DASK

FORECASTING METHODS

There are several different methods that can be used to create a forecast. The method a forecaster chooses depends upon the experience of the forecaster, the amount of information available to the forecaster, the level of difficulty that the forecast situation presents, and the degree of accuracy or confidence needed in the forecast.

Persistence Method : (today equals tomorrow) The persistence method assumes that the conditions at the time of the forecast will not change. The persistence method works well when weather patterns change very little. It may also appear that the persistence method would work only for shorter-term forecasts.

Trends Method : The trends method involves determining the speed and direction of movement for fronts, high and low pressure centers, and areas of clouds and precipitation. Using this information, the forecaster can predict. For example, if a storm system is 1000 miles west of your location and moving to the east at 250 miles per day, using the trends method you would predict it to arrive in your area in 4 days.

Climatology: The Climatology Method is another simple way of producing a forecast. This method involves averaging weather statistics accumulated over many years to make the forecast. The climatology method only works well when the weather pattern is similar to that expected for the chosen time of year. If the pattern is quite unusual for the given time of year, the climatology method will often fail.

Analog Method: The Analog Method is a slightly more complicated method of producing a forecast. It involves examining today's forecast scenario and remembering a day in the past when the weather scenario looked very similar (an analog). The forecaster would predict that the weather in this forecast will behave the same as it did in the past.

Numerical Weather Prediction: Numerical Weather Prediction (NWP) uses the power of computers to make a forecast. Complex computer programs, also known as forecast models, run on supercomputers and provide predictions on many atmospheric variables such as temperature, pressure, wind, and rainfall. A forecaster examines how the features predicted by the computer will interact to produce the day's weather.

TEMPERATURE FORECAST

Effects of Cloud Cover (on forecasted temperatures): During the day, the earth is heated by the sun. If skies are clear, more heat reaches the earth's surface (as in the diagram below). This leads to warmer temperatures. However, if skies are cloudy, some of the sun's rays are reflected off the cloud droplets back into space. Therefore, less of the sun's energy is able to reach the earth's surface, which causes the earth to heat up more slowly. This leads to cooler temperatures.

When forecasting daytime temperatures, if cloudy skies are expected, forecast lower temperatures than you would predict if clear skies were expected.

At night cloud cover has the opposite effect. If skies are clear, heat emitted from the earth's surface freely escapes into space, resulting in colder temperatures. However, if clouds are present, some of the heat emitted from the earth's surface is trapped by the clouds and reemitted back towards the earth. As a result, temperatures decrease more slowly than if the skies were clear.

When forecasting nighttime temperatures, if cloudy skies are expected, forecast warmer temperatures than you would predict if clear skies were expected.

High and Low Pressure Centers: The positions of high and low pressure centers can greatly influence a forecast. Fair weather generally accompanies a high pressure center and winds flow clockwise around a high. This means that winds on the back (western) side of the high are generally from a southerly direction and typically mean warmer temperatures. On the front (eastern) side of a high, winds are generally from the north and this typically results in colder temperatures.

If a region is expected to be located west of a high pressure center then warmer temperatures are likely. However, if the region is expected to be in the northerly winds of a high pressure center, then forecast colder temperatures. Cities under the influence of high pressure centers can expect generally fair weather with little or no precipitation.

In contrast, clouds and precipitation generally accompany a low pressure center and winds flow counterclockwise around lows. This means that winds on the back (western) side of the low are generally from a northerly direction and typically mean colder temperatures.

PRECIPITATION FORECAST

Clouds and precipitation may not occur if the low levels of the atmosphere do not contain sufficient moisture. If the dew point is close to the corresponding temperature, the air is nearly saturated, so precipitation is quite possible.

If there is sufficient moisture in the air and a forcing mechanism is approaching the area, then there is an increased probability that precipitation will occur. Clouds and precipitation are formed by the upward motion of air. Therefore, there must be a mechanism present to lift the air. Fronts often serve as such a mechanism.

Rain or Snow? Most precipitation that reaches the ground actually begins as snow high in the atmosphere. These snow flakes develop somewhere above the freezing level where the air temperature is less than 32 F, and begin to fall toward the earth as snow. If ground temperature is above 32 F, the freezing level must be located somewhere above the ground. The falling snow passes through the freezing level into the warmer air, where it melts and changes to rain before reaching the ground. When forecasting precipitation type, if temperatures are expected to be above freezing, then rain is most likely. If temperatures are expected to be below freezing, then forecast for snow.

PRACTICAL WEATHER FORECAST

Depending on when and where you are:

Look for cloudy, unsettled weather	When: ØThe barometer falls steadily ØThe wind blows strongly in the early morning ØThe temperature at night is higher than usual owing to clouds ØClouds rapidly move in various directions at different levels ØThe temperature is far above or below normal for the time of year high, thin, wispy clouds (cirrus) increas e in amount, thicken and lower, sometimes producing a ring or halo around the sun or moon ØClouds darken on a summer afternoon ØHigh- and/or middle-level clouds darken and move from the south and southwest ØThe sunrise is red
Look for steady rain or snow	When: ØThe barometer falls steadily (if the pressure falls slowly, rain or snow will come within a day; if it falls rapidly, expect precipitation any minute) ØWinds blow from the southeast to northeast and north ØClouds are low and uniformly flat and grey ØLeaves show their undersides as strong south wind in advance of the rain will flip the leaves over ØThere is a ring around the sun or moon
Look for more bright weather	When: ØThe barometer is steady or rising slowly ØA gentle breeze blows steadily from the west to north ØWinds swing from south to southwest or from east or northeast to the northwest ØCloud (amount and number) decreases in the late afternoon ØCloud base rises and humidity decreases ØEvening sky is clear and you can look directly at the setting sun which looks like a ball of fire ØMorning fog breaks within two hours of sunrise ØThe night before heavy dew or frost occurs ØThe moon shines brightly and the wind is light ØA bright blue sky with high thin wisps of cloud
Look for clearing skies	When: ØThe barometer rises Ø The wind shifts to any westerly direction (especially from east through south to the west) ØThe temperature falls rapidly, especially in the afternoon ØIncreasing breaks in the overcast ØClouds become lumpy ØDark clouds become lighter and steadily rise in altitude ØFFog lifts before noon ØFrost or dew is on the grass ØLook for showers (thundershowers) when: ØThe barometer falls ØWinds blow from the south or southeast ØThe morning temperature is unusually high, air is moist and sticky, and you see cumulus clouds building (rain within six hours) ØDark, threatening thunderclouds develop in a westerly wind ØThick, fluffy (cumulus) clouds develop rapidly upwards during early afternoon ØYou hear loud static on your AM radio (thunderstorms within the hour)
Look for heavy snow	When: ØThe barometer falls rapidly ØWinds blow from the east or northeast ØThe air temperature is between -10 oC and -1oC ØA storm lies to the south and east of you
Look for temperatures to rise	When: ØThe wind shifts from the north or west to the south ØThe night time sky is overcast with a moderate southerly wind ØThe sky is clear all day
Look for temperatures to fall	When: ØThe barometer rises steadily (in winter) ØThe wind shifts into the north or northwest from the south ØThe wind is light and the sky is clear at night ØSkies are clearing, especially in the winter Ø Snowflurries occur with a west or north wind
Look for fog	When: ØThe sky is clear, the winds are light, and the air is humid the night before ØWarm rain is falling ahead of the warm air ØWater temperatures are warm and the air is much colder

REFERENCE WEB PAGES

http://weather.about.com

USATODAY.com

www.ncdc.noaa.gov/jmdocs

http://www.hochgebirgstherapie.de/hochsten.htm

http://www.pcisys.net/~melanson/barrtrail

http://zephyr.unr.edu/outdoors/archives/out_pick_safety2.html

http://www.ciggrubu.org/cig-en.htm

http://www.ciggrubu.org/neden-en.htm

http://www.princeton.edu/~oa/winter/wintcamp.shtml#Avalanche%20Basics

http://www.math.utah.edu/~eyre/skiing/avalanche.html

http://www.standardfilms.com/photos.html

http://www.tuckerman.org/avalanche/terrainandconditoins.htm

http://www.bom.gov.au/info/ftweather/contents.shtml

http://www.uga.edu/srel/ecoviews4-23-00.htm

http://www.planet-sailing.net/2001/english/ge

http://www.blueplanetbiomes.org/climate.htm

http://www.doc.mmu.ac.uk/aric/eae/Climate/Older/Mediterranean_Climate.html

http://www.allaboutturkey.com/iklim.htm

http://www.meds-sdmm.dfo-mpo.gc.ca/cmos/beyourforecaster.html

http://www.geocities.com/CapeCanaveral/7742/

DASK	NATURE RESEARCHES, SPORTS AND RESCUE SOCIETY
		WEATHER FORECAST	Pre: Dr. Ahmet KILIÇ


WEATHER FORECAST
FORECASTING METHODS	The most straightforward way to predict the weather is to simply look out the window. However, for more accurate weather forecasts, you need something more than your eyes. New observational tools like radars, satellites, together with specially designed computers can give a clearer picture to the ever-changing weather conditions for better forecasting results. Weather has always been a significant concern to humankind, and our inability to control it has led us down through the ages to try to measure it, compare it to previous years, and predict it. Weather forecast is something that cannot be understand just by a few lines.
TEMPERATURE FORECAST
PRECIPITATION FORECAST
PRACTICAL FORECAST

WEATHER FORECAST Modern forecasting involves technology, science and advanced math to accurately predict the weather.