Advanced Weather Forecasting

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 in Figure 2, along with the 500 mb.map in Figure 2b.

In Figure 2b, 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.

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 500 millibar map in figure 4a shows a trough in the central part of the united states. 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 eastward very slowly, if it moves at all. We call this a “digging trough”.

In figure 4b 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.

Figure 4c 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.

The 700 MB. wind flow gives a good indication of both the location and amounts of expected precipitation associated with fronts. For example, the precipitation occurring with a cold front (Thunderstorms) usually occurs well in advance of the front itself if the 700 MB. winds are PERPENDICULAR to the front.

The stronger the 700 MB. winds become, the further in advance of the front the precipitation occurs. Briefly, this can be explained as follows. Winds blowing perpendicularly across the front flow downward over the frontal surface and will be warmed adiabatically. No precipitation will occur. Further in advance of the front, warm air will be converging with warm air already established in the warm sector, rising as it does. Prefrontal clouds and thunderstorms will occur where this UPWARD MOTION takes place. Since no "weather " is occurring along the front itself, such a front is classified as INACTIVE. and generally moves very rapidly.

Warm frontal precipitation becomes extensive when the 700 MB winds are PERPENDICULAR to the front and the wind flow above the front is cyclonic.

If the wind flow over the front is ANTICYCLONIC, very little precipitation occurs. In fact, many times it is only partly cloudy.

In Figure 11, surface winds around the back of the high-pressure system are blowing from the southeast, while winds at the 500 MB. level are blowing from the southwest in advance of the trough in the Mississippi Valley. If abundant moisture is available (check dew point depressions at the surface, 850 mb.700 MB and 500 MB. levels) excessive rainfall can be expected in the northeastern part of the United States.

To determine whether an area will experience rain, snow or a mixture of the two, simply MOVE the precipitation in the direction of the 500 mb. winds, at 1/2 their speed. If a "snow area" is to pass over the forecast location, forecast snow. If rain is indicated, forecast rain. If the boundary line between snow and rain will be passing over the forecast area, forecast a MIXTURE of snow and rain. ALTHOUGH THIS METHOD SEEMS TOO SIMPLE, IT IS NONE-THE-LESS EXTREMELY ACCURATE.

Snow often changes to rain. This usually appears on a weather map as follows. In this case, the N.Y.C. area would experience snow at the onset. Snow would continue for about six hours (150 miles divided by ½ the 500 mb. wind speed.) Rain would then take over and last for another six hours. Again, this method is simple but accurate.

If the storm takes PATH A, the moisture from the storm will fall over the ocean. A storm following this track would bring a partly cloudy sky, blustery northwest winds and probably cooler temperatures to the east coast.

If the storm takes PATH B, the surface winds along the coast will be from the southeast. In the winter season, since the ocean is warmer than the land, the winds would transport warm, moist air to the coast, causing the precipitation to fall in the form of rain.

If the storm takes PATH C, the warm air of the storm itself will move along the east coast causing rain instead of snow to fall along the entire coastline. (Or snow changing to rain).

For example, if the "540" thickness line passes through your forecast area (meaning that the calculated 1000-500 mb. thickness of the atmosphere over you is 5,400 meters) then rain or snow is equally probable. If, however, the thickness values over you are lower (meaning a colder atmosphere) then snow is more probable. Higher thickness values usually produce rain.

Figure 16 approximates the probability for UNFROZEN PRECIPITATION (rain) as the departure of observed thickness varies from the EQUAL PROBABILITY THICKNESS.

For example, if the equal probability thickness is 540 (5400 meters) and the observed thickness is 60 meters greater (5400+60=5,460 or546) then using Figure 16 it can be determined that there is about a 78% probability that the precipitation will be unfrozen (rain).

How much are we going to get? A very important and often asked question. Below is a simple but accurate method of forecasting snow accumulations. Again we use the UPSTREAM METHOD.

Referring to Fig. 17, we notice an area of snow (light, moderate and heavy) heading for Chicago at 20 Kts. (1/2 the 500 mb. wind speed). Using this speed, light snow will continue for five (5) hours (100 miles divided by 20 mph). MODERATE SNOW will then commence and continue for five (5) more hours. Heavy snow will fall for five (5) hours and then taper back to light snow for an additional five (5) hours. In all, about 20 HOURS of snow will occur. How many inches will accumulate?

Winds in the upper levels of the atmosphere (18,000 ft) do not just streak across the sky from west to east. Generally, there are waves in these wind patterns.

A RIDGE is a pocket of WARM air and appears as a mountain in Figure 18, while a TROUGH is a pocket of COLD AIR and appears as a valley. The winds (arrows) follow the contours formed by the troughs and ridges. WINDS INCREASE in speed when the PRESSURE HEIGHT LINES are close together.

The winds at the upper levels (500 mb. or 18,000 ft) are the STEERING FORCES for the surface weather.(Figure 19) The surface storm located in the Georgia-South Carolina area will move northeastward up the Atlantic seaboard, spreading its associated rain or snow up the coast. The storm will move at a speed of ONE HALF the speed of the winds steering the storm on the 500 MB. map. For example, if the wind speed on the 500 MB. map averages about 50 miles per hour, then the surface storm and its associated precipitation (weather) will advance up the coast at a speed of about 25 miles per hour.

Storms need support from the upper air. This support comes from the ridges and troughs on the 500 MT. map. (Diagram #3)

LOW PRESSURE SYSTEMS located on the eastern side of the 500 mb. trough have upper air support. Deep troughs with strong winds give the greatest support to developing storms.

HIGH PRESSURE SYSTEMS located on the western side of the 500 mb. trough have upper air support. A fair weather system (high pressure) will grow larger if located in this part of the trough. A high pressure system on the eastern side of the trough will weaken. A low pressure system on the western side of the trough with weaken. Referring to Figure 20, system number #1 will weaken, while systems numbered #2 & #3 will strengthen.

Baroclinicity basically means temperature differences. Extreme temperature contrasts over a relatively small surface area often indicate "trouble". Strong frontal zones usually exhibit high baroclinicity.

TROUGHS in the upper air can be either long wave troughs (B) or short wave troughs (A).

A LONG WAVE TROUGH (B) is usually very deep. It generally shows little or no movement. When it does begin to move, it usually moves very SLOWLY.

A SHORT WAVE TROUGH (A) is usually quite shallow and moves rather rapidly (20-30 MPH) across the map. These short wave troughs are usually associated with fronts or storms on the surface.

Extrapolation: is generally the most effective way of forecasting the movement of troughs. By calculating the distance traveled between yesterday and today, it is generally possible to determine the troughs probable location tomorrow. (See Figures 23A, 23B & 23c.)

When an area of strong baroclinicity is present (front) an approaching upper level disturbance (short wave trough) is often the "trigger" necessary to initiate storm development. Refer to Figure 21.

Surface storms horizontally transfer their energy when the upper air support for the storm advances ahead of the system. For example, surface storms get their greatest support when they are approximately 5 degrees (300 miles) east of the trough axis and about HALFWAY "UP" the eastern portion of the trough.

As the upper level trough moves eastward across the country, it may many times "over run" the surface storm. As a result, the surface storm becomes VERTICAL (directly underneath) the trough axis. The storm therefore loses it support. It begins to "FILL", meaning the central pressures begin to rise. The storm is "dying". However, the upper level energy is alive and well, still lending surface support 5 degrees EAST OF THE TROUGH.

The primary storm fades from the map while pressures begin to fall rapidly along the mid-Atlantic coast, heralding the birth of a new storm, the SECONDARY STORM.

This is an extremely common occurrence especially during the winter months and should be anticipated by the meteorologist each time a short wave trough over runs a surface system. East coast storms usually intensify rapidly when the 500 MB. trough axis reaches LONGITUDE 80 West.

Front "A" shows a typical pattern for a "Moving" cold front. Storm development on this front is unlikely at this time. Front "B" however, exhibits a "suspicious" area in the states of Colorado and Utah. A developing "wave" or storm seems likely in the indicated area.

Very cold high pressure systems moving eastward off the coast of New England during the winter months often heralds the onset of CYCLOGENESIS (storm development) along the mid-Atlantic coast in the vicinity of Cape Hatteras, North Carolina. Since this is a favored location for storm development, it should be watched carefully.

Numerous conditions favor storm development. However, the ones cited are most common and should be scrutinized carefully.

Storms intensify rapidly if the trough (short wave or long wave) possesses cold air. Generally, a temperature of -25℃ to -30℃ at the southern extremity of the supporting trough is sufficient for rapid intensification on the surface.

Generally speaking, the colder the air at the 500 MB. level, the greater the potential for storm deepening.

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. 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 STRONG SURFACE CYCLOGENESIS. Therefore, troughs with very cold air and strong vorticity possess the greatest potential for explosive surface storm development.

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. 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 700 MB. map.

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.).

In Figure 32, divergence is indicated in areas where the contour lines diverge or SPREAD APART. This is known as the DELTA EFFECT. A storm moving northeastward along the Atlantic coast would deepen rapidly as it approached the Delaware, New Jersey shores. The greater the divergence, the greater the storm deepening.

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 VELOCITY MINIMA.

ISOTACHS are lines connecting places that have equal wind speeds.

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 of Figure 35 in connection to the jet stream. AREA I is most favorable because it possesses CYCLONIC WIND SHEAR and also CYCLONIC CURVATURE, both favorable for storm development. AREA III, of Figure 35, also 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 is 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. 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. 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.

Cold fronts supported by a SHORT WAVE TROUGH most times produce thunderstorms, especially if they pass through during the late afternoon. However, if COLD AIR ADVECTION is occurring into the trough behind the cold front (Figure 37) thunderstorms become almost a certainty. During the summertime, if at least TWO (2) ISOTHERMS (10 degrees C cooling) were located within 60-90 miles apart, this cold air advection would be sufficient to trigger thunderstorms. The stronger the cold air advection at 500 MB the stronger the indication that thunderstorms would be more numerous and more severe.

Anticyclonic wind shear and curvature on the 850 MB. map often prevent thunderstorm formation even though favorable air mass conditions for thunderstorms exist. Figure 38 is an example.