GEOS 1002 Final Exam

Why does the top of a rotational slide tilt back into the slope?

The slide mass rotates on a circular curving surface, so everything on it rotates backward.

Why are flash floods more common in dry climates than wet climates?

The soil is less permeable

Why do braided streams form in a dry climate instead of meandering streams?

The stream has too little water to carry the total sediment supplied.

What is the greatest danger (what causes the most deaths) from a tornado?

being struck by flying debris

What channel pattern are less common and often occur in drier areas?

braided streams

To prepare for a tropical cyclone, the text recommends you do all of the following, except:

turn off your appliances, including your refrigerator

Where do tropical cyclones striking North America NOT originate?

the central Pacific Ocean, near the equator

What is "cohesion" as applied to slope failure?

the holding together of otherwise loose grains on a slope, often by a little moisture

Why is the peak season for tropical cyclones in August-September?

water takes a long time to heat up

Why is it so important to cover windows and doors with plywood or shutters in the event of a tropical cyclone?

wind-blown debris can easily break these parts of a structure allowing wind and rain to get into a structure and cause much greater damage

List ways to fight wildfires:

- *smokejumpers* parachute into remote areas to exterminate lightening fires before they spread out of control - bulldozers cut *firebreaks* to remove vegetation - aircraft drop *water/fire retardant* along advancing edges of fires - firefighters cut *firelines* to prevent spread of fire - *set burnout fires*

What are the 4 tsunami generation methods and where is each relatively important?

- Earthquakes (pacific subduction zones) - Landslides/volcanic flank collapse (Hawaii) - Volcanic eruption - asteroid impact

List some ways to mitigate tsunamis:

- Land use zoning - structures to resist wave erosion - streets perpendicular to shore - well-rooted street - large ditches/ concrete walls

Regarding lake-effect snow, the amount of moisture is determined by:

- air temperature - wind speed - area of exposed, unfrozen lake

Regarding lake-effect snow, the amount of snow is determined by:

- amount of moisture - topographic lift downwind of lake/body of water

list ways to mitigate landslides:

- avoid building in hazardous locations - reduce slope - remove water from slope - add a load to lower part of slope - spray shotcrete/ anchor with rockbolts - plant vegetation - artificially drain slopes with perforated pipes or geotextile fabric

List ways levee failure can occur:

- bank erosion from river currents or waves - slumps into channel - overtopping - piping or seepage through levee, sometimes reaching surface as a sand boil

What are the 3 components of the fire triangle?

- fuel - oxygen - heat

Local topography can:

- funnel air - accelerate fire - cause more rapid spreading - chimney-like funneling effect in canyons and on hillsides

List the hazards of thunderstorms:

- lightening (wildfires) - downbursts/straight-line winds - hail - tornadoes - flash floods

List the 3 types of channel patterns

- meandering streams - braided streams - bedrock streams

List the ways extratropical cyclones and nor'easters differ from hurricanes:

- most common October-April - gain their energy from large horizontal temperature gradient - lack clear, central eyes - named for direction from which winds come - bring heavy rain and snowfall - cold core systems that do not energy with height - damage is concentrated along coast

List the different types of landslides (5):

- rockfall - rotational shift - translational slide - soil creep - snow avalanche

Slope failure depends on:

- slope steepness - material weight - moisture content

What are the three different types of fuel fires?

- surface fires - ladder fuels - crown fires

The ability of a slope to resist sliding depends on:

- total *driving force* (gravity) pulling it down VS. - *resisting force* holding it up

As dictated by current restrictions, what is the maximum amount the level of a 100-year flood can be encroached (raised) as a result of building or development?

0.3 meters

What is the approximate lowest depth of a flowing stream that is dangerous to drive through?

30 cm (1 foot)

How long before a flood can someone purchase insurance from the National Flood Insurance Program? In other words, what is the waiting period between buying flood insurance and being able to file a claim?

30 days

How fast do tornadoes move along the ground?

50-80 km/hr

What causes a cold front?

A cold air mass moves in under a warm air mass

Following the May 1980 eruption of Mount St. Helens, Washington, thousands of trees lay on the ground, all parallel to one another. Why?

A lateral blast at the beginning of the eruption blew them all down.

What does a supercell look like on a radar image and in what part of the storm would a tornado be expected?

A supercell is shaped like a triangle pointing toward the west with a hook-like appendage extending from its back corner, and the tornado would be expected within this appendage.

What local circumstances also raise the height of a storm surge onshore close to the beach?

A surge moving into a bay funnels the water and raises the surge.

Why does a parcel of air get cooler as it rises over a mountain?

Air with a certain amount of heat expands as it rises and in so doing expends energy and cools

- Groundwater continuously feeds streams - changes in precipitation result in changes to groundwater levels, but stream inflow is generally constant *these things occur in what conditions?*

Areas of moderate to high annual rainfall

Why are the deposits of streams graded with the largest boulders or pebbles at the bottom?

As the water slows after a flood, the largest boulders drop out first

Which months comprise the typical peak (time of greatest storm activity) of the tropical cyclone season in the Atlantic, and why then? Select one: a. July, because that is the hottest month and rainfall is highest in the tropics b. July, because winds are the highest over the normally stagnant doldrums of the equatorial ocean c. July-August because these are the hottest months d. August-September because it takes all summer to warm the ocean

August-September because it takes all summer to warm the ocean

Three Sisters are located just west of which city?

Bend, Oregon

T/F Hurricanes arriving perpendicular to coastline have a lower storm surge.

False (higher)

T/F Small amount of water in pore space decreases cohesion.

False (increases)

What should you do to avoid being killed by lightning if caught out in the open in a storm with no available shelter?

Crouch down on the balls of your feet with your feet together

Why does Venice, Italy have canals for gondolas and boats instead of normal streets for cars?

Extraction of groundwater below the city caused subsidence of the city below sea level.

- cold weather storms that can cause damage similar to tropical cyclones

Extratropical cyclones and Nor'easters

After a major rainstorm in a humid climate and with no overland flow to a stream, why does the stream level rise almost immediately?

Groundwater seeps down to raise the water table to force older groundwater out into the stream

- depend on humid air and warmer temp. above cold air near the ground - occurs when snow above falls into a warm layer and melts

Ice Storms

If you are at the beach and feel a large earthquake, what should you do, and when should you do it?

Immediately run up slope as far as possible

Why does raising the groundwater level often lead to slope failure?

It increases the water pressure in the pore spaces between the grains, pushing them farther apart.

Why does removal of vegetation cause more surface runoff and thus more erosion?

It permits more water to directly strike the ground and run off.

Why does a stream bottom erode more deeply when its water level rises in a flood?

Its water level rises, so water velocity increases, and it can carry more sediment.

What is karst, and how does it form?

Karst is the ragged landscape that results when soil is eroded away revealing limestone bedrock that has been partially dissolved

Why does earthquake shaking of water-saturated sand often lead to ground settling?

The shaking of sand leads to closer packing of the grains

If an ash flow approaches you from across a kilometer-wide lake, are you likely to be safe or not? Explain why.

Not safe. Ash flows can cross much wider bodies of water.

What are oxbow lakes and how do they form?

Oxbow lakes are cut off meanders, left behind as a flooding stream takes a shorter path downstream.

What causes caverns to form in limestone?

Rainwater dissolves carbon dioxide to make weak carbonic acid that dissolves limestone

A big bulge sometimes slowly grows on the flank of an active Cascades volcano. Why?

Rising magma is pushing it up

- prolonged high winds - 90% of tropical cyclone deaths result from this - pile and rise up higher when approaching shallow coastline

Storm surges

How would a hydrograph for a drainage basin change if major urban growth were to occur upstream?

The hydrograph would be higher and narrower.

Why do ships in the open ocean NOT notice passage of a tsunami wave?

The long time between wave crests dictates that the wave flanks have almost no slope.

Why is storm damage of a westward-moving tropical cyclone generally less on the south side of the eye?

The winds are moving offshore there, so the waves are smaller and water is being pushed away from shore. The storm surge is primarily north of the eye.

- form over warm waters - dissipate over cooler waters or land

Tropical cyclones

T/F Building on top of previously unknown caverns can lead to sinkhole formation.

True

Why does swelling clay swell, potentially increasing the likelihood of a landslide?

Water gets into the clay layers, spreading them apart.

a weak layer, sloping down toward a road cut or hillside at a somewhat gentler slope than the hillside:

a daylighted surface or layer

What shape of roof is most susceptible to being lifted off by a tropical cyclone?

a gently slopping roof with one slope facing toward the wind

With urbanization of the upstream drainage area, what changes can be expected in the height of the 100-year flood?

a greater height

Freezing rain (an ice storm) occurs under which of the following scenarios? [Note that for this question, cold air refers to air with a temperature below freezing.]

a layer of cold air above warm air which lies above a shallow layer of cold air

What is often the first indication of the arrival of a tsunami at the coast?

a rapid drop in sea level

What is the significance of a sand boil?

a sign of seepage under a levee

A fan-shaped pile of rock fragment banked up against the base of a cliff:

a talus slope

The first tiltmeters were simple levels made with water tubes __________ long. Select one: a. 25 meters b. 25 feet c. 5 meters d. 50 meters

a. 25 meters

Which region has the highest density of lightning or thunderstorms on Earth? Select one: a. central Africa b. China c. the continental U.S. d. Australia

a. Central Africa

Which is NOT a method for recognizing the presence of swelling soils? Select one: a. Since they deform elastically, the ground feels spongy or bouncy underfoot. b. They form a popcorn-like surface when dried out. c. They deform concrete foundations and walls. d. They stick like gum to the bottom of your shoes when wet.

a. Since they deform elastically, the ground feels spongy or bouncy underfoot.

All of the following are reasons that lower sand dunes allow for more coastal damage in a tropical cyclone. Which is LEAST important? Select one: a. There is more erosion because wave energy is not used to move the dune sand. b. Waves can carry sand inland as overwash. c. Waves can go over the lower dunes easier to strike homes. d. The surge can move much farther inland with lower dunes.

a. There is more erosion because wave energy is not used to move the dune sand. Correct

Which of the following is a difference between tropical cyclones and nor'easters? Select one: a. Tropical cyclones gain their energy from evaporation from the sea surface. Nor'easters gain their energy from large temperature gradients. b. Damage is always much greater for tropical cyclones. c. Tropical cyclone are always cold-cored, whereas nor'easters can be cold- or warm-cored. d. Tropical cyclones are most common during October to January, whereas nor'easters tend to occur June-August.

a. Tropical cyclones gain their energy from evaporation from the sea surface. Nor'easters gain their energy from large temperature gradients.

What effects does the wind NOT have on buildings during a tropical cyclone? Select one: a. causes roofs to collapse into a building b. lifts and carries debris which amplifies damage to a building c. lifts off roofs d. blows out windows and doors

a. causes roofs to collapse into a building

Which weather-related hazard has led to the collapse of civilizations and war? Select one: a. drought b. desertification c. heat waves d. blizzards

a. drought

Which of the following would NOT cause the height of water in the ground to rise, thus increasing the danger of sliding? Select one: a. planting trees b. filling a reservoir behind a dam c. leaking pipes d. cracked swimming pools

a. planting trees

Which of the following submarine faults CANNOT cause tsunami waves? Select one: a. strike-slip faults b. normal faults c. thrust faults d. subduction zone faults

a. strike-slip faults

Why are the skies clear in the eye of a tropical cyclone?

air is descending, warming and drying as it descends

What pattern exists in the annual cost of U.S. flood damages from 1910 to 2010?

an increase due to population increases and floodplain development

If a 100-year floodplain was flooded in 2016, when should we expect the next 100-year flood?

anytime

What is tuff?

ash after deposition forms a rock

Where in a tropical cyclone is the atmospheric pressure lowest, and where is the wind speed highest?

at the center of the storm in the eye, just outside the eye in the eyewall

A flooding stream permanently leaves its old channel to form a new floodplain channel:

avulsion

Which material is most likely to move as a translational slide? Select one: a. homogeneous, cohesive materials b. 3-meter thick soil over bedrock c. a raised terrace of old beach sediments d. moist soil 300 meters thick

b. 3-meter thick soil over bedrock

The main factors that affect the chance that a slope will fail in a landslide do NOT include: Select one: a. amount of water b. age of the slope c. slope angle or steepness d. strength of the slope material

b. age of slope

Which is the best detection device to warn people of an already-moving debris flow? Select one: a. a visual film monitor b. an acoustic flow monitor c. a natural barrier d. a person listening to natural sounds created by the flow

b. an acoustic flow monitor

Human-built dams may directly fail for all of the following reasons EXCEPT: Select one: a. overtopping of the dam. b. build up of sediment behind the dam. c. seepage of water under the dam. d. subsurface erosion at the base of the dam. e. an earthquake.

b. build up of sediment behind the dam.

Which of the following states is NOT susceptible to landslides (Hint: The answer to this question is found in one of the figures in the chapter)? Select one: a. Pennsylvania b. Florida c. Montana d. Colorado

b. florida

Which of the following does NOT cause increased erosion in a stream channel? Select one: a. building a dam across the stream b. planting native trees in the flood fringe c. building a bridge across a stream, with roadway fill across the floodplain d. mining gravel from the stream channel

b. planting native trees in the flood fringe

Which of the following factors does NOT influence the amount of snow that falls in a lake-effect snow event? Select one: a. wind speed b. the depth of the lake c. air temperature d. how much of the lake is unfrozen

b. the depth of the lake

Which of the following CANNOT be used to determine the depth of a debris flow in a canyon it moved through? Select one: a. the highest rocks on natural levees deposited by the flow b. the size of the boulders left by the flow the maximum height of sand eroded from thevalley sides c. sand embedded in bark up to that height d. bark battered off the upstream sides of trees up to that height

b. the size of the boulders left by the flow the maximum height of sand eroded from thevalley sides

What channel patterns occur in mountainous areas of solid rock?

bedrock streams

Which of the following statements is NOT true about the effects of mining sand or gravel from a streambed or banks? Select one: a. Removal of sediment makes the stream erode its channel deeper. b. Deepening the channel can affect water supplies. c. There aren't any negative effects; the stream merely brings in more sand and gravel. d. Increased erosion by the stream can affect roads and bridges. e. Deepening the channel lowers the water table and affects agriculture.

c. There aren't any negative effects; the stream merely brings in more sand and gravel.

Which of the following areas is LEAST likely to have a flash flood? Select one: a. eastern Nevada b. southern Texas c. western Washington state d. southern Arizona

c. Western Washington state

Volcanic mudflows are caused when which of the following combine? Select one: a. water and dirt b. ash and lava c. ash and water d. rocks and lava

c. ash and water

Which of the following is LEAST effective in mitigating the impact of tsunami? Select one: a. plant trees with deep roots b. Limit building in places potentially flooded by tsunami c. orient streets and buildings parallel to waves d. design structures to resist erosion and scour

c. orient streets and buildings parallel to waves

Which of the following is NOT a feature of a modern debris flow trapping structure? Select one: a. sand and mud from the floodplain b. spillway c. permeable dams (i.e., dams that allow water through) d. water overflow pipes e. cross-linked steel pipes

c. permeable dams (i.e., dams that allow water through)

Which of the following does NOT lead to the formation of sinkholes? Select one: a. construction of a heavy building over a limestone cavern b. strong pumping of groundwater for crop irrigation in dry weather c. pumping waste fluids into the ground for disposal d. pumping groundwater for spraying on crops to prevent freezing

c. pumping waste fluids into the ground for disposal

What changes upstream from a site would NOT likely lead to a higher and narrower hydrograph? Select one: a. clear-cut logging b. wildfire c. reforestation for a Christmas tree farm d. increased urbanization

c. reforestation for a Christmas tree farm

What process does NOT lead to the failure of a river levee? Select one: a. piping or seepage through the levee b. erosion of the levee by river currents c. sandbagging the levee to raise its height d. water overtopping the levee e. slumping of sides of the levee into the river

c. sandbagging the levee to raise its height

All of the following diminish the reliability of recurrence intervals EXCEPT: Select one: a. population growth and urbanization. b. modifying the stream by channelization. c. separating different meteorological conditions or causes of flooding. d. using data from a short time interval.

c. separating different meteorological conditions or causes of flooding

What is in water in the ground that helps it dissolve limestone?

carbonic acid formed from carbon dioxide in air combining with rainwater

_______ form where carbonate rocks near water table dissolve in groundwater - the roof collapses if left unsupported where water table has dropped

caverns

What is the best way to avoid a mudflow?

climb up the valley wall

What creates the electrical charges in clouds that characterize lightning storms?

collisions between ice particles of different sizes in the presence of liquid water and subsequent separation of the ice by the cloud updraft

Which way is the flow through tropical cyclones in the northern hemisphere?

counterclockwise and inward toward the eye at the surface, upward outside the eye, and outward clockwise at the top of the storm plus some downward motion in the eye

burns through treetops with incredible speed and heat - most dangerous because spreads the easiest

crown fires

Which of the following is NOT a good use for floodplains? Select one: a. agriculture b. golf courses c. parks d. shopping centers

d. shopping centers

Aside from flooding, what damages do NOT accompany the surge of a tropical cyclone? Select one: a. Buildings, bridges, and piers are washed away. b. Houses not on high posts are floated and carried farther onshore. c. Saltwater invades and contaminates groundwater. d. Heavy sand is deposited on beaches, partly burying some homes.

d. Heavy sand is deposited on beaches, partly burying some homes.

Which of these is NOT a Cascade volcano of western North America? Select one: a. Three Sisters b. Mt. St. Helens c. Mt. Rainier d. Mt. Vesuvius

d. Mt. Vesuvius

Which effect do levees NOT have on a stream? Select one: a. They raise the water level upstream of the levee. b. They raise the flood level. c. They generally increase the water velocity. d. They prevent the stream or river from ever changing course. e. They raise the water level just downstream of the levee.

d. They prevent the stream or river from ever changing course.

What physical effect do most bridges have on the streams they cross? Select one: a. They minimize flooding upstream because water upstream moves more slowly. b. They decrease erosion under the bridge because they slow water. c. They decrease water level just downstream because the water there moves more slowly. d. They restrict flow on the floodplain, so water under the bridge is deeper and faster.

d. They restrict flow on the floodplain, so water under the bridge is deeper and faster.

Tsunami waves nearing the open coast: Select one: a. speed up because the wave launches forward as it breaks. b. speed up because they finally begin to feel bottom. c. slow down because they don't want to hit the coast too fast. d. slow down because they drag more on the shallower bottom than in the deep ocean.

d. slow down because they drag more on the shallower bottom than in the deep ocean.

Which of the following is NOT distinctive about a cloud that might develop a tornado? Select one: a. a broad anvil-shaped top b. a narrower, steep-sided, rotating cloud descending below the main cloud base c. an overall flat base shedding torrential rain and moving rapidly northeast d. a shallow cloud that extends over a large horizontal distance with a cauliflower or cotton ball-like top

d. a shallow cloud that extends over a large horizontal distance with a cauliflower or cotton ball-like top

Which of the following is the LEAST important factor for determining the risk of a snow avalanche? Select one: a. slope steepness b. wind speed and direction c. temperature d. angle of the sun in the sky

d. angle of the sun in the sky

In contrast to an old ash-flow deposit, an old ash-fall tuff does NOT include which of the following? a. layers that become coarser-grained toward the base b. ash that evenly coats hills and valleys c. thin layers d. differences in thickness based on valleys and hills

d. differences in thickness based on valleys and hills

What CANNOT be used to recognize the action of soil creep? Select one: a. fences that lean downslope b. walls that lean downslope c. tree trunks that bend out from a slope before curving up d. fragments of rocks that clearly came from farther upslope

d. fragments of rocks that clearly came from farther upslope

Which of these effects on coastal areas is NOT caused by heavy rain during a tropical cyclone? Select one: a. drowned people b. flooded streams c. landslides (from saturated slopes) d. large storm surges

d. large storm surges

What major factor does NOT control the maximum height of water downstream from failure of a dam? Select one: a. distance downstream from the dam b. volume of water c. water height behind a dam d. length of the reservoir upstream of the dam

d. length of the reservoir upstream of the dam

Old landslides are NOT normally reactivated by: Select one: a. steepening the slope by adding material at the upper part of the slope. b. earthquakes. c. steepening the slope by removing material at the lower part of the slope. d. piling heavy rocks on the lower part of the slope.

d. piling heavy rocks on the lower part of the slope.

What human action does NOT negatively impact stream processes and cause excessive erosion? Select one: a. overgrazing b. logging c. fire d. planting trees e. deforestation

d. planting trees

Hurricane Ike in 2008 was the _______ costliest tropical cyclone on record in the U.S. Select one: a. fourth b. most c. tenth d. third

d. third

What weather conditions are most likely to foster the formation of sinkholes?

dry weather because that lowers the water table so the cavern roofs are not supported and may collapse

Which of the following is NOT a common reason cited for building large dams on rivers? Select one: a. recreation b. water supply or irrigation c. flood control d. hydroelectric power e. raise groundwater levels upstream

e. raise groundwater levels upstream

Where does a tropical cyclone get its energy?

evaporation from the sea surface (latent heating)

Although the Atlantic coast of North America experiences few large earthquakes, what specific other event could generate a large tsunami wave that could cause catastrophic damage there?

flank collapse of a volcano in the Canary Islands

Where do people most commonly get the material to build an artificial river levee?

from the river channel, by dredging

What force contributes the most to the driving force of a landslide?

gravity

For a rockfall, what two factors lead to a greater distance of travel of the moving debris?

greater height of fall and greater volume of falling mass

What keeps a stream flowing year-round, even though it may not rain for many months?

groundwater flow from the adjacent ground into the stream

Land subsidence and sinkholes can develop due to what?

groundwater usage

What is the largest contributing factor to creating a tropical cyclone storm surge?

high winds pushing water causing it to "pile up"

What significant effects does faster forward speed of a tropical cyclone have on coastal areas?

higher surge and less rain

What kind of material is most likely to fall in a rotational slide?

homogeneous, cohesive materials

Where in North America are sinkholes most prevalent and why?

in Florida because it is almost all limestone and has abundant groundwater

Where is the safest place to be in a lightning storm?

in your house

As air temperature rises, its ability to hold water:

increases

low brush and branches allow fire to climb into higher treetops

ladder fuels

Regarding rate of runoff: Because less water infiltrates, the flood volume after urbanization will be a little ________.

larger

If you do not have a storm shelter or a basement, where is the safest place in a house to be during a tornado?

lying in a bathtub in an interior room without windows

Whats is the most common channel pattern?

meandering streams

Highest surge levels of storms occur in the _____ to __________ ____________

north to northeast

In what direction do most mid-continent tornadoes travel along the ground?

northeast

What is the most dangerous location in a tsunami hazard?

on a sandy beach at the end of a bay

A study of more than 8000 U.S. dams by the Army Corps of Engineers indicated that what proportion of the dams were unsafe?

one third

One way in which water CANNOT be removed from a wet slope that may slide is to:

place a load on the slope to help squeeze water out of it.

Which of the following human activities would NOT contribute to desertification? Select one: a. intensive farming and cultivation on land that is only marginally productive for farming b. allowing livestock to overgraze c. clearing the land to build houses to accommodate a larger population d. planting trees

planting trees

Fires generally move _______ upslope and ________ downslope.

rapidly; slowly

- depends on observation record - record flood frequencies - must cover interval long enough to be representative - assumes that upstream conditions were similar through time

recurrence intervals

As it enters a bay, the height of tsunami wave:

rises because more water from each wave is forced into less space.

Other than limestone, what other types of rocks are soluble and can form cavities that collapse?

salt and gypsum

What common sedimentary rocks are soluble in water?

salt, gypsum, limestone, and other carbonates

- water from losing streams sinks into ground and streams may dry up between storms - flash floods can occur after any major or prolonged rainfall - losing streams may dry up completely between rains *these things occur in what conditions?*

semi-arid regions

The main reason Santa Ana winds are warm is because:

sinking air warms by compression

What are the two main causes of tsunami?

subduction-zone earthquakes and landslide into the ocean

What causes the loud boom that we hear as thunder?

supersonic expansion of air heated by a lightning bolt

spread along ground, fueled by grasses and low shrubs - vegetation with large relative surface area accelerates ignition and burning - heavy fuels with small surface area ignite with difficulty and burning slowly

surface fires

What caused the catastrophic 1997 flood along the Red River in North Dakota?

thawing to the south of the frozen north-flowing river that backed up water behind the ice to the north

What are the main characteristics of soil above a limestone cavern that leads to the formation of a cover collapse sinkhole?

thick, clay-rich, and cohesive enough to develop a large cavern before it collapses

Why are houses built close to or on the beach in tropical cyclone-prone areas generally on tall posts (i.e., elevated off the sand)?

to keep them above the level of the storm surge

Why do highway departments sometimes pile huge boulders near the base of a sloping road cut?

to prevent slumping of the road cut by loading the toe of the potential slump

Which part of a storm is most likely to form tornadoes?

towards the trailing edge of the storm behind the heaviest rain