extreme weather conditions

Jet Stream and Low-Pressure Systems

Air rises and cools, holds less moisture, higher relative humidity, precipitation

Can predict weather by knowing where the low-pressure system is going to go 

Jet Stream and High-Pressure Systems

Air compresses and heats, holds more water, lower relative humidity

Ridge in jet stream can cause heat waves or domes (high-pressure)

Omega Block

Formation in jet stream, named for the shape of the ridge, looks like Greek letter, omega

If Omega Block is stationary, heat waves become heat domes because descending air compresses, warms air, and stalled heat dome diverts other weather around it

How does a heat dome self-sustain?

Omega Block traps a pocket of warm air, ground warms, loses moisture (easier to heat), absence of clouds causes sunlight to further heat the air, cloudy and cool air masses are re-routed around the dome

Sensible Heat

All heat energy is used to raise the surface temperature, can be “sensed”, DRY

Latent Heat

Some energy used to evaporate, some used to heat the surface, WET

Hurricane Season

June 1st - November 30th

Rules for Naming Hurricanes

6 Lists, rotated yearly, alternating male-female, organized alphabetically

Storms that cause exceptional damage have their names retired and replaced on their lists. 

What are the necessary conditions for hurricane formation?

Warm waters, lifting mechanism, Coriolis Force (spin), low wind shear

Intertropical Convergence Zone (ITCZ)

Hot and wet air, changes location based on season

Spring Equinox: located over equator, no Coriolis, no hurricanes

Summer Solstice: Northern Hemisphere —> warm waters + Coriolis + lift = hurricanes

Winter Solstice: Southern Hemisphere —> warm waters + Coriolis + lift = hurricanes

Why don’t hurricanes form at the equator?

ITCZ has excellent lift + hot, wet air, but…

There is no Coriolis Force at the equator, no spin = no hurricane

Easterly waves in Trade Wind flow

Another lifting mechanism

Fast-moving trade winds converge with slow-moving trade winds, creates lift!

Spaghetti Models

Model showing the different potential paths for the eye of a hurricane, also shows forecast confidence

Narrow spread = high confidence

Wide spread = less certainty

How does wind shear impact tropical storm growth?

Low wind shear means surface and top winds are flowing in the same direction, high means different directions

If wind shear is high, storm cannot grow and loses power

What is the role of the Coriolis Force on a hurricane’s path?

Brings hurricanes across Atlantic Ocean, makes them turn and go up the coast or out to sea

Bermuda High

Semi-permanent high-pressure system, at center of BH air sinks, warm dry air, clear skies

Directs tropical systems, including hurricanes, acts like guide or wall, clockwise rotation

Hurricane Structure

Eye (relatively clear), eye wall (most intense winds and rains), spiral rain bands (due to outflow and cyclonic motion)

Eye Formation

Air in the eyewall is carried to the top of the storm, some flows outward, some at the very center sinks

Descending air is warm and dry, suppresses cloud formation, causes clear conditions

What regions of the hurricane are windiest?

Eye wall most intense winds, spiral rain bands can also be intense, wind from Southeast is much more intense due to rotational speed and forward motion

What is a hurricane’s cool wake and its impact? 

Strong winds mix cold water from deep in ocean coupled with evaporation of warm water by the storm, leaving a trail of cool water in its path, makes it harder for following storms to develop/grow stronger

What is the impact of dry air and Saharan dust?

Saharan Air Layer (SAL) is an extremely hot, dry, and sometimes dusty layer of the atmosphere

As SAL is pushed out over the Atlantic, it creates an inversion, where air gets hotter with height, acts as a cap and prevents lift below it, limits moisture needed for development

Air Masses

Wet = Maritime

Dry = Continental

Tropical, Polar, Arctic

Stable and Unstable Air

Stable air —> resists vertical motion, meaning if a pocket of air is pushed up, it will sink back to its original position, creates relatively horizontal movement, fair weather, layer clouds (stratus), smooth air, poor visibility, and steady precipitation, involves temperature inversion (air gets warmer with altitude)

Unstable air —> warmer and less dense than surrounding environment, continues rising, vertical motion promotes vertically developed clouds, can lead to showers and thunderstorms, temperature decreases with height

Tropical Storm Categories

Disturbance (organized) —> Depression (38mph) —> Storm (39-73mph) —> Hurricane (74+ mph, Cat 1-5 depending on wind speed)

Hurricane Dangers

Wind damage

Storm Surge: water being pushed toward shore by winds, low pressure raises water level, onshore winds pile up water, wave action, high tide

Inland Flooding: may affect places for hundreds of miles inland many days after hurricane has weakened or moved on

Hurricane watch vs. warning

Watch: hurricane possible in area, issued 48 hours in advance of anticipated onset of winds, prepare home and review evac plan

Warning: expected in area, hurricane preparation becomes difficult once winds reach tropical storm force, issued 36 hours in advance, complete preparations and immediately leave threatened area

Fujiwara Effect + Hurricane Sandy

Two hurricanes passing close by each other, usually stronger storm absorbs weaker one, similar strength will be pulled to each other, spin around and continue on their paths

Sandy: October 2012, storm merged with low pressure cell, created “superstorm” that generated powerful winds, heavy rainfall, record-breaking storm surge

2025 Hurricane Season

All Cape Verde Hurricanes!

Fernand: Early August, tropical storm, never made hurricane status, stayed out at sea

Gabrielle: Mid-September, tropical storm, warm waters led to rapid intensification, Hurricane! Cat 1 to Cat 4 in less than 24 hours, Bermuda High caused early turn (fish storm)

Humberto: Late September, HUGE, Late September, tropical storm, reached Cat 5 Hurricane status, followed closely by Imelda (Fujiwara Effect), Humberto caused dangerous surf, affected Northern Caribbean and US East Coast

Imelda: Late September, reached category 2, weakened, being pulled out to sea by Humberto due to Fujiwara effect, hit Bermuda early October

What is a mid-latitude, low pressure system?

a large-scale weather system forming between 30 and 60 degrees latitude, characterized by a center of low air pressure, form when cold, dense air from the poles meets warm, moist air from the tropics, creating a rotating storm with warm and cold fronts

How are low-pressure systems linked to the jet stream?

Jet stream guides location of low-pressure systems, divergence in jet stream  causes air to rise, cyclonic flow

Structure of a surface low

Cold front, warm front, cyclonic (counter-clockwise) rotation

Thunderstorm

A cumulonimbus cloud that produces precipitation, lightning, and thunder

May cause other severe weather

Rainstorms without thunder (absence of lightning) cannot be classified as a thunderstorm

What is needed for a thunderstorm to develop?

1. Source of moisture (warm and wet airmass)

2. Unstable environment (air needs to rise)

3. Trigger for the storm’s updraft 

Conditional Instability

High Convective Available Potential Energy (CAPE) exists with slightly warmer layer overlaying a warm, moist layer

Acts as a weak lid on updrafts forming in lower layer, keeps thunderstorms from forming early in the day

Inversion acts like lid on a tea kettle, storm rapidly builds

What are the various updraft triggers for a thunderstorm?

1. Heating of the surface —> warms air above it, air rises, convection = thunderstorm

2. Boundaries (Frontal, Subtle) —> dense cold front can propel warm air up quickly, leading to fast development, convection = thunderstorm

3. Low Pressure Cell —> associated with rising air, convection = thunderstorm

Stages of thunderstorm development

a. Cumulus stage - developing stage, rising updraft, growing cumulus clouds

b. Mature stage - updraft and downdraft coexist, heavy rain, lightning, hail

c. Dissipating - downdraft cuts off the updraft, storm weakens and eventually disappears

Structure of and within a thunderstorm cloud

Updraft —> rising warm, moist air that fuels vertical growth

Anvil —> flat, spreading top caused by strong hitting the tropopause

Downdraft —> caused by forming precipitation, falling rain and hail mixes with updraft 

Charge separation —> ice crystals colliding creates negatively charged base and positively charged top, can cause lightning

Mammatus —> pockets of dense, sinking air within a thunderstorm's anvil or cloud base descend into warmer, drier air, sinking air descends, it cools due to evaporative cooling and forms pouch-like, rounded structures, often signal severe weather conditions

Gust front —> cool air spreading out from the downdrafts causes boundary of sudden, strong winds found at surface

Overshooting top —> an extremely strong updraft penetrates the tropopause, resulting in a dome over the troposphere

What is the mesocyclone and how does it form?

Mesocyclone: tilted, rotating updraft, tilt allows storm to last longer because the downdraft doesn’t choke out the updraft, rotation can lead to tornado formation, forms when changes in wind shear creates spinning layers

What are the two types of rain processes?

Collision Coalescence - tiny droplets accumulates on larger droplets, increasing in size until they fall as rain

Cold-Rain Process - Super-cooled liquid, water vapor, and ice crystals accumulate and form larger ice crystals that thaw as they fall from clouds

Heat lightning

Lightning close enough to see, but too far to hear, might head in your direction!

Hail formation

forms inside the updraft when cloud droplets are carried above the freezing level,  strong updraft + long time in cloud = large hailstone, falls when the thunderstorm’s updraft can no longer support the weight of the hailstone

Wall cloud

downward extension of the mesocyclone, tornadoes extend from there

What is the significance of the radar hook?

signifies a supercell thunderstorm has a mesocyclone, that often leads to tornado formation

Funnels and scud clouds

Funnel clouds can lead to tornadoes, they have rotation

Scud clouds are non-rotating, more ragged appearance

Dynamic Pipe Effect tornado formation

Mesocyclone stretches, causes air below it to constrict its circulation leading to a tighter vortex, process proceeds earthward until the vortex is narrow at the ground, forming a tornado

Types of tornadoes

Rope Tornadoes —> smallest, most common

Cone Tornadoes —> narrower where they touch the ground than where they meet the base

Wedge Tornadoes —> can appear wider than they are tall, may contain multiple vortices, most destructive

Bow Echo 

Line of thunderstorms shaped like an archer’s bow, can produce dangerous straight-line winds and occasionally tornadoes caused by rotations at the ends of the bow

Tornado Alley, why is it there?

Southern plains region prone to tornado formation, boundary between cold and warm air masses, warm, humid air from Gulf of Mexico meets cooler air from the North to create ideal environment for tornadoes to grow, the “dry line”

Tornado Watch + Warning Polygons

Watch —> the possibility of tornadoes over a specified area for a particular time frame (covers many counties for periods up to 6 hours)

Warning Polygon —> geographic shape drawn by the National Weather Service (NWS) to show the specific area where a tornado is occurring or is imminent

Enhanced Fujita Scale

Categorizes tornadoes by strength using the damage they cause as a gauge

Reasons: Better correlates damage with specific wind speeds

What are waterspouts and how are they formed?

rotating column of water-filled air, either "fair-weather” or “tornadic”, fair-weather form from non-thunderstorm clouds and build up from the water surface, strong temp gradient between warm water and cold overriding air and shifting winds near the surface, warm air rises, expands and cools, holds less water

Microbursts and related dangers

localized, powerful downdrafts in thunderstorms, cause intense outward-spreading winds upon impact with ground

Can cause aircraft accidents, building damage, downed trees and power lines