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1

Maximum temperatures usually occur at about

2 P.M. or 3 P.M

2

while minimum temperatures occur

around sunrise or just after sunrise. (6 A.M.)

3

To determine tomorrow's maximum temperature

examine today's 2 P.M. or 3 P.M. surface map. Calculate, using ½ the speed of the 500 MB winds the parcel of air that will be influencing your area 24 hours later. The temperatures in that parcel will very closely resemble tomorrow's maximum temperature in your area.

4

To determine tomorrow's maximum temperature, examine today's 2 P.M. or 3 P.M. surface map. Calculate, using ½ the speed of the 500 MB winds the parcel of air that will be influencing your area 24 hours later. The temperatures in that parcel will very closely resemble tomorrow's maximum temperature in your area. .......................... must be studied and then introduced into each forecast.

Local variations

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Since the 500 MB winds are from the

southwest at 50 kts

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Since the 500 MB winds are from the southwest at 50 kts. weather which is now ................................ will influence our area tomorrow at ......... 

600 miles (1/2 50 X 24 hours) to our southwest

3 p.m.

7

Since the 500 MB winds are from the southwest at 50 kts. weather which is now 600 miles (1/2 50 X 24 hours) to our southwest will influence our area tomorrow at 3 p.m. Since most of the upstream temperatures are in

the upper 60's, temperatures in the upper 60's are likely for the forecast area the next day.

8

The procedure for minimum temperatures is

exactly the same as for maximum temperatures with one exception. Instead of using a 3 P.M. surface map, the 6 A.M. or 7 A.M. surface map should be used. Employ the same upstream procedures and introduce the local variations experienced in your area.

9

If an increase in clouds, moisture or wind is expected in the parcel of air being transported into your area,

certain modifications must be made. 

10

If an increase in clouds, moisture or wind is expected in the parcel of air being transported into your area, certain modifications must be made. Generally speaking, any or all of these will tend to

to lower the maximum temperature expected as well as raise the minimum temperature expected

11

Morning cloud cover can be very

misleading

12

Morning cloud cover can be very misleading. For example,

"upstream" weather may indicate overcast skies at all reporting Stations causing a forecaster to "lower" his estimate on forecasted maximum temperatures.

13

Morning cloud cover can be very misleading. For example, "upstream" weather may indicate overcast skies at all reporting

Stations causing a forecaster to "lower" his estimate on forecasted maximum temperatures. However, upon investigation,

the forecaster may notice that the HEIGHT of these clouds is below 1000 feet

14

Morning cloud cover can be very misleading. For example, "upstream" weather may indicate overcast skies at all reporting

Stations causing a forecaster to "lower" his estimate on forecasted maximum temperatures. However, upon investigation, the forecaster may notice that the HEIGHT of these clouds is below 1000 feet. These .......... clouds, probably .............

low clouds, probably stratus, will most likely "burn off" by mid morning, thus not preventing temperatures from reaching the expected maximum.

15

These low clouds, probably stratus, will most likely "burn off" by mid morning, thus not preventing temperatures from reaching the expected maximum. However,

, if these low clouds have a higher deck of clouds above them (two or more decks) the sun will probably not be able to penetrate. Overcast skies will persist all day. Lower maximum temperatures can thus be expected.

16

The above factors, yes or no, when and how much can usually be determined very accurately for the

next 12 hours by using the surface map along with the 500 mb

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the figure indicates that

the 500 MB. winds indicate the rain area will effect station "A".  In fact, light rain (..) should move into the area in approximately six 6 hours. (150 miles divided by 25 MPH).  However, the rain should become moderate (...) and heavy (….) after about four (4) hours. (100 miles divided by 25kts) of light rain.  Moderate and heavy rain should continue for about 4 hours, tapering off to very light rain showers, then ending.

Station "B" would not be affected by this rain pattern, since all the rain would be passing to the north of the area.

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RAIN, YES OR NO, WHEN AND HOW MUCH?

Here are some factors that may complicate matters:

 

1.    A MOVING 500 MB. TROUGH

2.    A "LIFTING OUT" TROUGH

3.    A "DIGGING" TROUGH

4.    A DEVELOPING STORM

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this will work if 

the trough is stationary

20

The example given above works well if the trough is stationary. However, if

the trough is moving, modifications must be introduced.

21

The example given above works well if the trough is stationary. However, if the trough is moving, modifications must be introduced. We must use

vectors

22

The example given above works well if the trough is stationary. However, if the trough is moving, modifications must be introduced. We must use vectors.

The rain pattern will move according to the

"resultant of the two vectors, the vectors being 1) direction of the upper air winds plus 2) the eastward movement of the trough

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The faster the trough is moving, the

greater the eastward component will be in the resultant vector showing the actual movement of precipitation, taking the trough movement into account

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The STRONGEST WINDS are

“digging” down the western side of the trough.  

25

.  The STRONGEST WINDS are “digging” down the western side of the trough.  Since the winds are weak at the “bottom” of the trough, this trough will 

DEEPEN AND MOVE SOUTH-EASTWARD VERY SLOWLY, IF IT MOVES AT ALL.  WE CALL THIS A “DIGGING TROUGH”.

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the trough in the central part of the United States with the STRONGEST WINDS AT THE BOTTOM OF THE TROUGH.  This trough will REMAIN ABOUT THE SAME IN INTENSITY BUT WILL MOVE EASTWARD

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shows the trough in the center of the United States with the STRONGEST WINDS MOVING UP THE EASTERN SIDE OF THE TROUGH.  This trough will weaken and move rapidly towards the NORTHEAST.

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A "LIFTING OUT" TROUGH

When a trough is lifting out, the area of precipitation, along with the intensity of the precipitation, generally DECREASES

29

When a trough is lifting out, the area of precipitation, along with the intensity of the precipitation, generally DECREASES.  This should be taken into account when determining

when precipitation will start,

how much will fall, and

when it will end.

30

.    A "DIGGING" TROUGH

When a trough is "digging" 

the area of precipitation, along with the intensity of the precipitation generally INCREASES. This again must be considered when making a forecast.