Forecasting Final

What factors do the intensity and duration of floods depend on (including direct weather related, indirect weather related, non-weather related factors)?

Direct weather related factors

1. intensity

2. duration

3. number of events

4. size of rainfall area

Indirect weather related factors

1. snowmelt from previous storms

2. ice jams on rivers (pieces of floating ice accumulating and obstructing river flow)

3. saturated soil

Non-weather related factors

1. land use

2. levees and dams

3. topography

Explain what is meant by the term “100-year flood”?

in any given year, there is a 1 in 100 chance of a flood of that particular magnitude. the probability of a flood of that magnitude in any given year is 1%.

Flash floods

• occur rapidly with little to no warning

• short duration and limited area - primarily small rivers and streams

• injuries and fatalities - many in cars

• most often due to slow moving thunderstorms in the summer months

• urban areas - blocked or overloaded storm drains back up water in the streets

When considering the flash flood climatology for central NY, what are some of the reasons we experience flash floods even in the winter months?

frontal squall lines / stationary fronts

frontal overrunning

snowmelt

Susquehanna River Basin is one of the most flood-prone areas in the US. Physical location, basin topography, highly erodible soils, ~50k miles of waterways.

How does topography enhance rainfall from tropical cyclones that make landfall in the eastern US?

Floods are enhanced by the Appalachian Mountains. Inland flooding due to torrential rains. Moisture from dissipating tropical cyclones can be incorporated into extratropical cyclones, causing heavy rain and flooding.

What typically causes floods in tropical regions, other than tropical cyclones?

Thunderstorms that set up when the atmosphere is moist and conditionally unstable. occurs during the passage of easterly waves or when cold fronts stall.

Where in the US does flooding associated with squall lines and MCSs typically occur?

Between the Rockies and the Appalachian Mountains

What is meant by the term “training”?

Repeated areas of rain, typically associated with thunderstorms, that move over the same region in a relatively short period.

What is frontal overrunning?

• During cold season, east-west oriented warm front develop in the southeast U.S.

• Sometimes stall and move slowly that they are stationary
  Warm, moist air from the Gulf moves northward over a cool airmass north of the front – frontal overrunning.

Where does heavy precipitation fall relative to a front during floods associated with frontal overrunning?

• Produces t-storms and heavy precipitation north of the frontal boundary, if the air rising over the front is conditionally unstable.

Why are floods associated with frontal overrunning more likely to occur in the east-central US than in other regions?

Geography and where two air masses meet

How does snowfall in February and March contribute to flooding in April?

rapid melt of snowpack enhances runoff from heavy rain in the spring months.

• Ground frozen beneath which prevents infiltration.

• Extreme flooding can also occur from snowmelt, even without heavy rain.

• Series of extratropical cyclones following a similar track during late winter.

How does the North American monsoon develop?

A seasonal change in the atmospheric circulation that occurs as the continental land mass is heated by the summer sun.

During much of the year, the prevailing wind over AZ, NM, & northwestern Mexico is westerly and dry.

How do the Rocky Mountains enhance flooding during the summer months?

The flow on the plains east of the Rocky Mountains becomes easterly during the summer months.

When air is conditionally unstable, afternoon heating can produce thunderstorms just east of the Rockies.

Can remain nearly stationary, result in flash floods.

Why is the Central Valley of CA susceptible to floods?

Wintertime flooding is a great danger in the mountains.

Flash flooding in Sierra Nevada, Cascade, and Coastal Ranges leads to widespread flooding in the low-lying Central Valley of California.

Most precip during winter months.

What is the Pineapple Express?

Split flow in the middle troposphere.

• Lower branch of the flow brings tropical moisture to the coastal mountains and the Sierra Nevada.

• Storms develop within the southern branch of the jet and can move into the coast every 36 – 48 hours...

• Mountains force air to rise, resulting in heavy precipitation.

• Air stream = tropical in origin, so freezing level is high – most precipitation falls as rain.

What is the difference between a flash flood watch and a flash flood warning?

Flash flood watch - potential, but flood is not certain or imminent.

Flash flood warning - occurring or imminent.

What was the general synoptic setup that resulted in the historic flood of 2006 (surface to 250mb)? What were conditions like prior to this event? How did the placement of high and low pressure centers affect the movement of tropical moisture? What was precipitable water like over NY during this event? How were the forecasts for this event?

Drought watches across all of PA prior to June. Streamflow in the Mohawk and Delaware Basins in the normal range. Streamflow in the Susquehanna River Basin in the dry range. Storm activity picked up during the first part of June: precipitation generally less than 0.75”. June 16-22 was relatively calm and dry, rainfall across Mohawk, Delaware, and Susquehanna basins were less than 0.5”.

Cold front moved through PA, NJ, and SE NY on June 23; rainfall light to moderate throughout CNY. Scattered strong storms across PA. Pattern continued over the next several days, with daily rainfall totals of 1-3” in SE NY.

Area of low pressure over the Midwest; and high off coast stalled. Opposite rotations of the systems bought tropical moisture northward directly over CNY. Front was stationary until it backed up with the influx of tropical air.

Nearly stationary jet stream oriented from S. Upper air trough slow to move through, keeping NY on its warm east side. Wind in the NE becomes strong and southerly.

NAM forecasts and surface progs tended to push the fronts and precipitation to far west, under forecasting the impacts on CNY. Meteorological conditions weren’t impossible to forecast - just atypical for NY.

What is a thermal low? What role does this feature play in transporting moisture from the Gulf of California?

Thermal low - area of low pressure near the surface from intense heating of the lower troposphere = low level moisture transport to the southwest. Air pressure is relatively high over adjacent ocean areas (Gulf of CA and Pacific) - air flow brings much more humid air toward the thermal low.

What role does an upper-level ridge play in transporting moisture from the Gulf of Mexico aloft?

moves into the Southern Plains which allows moisture to move from the Gulf of California, Gulf of Mexico, and eastern Pacific. Position of ridge determines what areas will get precipitation. When high expands westward, it nudges the moisture westward.

What is unique about the Sonoran Desert?

Wettest desert in the world - only place where the saguaro cactus grows in the wild. Frequent, low-intensity winter rains. Large-scale surface low pressure systems that traverse the SW, pulling in moisture from the Pacific. Fueled by upper-level jets. violent summer convective thunderstorms.

When is North American monsoon season?

Monsoon begins to set up in Mexico in June, moves to SW US in July. By mid-September, wind pattern has returned to the westerly flow, marking the end of the monsoon.

How did drought conditions and wildfires impact the flash floods across the Southwest US in 2021?

Drought conditions in mid-June

Exceptionally dry: Limits infiltration into the soil...water runs off very quickly. Any precip that does get into the soil evaporates quickly due to hot temps.

Make ground more resistant to absorbing water. Charred vegetation & ash are hydrophobic. Areas burned by wildfire take much less rainfall to cause flash flooding. T-storms over these areas can produce flash floods as fast as the NWS radar can detect the rainfall. Debris flows

What is an atmospheric river?

A long, narrow, and transient corridor of strong horizontal water vapor transport associated with a LLJ ahead of the cold front of an extratropical cyclone.

What is a baroclinic weather system? How does this relate to atmospheric rivers?

Baroclinic weather system driven by baroclinic instability (horizontal differences in temperature). Thermal contrast = unstable, cold air sinks and spreads out beneath the warm air, and warm air moves to occupy region previously occupied by cold air.

What is the Fujiwhara Effect?

2 low pressure systems pinwheeling around a central point. Circulation patterns begin to interact and that causes them to move closer to each other. Two lows become dislodged due to jet stream movement.

What is numerical weather prediction?

A crucial part of a sophisticated process by which weather forecasts are produced. Collection, transmission, and synthesis of observation = production of weather maps. Simulates the atmosphere, solves complex governing equations of the atmosphere to predict the future state of the atmosphere based on the present state.

Explain the difference between a deterministic and probabilistic forecast.

Deterministic - 1 solution

Probabilistic - one that conveys uncertainties by expressing forecasts as probabilities of various outcomes

What is an ensemble forecast?

• comprised of many forecasts

• gives probabilistic information, showing where models are confident and where they are not

• presents a range of wx possibilities that may occur as the atmosphere evolves

How is absolute error and mean absolute error used to assess forecast accuracy?

the absolute value of the difference between the observed and forecast value.

Absolute error = forecast - observed

Mean absolute error - absolute errors over a number of forecast periods and average them

How is the Brier Score used to assess forecast accuracy?

Assesses the accuracy of a probabilistic forecast.

BS = (p-o)²

p = the forecast probability of an event occurring

o = the occurrence of the event (0 if the event does not occur, 1 if the event does occur)

Perfect BS: 0

Worst BS: 1

How is the Threat Score used to assess forecast accuracy?

Used by WPC to assess their aerial forecasts. Ratio of the area where the forecast was accurate to the area where it didn’t verify correctly.

TS = correct / (forecast+observed-correct)

TS = 1 is a perfect forecast

TS = 0 is the worst possible forecast

What is the difference between global domain and limited domain models? Which has a higher resolution?

Global domain covers the entire Earth (GFS)

Limited domain covers a certain region of the Earth (NAM). They have a higher resolution.

What are nested grid models?

A finer resolution small domain will be nested inside a coarse resolution larger domain. Large domain captures the large-scale wx systems and that information is fed into the finer grid.

widespread floods

• when large amount of rain falls over watershed for many days

• develop more slowly and often last a week or more

• ‘leisurely disasters’ - slowly and predictably destroy homes and property

• greater economic losses than flash floods

• fatalities minimal thanks to warnings

• commonly occur when multiple extratropical cyclones follow the same storm track

coastal floods

• rise in the ocean surface as a result of storm surge during tropical cyclones and strong extratropical cyclones

• more extreme near the Gulf and East Coasts of the US - greater number of cyclones and flatter coastal topography