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| FORECAST CONCEPTS |
In this webpage you will find 20 forecasting situations. Each one will be expressed as a multiple choice question. The answer explanation for each question is at the bottom of this webpage.
1. You are a forecaster in Savanna, Georgia. It is 7 pm in June. The surface air temperature is 86 F with a dewpoint of 74 F. In the over night hours, clear skies are expected along with light and variable winds. What would you expect the over night low temperature to be from the choices below?
a. 80 F
b. 77 F
c. 74 F
d. 71 F
e. 68 F
2. You are a forecaster in Wichita, Kansas. The 7 am temperature in January is 35 F with a dewpoint of 20 F. Cloud cover is expected all day. Snow is also expected to begin around noon and continue for the remainder of the day. The wind is light from the northwest and will continue at this speed and direction the remainder of the day. What should the surface air temperature be at 5 pm on this day?
a. 38 F
b. 35 F
c. 32 F
d. 29 F
d. 26 F
3. It is December in Omaha, Nebraska and the morning surface temperature is 25 F with a dewpoint of 18 F. According to the forecast models, the UVV's will be high over the forecast region in the afternoon hours. Precipitation if it occurs will fall as snow. The lifting is due to a strong 700 to 500 mb shortwave. However, no measurable precipitation occurs that afternoon. From the choices below, choose the most logical explanation to why no precipitation occurred.
a. The lower troposphere was too dry and the lifting not strong enough to generate precipitation
b. The surface dewpoint was too low
c. The lifting from the shortwave lasted longer than expected
d. A strong capping inversion just above the PBL prevented precipitation
e. The troposphere was not unstable enough
4. You are a forecaster in Orlando, Florida is June. On a fairly barotropic day strong thunderstorms develop in the afternoon along two merging sea breeze boundaries. The storms produce vivid and intense lightning. The lightning from these storms is more intense than is normally associated with storms in this region. What are the two most significant factors responsible for the unusually intense lightning?
a. Low level convergence and strong vertical wind shear
b. Strong Warm Air Advection and upper level divergence
c. Strong dynamic lifting and high Helicity
d. Frozen anvil and convective turbulence
e. Large instability and climatologically high values of Precipitable Water
5. At 6 pm in Memphis, Tennessee on a February day the temperature is 38 F with a dewpoint of 36 F. Light rain that had been falling during the day changes over to sleet and then to snow by midnight. What is the most likely cause of the precipitation changing from rain, then to sleet, then eventually to snow?
a. Radiational cooling from the earth's surface
b. Latent heat release from snow
c. Soil temperature that is colder than the surface temperature of the air
d. Evaporative cooling
e. Low level Cold Air Advection
6. The temperature is 42 F with a dewpoint of 34 F at 8 pm. On a long winter night with clear skies and light wind, the temperature drops to a low of 30 F. Which of the following processes released latent heat during the course of the night assuming the dewpoint only drops when temperature drops?
a. Condensation
b. Freezing
c. Deposition
d. All of the above
7. When skies are clear, surface snow cover compared to no surface snow cover causes which answer below to be most true:
a. Surface snow cover generally decreases the high temperature and decreases the low temperature
b. Surface snow cover generally increases the high temperature and decreases the low temperature
c. Surface snow cover generally decreases the high temperature and increases the low temperature
d. Surface snow cover generally increases the high temperature and increases the low temperature
8. On a Friday in July in Louisville, Kentucky the high temperature is 97 F. The weather should be similar tomorrow except the weather will be windier. A forecaster would expect the high tomorrow to most likely be:
a. 103 F
b. 100 F
c. 97 F
d. 94 F
e. 91 F
9. Which of the following is generally most true when precipitation ends and the skies clear after the passage of a frontal boundary?
a. Surface temperature falls
b. Surface temperature rises
c. Surface pressure falls
d. Surface pressure rises
10. The forecast models show that a surface Cold Air Advection pattern will occur over the forecast area during the daytime hours. However, the surface temperature actually warms during the daytime hours over the forecast area. What could best explain this?
a. Unforeseen precipitation
b. Upslope flow
c. Solar radiation
d. Positive Vorticity Advection
e. Jet stream divergence
11. You are a forecaster in Garden City, Kansas. On January 15 the temperature is 43 F with a dewpoint of 27 F at 4 pm. The skies are expected to be clear over night and the winds will average 13 knots from the east. Assume no thermal advection or moisture advection takes place over night. Which of the following is most likely to be seen at sunrise?
a. Dew
b. Frost
c. Frozen dew
d. Fog
e. Ice fog
12. The temperature is 55 F in the morning (8 am) with a Relative Humidity of 100% on a late fall day in St. Louis, MO. It is completely overcast during the day and the clouds block out most of the incoming solar radiation during the day. Light to moderate stratiform rain falls continuously during the day. No thermal advection takes place. The soil temperature is 53 F at 8 am. The most likely high temperature for this day using the choices below will be:
a. 53 F
b. 55 F
c. 58 F
d. 61 F
e. 64 F
13. On a June afternoon severe storms produce large hail in eastern Colorado. What two factors are most responsible for producing this large hail?
a. Strong vertical wind shear and strong low level convergence
b. Large instability release and a freezing level relatively close to the surface for a severe storm situation
c. Strong dynamic lifting through upper level divergence and climatologically large values of Precipitable Water
d. Initially dry air in the middle troposphere and high surface dewpoints
e. Upslope flow and a jet streak in the upper troposphere
14. The temperature (on a clear day with light wind) tends to rise much more rapidly in the morning hours (6am to noon) as compared to the afternoon hours (noon to 6pm). What do you suppose causes this tendency?
a. The shallow boundary layer inversion mixes out in the morning hours
b. The sun's radiation is stronger in the morning hours
c. Latent heat absorption from vaporizing moisture on the ground occurs in the morning
d. The colder the air is initially, the faster it will warm with the same energy input
e. All of the above
15. The afternoon temperature is 55 F with a dewpoint of 20 F. A local forecaster predicts a low of 40 F overnight with mostly cloudy skies. However, the actual low ends up being 32 F. Any cooling overnight was not due to temperature advection. Which choice(s) below could explain this incorrect forecast?
a. There was more cloud cover than anticipated
b. Precipitation occurred when none was expected
c. Wind was stronger than anticipated
d. Skies cleared overnight
e. Both b and d
f. Both a and c
16. Which of the following is most UNLIKELY to occur on a night when a record low temperature occurs in Denver, Colorado in winter?
a. Light wind
b. Clear skies
c. Surface snow cover
d. Colder than normal soil temperature
e. Snow falling throughout the night
17. It is early spring in Dallas, TX and thunderstorms are possible in the afternoon. According to the models, the 700 mb dynamic upward vertical motion is high over the Dallas forecast region during the afternoon. The lifting is due primarily to low level Warm Air Advection and convergence along a cold front. However, no thunderstorms occur. From the choices below, give a logical explanation why no thunderstorms occurred:
a. The capping inversion ended up being too strong to support thunderstorms
b. The air was too dry in the lower troposphere to support thunderstorms
c. Cloudy skies prevented instability from occurring
d. All of the above
e. None of the above
18. A farmer's citrus crop in south Texas is in danger from freezing. The 5 pm temperature is 35 F with a dewpoint of 15 F. The low temperature overnight is expected to be 22 F with strong winds out of the North. The dewpoint will be around 10 F overnight. The farmer calls you for advice on saving the crop. You tell the farmer:
a. It is hopeless since the temperature will be too cold; Hope you have that insurance
b. Use wind machines and helicopters overnight to mix out the low level inversion and warm the air above freezing
c. Use sprayers to spray the crops with a fine mist of water overnight. The latent heat of freezing will keep the crops above freezing as ice coats and insulates the crops
d. Burn piles of wood since the smoke will act like clouds and keep the surface air temperature warmer
e. Perform animal sacrifice and meditate to the weather gods
19. It has been foggy for several hours. The surface air temperature is 54 F. What is the most likely surface air dewpoint at this time?
a. 60 F
b. 56 F
c. 54 F
d. 50 F
e. 46 F
20. There is an 80% chance of precipitation in the forecast area. What does this mean?
a. Any particular point location in the forecast area has an 8 in 10 chance of receiving measurable precipitation
b. 80% of the forecast area will receive measurable precipitation
c. All point location in the forecast area will have measurable precipitation
d. There is an 80% chance that measurable precipitation will occur anywhere within the forecast area
e. There is an 80% chance that the forecast will be wrong for any particular point location in the forecast area
Answer Explanations:
1. With a high dewpoint, clear skies and light winds a forecaster would expect the temperature to fall toward the dewpoint fairly quickly in the over night hours. There will be enough cooling for the temperature to fall to the dewpoint and for dew to form. Cooling slows down once the temperature falls to the dewpoint since the temperature and dewpoint will fall together as latent heat is released to form the dew. The latent heat release slows the rate the temperature falls once the air is saturated. Answer choices a, b and c will likely end up being too warm. It will also be difficult for the dewpoint to drop from 74 to 68 since at warm temperatures dew formation releases a large amount of latent heat. The temperature and dewpoint should only be able to drop about 3 degrees lower than the 7 pm dewpoint. A forecaster will use the dewpoint temperature as an aid in making an over night low temperature forecast in this type of situation. The over night low temperature will generally only fall a few degrees below the late afternoon dewpoint in this type of situation. The best answer is d.
2. The cloud cover will reduce warming from solar radiation. Even on cloudy days though, some solar radiation manages to make it to the surface (if it didn't it would still be dark outside). Thus a forecaster would only count on minor warming from solar radiation. The dewpoint depression before the snow begins is delta 15 F degrees. In temperatures near freezing, wet bulb cooling will generally cool the temperature 1/3rd the dewpoint depression. Thus, about delta 5 F of cooling would be expected from vaporization. This would cool the temperature from 35 F to about 30 F by itself. With light winds, not much temperature advection would be expected. The snow should fall as a wet snow since the surface is just above freezing before the snow starts. The temperature will often be right at the freezing point during wet snow events. With cooling from vaporization, slight warming from solar radiation and being a wet snow event the temperature should be around 32 F at 5 pm. The best answer is c.
3. Answer b is not correct since the lifting occurs above the surface. The relative humidity in layers aloft is much more important than the surface dewpoint when forecasting precipitation in snow events. Lifting of air with a relative humidity of 100% will generate precipitation. Answer c is not correct since a shorter than expected lifting would be favored over longer than expected lifting when it comes to precipitation not being likely. The lifting needs to be in place long enough in order to saturate the troposphere. Answer d and e would be relevant in a convective thunderstorm situation but not in a dynamic lifting winter snow situation. That leaves a as the best answer. The relative humidity in layers aloft was initially too dry for precipitation to be generated. Another way of saying this is that the lifting was not strong enough to increase the RH to 100% in layers aloft to generate precipitation.
4. A frozen anvil will be associated with any thunderstorm thus it is not a discriminating factor. The atmosphere is fairly barotropic thus strong Warm Air Advection would not be expected. High Helicity and convective turbulence are not discriminating factors. While dynamic lifting (through low level convergence and/or upper level divergence) is important to get storms started, the amount of instability is more important in determining how strong the storms will be once they develop. Large instability is one factor to producing storms with vivid lightning. Another important factor to the generation of lightning is for the storm to have abundant moisture. High Precipitable Water will supply a thunderstorm with abundant moisture. Large instability combined with High Precipitable water maximize the amount of charge separation that occurs within a storm and thus maximizes the lightning a storm will produce. The best answer is e.
5. A process that cools the lower troposphere will be needed. Radiational cooling can do this but since this is a dense cloudy night, radiational cooling will be greatly inhibited. Latent heat release is a warming process and thus that can not be the answer. Since the troposphere is already basically saturated, evaporative cooling will be inhibited. Cold soil temperatures in this situation will not have much of an effect since the soil temperature is likely not much colder than the air surface temperature due to the rain all day. Advection of colder air into the forecast area is needed to change the precipitation from a liquid to a solid. The best answer is e. As an aside, two other factors not listed in the answers that could also change the precipitation from rain to snow in this situation would include increasing dynamic lifting and cold air aloft being filtered toward the surface by the precipitation. As dynamic lifting occurs, the air cools adiabatically. Strong dynamic lifting will lower the freezing level closer to the surface. Stronger dynamic lifting also tends to produce heavier precipitation. Large snowflakes have less of a chance of melting aloft compared to smaller snowflakes. Melting precipitation absorbs some latent heat. Conduction between icy precipitation and above freezing air will cool the surrounding air even if conditions are saturated. Change over from rain to heavy wet snow is not uncommon in these situations. Evaporative cooling will become more significant if the advection of drier air along with the Cold Air Advection occurs at the same time.
6. First, the temperature will drop to the dewpoint. At this point the temperature and dewpoint will both be 34 F. Since this is above freezing, liquid dew will develop on the ground as the temperature tries to cool further. When liquid dew forms, the latent heat of condensation is released. This slows the rate of cooling after the temperature and dewpoint become equal. New dew will form as both the temperature and dewpoint cool together to a temperature of 32 F. At this point, the liquid dew on the ground will begin to freeze. This freezing releases the latent heat of freezing. Eventually all the liquid dew will freeze. Now there is frozen dew on the ground. As the temperature and dewpoint continue to fall with each other from 32 F to 30 F, frost will be deposited on top of the frozen dew. Frost formation releases the latent heat of deposition. While frozen dew is a liquid turning to a solid, frost is a gas turning to a solid. Frozen dew and frost will have a differing appearance thus a forecaster can determine if icy dew is the product of frozen dew or frost. The best answer is d since all three forms of latent heat release occur on this night.
7. Snow generally has a higher albedo than the land surface it covers. Because of this during the day, less solar radiation is absorbed by the earth's surface since more is reflected away. Less absorption of solar radiation during the day will decrease the high temperature from what it would be without the snow cover. Snow is also efficient at emitting longwave radiation. At night, the enhancement of longwave energy emission from the snow cover causes the overnight low to be colder with snow cover than if the snow cover was not there. The best answer is a.
8. The solar radiation from the sun warms the earth's surface. This air is then mixed with air aloft. When winds are calm or light, the air can not be mixed as efficiently. Thus, the heat builds up and remains at and near the earth's surface. This results in days with light wind being warmer than days with stronger wind. Stronger wind mixes the heated air through a increasing depth of the troposphere and mixes it faster. In the question, the winds are stronger the next day while the other weather parameters remain similar. This will result in the afternoon high temperature being a little cooler than the previous day. 94 F and 91 F are the two possible answers. The effect on temperature will be to drop it a few degrees. A full 6 degree difference is probably a bit much. The best answer is d.
9.The frontal type is not specified thus the temperature answers can be eliminated. If you assume the front is a cold front or a warm front then one of the temperature answers will work. Often higher pressure will build in behind a frontal passage. Higher pressure is certainly building in since skies are clearing out in this case. Surface pressure is lowest along a frontal boundary. As a frontal boundary approaches the surface pressure will fall and once the front passes the pressure will rise. This happens regardless if it is a cold or warm front. The best answer is d.
10. Precipitation would cool the air thus that answer can be eliminated. Upslope flow is also a contribution to cooling since the air would cool adiabatically. PVA and divergence within the jet stream would likely produce cloud cover and perhaps precipitation. The one answer that will warm the surface air is solar radiation. Solar radiation will partly or completely cancel out a surface Cold Air Advection pattern. In this case it completely cancels it out and even increases the temperature somewhat. The best answer is c.
11. With clear skies and a long winter night the temperature should be able to drop to the dewpoint. Since the dewpoint is below freezing then dew or frozen dew would not be expected. Deposition will occur once the temperature drops to the frost point. The frost point is slightly higher in temperature than the dewpoint for temperatures below freezing since the vapor pressure of water vapor is less over an icy surface as compared to a liquid surface. Temperature have to be extremely cold for ice fog (fog composed of ice crystals) to occur. With temperatures in the 20s F, the fog would be composed of supercooled droplets and not ice crystals. We are left now to choose between frost and fog. With a wind speed of 13 knots (15 mph), fog will have a difficult time forming since drier air aloft will mix the fog out as it tries to develop. Stronger winds will also vaporize some of the frost that forms on the ground. An east wind will produce an upslope flow. These mixing winds along with upslope flow could reduce the dewpoint some over night since the dewpoint can decrease somewhat when air upslopes and strong winds aloft will mix out some of the moisture near earth's surface. The wind also will reduce the rate that cooling takes place over night since a stronger wind reduces the morning surface inversion that sets up. However, with a long winter night, clear skies, adiabatic cooling from upslope flow, and the afternoon dewpoint depression not impressively large, look for a light frost by sunrise especially in vegetated areas. The best answer is b.
12. A forecaster will weight the cooling factors versus the warming factors. The cooling factors are precipitation, cooler soil than air temperature and overcasting sky. The only warming factor is that percentage of solar radiation that is able to penetrate the clouds. Being that it is late fall the sun angle will be low and the day short. The cooling factors will limit the amount of warming that occurs during the day. Evaporative cooling only becomes relevant when the temperature tries to increase above the dewpoint. Since it is raining this day, the temperature and dewpoint will tend to rise together during the day. Each time the temperature tries to warm above the dewpoint the temperature and dewpoint will wetbulb toward each other again. Expect only a very slow rise in temperature. A 58 F high is reasonable. There are likely too many cooling factors to get the temperature up into the 60s. There should be enough diffuse solar radiation to warm the soil and air temperature a few degrees during the day. Answer is c. As an aside, temperatures aloft can be important in this type of situation also. The temperature of the rain can act to warm or cool the temperature of the air at the surface. I have seen freezing rain change to rain just from the fact that the rain was falling through a much warmer layer aloft. The addition of heat from the rain gradually crept the air temperature above freezing during the night.
13. The growth of hail depends upon a strong updraft and a significant growth region. Updraft strength is proportional to instability release. A freezing level closer to the surface in a severe weather situation will give the hail stones more distance to grow in the updraft and less time to melt once they make their final approach toward the surface. Several of the answers will enhance instability release but a large magnitude of instability release needs to be expressed in any correct answer. Dry air initially in the troposphere during severe storm development is important to the production of strong convective wind gusts. Dry air in the middle troposphere with high surface dewpoints can enhance instability when dynamic lifting occurs since potential instability would be present. Strong vertical wind shear can also enhance instability. Dynamic lifting through low level convergence or upper level divergence would not lift the air at nearly the rate as large instability release. The best answer is b since it has two critically important factors and expresses then explicitly.
14. On a clear night with light winds the earth's surface cools significantly through longwave energy emission. The air closest to the earth's surface will be cooled the most. This creates a temperature inversion from the surface through the boundary layer (temperature increase with height). In the morning hours solar radiation mixes out this inversion since the warming of the earth's surface eliminates the temperature increase with height that was in place. Wind and turbulence generally increase throughout the day since warmer air under cooler air will convectively mix while colder air under warmer air will not convectively mix. During the afternoon, the stronger wind and increased convective mixing reduces the amount of heat that can build up at the surface before it is mixed with air aloft. Examine the sounding at the link below. It shows a strong shallow inversion at the surface. Once the sun comes out, the inversion will quickly erode and the temperature will increase rapidly toward the temperature at 850 mb on the diagram. Generally, once air warms to the dry adiabatic lapse rate in the boundary layer further warming during the day will be slow. This is why on days such as this the temperature will climb rapidly in the morning but will only climb slowly in the afternoon. The best answer is a. Answer b and d are basically false while answer c is a cooling process that the solar radiation overpowers very quickly in these type of situations.
http://theweatherprediction.com/thermo/soundings/inversion/
15. Increasing cloud cover overnight would tend to make the temperature warmer than expected since cloudy skies insolates longwave radiation (clouds act like a blanket at night). There is a large dewpoint depression. There will be significant wet bulb cooling if precipitation occurs. A stronger wind at night will reduce the strength of the boundary layer inversion and that would tend to make the low temperature warmer. Skies clearing overnight would allow more longwave radiation to escape and thus that would make conditions cooler than expected. e is the best answer. Both b and d would make the temperature cooler than expected.
16. With clear skies, light wind and surface snow cover a very strong surface temperature inversion will set up. Many nights with record low temperatures in winter have these conditions for locations than can receive snow in winter. If snow falls throughout the night then that would mean the skies are cloudy. Cloudy skies overnight will greatly reduce the amount of overnight cooling that could occur.
17. For a thunderstorm to occur there needs to be adequate moisture to support a thunderstorm for a given situation and instability release generally needs to be present in order for lightning/thunder to occur. Without convective uplift, if precipitation occurs it will tend to be stratiform in nature. Even if instability and adequate moisture are present, a capping inversion can prevent the instability from being released. The dynamic lifting needs to be strong enough in order to break the capping inversion. Answers a, b and c are all possibilities. A forecaster should find out the specific reasons why thunderstorms do not occur in the forecast area when they are forecasted to occur.
18. Spraying the crops with water can help prevent crop damage in situations where the air temperature only goes slightly under freezing overnight. In this situation the wet bulb cooling from spraying the water and the fact that the temperature will be well below freezing reduce the effectiveness of this option. Wind machines and smoke work well at preventing crop damage if a strong surface temperature inversion is forecast to set up. Since the wind will be strong overnight, the wind machine will not be effective since there will be no inversion to mix out. The strong wind will blow the smoke away and thus the effectiveness of the smoke will be reduced. Animal sacrifice and praying are not proven to work scientifically. That leaves this as basically a hopeless situation. This is what insurance is for.
19. The dewpoint can not be higher than the temperatures. Thus, 60 F and 56 F can be eliminated. Since it has been foggy for several hours, likely the relative humidity is 100%. Thus, the temperature will be equal to the dewpoint. The best answer is c.
20. The correct answer is a. Answer b is not correct because a forecast does not assume the exact areal coverage of the precipitation. Answer c would be a 100% chance. Answer d is incorrect since it mentions anywhere in the forecast area and this is different from any particular point location in the forecast area. Answer e is wrong since percentages are not meant to mean if a forecast is right or wrong. See webpage below:
http://www.theweatherprediction.com/habyhints/284/