SurfaceSurface temperature and wind
Temperature: °C / °F; wind barbs: kt
1. Why it matters
Temperature near 2 m above the model ground describes the air people experience. The wind overlay describes near-surface flow, not temperature movement by itself.
2. How it helps forecasting
Use it to follow daily heating, cold fronts, freezes, sea breezes, and the transport of warm or cold air. Compare the same valid time with dewpoint, cloud cover, and pressure.
3. How to read the map
Match the shading to the displayed temperature legend. Locate sharp temperature changes, then inspect wind direction and animate several hours. Read barbs using the knot guide below; a temperature unit change does not necessarily change the embedded barbs.
Example
A wind shift followed by colder shading advancing southeast suggests a cold front. A valley can still be colder than its surrounding 14 km model cell.
Limits and common mistakes
This is model air temperature, not pavement temperature, heat index, wind chill, or an observed thermometer reading. A 0 °C / 32 °F contour alone does not determine precipitation type.
Return to the forecast map →SurfaceSurface dewpoint temperature
°C / °F
1. Why it matters
Dewpoint is the temperature at which air would become saturated if cooled at approximately constant pressure and moisture content. Higher values generally mean more water vapor near the ground.
2. How it helps forecasting
Find humid air returning ahead of a storm, dry air behind a front, and strong moisture boundaries. Compare dewpoint with temperature and the vertical sounding before judging fog or storm potential.
3. How to read the map
Read dewpoint shading from the legend. A small temperature–dewpoint difference suggests air near saturation; a large difference suggests relatively dry air. Follow changes in the same location rather than comparing dewpoint with air temperature from a different time.
Example
Air temperature 20 °C and dewpoint 19 °C is near saturation. Air temperature 20 °C and dewpoint 5 °C is much drier.
Limits and common mistakes
Dewpoint alone does not guarantee fog, clouds, rain, or thunderstorms. Surface moisture can be shallow, and humidity can differ substantially higher in the atmosphere.
Return to the forecast map →SurfaceSurface relative humidity
%
1. Why it matters
Relative humidity compares water vapor with saturation at the current temperature. It depends on both temperature and moisture.
2. How it helps forecasting
Use it with temperature and dewpoint for drying trends, nighttime saturation, and broad fire-weather context. It helps explain why RH often rises after sunset even without new moisture arriving.
3. How to read the map
Values close to 100% mean near saturation at 2 m; low values indicate a larger saturation deficit. Read the numeric legend and compare the daytime minimum with the overnight maximum.
Example
If dewpoint holds steady while temperature falls overnight, RH can rise substantially without additional moisture moving into the area.
Limits and common mistakes
100% RH is not a rain probability. Near-surface RH does not describe cloud cover through the whole atmosphere, and low RH alone does not establish fire danger.
Return to the forecast map →SurfaceTotal cloud cover
%
1. Why it matters
Total cloud cover estimates how much of the model sky is covered by clouds somewhere in the atmospheric column.
2. How it helps forecasting
Use it to anticipate sunshine versus overcast periods and to interpret temperature trends or the visibility of astronomical events. Compare with satellite observations for the early forecast hours.
3. How to read the map
Near 0% indicates little modeled cloud cover; near 100% indicates nearly complete cover. Animate to follow clearing or thickening and use the legend rather than assuming that a particular map color always means the same percentage.
Example
Cloud cover falling from 95% to 20% during the afternoon supports increased sunshine if the model timing is right.
Limits and common mistakes
Total cover does not supply cloud-base height, aviation ceiling, optical thickness, or fog visibility. Thin high cloud and thick low cloud can both contribute to a high percentage.
Return to the forecast map →PrecipitationAccumulated precipitation
mm / in of liquid equivalent
1. Why it matters
This is water accumulated over the product’s stated forecast accumulation window. Frozen precipitation is expressed as the equivalent depth of liquid water.
2. How it helps forecasting
Use it to locate broad wet and dry regions and estimate storm-total water. Compare successive forecast times and runs to assess timing and placement uncertainty.
3. How to read the map
Read the valid time, initialization, units, and accumulation window first. Cumulative totals generally increase within one continuous run; a new initialization resets its own accumulation. Difference two totals only when they use the same run, definition, and uninterrupted accumulation.
Example
If a cumulative total is 12 mm at F12 and 20 mm at F18, the intervening amount is 8 mm, provided both frames share the same accumulation origin.
Limits and common mistakes
An accumulated amount is not an instantaneous rainfall rate or a rain probability. The supplied frontend does not establish the generator’s reset convention; verify the index/product metadata before calculating an interval amount. Flooding also depends on duration, terrain, soils, and drainage.
Return to the forecast map →PrecipitationComposite reflectivity
dBZ
1. Why it matters
Composite reflectivity is the strongest simulated radar return in a vertical column. Larger values often indicate larger or more numerous precipitation particles.
2. How it helps forecasting
Use it to examine the model’s broad storm organization, precipitation bands, and approximate timing. Compare early forecast frames with observed MRMS radar.
3. How to read the map
Read the dBZ legend and animate the predicted echoes. Follow the overall pattern and movement rather than treating individual small peaks as exact future storm cells. Composite reflectivity can show strong echoes aloft even when surface precipitation is weaker.
Example
A modeled line of stronger echoes crossing a region suggests a possible organized precipitation band, while its precise arrival time and intensity remain uncertain.
Limits and common mistakes
This is simulated radar, not a live observation. Reflectivity alone cannot determine surface rainfall rate, hail size, wind damage, or tornado occurrence. A 14 km model does not resolve individual tornadoes or fine thunderstorm structure.
Return to the forecast map →SurfaceSea-level pressure
hPa (equivalent numerically to mb)
1. Why it matters
Sea-level pressure adjusts pressure to a common reference elevation so weather systems can be compared across terrain.
2. How it helps forecasting
Find lows, highs, troughs, and the broad pressure gradient associated with wind. Follow the position and depth of a low over time and compare with surface temperature and wind.
3. How to read the map
Read the pressure values and identify relative minima and maxima. Where pressure contours are displayed, closer spacing over the same distance means a stronger pressure gradient. Use wind and temperature changes to help locate fronts.
Example
A low falling from 1000 to 992 hPa while moving northeast suggests intensification, but a location’s wind depends on the surrounding gradient and storm track.
Limits and common mistakes
SLP is neither station pressure at the local elevation nor a METAR altimeter setting. Terrain reduction can create artifacts over high mountains; interpret those areas cautiously.
Return to the forecast map →Severe weatherConvective available potential energy
J/kg
1. Why it matters
CAPE measures the positive buoyant energy available to a specified rising air parcel. It is a measure of potential instability, not a storm forecast by itself.
2. How it helps forecasting
Use it with moisture, a lifting mechanism, CIN, and vertical wind shear to assess whether the environment could support buoyant updrafts.
3. How to read the map
Locate greater CAPE in the shading and follow its evolution near fronts or other lifting zones. Compare neighboring areas and soundings. The parcel definition matters; the supplied map label does not document whether the generator uses surface-based, mixed-layer, or most-unstable CAPE.
Example
CAPE increasing ahead of a front may support stronger updrafts if storms develop. High CAPE under a strong cap can remain unused.
Limits and common mistakes
CAPE does not equal storm probability or severity. Some severe environments have modest CAPE, especially with strong shear. Do not compare numerical thresholds across different parcel definitions without checking the diagnostic.
Return to the forecast map →Severe weatherConvective inhibition
J/kg
1. Why it matters
CIN describes the energy barrier that a rising parcel must overcome before reaching freely buoyant ascent. It is often called a cap.
2. How it helps forecasting
Compare CIN with daytime heating, incoming fronts, convergence, and CAPE to assess whether storm initiation may be suppressed or delayed.
3. How to read the map
Check the displayed sign convention. This viewer’s fallback scale is 0–500 J/kg, consistent with plotting inhibition magnitude: larger magnitude means a stronger barrier. If published metadata uses negative CIN, values farther below zero mean stronger inhibition.
Example
A decrease in inhibition while CAPE stays substantial can make initiation easier when a lifting mechanism arrives.
Limits and common mistakes
Weak CIN does not supply a trigger or moisture. Strong CIN can be overcome by sufficient lifting. Exact values depend on parcel choice and calculation details, which are not defined by the frontend alone.
Return to the forecast map →Surface10 m wind speed and 2 m potential temperature
Wind: kt / m/s / mph; θ₂ contours: use contour labels (usually K)
1. Why it matters
Wind-speed shading shows modeled near-surface flow strength. Potential temperature is temperature adjusted to a reference pressure; it helps distinguish air masses across different elevations.
2. How it helps forecasting
Use the wind field for broad windy corridors and θ₂ gradients for fronts or boundaries. Compare with ordinary temperature and sea-level pressure to understand the cause of a wind change.
3. How to read the map
Read wind speed from the color legend. Read θ₂ from its own contour labels; those contours do not use the wind-speed units. A tight θ₂ gradient marks a strong air-mass contrast, and an associated wind shift can strengthen the frontal interpretation.
Example
A band of stronger winds along a sharp θ₂ gradient may indicate a frontal zone rather than simply the warmest surface air.
Limits and common mistakes
10 m mean wind is not a gust forecast or wind at treetop/roof height. Potential temperature is not the actual thermometer temperature. Local exposure and terrain can strongly modify observed winds.
Return to the forecast map →Severe weatherSignificant tornado parameter: fixed layer
Dimensionless index
1. Why it matters
STP combines ingredients associated with environments that can support significant tornadic supercells. The fixed-layer formulation uses prescribed atmospheric layers rather than storm-dependent effective layers.
2. How it helps forecasting
Use it to identify areas where instability, low-level rotation, shear, and cloud-base-related ingredients overlap. Cross-check soundings, CAPE/CIN, SRH, and the observed storm mode.
3. How to read the map
Read the index shading and compare the location and trend of maxima. A value near or above 1 is often used as environmental context in SPC guidance, but it is not a universal threshold. The plotting package does not include the diagnostic generator, so its exact normalization cannot be certified here.
Example
An STP maximum near a warm front warrants examining the sounding and current storms; it does not establish that a tornado will form there.
Limits and common mistakes
STP is not a percent chance, tornado count, intensity forecast, or warning. Small placement errors in its component fields can shift the maximum. Fixed and effective versions should not be assumed identical.
Return to the forecast map →Severe weatherSignificant tornado parameter: effective layer
Dimensionless index
1. Why it matters
This version uses an effective inflow layer intended to represent air that can feed a storm. It combines multiple environmental ingredients rather than depicting a tornado directly.
2. How it helps forecasting
Use it to evaluate whether the air actually available to storms has a concerning combination of buoyancy, low-level rotation, shear, and other ingredients. Compare with the fixed-layer product when a stable surface layer or elevated storms are possible.
3. How to read the map
Follow the index legend and examine the sounding to understand the effective layer. A difference from fixed-layer STP can be physically meaningful; check the inflow depth and whether storms can access near-surface air.
Example
Fixed STP may appear elevated while effective-layer STP is smaller if the modeled inflow and buoyancy structure do not support the same near-surface ingredients.
Limits and common mistakes
This is not a tornado probability, and values are sensitive to parcel, storm-motion, and layer calculations. The exact generation formula is outside the supplied frontend. Use official warnings and current observations for immediate decisions.
Return to the forecast map →Severe weather0–3 km storm-relative helicity
m²/s²
1. Why it matters
SRH measures how the wind profile and an assumed storm motion combine to supply streamwise rotation to a potential updraft through the lowest 3 km above ground.
2. How it helps forecasting
Use it with CAPE, CIN, shear, cloud-base height, and an appropriate storm motion to assess organized rotating-storm environments.
3. How to read the map
Read the value and sign from the legend. Larger positive values often support more cyclonic rotation for the assumed right-moving storm in the Northern Hemisphere. Examine a sounding/hodograph to understand why the value is large.
Example
A rise in low-level SRH as evening winds strengthen can increase environmental concern if surface-based storms persist and have sufficient instability.
Limits and common mistakes
SRH is not wind speed, vorticity at one level, or a tornado probability. Negative values can be relevant to other storm motions. The storm-motion assumption materially affects the result; 0–3 km SRH is not identical to 0–1 km or effective-layer SRH.
Return to the forecast map →Upper air850 hPa temperature and wind
Temperature: °C / °F; wind barbs: kt
1. Why it matters
This shows air temperature and flow on the 850 hPa pressure surface, typically around 1.5 km / 5,000 ft above sea level. Its actual height varies with the weather pattern.
2. How it helps forecasting
Use it to follow warm or cold air transport and broad thermal boundaries. The 850 hPa layer is useful for low-level air masses and moisture transport.
3. How to read the map
Read temperature from the shading, then check whether the wind carries air from the warmer or colder side of the gradient. Follow the pattern through time and compare with surface temperature and a full sounding.
Example
Wind crossing from warmer toward colder temperature shading supports warm-air advection at that level; wind parallel to the temperature gradient transports less heat across it.
Limits and common mistakes
The 0 °C line at one pressure level does not by itself separate rain from snow. 850 hPa can be below ground over high terrain; missing or extrapolated areas need caution.
Return to the forecast map →Upper air700 hPa temperature and wind
Temperature: °C / °F; wind barbs: kt
1. Why it matters
This shows air temperature and flow on the 700 hPa pressure surface, typically around 3 km / 10,000 ft above sea level. Its actual height varies with the weather pattern.
2. How it helps forecasting
Use it to follow warm or cold air transport and broad thermal boundaries. The 700 hPa layer helps diagnose middle-level warmth, caps, and the thermal environment above the boundary layer.
3. How to read the map
Read temperature from the shading, then check whether the wind carries air from the warmer or colder side of the gradient. Follow the pattern through time and compare with surface temperature and a full sounding.
Example
Wind crossing from warmer toward colder temperature shading supports warm-air advection at that level; wind parallel to the temperature gradient transports less heat across it.
Limits and common mistakes
The 0 °C line at one pressure level does not by itself separate rain from snow. 700 hPa can also intersect high terrain. A warm layer here may cap storms, but the whole parcel path matters.
Return to the forecast map →Upper air700 hPa wind speed and geopotential height
Wind: kt / m/s / mph; height contours: read labels (m or dam)
1. Why it matters
Wind shading identifies faster flow on the 700 hPa surface. Height contours show how high that constant-pressure surface lies above sea level.
2. How it helps forecasting
Use it to locate lower to middle tropospheric wind corridors and short-wave disturbances. Compare the system across multiple pressure levels and times.
3. How to read the map
Read the color legend for wind speed and the separate contour labels for height. Height contours bending southward generally mark a trough; northward bulges generally mark ridges in the Northern Hemisphere. Faster flow often follows tighter contour spacing. One decameter (dam) equals 10 m.
Example
A lower-level wind maximum feeding moist air into a storm system can help sustain precipitation when lifting is also present.
Limits and common mistakes
Upper-air wind is not a surface gust prediction. Heights describe a pressure surface, not terrain elevation or sea-level pressure. Storm movement can differ from the wind at one level.
Return to the forecast map →Upper air700 hPa relative vorticity and wind
Relative vorticity: 10⁻⁵ s⁻¹; wind barbs: kt
1. Why it matters
Relative vorticity describes local spin in the horizontal wind field, excluding the background rotation of Earth. It can arise from curvature and horizontal wind shear.
2. How it helps forecasting
Use 700 hPa vorticity to follow middle-level disturbances that can influence bands of cloud and precipitation.
3. How to read the map
In the Northern Hemisphere, positive relative vorticity generally indicates cyclonic spin. Read the scaled legend: 20 in units of 10⁻⁵ s⁻¹ means 0.00020 s⁻¹. Compare the wind field and watch whether a maximum approaches your location. The change of vorticity with height and its transport are important for diagnosing larger-scale ascent.
Example
A vorticity maximum moving toward a moist region may help identify a passing disturbance. Confirm cloud and precipitation development with the other products.
Limits and common mistakes
A maximum is not automatically a surface low, upward motion, or a tornado. Positive vorticity at one level alone does not establish rising air. Small peaks can be sensitive to grid spacing and model noise.
Return to the forecast map →Upper air700 hPa water vapor mixing ratio and wind
Water vapor: g/kg; wind barbs: kt
1. Why it matters
Water vapor mixing ratio is the mass of water vapor per mass of dry air. It describes moisture amount at a particular pressure level.
2. How it helps forecasting
Use 700 hPa moisture to track the depth and movement of middle-level moist and dry layers.
3. How to read the map
Read g/kg from the shading and use the barbs to see where moist or dry air is being transported. Compare the same pressure level through time and combine it with the adjacent levels and sounding to judge moisture depth.
Example
A moist 850 hPa layer beneath dry 700 and 500 hPa air describes a layered atmosphere, not a uniformly humid column. Temperature determines how close each layer is to saturation.
Limits and common mistakes
Mixing ratio is neither relative humidity nor total precipitable water. A low g/kg value in very cold air can still coexist with high RH. Moisture at this level alone does not guarantee a cloud layer or surface rain.
Return to the forecast map →Upper air850 hPa relative vorticity and wind
Relative vorticity: 10⁻⁵ s⁻¹; wind barbs: kt
1. Why it matters
Relative vorticity describes local spin in the horizontal wind field, excluding the background rotation of Earth. It can arise from curvature and horizontal wind shear.
2. How it helps forecasting
Use 850 hPa vorticity to follow low-level disturbances and cyclonic flow near developing weather systems.
3. How to read the map
In the Northern Hemisphere, positive relative vorticity generally indicates cyclonic spin. Read the scaled legend: 20 in units of 10⁻⁵ s⁻¹ means 0.00020 s⁻¹. Compare the wind field and watch whether a maximum approaches your location. The change of vorticity with height and its transport are important for diagnosing larger-scale ascent.
Example
A vorticity maximum moving toward a moist region may help identify a passing disturbance. Confirm cloud and precipitation development with the other products.
Limits and common mistakes
A maximum is not automatically a surface low, upward motion, or a tornado. Positive vorticity at one level alone does not establish rising air. 850 hPa can be below ground over mountains.
Return to the forecast map →Upper air850 hPa water vapor mixing ratio and wind
Water vapor: g/kg; wind barbs: kt
1. Why it matters
Water vapor mixing ratio is the mass of water vapor per mass of dry air. It describes moisture amount at a particular pressure level.
2. How it helps forecasting
Use 850 hPa moisture to track low-level moisture transport and returning humid air ahead of weather systems.
3. How to read the map
Read g/kg from the shading and use the barbs to see where moist or dry air is being transported. Compare the same pressure level through time and combine it with the adjacent levels and sounding to judge moisture depth.
Example
A moist 850 hPa layer beneath dry 700 and 500 hPa air describes a layered atmosphere, not a uniformly humid column. Temperature determines how close each layer is to saturation.
Limits and common mistakes
Mixing ratio is neither relative humidity nor total precipitable water. A low g/kg value in very cold air can still coexist with high RH. Below-ground 850 hPa data over mountains may be unavailable or unrepresentative.
Return to the forecast map →Upper air500 hPa relative vorticity and wind
Relative vorticity: 10⁻⁵ s⁻¹; wind barbs: kt
1. Why it matters
Relative vorticity describes local spin in the horizontal wind field, excluding the background rotation of Earth. It can arise from curvature and horizontal wind shear.
2. How it helps forecasting
Use 500 hPa vorticity to follow short-wave disturbances embedded in the larger trough-and-ridge pattern.
3. How to read the map
In the Northern Hemisphere, positive relative vorticity generally indicates cyclonic spin. Read the scaled legend: 20 in units of 10⁻⁵ s⁻¹ means 0.00020 s⁻¹. Compare the wind field and watch whether a maximum approaches your location. The change of vorticity with height and its transport are important for diagnosing larger-scale ascent.
Example
A vorticity maximum moving toward a moist region may help identify a passing disturbance. Confirm cloud and precipitation development with the other products.
Limits and common mistakes
A maximum is not automatically a surface low, upward motion, or a tornado. Positive vorticity at one level alone does not establish rising air. Small peaks can be sensitive to grid spacing and model noise.
Return to the forecast map →Upper air500 hPa wind speed and geopotential height
Wind: kt / m/s / mph; height contours: read labels (m or dam)
1. Why it matters
Wind shading identifies faster flow on the 500 hPa surface. Height contours show how high that constant-pressure surface lies above sea level.
2. How it helps forecasting
Use it to locate the main middle-tropospheric steering pattern, troughs, and ridges. Compare the system across multiple pressure levels and times.
3. How to read the map
Read the color legend for wind speed and the separate contour labels for height. Height contours bending southward generally mark a trough; northward bulges generally mark ridges in the Northern Hemisphere. Faster flow often follows tighter contour spacing. One decameter (dam) equals 10 m.
Example
A trough approaching from the west can help organize a surface system, but the surface response depends on its structure and evolution.
Limits and common mistakes
Upper-air wind is not a surface gust prediction. Heights describe a pressure surface, not terrain elevation or sea-level pressure. Storm movement can differ from the wind at one level.
Return to the forecast map →Upper air500 hPa height with sea-level pressure contours
Height shading: m; pressure contours: hPa
1. Why it matters
This overlays the middle-atmosphere height pattern and near-surface pressure systems. They describe different physical quantities on the same map.
2. How it helps forecasting
Use it to see how surface lows and highs are positioned relative to a larger trough or ridge and to follow their development together.
3. How to read the map
Read shaded 500 hPa height from its legend. Read SLP from contour labels, not from the height legend. Track whether a surface low lies ahead of a trough and whether the upper and lower circulations become more vertically aligned.
Example
A surface low east of a 500 hPa trough may be in a developing storm configuration. A vertically stacked mature low may evolve differently as it moves.
Limits and common mistakes
Low height is not automatically low surface pressure. The overlay does not display vertical velocity or prove that a storm will intensify. Contour spacing and units must be read separately.
Return to the forecast map →Upper airOutgoing longwave radiation and 500 hPa wind
OLR: W/m²; wind barbs: kt
1. Why it matters
OLR is thermal energy emitted upward from the Earth–atmosphere system. Cold, high cloud tops often emit less outgoing longwave energy than warm clear scenes.
2. How it helps forecasting
Use broad lower-OLR areas as context for high cloud shields and convection, and compare their evolution with simulated reflectivity, cloud cover, and satellite imagery.
3. How to read the map
Read the radiation legend numerically: lower values often accompany colder, higher emitting surfaces. Use 500 hPa wind as middle-level flow context, while remembering that upper cloud can move differently.
Example
A growing low-OLR area overlapping modeled strong reflectivity can suggest a developing deep cloud shield. Low OLR without surface echoes may instead represent high cloud spreading away from storms.
Limits and common mistakes
OLR is not a direct cloud-top temperature measurement or a rainfall-rate product. Surface temperature, atmospheric moisture, and cloud properties all matter. It is model output, distinct from observed infrared satellite brightness temperature.
Return to the forecast map →Upper air500 hPa water vapor mixing ratio and wind
Water vapor: g/kg; wind barbs: kt
1. Why it matters
Water vapor mixing ratio is the mass of water vapor per mass of dry air. It describes moisture amount at a particular pressure level.
2. How it helps forecasting
Use 500 hPa moisture to track middle-tropospheric dry intrusions or moist plumes above the lower atmosphere.
3. How to read the map
Read g/kg from the shading and use the barbs to see where moist or dry air is being transported. Compare the same pressure level through time and combine it with the adjacent levels and sounding to judge moisture depth.
Example
A moist 850 hPa layer beneath dry 700 and 500 hPa air describes a layered atmosphere, not a uniformly humid column. Temperature determines how close each layer is to saturation.
Limits and common mistakes
Mixing ratio is neither relative humidity nor total precipitable water. A low g/kg value in very cold air can still coexist with high RH. Moisture at this level alone does not guarantee a cloud layer or surface rain.
Return to the forecast map →Upper air300 hPa wind speed and geopotential height
Wind: kt / m/s / mph; height contours: read labels (m or dam)
1. Why it matters
Wind shading identifies faster flow on the 300 hPa surface. Height contours show how high that constant-pressure surface lies above sea level.
2. How it helps forecasting
Use it to locate upper-tropospheric jet streams and jet streaks. Compare the system across multiple pressure levels and times.
3. How to read the map
Read the color legend for wind speed and the separate contour labels for height. Height contours bending southward generally mark a trough; northward bulges generally mark ridges in the Northern Hemisphere. Faster flow often follows tighter contour spacing. One decameter (dam) equals 10 m.
Example
A localized maximum within the jet is a jet streak. Use its position as context, then assess divergence and the full flow before inferring upward motion.
Limits and common mistakes
Upper-air wind is not a surface gust prediction. Jet-streak entrance/exit rules depend on hemisphere and flow curvature; a colored wind maximum alone does not establish surface severe weather.
Return to the forecast map →PrecipitationSnowfall using a 10:1 ratio
in / mm of estimated snow depth
1. Why it matters
This estimates snowfall depth with a fixed relationship of 10 units of snow depth for one unit of snow water equivalent.
2. How it helps forecasting
Use it for the broad area and approximate scale of modeled snow potential. Compare with precipitation type, total liquid precipitation, temperature profiles, and newer forecast runs.
3. How to read the map
Read the snow-depth legend and accumulation window. Under a 10:1 assumption, 0.50 inch of liquid equivalent assigned to snow gives 5 inches of snow before compaction, melting, or other adjustments.
Example
An area showing 6 inches on this map represents the fixed-ratio diagnostic, not a guarantee that 6 inches will accumulate on a road or lawn.
Limits and common mistakes
Actual snow-to-liquid ratios vary with temperature and crystal growth. Settling, melting, warm ground, wind, and mixed precipitation can change measured accumulation substantially. Do not treat this as a snow-depth observation.
Return to the forecast map →PrecipitationFreezing-rain accumulation
mm / in; verify the published accumulation definition
1. Why it matters
This identifies accumulated precipitation diagnosed as freezing rain by the model processing. Freezing rain falls as liquid and can freeze on exposed surfaces.
2. How it helps forecasting
Use it with precipitation type, temperature, the full sounding, and timing to locate a possible icing corridor or transition zone.
3. How to read the map
Read the numeric amount, units, and accumulation window. Look for an above-freezing layer aloft over a shallow near-surface cold layer. Examine neighboring times because a brief change in that profile can change the precipitation type.
Example
If the freezing-rain band shifts between runs, local icing placement remains uncertain even if the broad winter-storm signal persists.
Limits and common mistakes
The supplied frontend does not document a physical ice-accretion model. Do not equate the displayed amount with radial glaze thickness on branches or power lines. Surface temperature, droplet collection, runoff, and wind affect actual ice buildup.
Return to the forecast map →PrecipitationPrecipitation type
Categories: rain, freezing rain, sleet, snow
1. Why it matters
This classifies the modeled precipitation reaching the surface. It summarizes the effects of the atmospheric temperature profile on falling precipitation.
2. How it helps forecasting
Use it to follow rain–snow boundaries, mixed-weather transitions, and the timing of possible freezing precipitation. Compare with the sounding, accumulated precipitation, and observed surface temperatures.
3. How to read the map
Match each map color to its category legend; the colors are labels, not a continuous intensity scale. Follow several times before and after an event. Rain occurs as liquid, sleet consists of ice pellets, snow consists of ice crystals, and freezing rain freezes on suitable surfaces.
Example
A snow-to-sleet-to-freezing-rain transition can reflect a strengthening warm layer aloft with changing cold-layer depth near the ground.
Limits and common mistakes
One grid cell may contain a real mixture or rapid changes that a single category cannot depict. A type map does not provide accumulation amount, road condition, or certainty. Temperature at the surface or 850 hPa alone cannot replace the full vertical profile.
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