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INTERPRETATION OF THE SIGNIFICANT LEVELS ON THE GRAPHICAL FORECAST MODELS

METEOROLOGIST JEFF HABY

1. THE SURFACE PRESSURE/PRECIPITATION/THICKNESS PROG

This graphical prog has a wealth of meteorological information. The surface pressure is plotted for zero geopotential meters. This standardization allows pressures from high elevation regions to be compared with low elevation regions. Minimum pressure regions at zero geopotential meters are denoted as low pressures and maximum regions as high pressure. A low pressure that is less than 1000 millibars is very significant. High pressures tend to cover a large spatial area. The precipitation is colorized on the panel in a variety of increments. This precipitation product represents general synoptic scale precipitation and general areal coverage by the different precipitation categories. It is not a good indicator of the exact amount of precipitation any one point will receive. Precipitation amounts are highly influenced by mesoscale and microscale processes. High precipitation maximums will be found in those regions with high upward vertical velocity coupled with high relative humidity. The 1000 to 500 millibar thickness is shown in dashed lines with the exception of the 5100 thickness, 5400 thickness, and 5700 thickness shown as a solid line. The 5,100 thickness line generally separates arctic air from polar air. The 5,400 line is used as a general cutoff between rain and snow. The 5,700 geopotential meter thickness generally divides mid-latitude air from tropical air. The thickness lines are in 60 geopotential meter increments. This prog is available on UNISYS weather at:

http://weather.unisys.com/nam/pres.php

The combination of the zero geopotential meter surface pressure and thickness lines will show regions experiencing WAA and CAA. Lower thicknesses being advected toward a fixed location indicates CAA. Cold air has a lower thickness while warm air has a higher thickness (warm air expands and is less dense). Advection of higher thicknesses is indicative of low level WAA. Low level WAA leads to rising air on the synoptic scale. It is important to notice if the wind is flowing from a moisture source or a dry source. The wind direction near the surface crosses the isobars at about a 30-degree angle toward low pressure. WAA from a moisture source will cause more instability than WAA from a dry source because the combination of moisture and the dynamical rising caused by WAA can cause saturation of the air.

2. THE 1000 mb FORECAST MODEL PROG

The 1000-mb prog is important for assessing warm/cold air advection, convergence, moisture and the low level wind among others. This prog shows four meteorological variables, which are temperature, dewpoint, wind and convergence. The 1000 mb forecast prog is available at:

http://weather.unisys.com/nam/1000.php

The temperature in Celsius is given using colors. The colors that are assigned to each temperature inclement are given below the panel. The aqua color represents temperatures from freezing to 4 degrees Celsius above freezing. Temperatures are given in 4-degree increments. Dewpoint is a little more difficult to interpret. Dewpoint is coded using different color lines and thicknesses of lines. You will see a thick solid white line and a thick red line and several other thinner lines of varying colors. The white thick line is the 15-degree Celsius isodrosotherm. The thick red line is the 0-degree isodrosotherm. The thick pink line is the -15 degree Celsius isodrosotherm. There are "thinner" colored lines between these thicker lines. Each line (thick or thin) represents a 5 degree Celsius dewpoint change. In general, temperatures and dewpoints will decrease when moving south to north across the forecast panel. Convergence on the prog is shown by a dark murky speckled color (this hides the temperature colors below it). Convergence is the coming together of air streams. Air that piles together in the low levels of the atmosphere (which convergence does on the 1000 mb chart) results in UVV. Often convergence will be noticed in association with fronts, low-pressure systems and topographic convergence. The wind vectors on the panel can be used to assess (1) direction of windflow, (2) relative magnitude of thermal advection, (3) relative magnitude of moisture advection, (4) locating convergence and frontal boundaries, and (5) relative strength of low level wind flow.

3. THE 850 MB MODEL PROG

The 850 model prog is a low level layer prog which can be used to forecast thermal advection, upslope/downslope wind, cyclogenesis and dynamic lifting or sinking due to WAA and CAA. This model prog can be found at:

http://weather.unisys.com/nam/850.php

The prog displays isotherms (in colored 2 degree increments), wind vectors (longer wind vectors indicate stronger wind) and the lines we see on each upper level prog: height contours. The number 1 item a forecaster looks for on the prog is WAA and CAA. Strong WAA can create instability especially when associated with moisture advection. Strong CAA, especially when associated with dry air, causes a dynamic sinking of air and the promotion of stability. Thermal advection is analyzed by using the wind vectors and the temperature gradient at 850 millibars. If strong winds (long wind vectors) and blowing through a "tight" temperature gradient (isotherms close together), significant thermal advection will occur. If warmer temperatures are moving toward a fixed point then it is WAA, if colder temperatures are moving toward a fixed point then it is CAA. In the cool season, an important 850 temperature is the zero degree isotherm. If the temperature at 850 is above freezing (especially if more than 2 C above freezing), precipitation is likely to NOT fall as snow according to that forecast model. Freezing temperatures at 850 mb will support snow or some other form of frozen precipitation. Temperatures can be above freezing at 850 mb and have sleet or freezing rain at the surface. Check the surface and 1000 mb temperatures for this potential. Downslope and Upslope winds are common near mountainous locations (i.e. Rocky Mountains, Laramie Mountains, Bighorn Mountains, Appalachian Mountains, etc.) and across the sloping elevation of the High and Great Plains. Downsloping winds will lead to warmer 850 mb temperatures than surrounding regions. Strong downslope winds in the winter are referred to as the Chinook in the High Plains region of the US. Upslope winds can cause snowfall and/or cooling temperatures in the cool season. Cyclogenesis can be located on the prog by noticing a gradual curving of the isotherm colors and the beginnings of a counterclockwise rotating 850 wind vector field. Low level cyclogenesis occurs often in a region with closely spaced isotherms. It can occur along a frontal boundary and along or near a coastline (often a strong thermal gradient will exist near a coastline, especially if the coastline separates warm and moist air from a synoptic scale polar like air mass). The prog can be used to help forecast temperatures. Strong WAA will lead to a warming trend in temperatures and strong CAA will lead to a cooling trend.

4. VERTICAL VELOCITY ON FORECAST MODEL PROGS (700 mb PROG)

Wind flows in the horizontal at a much higher average wind speed than in the vertical. Vertical motion is roughly two orders of magnitude smaller than horizontal motion. Wind speeds in the horizontal are commonly over 50 knots at some point in the atmosphere above a location. The average vertical wind speed is only a few centimeters per second!!! This seems outrageous considering thunderstorm updrafts can have vertical velocities over 100 miles per hour. The deal is, thunderstorms only encompass a tiny surface area of the earth compared to regions not having thunderstorms. Well less than 1% of the time are vertical velocities greater than 1 mile per hour above any point location. All the uplift from low pressure and fronts only produce vertical upglide of a few to sometimes greater than 20 centimeters per second on the synoptic scale. That's it. Why then does it rain? Well, an uplift of 6 centimeters per second leads to a pretty significant distance given enough time. In fact, moving 6 centimeters per second in one hour produces 216 meters of vertical distance. Give it a few hours, and that parcel of air can rise in the vertical over a kilometer. An upward vertical velocity of just 6 centimeters per second can produce a large volume of precipitation if the moisture is present to be condensed. Let us apply upward vertical velocity to interpreting the synoptic scale forecast models (ETA, NGM, AVN, ect.). Upward vertical velocity is plotted on the 700-mb prog. Here is one place to get it:

http://weather.unisys.com/nam/700.php

You will see a panel full of colors, wind vectors, and height contours. The colors are the upward and downward vertical velocities. Notice the color scale below the panel. The scale will range from the lowest to highest forecasted synoptic scale vertical velocity on the panel. On the legend at the top you will notice a -ub/s. This stands for negative microbars per second. The negative sign is used because pressure decreases with height in our atmosphere (usually when graphing, up is positive, but in this case, up leads to LOWER pressure). ub/s is made negative so upward vertical velocity can be given a positive sign. What is a ub/s anyway? A bar of pressure is equal to 1000 millibars. A ub (called a microbar), is a millionth of a bar and a thousandth of a millibar. You probably know that a thousandth of a bar is a millibar. This is a fairly small pressure change over time but can lead to large changes in pressure given enough time. The model is using pressure as vertical distance instead of height. Conveniently as the math works out, a ub/s is just about the same vertical velocity as a centimeter per second. A vertical velocity of 6 -ub/s is significant while a vertical velocity of 10 or greater is very significant (also need moisture!). As you look at the 700 mb forecast panel you will notice bullseyes of upward vertical velocity (denoted UVV for short). These are regions where mechanisms such as low level WAA, low level convergence, PVA, jet streak divergence, orographic uplift, etc. are causing the air to rise in the vertical. Sinking mechanisms such as CAA, downsloping and NVA causes downward VV's. The vertical velocity value for any one point is the compilation of ALL upward and downward vertical velocities added and subtracted at that point. UVV will be maximized in regions lacking downward VV mechanisms while having uplift mechanisms in place. The average of all upward and downward motions is zero averaged across the entire earth. If upward motion constantly were larger than downward motion, then the atmosphere would lose its mass. There is a conservation of mass for the atmosphere; What air goes up, eventually has to come back down. You will notice that by areal coverage, the regions of near zero and negative vertical velocity (downward motion) encompass a larger area than the regions experiencing UVV. Also, the UVV maximums tend to be higher in magnitude than the DVV maximums. This is partly because high pressure encompasses a larger region than low-pressure regions. Having a larger area of downward motion is offset by a smaller but more intense upward motion; In the end, the mass of the atmosphere is conserved.

5. THE 500 MB MODEL PROG

The 500-millibar prog represents the level where about one half of the earth's atmospheric mass is below it and half the earth's atmospheric mass is above it. The number 1 item a forecaster looks for on this prog is vorticity. Vorticity is a spinning motion (or eddy) created by directional and/or speed changes in the wind field. This model prog is available on UNISYS weather at:

http://weather.unisys.com/nam/500.php

This prog shows colorized 500-millibar absolute vorticity, height contours, and wind vectors. Absolute vorticity is a combination of curvature, shear and coriolis vorticity. Coriolis vorticity is always positive because the earth rotates counterclockwise when viewed from the North Pole. Curvature or Shear vorticity can be positive OR negative. Since the model shows absolute vorticity, the values of vorticity will be skewed toward positive values (because Coriolis vorticity is always positive). The model determines the value of vorticity for numerous locations across the model panel. These vorticity values are then colored using the scale below the panel. The normal range of vorticity values are from near zero (or single digit negative) to sometimes more than +30units. A unit of vorticity is equal to 1 times 10 to the negative 5th with units of seconds to the negative 1. The units of vorticity are derived from the change is wind speed over a horizontal distance. Since wind speed has units of m/s and distance has units of m, the units cancel to seconds to the negative 1. The highest vorticity regions on the panel (caused by the greatest combination of positive shear and positive curvature) are termed vorticity maximums. Positive curvature is a counterclockwise windflow (such as in a trough) and positive shear is a horizontal speed change of wind with distance that causes a counterclockwise rotation. An eddy of vorticity spins like a low pressure in that it spins counterclockwise. This counterclockwise spin-up causes the air the stretch and rise just downwind from the vorticity maximum. A rough guide to the intensity of a vort max is: less than 14: small vorticity; 14 to 20: moderate; 22+ large. The value of the vorticity maximum does not tell the whole story. For strong upward motion to result with a vort max, there must also be a strong wind flow (long wind vectors) flowing through the vort max. Strong wind flowing through the vort max will create a region of NVA and PVA, where NVA stands for negative vorticity advection (vort max moving away from a fixed location) and PVA stands for positive vorticity advection (vort max approaching a fixed point). It is the region where a vort max approaches a fixed point that has upper level divergence and rising air (also called the downwind region of the vort max). The 500-millibar chart is also important for forecasting the trough/ridge pattern propagation across the forecast area as well as the forecasting of "shortwaves". A shortwave is a lower height kink in the height contours that is about the size of a U.S. state or a little bigger. The kinking creates vorticity due to the counterclockwise curvature the kinking produces. The trough/ridge pattern will determine which regions are having above or below normal temperatures and if the flow is conducive to storms systems or quiet (zonal) weather.

6. THE 300 MILLIBAR PROG

The 300-millibar prog is located at or near the core of the jet stream. Winds within the jet stream pattern are commonly over 100 knots. A 100-knot wind does not have the same force as a 100-knot wind at the surface because the density of the air is about 70% smaller at 300 millibars than at the surface. The two jet streams forecasters are interested in at this level are the polar jet and the subtropical jet. The winds associated with the polar jet tend to be stronger than the subtropical jet because there is a larger temperature gradient between polar and midlatitudinal air than between tropical and midlatitudinal air. The forecaster also looks for jet streaks. These are high velocity segments of wind within the jet stream. The 300-millibar prog is available on the UNISYS weather website at:

http://weather.unisys.com/nam/300.php

The colors on the chart represent wind velocity in knots using 10-knot wind speed intervals. A significant jet streak has winds over 100 knots. A jet streak itself may only move 20 to 30 knots. It is the winds WITHIN the jet streak that are moving at the speeds portrayed on the prog. Jet streaks help carve the trough and ridge pattern. If a jet streak is on the left side of a trough, that trough will tend to become more amplified with time and "dig". If a jet streak is on the right side of a trough, that trough will tend to become less amplified with time and "lift". Jet steaks tend to develop at those locations in the atmosphere that have the greatest horizontal temperature gradient. A cold dome of air next to a warm dome of air (synoptic scale) will cause the pressure gradient to flow from the warmer toward the colder air. The Coriolis force then turns the airflow to the right of the path of motion. If cold air is to the north and warm air to the south, the airflow will be south to north then turned to the east by the Coriolis force. Therefore, the jet stream tends to flow from west to east. Jet streaks, as you will learn or have learned in synoptic meteorology, have regions of divergence and convergence. Upper level convergence leads to sinking air while upper level divergence leads to rising air. Rising air is favorable in the right rear and left front quadrants of a jet streak.

7. THE RELATIVE HUMIDITY PROG

The relative humidity prog helps a forecaster determine how close a deep layer of the atmosphere is to saturation. Regions of the atmosphere that are close to saturation have low vapor deficits. The vapor deficit is the number of millibars a parcel of air must rise in order to achieve saturation. The relative humidity given on graphical model progs is the average 850 mb to 500 mb relative humidity. If air is rising through a deep layer of the atmosphere (low levels and mid levels), the relative humidity will increase. If the vapor deficit is low, a trigger mechanism such as a front or upper level divergence will be able to condense clouds and produce precipitation. The relative humidity prog is available on UNISYS weather at:

http://weather.unisys.com/nam/rh.php

The highest relative humidities will tend to be located near strong low-pressure regions and fronts. The lowest relative humidities will occur in regions experiencing downsloping wind, behind strong/deep cold fronts, upper level convergence due to high pressure, and over dry interior regions of the U.S. which currently have no UVV mechanisms. At times you may notice the forecast model NOT producing precipitation over a region where the UVV's are moderately high. Most likely, the cause of the model not producing precipitation is high vapor deficits (the lifting in place is not enough to condense water vapor into precipitation). Dry air, near the surface or aloft, can also evaporate precipitation that does occur (producing virga). The relative humidity value says nothing about the amount of moisture in the air or how much precipitation will occur. It can be used for cloud forecasting. UVV combined with low vapor deficits has a high potential to produce clouds (especially those regions in the atmosphere with the lowest vapor deficits). Relative humidity is contoured on the prog in increments of 10%'s. The color code of interpretation is given under the model panel. 80 to 100% RH indicates nearly saturated air (low vapor deficit), less than 60% indicates air that, when averaged from 850 mb to 500 mb, is fairly far from saturation (high vapor deficit). However, an average RH may misrepresent layers of dry or moist air. An average can be misleading. Example: the atmosphere may be near saturation from the surface to 700 millibars and dry above 700 millibars. The average will yield a low RH that is not indicative of the nearly saturated air in the low levels. Generally, the RH will be high only when lifting is occurring through a deep layer of the atmosphere and vapor deficits are fairly low.

8. THE LIFTED INDEX PROG

The Lifted Index is a thermodynamic parameter that compares the theoretical 500 mb parcel temperature (temperature of a parcel of air lifted from the lower PBL to the 500 mb level) to the actual (environmental) temperature at 500 millibars. Lifted Index is plotted with the relative humidity on model progs and is available at:

http://weather.unisys.com/nam/rh.php

A lifted Index value that is positive represents bulk stability of the atmosphere. A negative LI indicates bulk instability. A LI of 0 is neutral. LI should only be used when forecasting in the warm season or in the warm sector of a mid-latitude cyclone. Parcels of air will NOT rise from the lower PBL behind a frontal boundary or within dense PBL polar air. The LI is best used when the atmosphere has the potential to produce thunderstorms (excluding thundersnow/sleet). The more negative the LI, the more potential acceleration an air parcel has if lifted to the Level of Free Convection (LFC). LI values from -1 to -3 are unstable, -4 to -6 are very unstable, -7 or less are extremely unstable. CAPE and LI are related in that as a general rule: when the LI decreases, CAPE increases. The LI value says nothing about if storms will occur. It gives a forecaster a general idea of convective forcing if thunderstorms do develop. Why is LI plotted with RH? Because unstable LI values combined with a high RH indicates the atmosphere is near saturation and has bulk instability. A "trigger mechanism" such as a front will be able to produce thunderstorms and heavy rain in this high RH low LI environment. Again, LI is not of much use in the winter because the PBL tends to be dry (low dewpoints) and cold (stable). "Elevated convection", "dynamic forcing without thermodynamic forcing" and "isentropic lifting" do not mesh well with using the LI. The LI can be very stable but the atmosphere produces precipitation because of the three terms in parenthesis above.