Forecasting weather elements and phenomena Flashcards
Maximum temperatures usually occur at about
2 P.M. or 3 P.M
while minimum temperatures occur
around sunrise or just after sunrise. (6 A.M.)
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.
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
Since the 500 MB winds are from the
southwest at 50 kts
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.
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.
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.
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.
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
Morning cloud cover can be very
misleading
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.
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
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.
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.
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
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.
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
this will work if
the trough is stationary
The example given above works well if the trough is stationary. However, if
the trough is moving, modifications must be introduced.
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 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
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
The STRONGEST WINDS are
“digging” down the western side of the trough.
. 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”.
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
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.
A “LIFTING OUT” TROUGH
When a trough is lifting out, the area of precipitation, along with the intensity of the precipitation, generally DECREASES
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.
.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.
A DEVELOPING STORM
If rapid storm development (cyclogenesis) is expected, the size of the precipitation area, as well as the precipitation intensities, will increase significantly
If rapid storm development (cyclogenesis) is expected, the size of the precipitation area, as well as the precipitation intensities, will increase significantly. This data should then be
utilized in modifying amounts and duration of precipitation expected.
.STORM INTENSIFICATION:
Certain conditions favor rapid storm intensification. A few are given below:
- COLD AIR AT THE 500 MB. LEVEL
- VORTICITY:
- COLD AIR ADVECTION AT THE 700 MB. & 500 MB. LEVEL
- THE “DIGGING” TROUGH ->
- DIVERGENCE AT THE 200 MB. LEVEL >
- BAROCLINICITY AT THE SURFACE
- THE JET STREAM AND ITS INFLUENCE ON STORM DEVELOPMENT
COLD AIR AT THE 500 MB. LEVEL:
Storms intensify rapidly if
the trough (short wave or long wave) possesses cold air.
Storms intensify rapidly if the trough (short wave or long wave) possesses cold air. Generally, a temperature of ……………… at the southern extremity of the supporting trough is sufficient for rapid intensification on the surface.
-25°C to -30°C
Generally speaking, the colder the air at the 500 MB. level, the
greater the potential for storm deepening
Storm deepening is usually favored where
500 MB. CYCLONIC WINDSHEAR & CURVATURE EXIST.
Storm deepening is usually favored where 500 MB. CYCLONIC WINDSHEAR & CURVATURE EXIST. These are the components of
POSITIVE VORTICITY
A storm developing near station “A” would
deepen rapidly.
A storm developing near station “A” would deepen rapidly. 500 MB. wind speeds to the east of “A” are ->
100 Kts
while to the west of “A” near the center of the trough, they are
only 25 Kts
A storm developing near station “A” would deepen rapidly. 500 MB. wind speeds to the east of “A” are 100 Kts. while to the west of “A” near the center of the trough, they are only 25 Kts. This creates a
counterclockwise (cyclonic) wind shear
A storm developing near station “A” would deepen rapidly. 500 MB. wind speeds to the east of “A” are 100 Kts. while to the west of “A” near the center of the trough, they are only 25 Kts. This creates a counterclockwise (cyclonic) wind shear. The directional curvature at the bottom of the trough
also induces cyclonic wind shear.
A storm developing near station “A” would deepen rapidly. 500 MB. wind speeds to the east of “A” are 100 Kts. while to the west of “A” near the center of the trough, they are only 25 Kts. This creates a counterclockwise (cyclonic) wind shear. The directional curvature at the bottom of the trough also induces cyclonic wind shear. Cyclonic wind shear then is
a combination of directional shear and speed shear
Cyclonic wind shear then is a combination of directional shear and speed shear. This total shear induces
rotational motion or circulation
Cyclonic wind shear then is a combination of directional shear and speed shear. This total shear induces rotational motion or circulation. This is commonly referred to by the meteorologist as
vorticity
The greater the shear, the
greater the vorticity.
STRONG VORTICITY INDUCES
INDUCESSTRONG SURFACE CYCLOGENESIS
STRONG VORTICITY INDUCESSTRONG SURFACE CYCLOGENESIS. Therefore,
troughs with very cold air and strong vorticity possess the greatest potential for explosive surface storm development
COLD AIR ADVECTION AT THE 700 MB. & 500 MB. LEVEL:
Since a trough is a pocket of
cold air
Since a trough is a pocket of cold air, deepening of the trough will occur if
more cold air is fed into it.
STRONG COLD AIR ADVECTION causes
rapid intensification of the trough
STRONG COLD AIR ADVECTION causes rapid intensification of the trough. This, in turn, often triggers
rapid surface intensification east of the trough axis.
STRONG COLD AIR ADVECTION causes rapid intensification of the trough. This, in turn, often triggers rapid surface intensification east of the trough axis. Generally,
2 or 3 isotherms (less than 60 miles apart) perpendicular to winds stronger than 50 Kts. on the 500 MB. map would be considered favorable for rapid surface cyclogenesis (STORM FORMATION.)
*THE SAME RULES APPLY TO COLD AIR ADVECTION ON THE’
When winds on the western side of the trough are strong and from the north or northwest, trough
deepening can be expected
When winds on the western side of the trough are strong and from the north or northwest, trough deepening can be expected. (Especially if
weak winds prevail at the base of the trough.)
When winds on the western side of the trough are strong and from the north or northwest, trough deepening can be expected. (Especially if weak winds prevail at the base of the trough.) This, in turn, often creates
surface cyclogenesis.
Surface storms deepen rapidly when
DIVERGENCE is present at the 200 MB. level (approximately 38,000 ft.)
DELTA EFFECT
divergence is indicated in areas where the contour lines diverge or SPREAD APART
The greater the divergence, the
greater the storm deepening.
Earlier we mentioned that storms form in zones of strong baroclinicity (strong
temperature gradient).
Earlier we mentioned that storms form in zones of strong baroclinicity (strong temperature gradient). Storms may also
deepen under the same conditions.
RAPID INTENSIFICATION of a storm can be expected if
very cold and dry air is fed into the storm from the NORTH or NORTHWEST while very warm and humid air is introduced into the storm from the SOUTH OR SOUTHEAST.
RAPID INTENSIFICATION of a storm can be expected if very cold and dry air is fed into the storm from the NORTH or NORTHWEST while very warm and humid air is introduced into the storm from the SOUTH OR SOUTHEAST. Of course, favorable support must be present at
500 MB. for any good development to take place.
The jet stream may be depicted as a
tube with high wind speeds meandering through the upper atmosphere
In order to locate the jet stream it is best to use the
300 MB. map (32,000 ft.) during the summer season and the 200 MB. map (38,000 ft.) during winter.
The jet stream often meanders in the general vicinity of
the polar front
Wind speeds in the jet stream are
not constant
Wind speeds in the jet stream are not constant. Some areas have much higher wind speeds than adjacent locations. Areas having the highest wind speeds are called
VELOCITY MAXIMA
Wind speeds in the jet stream are not constant. Some areas have much higher wind speeds than adjacent locations. Areas having the highest wind speeds are called VELOCITY MAXIMA and those showing the lowest speeds are called
calledVELOCITY MINIMA.
ISOTACHS
are lines connecting places that have equal wind speeds.
Associated with the jet stream is
WIND SHEAR.
The air at “A” in Figure 34 is located in an area of
wind shear and is being induced to circulateCOUNTERCLOCKWISE.
. The air at “A” in Figure 34 is located in an area of wind shear and is being induced to circulateCOUNTERCLOCKWISE. This occurs
NORTH of the jet stream and is known as CYCLONIC WIND SHEAR
The jet stream provides a clue to the
development of lows along the polar front
their intensification as well as the location of their associated precipitation.
Generally, storms develop and intensify in
AREA I is most favorable because it possesses CYCLONIC WIND SHEAR and alsoCYCLONIC CURVATURE,
AREA I is most favorable because it possesses CYCLONIC WIND SHEAR and alsoCYCLONIC CURVATURE, both favorable for
storm development.
AREA III,
possesses CYCLONIC CURVATURE, favorable for storm development, BUT it also exhibits ANTICYCLONIC WIND SHEAR, unfavorable for storm development, therefore, strong storm development in AREA III would not be expected.
AREA II
is also not a favored area for storm development because although it has CYCLONIC WIND** **SHEAR**, it also has **ANTICYCLONIC CURVATURE.
AREA IV
the least likely area for storm development.
In summary then, greatest intensification of storms occurs when
the surface storm is located on the eastern edge of a 500 MB. trough, to the left of the jet stream and with a velocity maxima approaching from upstream.
The jet stream also influences the
amount of precipitation.
The jet stream also influences the amount of precipitation. Generally, the maximum amount of precipitation coincides with
the center of the jet stream
The jet stream also influences the amount of precipitation. Generally, the maximum amount of precipitation coincides with the center of the jet stream. Rainfall usually decreases
north and south from this center,
The jet stream also influences the amount of precipitation. Generally, the maximum amount of precipitation coincides with the center of the jet stream. Rainfall usually decreases north and south from this center, but much more rapidly on the
south side
Storms become more likely in the United States when the jet stream
“digs” deep into the low latitudes, such as it does during the winter.
. Storms become more likely in the United States when the jet stream “digs” deep into the low latitudes, such as it does during the winter. During the summer, when the jet stream is usually located
north in Canada, very little storm activity or prolonged rainfall occurs in the continental United States.
