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The following material is given out to my ES17 (Intro to Weather) students to aid them in their weather forecasting. Keep in mind that this is an introductory guide written for students who have no previous knowledge of weather maps or forecasting techniques.

ALWAYS BE THINKING:

1) Amount of Sun vs. Clouds

2) Wind Direction & Speed

Since you are forecasting temperatures, the amount of cloud cover plays a significant role. All else being equal:

CLOUDS CAUSE COOLER HIGH TEMPERATURES WHILE

CAUSING WARMER LOW TEMPERATURES.

Wind direction determines whether or not warm vs. cold or wet vs. dry air is moving into the forecast city. All else being equal:

SOUTHERLY WINDS BRING WARMER & MORE HUMID AIR WHILE

NORTHERLY WINDS BRING COOLER & DRIER AIR.

HIGHER WIND SPEEDS CAUSE COOLER HIGH TEMPERATURES WHILE

CAUSING WARMER LOW TEMPERATURES.


Normal Temps: These temps are based upon a 30 year average for that particular location. The normal temps are a good starting point to give you a feel for the normal weather conditions during the forecast period. Keep in mind that the actual temps may vary from normal quite dramatically in changeable weather situations.


Past Temps: High and low temps from the day before can reveal how much different the last day was from normal and from today (Day 0). Keep in mind that the first day you forecast (Day 1) will be two days after yesterday so temps can change dramatically in that time. Temperatures are usually analyzed in intervals of 10 oF. Lines of equal temperature are called isotherms.


Current Conditions: Note the current temps, wind direction/speed, cloud cover. How much different are the numbers from yesterday? From normal? Keep in mind that the high temperature from the current day may determine how cool the low temperature is for Day 1. For example, if you believe that there will be about 20 degrees of cooling overnight, the low should be today’s high temp minus 20 degrees. Also note the current conditions for surrounding states. Since weather moves with the wind direction, keep an eye on conditions upwind from the forecast city.


Dewpoint Temp: You will eventually learn more about dewpoint temperature. Dewpoint temperature is related to the amount of water vapor in the air. Fog will form when the air temperature cools to the dewpoint and further cooling is limited. Therefore, dewpoint temperature can be considered the "lowest temperature" that the air can cool overnight. Dewpoint temperatures do not change much unless a weather front passes. Do not forecast temperatures that are lower than the regional dewpoints unless you think a front will pass.


Air Pressure: Air pressure is measured in millibars (mb). Normal sea-level air pressure measures 1013.2 mb. Under normal circumstances, air pressure will range from 960 mb (very low) to 1050 mb (very high). Stormy weather is usually preceded by falling air pressure while fair weather usually results when pressure is rising. Keep an eye on the pressure changes predicted by the forecast models. Pressure is usually analyzed in intervals of 4 mb. Lines of equal pressure are called isobars. Wind tends to blow close to parallel between isobars with lower pressure to the left of the wind.


Radar Maps: Radar maps detail precipitation intensity by colors (lt. green, dk. green, yellow, orange, red, magenta), height of cloud tops given in hundreds of feet above se-level (i.e. 340 = 34,000 ft.), and direction of movement (indicated by an arrow for direction and speed in knots).


Satellite Maps: There are two main types of satellite maps: visible and infrared (IR). Visible satellite maps are only available during daylight hours since they show reflected sunlight. Think of one of these maps as a "snapshot" of the clouds from a camera in space. IR maps measure the heat (radiation) emitted by clouds so they can be utilized 24 hours per day. Shades of grey are used to indicate cloud temperatures. Clouds that are low (close to the earth’s surface) are relatively warm and appear as dark grey on an IR map. Clouds that are very tall (high tops) are relatively cold and appear as bright white on an IR map. Keep in mind that the thicker the cloud is, the more moisture it holds. Thus, it appears brighter on an IR map.


Low-Level Jet Stream: The low-level jet stream (850 mb) reveals wind speed and direction at an average of 5,000 ft. above sea-level. Since surface winds can be greatly affected by topography, the low-level jet stream may be a better indicator of the general wind flow in the region. Look for direction to indicate temperature change (south-warm, north-cold) and see if the source of the air is from land (drier) or ocean (wetter).


Upper-Level Jet Stream: The upper-level jet stream (200-300 mb) reveals wind speed and direction at an average of 30,000 ft. above sea-level. This is the "jet stream" that most forecasters speak of. This jet stream is considered to be the steering current for low and high pressure systems. Generally, storms move with the direction of the upper-level jet stream. Keep in mind that the "shape" of the jet stream changes so it may only be useful for Day 1, however, long range forecast models predict the jet stream shape out to 10 days!


100-500 mb Thickness: The "thickness" of air between these two pressure levels is determined by the average air density in this vertical region. Since warmer air is less dense, it will take up more space causing a taller (thicker) atmosphere. Cooler air is more dense so it takes up less space. The result is a shorter (less thick) atmosphere. Therefore, higher thickness usually means a warmer atmosphere while lower thickness means a cooler atmosphere. Thickness is measured in decameters (Dm). A general rule of thumb is that air with thickness below 540 Dm is cold enough to support snow while air above 540 Dm thickness is not. This rule is NOT always reliable. More importantly, the TREND in thickness should be noted. Observe if the models are predicting increasing thickness values (warming trend) or decreasing values (cooling trend).


RAW ETA FOUS: The ETA forecast model as well as the medium-long range models are generated by predicting a variety of weather variables at points around North America. These "points" are on an imaginary grid covering the continent. Therefore, it is useful to have all of the raw data for the grid point closest to the forecast city. Example from LGA airport:

A

B

C

D

E

F

G

H

I

J

K

L

M

N

HR

Valid

Time

Temp

Dewpt

Pres

Wind

Prcp

V.V.

L.I.

Thick

R1

R2

R3

00

00Z

FEB 1

31

24

1018

347/13

.00

-1.5

14

532

75

30

42

06

06Z

FEB 1

27

24

1018

353/13

.00

-2.7

12

535

88

66

43

12

12Z

FEB 1

27

24

1019

26/8

.00

-1.7

10

537

89

60

41

18

18Z

FEB 1

32

29

1020

52/1

.00

1.5

10

538

90

55

45

24

00Z

FEB 2

30

28

1023

94/0

.00

-.1

10

539

90

51

53

30

06Z

FEB 2

28

24

1021

166/5

.00

.1

10

539

83

47

51

36

12Z

FEB 2

28

22

1022

180/6

.00

.8

11

539

77

45

50

42

18Z

FEB 2

32

26

1019

182/6

.00

1.2

12

539

79

46

53

48

00Z

FEB 3

32

27

1020

175/11

.01

1.3

8

537

90

75

68

A: The number of hours into the future. 00 is considered the initial time with each successive row being 6 hours later.

B & C: The GMT time & date for the forecast data. Many of the maps that you will be viewing will display the time in Zulu or GMT (Greenwich Mean Time). This time system is based upon the time along the Prime Meridian (zero degrees longitude) which passes through Greenwich, England (and many other places.) Therefore, the 0 to 24 hour time scale needs to be adjusted for every time zone. It is quite simple once one gets the hang of it. For EST one must subtract 5 hours from GMT (subtract 4 hours for EDT). Therefore, a time displayed as 18Z or 18GMT would be 1300 hours EST or 1:00 PM. Some key times to know are:

  • 00Z = 7 PM EST,
  • 05Z = 12 AM EST (midnight),
  • 12Z = 7 AM EST.

    Please note that a map with a time displayed as 00Z THU would be 7 PM WED EST. It would be midnight in England, but not on the east coast of the U.S. More info on the Web.

    D & E: Boundary temperature and dewpoint in oF.

    F: Sea-level pressure in mb.

    G: Boundary wind direction/speed (kt). Wind direction is given as the direction the wind is coming from along a compass direction. Therefore, 0 means from the north, 90 from the east, 180 from the south, and 270 from the west. Speed is given in knots (n.m. per hour) and can be converted to mph by multiplying by 1.15. Ex. 10 kt = 11.5 mph, 20 kt = 23 mph, etc. A boundary wind predicted to be 166/5 would be from 166o (SSE) at 5 kt.

    H: Liquid-equivalent precipitation in the previous 6 hours. Notice the last time block has .01. That means .01 inches of liquid-equivalent precipitation is forecast between the 42nd and 49th forecast hour. The column is cumulative meaning that the total precipitation for the 48 period is the sum of all forecast times. (Note: snowfall can be estimated by using a 10:1 ratio. 1.00 inches of rainfall usually corresponds to 10 inches of snowfall.)

    I: Vertical velocity in cm/s. Positive numbers indicate rising air and possible clouds/precipitation while negative numbers mean sinking air and improving conditions. –2.7 corresponds to air sinking at 2.7 cm/s.

    J: Lifted index (LI) value. This number is used to estimate thunderstorm and severe weather potential. Negative numbers imply increasing probability of thunderstorms and/or severe weather. Values are subtracted from 100 where 98 means –2, 96 means –4, etc.

    K: Thickness value in decameters (Dm). 534 equates to 534 Dm thickness.

    L, M, N: These are mean relative humidity values in percent for that time period. Ex. 56 means the humidity averages 56 percent in that level.

    R1 = Mean RH (in%) of model layer 1 (approx. 35mb in thickness)

    R2 = Mean RH (in%) of model layers 2-9 (approx. 35mb above the surface to 500mb)

    R3 = Mean RH (in%) of model layers 10-13 (approx. 500mb to tropopause)


    Weather Fronts: A weather front is a boundary between two distinct air masses. The weather conditions on either side of a front are usually quite different. Most of the cities you will be forecasting for will experience the passage of at least one weather front. Therefore, some basic knowledge of fronts and their associated weather is needed. Refer to the illustration above.

    Since you will be viewing forecast maps, frontal passage can be determined by observing shifts in wind direction. If the wind is from the south and then shifts to a northwest direction, it usually means a cold front has passed. Conversely, if the winds are from an easterly direction and then shift to a more southerly direction, it is likely a warm front has passed.



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