What this page is for. It is for the synoptic scale: reading the pattern across the whole ensemble, where the members agree, where they split, and which scenarios lie behind the split. That is why its grids are 0.25° near land and 1° elsewhere, and why every map is a statistic of 50 members rather than one model run. It is not for the smallest detail, such as the deepest snow at one point, one run's heaviest shower or a valley's lowest temperature: members cannot place that detail, and fields that only make sense at that scale (snow depth among them) are left out on purpose. Use a high-resolution deterministic model for those.
The explorer shows the 50-member ECMWF ensemble (ENS) over the northern hemisphere, for the two newest runs. It is built for forecasters: every map is worked out on request from every member, so you choose the field, the period, the statistic and the threshold.
- The data
- The map and the grids
- Layers
- Worked-out fields
- Displays (keys 1–5)
- Probabilities and neighbourhoods
- Periods
- Boxes, thirds and x–y
- Plumes
- Fronts
- Cyclones
- Below the ground
- Setups and colour scales
- Keys
- Saving pictures and animations
- Sharing a view
- Hovmöllers
The data
Each 00Z and 12Z run is downloaded from ECMWF's open data (CC-BY-4.0) and held in one Icechunk store. The three newest runs are kept. Fields are 6-hourly. The Hovmöller tab has its own, much smaller archive that goes back 60 days (below).
| Grid | Area | Steps | Fields |
|---|---|---|---|
| 1° | 0–90°N, every longitude | T+0 to T+240 | Every field |
| 0.25° | 12–70°N, 130°W–60°E, over land and within 250 km of it (MSLP everywhere in the area) | T+0 to T+120 | MSLP, 2 m temperature and dew point, gust, 100 m wind, precipitation, MUCAPE, 1000–850 hPa thickness, 850 hPa temperature, humidity and wind (so θw and 850 hPa wind speed) |
The ensemble mean and spread of every field are stored as each run comes in, so those maps are instant; probabilities, percentiles, periods and thirds are worked out from all the members when you ask.
The map and the grids
The map is equirectangular (Lat–lon), true to scale at 45°N, and repeats round the dateline, or Polar stereographic, chosen under Map. The polar map looks down on the pole, has no break at the dateline and keeps shapes and angles locally, so the flow round the hemisphere (jets, troughs, the paths of cyclones) reads as one. Its upright longitude is the meridian that runs straight up the screen to the pole: set it to the region you care about (30°W for the Atlantic, 0° or 20°E for Europe), so that features upstream sit to its left and the flow over it runs left to right. Changing projection or upright longitude keeps the middle of the map. Wind barbs are turned to each projection's north. The side panel's thirds map and the member stamps stay lat–lon.
The colour scales and keys of everything on the map sit in its lower right corner; fold them away with the arrow on their heading (your browser remembers it). The full legend, with notes, is at the foot of the left panel. Drag the map to move it; click it for plumes at that point.
Drag the left edge of the side panel to make it wider or narrower (your browser remembers it); double-click the edge to put it back.
Every field is drawn on one 0.25° grid. Where only 1° values exist they are interpolated (bilinear). Inside the 0.25° area the 0.25° values are blended in: fully where they are kept, fading to the 1° values over the outermost 1.5° (6 points) of what is kept, so there is no seam where one grid meets the other. The ensemble statistic is worked out on each grid's own points and then blended. The dashed outline shows the 0.25° area. The badge by the time bar says which grids the map is using: after T+120 everything is 1°.
Relief, coastlines, borders, rivers and place names come from the ERA5 climate explorer's basemap (ETOPO 2022 and Natural Earth). Turn them on and off under Map.
Layers
A map is a stack of layers; the top of the list draws on top. Click a layer for its settings. ⛓ makes a layer follow the field and period of the layer above (barbs follow its level), so changing the top layer changes both.
| Type | Shows |
|---|---|
| Shaded | Colour fill of any display below |
| Shaded + isolines | The same, with isolines of the shaded map on top (every 10% for a probability) |
| Isolines | Contours of the mean, a percentile, or one member |
| Wind | Ensemble-mean wind in knots at 10 m, 850, 700, 500, 250 or 10 hPa, or the 850–200 hPa steering flow, drawn as barbs, arrows or streamlines; the whole wind or only its divergent part (see below) |
| Numbers | Values printed at evenly spaced points |
| Fronts | Automated 850 hPa fronts: one member's, every member's, or how often members have one |
| Cyclones | Every member's cyclone centres, coloured by stage |
Isolines take a colour, a width and a style (solid, dashed or dotted), and an interval if the field's own does not suit. Every wind (gusts, speeds, components and barbs) is in knots.
Arrows point the way the wind blows, as long and as thick as it is strong: the legend gives the speed of a full-length arrow (about the strongest 5% over the map, rounded). Hide arrows below leaves out the weak winds so the strong ones stand out (5 kt by default for the divergent wind, where it picks out the outflow over rising air). Streamlines follow the wind's direction only, with an arrowhead halfway along each; read the speed from shading beneath them. Both are drawn from the ensemble mean.
Label extremes (isolines and shading) marks highs and lows (H and L, in red and blue, with the value) or only the maxima or minima, such as the core of a jet ("Max 180"). A centre must be the highest or lowest point for about 900 km and stand out by at least one isoline interval; where two of a kind are close, the more prominent is kept, and labels that would overlap on screen are dropped, weakest first. MSLP isolines label highs and lows from the start.
Worked-out fields
These are worked out from the stored fields member by member, so every display (probabilities, percentiles, thirds) works on them.
| Field | How |
|---|---|
| 2 m dew-point depression | 2 m temperature − 2 m dew point: small where the air near the ground is nearly saturated (fog, low cloud) |
| 10 m–700 hPa shear (about 0–3 km) | The size of the 700 hPa wind minus the 10 m wind, as vectors |
| 10 m–500 hPa shear (about 0–6 km) | The same with the 500 hPa wind: 40 kt or more favours organised convective storms |
| Relative vorticity | ∂v/∂x − ∂u/∂y on the sphere, at every level with a wind, in 10⁻⁵ s⁻¹; positive is cyclonic |
| 250 hPa velocity potential | χ, whose slope is the divergent wind. Negative where the upper air spreads out over rising air (the tropical convection that drives the MJO, or ahead of a trough) |
| Divergent wind | The irrotational part of the wind: the slope of χ, so it blows from low χ to high. Small (a few knots) next to the whole wind, but it shows where air rises and sinks. Offered on wind layers as "Wind: divergent" |
Vorticity, χ and the divergent wind need each member's whole map, so they use the 1° grid only. χ is solved over the northern hemisphere taking the southern as its mirror image, as the store holds 0–90°N only: it is right in mid and high latitudes but only roughly so in the tropics. The 10 m and 700 hPa winds (and so the shear) are stored only by runs that came in after they were added; an older run says so if you ask for them.
Displays (keys 1–5)
| Key | Display | What it is |
|---|---|---|
| 1 | Mean & spread | Spread (standard deviation of the members) shaded, the mean as isolines |
| 2 | Mean | Ensemble mean |
| 3 | Probability | % of members at or beyond a threshold (see below) |
| 4 | Percentile | A percentile of the members at each point; 50 is the median, 90 a reasonable worst case for high values |
| 5 | Members | Postage stamps of every member over the part of the map on screen; click one to see it full size, then [ ] to step through members |
For fronts the keys pick one member (1), every member (2) or the frequency (3). Box thirds are in the Box tab.
Probabilities and neighbourhoods
A probability is the share of members at or above (or at or below) the threshold at each point. With a neighbourhood it is averaged over a disc of that radius around each point (true distance, so the disc covers more longitude further north): the chance of the event somewhere nearby rather than exactly here. Use one for showery precipitation or gusts, where the members cannot place the detail.
+ and / or adds a second condition, on any field with its own threshold and period. With and the map is the share of members where both happen in the same member, such as 24 h precipitation of 1 mm or more and a 2 m dew point of 1 °C or less (a guide to snow); with or, where either (or both) does. That is not the product of the two probabilities: it counts the members, so it keeps how the two go together in each one.
Periods
A period combines each member's steps over the hours before the valid time, before any statistic is taken: the probability of a 24 h maximum gust over 50 kt is the share of members whose highest gust in those 24 hours is over 50 kt. Precipitation over a period is each member's total. With no period, precipitation is the total since T+0. The gust is ECMWF's highest gust since the previous output (1 h to T+90, 3 h to T+144, 6 h after), so a period's maximum gust covers the whole period. For other fields only the 6-hourly steps are used, so peaks between them are missed.
Boxes, thirds and x–y
Draw a box with ▭ or b and drag on the map (drag inside it to move it, a corner to resize). The Box tab works out a statistic over the box for every member: the area-weighted mean, the max, the min, or the % of the box beyond a threshold. That ranks the members into thirds. Pin it to a time (📌) to rank by one event while you step through others.
- Thirds: a small map, centred on the box, of how the members in one third differ from all of them, for any field at the map's time or a fixed one. Shading is the third's mean minus the all-member mean, shown only where it is larger than one standard error (the all-member spread ÷ √ the number in the third); solid contours are the third's mean, dotted ones the spread within it. One colour scale serves all three thirds, so they compare directly. Hover for values. The small map is drawn in the main map's projection: on the polar map it is fitted to the box (a ring round the pole is shown whole); on the latitude–longitude map it widens to fit a box wider than 240°.
- Through time plots each member's box statistic at every step, coloured by third.
In the polar projection a box can go all the way round: drag it past about 300° of longitude and it becomes a ring (or, up to the pole, a cap), and the box statistics are over the whole band, area-weighted, which is as valid as any box. Drag its inner or outer edge to change the latitudes.
The x–y tab plots one box statistic against another, one point per member, coloured by third, with the correlation r, a bootstrap range, the 5% critical value and r² (the share of how the members differ in y that goes with x). It answers questions such as whether the members with the deepest low (box minimum of MSLP) are the ones with the strongest gusts (box maximum of the gust): often they are not. "Every grid point" instead bins every point in the box for every member: the physical relationship between two fields, with no significance test (neighbouring points are not independent).
Plumes
Click the map for plumes of the fields on the map (barbs give their wind speed) at that point, through the run: every member, the 10–90% range shaded, the median bold, the mean dashed, the min and max dotted, and the other run's 10–90% range and median in grey, placed by valid time. The table gives the numbers at the map's time. The shaded stretch on the right is after T+120, where the point uses the 1° fields. Click a plume to go to that time.
Fronts
Fronts are found in each member at 850 hPa, on the 0.25° grid to T+120, with Hewson's thermal front parameter on the wet-bulb potential temperature θw. A front is kept only where a 100 m wind shift or an MSLP trough backs it. No fronts are drawn where 850 hPa is below the ground in most members. Each point is typed cold, warm or quasi-stationary from the wind across it, and each line is smoothed a little (over about 40 km) to take out the steps of the grid. The thresholds are set a little below Hewson's, so weaker fronts show too. For one member, triangles are drawn on the warm side of cold fronts and semicircles on the cold side of warm fronts (the way each moves), alternating on quasi-stationary ones, which are drawn red and blue in turn. Frequency is the share of members with a front within 100 km.
Troughs and occlusions (a tick box on the fronts layer) carry the analysis on where the fronts stop. A front needs a θw gradient; where that fades, as when a cold front runs into the occluded part of a low, the trough often goes on as a strip of cyclonic 850 hPa vorticity in an MSLP trough. Those strips' axes are drawn as lines, worked out from the 1° vorticity, θw and MSLP, so they reach 85°N, north of the fronts' 0.25° area (which stops at 70°N). A line must be at least 400 km long and fairly straight, mostly in an MSLP trough, and more than 180 km from a front (the vorticity strip just behind a front belongs to that front). Most are drawn as plain troughs (black, white in dark mode). One is an occlusion (purple, triangles and semicircles in turn on the side it moves to) only if it carries a front on (an end within 300 km of one), reaches to within 500 km of one of that member's cyclone centres, and mostly follows a ridge of θw, the tongue of warm air being lifted. Occlusions are found only where cyclones are tracked (20–80°N, 90°W–50°E). In Frequency, dashed lines give the share of members with a trough or occlusion within 100 km (25, 50 and 75%). They are worked out when asked for, so the first look at a time takes a moment longer.
Cyclones
Cyclones are tracked in every member from MSLP minima to T+240 over 20–80°N, 90°W–50°E (refined on the 0.25° grid to T+120). Each centre's stage comes from its tilt: how far, and which way, the nearest closed 500 hPa low is from the surface centre. A baroclinic low deepens while its upper low lies well to the west, and the two stack up as it matures and occludes.
| Stage | 500 hPa low |
|---|---|
| Open wave | No closed low within 1000 km |
| Developing | 500 km or more to the west |
| Mature | 200–500 km to the west |
| Occluded / stacked | Within 200 km |
| Upper low ahead | 200 km or more to the east |
Click a centre to follow that cyclone: the nearest centre within 500 km in every member at that time, followed back and forward along its track, in both runs. The Cyclone tab follows each member's cyclone through time: its central pressure (dots by stage), the 24 h change in it scaled to 60°N (× sin 60° ÷ sin latitude; below −24 hPa it is deepening explosively, a "bomb", after Sanders and Gyakum), or the strongest gust within 500 km of the centre. The x–y chart plots two of these against each other, a point per member, with the correlation: the lowest pressure against the strongest gust, for instance, asks whether the deepest members are the windiest. The table gives each member's lowest pressure and largest 24 h fall.
The centres can also be coloured by central pressure (920–1010 hPa), by the strongest 100 m wind, or by the strongest gust within 500 km of the centre in that member (keys 1–4). The scales are fixed, so a colour means the same from run to run, and the deepest or strongest are drawn on top, so the outliers stand out. The winds are from the 1° grid; 500 km can take in a neighbouring system's wind, and over land the 100 m wind is held back by friction. The gust is ECMWF's highest since the previous output, so it is noisier than the 100 m wind.
Below the ground
Over high ground the 850 hPa (and 1000 hPa) surface lies below the ground, and the model's values there are extrapolated. Pressure-level fields are hatched where the level is below the ground in most members, the plumes shade those times, and the hover note gives the share of members.
Setups and colour scales
Save as… keeps the layers (and, if ticked, the map's area) as a named setup. Setups and colour scales are kept in this browser, for you alone. The admin can also save them for everyone: shared ones are marked ★ and listed under "Shared with everyone". The Hovmöller tab has setups of its own, saved the same way.
Copy link copies the page's address, which holds everything on screen (layers, colours, time and area), so whoever opens it sees the same map. It is the quick way to show someone a map.
Download… saves the chosen setup (with any colour scales of yours it uses), or everything kept in this browser (map setups, colour scales and Hovmöller setups), as a small file. Import… adds the setups in such a file to this browser: use it to move your setups to another browser or computer, to keep a copy, or to pass setups on. Nothing is sent to the server. Layers or fields this explorer cannot show are left out, and it says which; where a name is already in use you can keep both (the new one gets "(2)") or replace yours. Clearing the browser's site data deletes the setups kept in it, so download them first.
Open with this in this browser makes a setup the one this page starts with when it is opened without a link. Otherwise it starts with the setup the admin has chosen for everyone (Make everyone's default, shown to the admin on a shared setup), and at first that is "Surface overview" (the maps) and "Waves round the hemisphere" (the Hovmöllers).
The ? beside a panel's or card's title opens a short note on it; click again, or anywhere else, to close it.
Keys
| ← → | Previous / next step; with Shift, a day |
| Home End | First / last step |
| Space | Play |
| 1–5 | Display of the open layer (else the top shaded one) |
| [ ] | Previous / next member, showing one member |
| r | Switch run, keeping the valid time |
| l | Link the open layer to the one above |
| b | Draw a box |
| Esc | Stop drawing; close the box, cyclone or point tab |
| ? | This page |
Saving pictures and animations
⤓ on the map opens Export. Save PNG saves the map as on screen. Its title is the saved setup's name ("(modified)" once you change something), or what the map shows if it is not a setup; below it are the valid time, T+ and run. Under the map are the colour scales and, on the right, what the map shows and the credits. Make GIF steps the map through a range of times (every step, or every 2nd or 4th) and saves them as an animated GIF at screen size (at most 960 pixels wide), with the frame time you pick; one palette serves every frame, so a colour means the same throughout. The map can't be used while it runs.
In the side panel, point at any chart (a plume, the box strip, the thirds map, the x–y plot, through time, a cyclone's pressure) and click ⤓ PNG: it saves the chart with its heading, where and when it is for, and, for the x–y plot, its key and correlation statistics.
Sharing a view
The address holds everything on screen (run, time, layers, box, point, cyclone, projection, map area), so copying it shares the view. "latest" in it follows the newest run.
Hovmöllers
The Hovmöller tab shows a latitude band's mean at each longitude through time, oldest at the top, so troughs and ridges show as sloping streaks: down to the right as they move east. Choose the band in 5° steps from 0 to 90°N and the longitudes in 10° steps; if west is east of east, the plot crosses the dateline. A map of the band sits underneath, on the same longitudes.
Analysis, then a forecast. Above the black line are ECMWF's analyses at 00Z and 12Z for the days before the run (the T+0 of its deterministic run, HRES, which starts from the analysis). Below it is the chosen run's ensemble mean every 6 hours to T+240, with the spread shaded grey. Pick an earlier run to see what it expected.
Departure from the time mean takes each longitude's mean over the whole plot off its values. Plain heights or pressures are dominated by the waves that stay put (the troughs over eastern Canada and Asia, the ridges over the Rockies and Europe), which hide the systems moving through. It is not an anomaly from climate: change the dates or the band and the baseline changes with them. Red is above the mean, blue below, green the zero line.
Spread shaded draws the values as contours over the grey spread. Untick it and the values themselves are shaded in colour instead, which draws the eye to the waves: 11 classes of a ColorBrewer diverging scale (red–blue, red–yellow–blue, spectral, brown–teal and others), the middle one around zero, with the interval chosen from the largest values unless you set one. Flip swaps the ends, for instance to make moist teal and dry brown. The spread is the band mean of each point's spread, so it is larger than the spread of the band mean would be. Daily means average each UTC day, which takes out the daily cycle of the near-surface fields.
Smooth shading interpolates between the cells (1° of longitude by 6 or 12 hours) as the contours do, so the colour bands have smooth edges; untick it to see the cells themselves. Height of a day keeps a day the same height however many days back you go (compact, normal or tall), and the plot scrolls when it is taller than the window, opening on the forecast's start; fit the window squeezes it all in instead. Save PNG saves the whole plot.
Forecast bias at a lead time. Each row is one run's ensemble mean at that lead time minus the analysis at the time it was for, so a column that stays red is a place the forecasts keep running too high, or too warm. The chart below gives, at each longitude over the period, the mean error, the RMS error and the spread: the spread should be about the size of the RMS error once the mean error is small. The rows fill in as the archive grows: a T+120 bias needs runs at least five days old.
| Fields | Levels |
|---|---|
| MSLP, total column water vapour | surface, column |
| Temperature, geopotential height | 850, 700, 500, 250 hPa (and height at 10 hPa) |
| u and v wind | 850, 700, 500, 250, 10 hPa |
| Divergent (irrotational) wind u and v; velocity potential χ | 850, 700, 500, 250 hPa; χ at 250 hPa |
The band means come from the 1° fields, weighted by the cosine of latitude. At 850 hPa, points below the ground in half the members or more (or in the analysis) are left out, so over the Rockies and Asia an 850 hPa band mean is of what is left. The divergent wind is solved over the northern hemisphere only, so it is rough near the equator (and the MJO, mostly south of 15°N, is beyond it).
The archive keeps, for each run, the ensemble mean and spread of every field in 5° bands at every step, and the analysis: about 30 MB a run. Runs older than 60 days are deleted. The analyses for the 60 days before the archive began were fetched from ECMWF's open data on Amazon S3; the forecasts start with the archive.
Saved setups. A Hovmöller setup keeps the field, the band, the longitudes, the mode and the display, and always opens on the latest run. Shared ones to start from: waves round the hemisphere (500 hPa height, 35–60°N), Rossby wave packets (250 hPa v wind, which shows them best without the time mean taken off), the Atlantic storm track (MSLP), Pacific moisture (total column water vapour) and the Rockies' 700 hPa temperature bias at day 5 (which fills in once the archive holds a week of runs with 700 hPa temperature). Links, downloads and imports work as on the maps (see Setups and colour scales).
Questions about the forecast belong to the forecaster; this explorer shows what the ensemble does, not what will happen.