Download Forecasting Outline

Forecasting Outline
The following outline may be useful while forecasting weather conditions for tonight and tomorrow.
The forecaster predicts future weather conditions by comparing current conditions to computer forecast
maps. The forecaster makes a forecast by comparing the differences they observe between current and
forecasted information. If no differences are observed, the forecaster expects conditions to remain the
same. For example, if the high temperature today was 50 °F and conditions are predicted to remain the
same for tomorrow, then you would forecast 50 °F for tomorrow’s high temperature.
All maps are found within the Netweather section of the Mahr Netweather Homepage.
http://teachers.oregon.k12.wi.us/mahr/
***Remember: All maps must be checked for time accuracy. Are all current maps really current?
For what time periods are the forecast maps valid?***
Forecast Preparation
1. Look outside- observe current cloud cover and any precipitation.
2. Record Averages and Records for Your City.
3. Record Yesterday’s High/Overnight Low Temp.
4. Record Current Conditions for your city.
5. Current Surface Map
a. What type of weather system (High or Low) is currently influencing your city’s weather?
b. Are any strong storms (many tightly packed isobars around the Low) located to the west?
c. What type(s) of precipitation is the storm producing and is any of the current precipitation close to your area?
d. What type of air mass (warm, cold, or cool) is currently influencing your weather?
e. What direction is the wind coming from? Air circulates clockwise around a High and counterclockwise around a
Low.
f. Are your winds strong, moderate, or light and how far apart are the isobars?
6. Satellite Loop
a. Cloud cover shows positions of Low pressure storm systems.
b. Are any clouds moving toward your city?
7. Current National Radar Loop
a. Is any precipitation headed toward your city?
b. What is the type of precipitation?
c. Is the precipitation heavy or light?
8. Current 500mb Jet Stream Map
a. Observe the position of the 5400m height line.
b. Is your city under the influence of a trough (cold), ridge (warm), or is the flow zonal (mostly west to east which
brings normal temperatures)?
c. Is the ridge building or is the trough deepening from yesterday’s observations?
d. Is a cut-off Low influencing your area? If so, clouds and temperatures will remain much the same.
9. Current 850 mb Map
a. Identify the current 850 mb temperature line over your city.
b. Is it cold enough to snow? If the temperature at 850 mb is 0 °C or colder, snow will likely fall to the surface without
melting.
Your Forecast Tonight (6 p.m. today – 6 a.m. tomorrow)
Clouds
***If you forecast precipitation, you must forecast cloud cover.***
The forecasting questions for clouds are “If currently cloudy, will the clouds move away? If currently not cloudy, will clouds
move in?”
• Satellite Loop - Is it currently cloudy? Do you observe any clouds to the west heading toward your city?
• Current Surface Map - Is a Low currently influencing your city’s weather? Is there a Low to the west? Lows located
in the Dakotas, Nebraska, and Kansas often take a day or longer to reach Wisconsin.
• This Evening Surface Forecast and Tomorrow Morning Surface forecast maps - Is the Low or High pressure system
currently influencing your city’s weather predicted to move away? Is a storm to the west predicted to move into your
area? Is the storm intensifying? If the number and closeness of the isobars around the Low increases, the area of cloud
cover and precipitation will likely expand. A cut-off Low remaining in the area likely means continued cloudy, cool
conditions.
Precipitation Quantity
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Current National Radar Loop - Does precipitation to the west appear to be moving fast enough to reach your city
tonight?
Precipitation Forecast- 12 hr, 24 hr, 36 hr, 48 hr (NGM) – Look at the map valid 12Z (6 a.m. tomorrow). How much
precipitable water is predicted for your area? If you expect snow, use the conversion of 10 inches of snow for each inch
of precipitable water. Multiply the inches of precipitable water by 10 to estimate the snowfall amount.
Precipitation Type
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850 mb- 12 hr, 24 hr, 36 hr, 48 hr (NGM) - Look at the maps valid 0Z (6 p.m. tonight) and 12Z (6 a.m. tomorrow).
Identify the location of the freezing (0°C) line. If the freezing line is north of your city, rain is likely. If the freezing line
is south of your city, snow is likely. If the freezing line is forecasted to move south of your city from 0Z to 12Z then
forecast rain changing to snow overnight. If the freezing line is expected to move north of your city from 0Z to 12Z then
forecast snow changing to rain overnight.
Low Temperature
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Current Surface Map – What type of air mass currently influences your city (warm, cool, cold)? What was last night’s
low temperature?
This Evening Surface Forecast and Tomorrow Morning Surface forecast maps – What type of air mass is predicted
for tonight? If the air mass is similar to last night’s air mass then forecast the same overnight low temperature. If a warm
air mass moves in by tonight then forecast warmer temperatures for tonight. If a colder air mass moves in by tonight then
forecast cooler temperatures. Clouds and moderate winds keep temperatures up overnight. If in the same air mass as last
night, but last night was clear, you would forecast temperatures 5-10 degrees warmer tonight because of cloud the cover.
If the sky was cloudy last night and predicted to be clear tonight then forecast temperatures 5-10 degrees cooler. If you
are forecasting late in the day the dew point temperature is often a good prediction for the overnight low temperature.
500mb Jet Stream Forecast Map - If a ridge builds in by tomorrow (12Z) then temperatures may tend to be warmer
tonight than last night. If a trough digs in by tomorrow then temperatures may tend to be cooler tonight than last night.
850 mb- 12 hr, 24 hr, 36 hr, 48 hr (NGM) – If warmer 850 mb temperatures are forecasted to move over your city by
tomorrow morning (12Z) expect temperatures to be warmer than yesterday. If colder 850 mb temperatures are forecasted
to move over your city by tomorrow morning (12Z) expect the overnight low temperature to be lower. You can estimate
how much warmer or colder by comparing current 850 mb temperature to the forecasted 850 mb temperature. A large
forecasted change in 850 mb temperature generally produces a big change in the surface temperature.
Wind Direction
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This Evening Surface and Tomorrow Morning Surface forecast maps – Compare the Evening forecast to the
Morning forecast to determine the wind direction tonight. If the two maps are essentially the same, the wind will not
shift overnight. If they are different, forecast the appropriate wind direction shift overnight. Wind blows clockwise
around a High and counterclockwise around a Low. Wind blows along the isobars slightly in towards a Low and
slightly out of a High.
Wind Speed
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This Evening Surface and Tomorrow Morning Surface forecast maps - Compare the spacing of the isobars on the
current surface map to the predicted spacing on the forecast maps. If the isobar spacing forecasted tonight is the same as
the spacing right now then forecast the same wind speed for tonight. If the isobars move slightly closer together then
forecast a slightly faster wind speed and vice versa. For example, if the wind is currently blowing 10 – 15 mph and the
isobars move slightly closer together on the forecast map then you would predict a wind speed of 15 – 20 mph.
Your Forecast Tomorrow (6 a.m. tomorrow – 6 p.m. tomorrow)
Clouds
***If you forecast precipitation, you must forecast cloud cover.***
The forecasting questions for clouds are “If currently cloudy, will the clouds move away? If currently not cloudy, will clouds
move in?”
• Satellite Loop, Tomorrow Morning Surface, and Tomorrow Evening Surface forecast maps – If skies are currently
cloudy and the Low is not forecasted to move far enough away to take the area of clouds with it then forecast clouds for
tomorrow. If High pressure is currently producing clear skies and the High pressure is forecasted to remain in the area
tomorrow then forecast clear skies. If currently clear and a Low is forecasted to move close enough to bring clouds into
the area then forecast clouds. If currently cloudy with a High expected to move into the area by tomorrow evening then
expect clearing skies throughout the day. A cut-off Low remaining in the area likely means continued cloudy, cool
conditions.
Precipitation Quantity
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Precipitation Forecast- 12 hr, 24 hr, 36 hr, 48 hr (NGM) - Look at the map valid 0Z two calendar days from today (6
p.m. tomorrow). How much precipitable water is predicted for your city? If you expect snow, use the conversion of 10
inches of snow for each inch of precipitable water. Multiply the inches of precipitable water by 10 to estimate the
snowfall amount.
Precipitation Type
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850 mb- 12 hr, 24 hr, 36 hr, 48 hr (NGM) - Look at the maps valid 12Z (6 a.m. tomorrow) and 0Z (6 p.m. tomorrow).
Identify the location of the freezing (0°C) line. If the freezing line is north of Oregon we will likely receive rain. If the
freezing line is south of Oregon we will likely receive snow. If the freezing line is forecasted to move north of Oregon
from 12Z to 0Z tomorrow then forecast snow changing to rain during the day and vice versa.
High Temperature
If the air mass and cloud conditions predicted for tomorrow are the same as today then predict the same high temperature. The
same type of air mass can be warmed 5-10 degrees by sunshine or kept 5-10 degrees colder by clouds.
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Departure from Normal High Temp, U.S. Hourly Data, and Current Dew Point and Temperature Contours - If a
cold front is predicted to pass through temperatures generally become cooler. Temperatures are generally warmer with
the passage of a warm front. You can get an idea of how cold or warm an air mass is by checking the current
temperatures of the air mass to the west. If you are forecasting in the afternoon, check the current temperatures of the air
mass currently to the west that is forecasted to be in your area by tomorrow afternoon.
500 mb Jet Stream Forecast - If a ridge builds in by tomorrow (12Z and 0Z) then expect temperatures to be warmer. If
a trough digs in by tomorrow then expect temperatures to be cooler than today.
850 mb- 12 hr, 24 hr, 36 hr, 48 hr (NGM) – If warmer 850 mb temperatures are forecasted to move over your city by
tomorrow evening (0Z) expect temperatures to be warmer than yesterday. If colder 850 mb temperatures are forecasted
to move over your city by tomorrow evening (0Z) expect daytime high temperature to be lower. You can estimate how
much warmer or colder by comparing current 850 mb temperature to the forecasted 850 mb temperature. A big
forecasted change in 850 mb temperature generally produces a big change in the surface temperature.
Wind Direction
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Tomorrow Morning Surface and Tomorrow Evening Surface forecast maps - Use the same method you used to
predict winds for last night.
Wind Speed
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Tomorrow Morning Surface and Tomorrow Evening Surface forecast maps - Use the same method you used to
predict winds last night.