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Storm · Forecasts split · Forecast to formSouth Atlantic

A new low is likely to form north of Montevideo on Tuesday morning

Montevideo, Uruguay — Could pass near Montevideo Tue afternoon; 1 in 10 forecasts bring it within 160 km.

Updated Mon., Sept. 28, 8:37 p.m. GMT-3 · 183 forecast tracks from NOAA, ECMWF and Environment Canada: low

Montevideo, Uruguay — Could pass near Montevideo Tue afternoon; 1 in 10 forecasts bring it within 160 km.

Quick answers

As of the Mon 28 Sep 12Z–18Z model runs · updated Mon., Sept. 28, 8:37 p.m. GMT-3

Where will the South Atlantic low southeast of Uruguay form?
Forecasts that develop it have the centre forming about 450 kilometres north of Montevideo on Tuesday morning. The forecasts split: 7 in 10 take a southern track, while 3 in 10 take a northern track.
When will the South Atlantic low southeast of Uruguay be closest to Montevideo?
Most likely Tuesday, Sept. 29, around 3 p.m. GMT-3. Across the forecasts the comes between Tue 10 a.m. and Tue 9 p.m. GMT-3. Most forecasts pass the centre 130–1,000 kilometres east of Montevideo.
What is the chance it passes within 160 kilometres of Montevideo?
12%: 1 in 10 forecasts bring the storm’s centre within 160 kilometres of Montevideo. Rain, wind and coastal flooding can reach well beyond 160 kilometres from the centre.
How much rain could Montevideo get while the South Atlantic low southeast of Uruguay passes?
From Mon 9 p.m. (when the forecast’s rain totals start) to Tue 9 p.m. GMT-3, the end of the window when its centre is closest to Montevideo, the middle of NOAA’s 31 GEFS forecasts brings 8.2 mm of rain and melted snow to Montevideo; 2 in 10 bring 13 mm or more. By Tue 9 a.m., when that window opens, the middle forecast already has 3.3 mm. That is all the rain in that time, not only the South Atlantic low southeast of Uruguay’s, from one model (GEFS) on a coarse grid; local official forecasts can differ. Montevideo: rain and wind this week →
How sure is the forecast?
Path certainty: low. 183 of 185 show the storm, and they on where it goes. Little has changed since the previous run.
Do the AI and physics models agree?
Largely, yes. 6 in 10 AI- (AIFS ENS, AIGEFS) take a southern track, against 7 in 10 physics-ensemble forecasts (GEFS, ECMWF ENS, GEPS). By Thursday morning the AI ensembles’ middle track is about 80 kilometres southeast of the physics ensembles’. At its deepest, the AI ensembles take it to about 979 and the physics ensembles to about 973 mb.
Will the South Atlantic low southeast of Uruguay become a bomb cyclone?
26% of forecasts (3 in 10) deepen it fast enough to count as a (a pressure fall of roughly 24 in 24 hours, adjusted for latitude).

New to these maps? Read how to read a spaghetti map, or see the week’s rain and wind around the world on the globe.

What we know

Where it forms. The forecasts that show it have the centre taking shape about 450 kilometres north of Montevideo on Tuesday morning, at about 1,004 .

How sure. 183 of 185 show the storm, and they on where it goes. Little has changed since the previous run.1

How strong. It is still strengthening: forecasts deepen it to about 972 by Friday night. Three in 10 forecasts make it a , deepening about 24 millibars or more in a day.

What’s uncertain

Whether it forms. 183 of 185 develop it; the rest do not. Storms that exist only in the forecast can change a lot from run to run.

Which way it turns. Seven in 10 forecasts take a southern track; the other three in 10 take a northern track. By Wednesday afternoon the two paths are about 350 kilometres apart.

How close it comes. For Montevideo, most forecasts put the between 130 and 1,000 kilometres away, most likely Tuesday 3 p.m.

Later on. The forecasts fan out after Tuesday. By Friday night, the is about 1,550 kilometres across.2

When it arrives

When the centre passes closest to each place, earliest to latest across the forecasts. Places are ordered by the most likely time.

Window of Most likely timeNight
is the share of forecasts that bring the centre within 160 kilometres. Shaded columns are night; times are in your time zone.

Rain and wind this week in Montevideo.

DetailsStrength, sources and the fine print

How strong it gets

Central pressure over time. The line is the middle forecast; the band holds 8 in 10 of them.

(lower is stronger)

Times are shown in

Where the forecasts come from

ModelTracksStatus
NOAA
31 of 3118Z
arrived 59 min ago
Latest run
ECMWF
51 of 5112Z
arrived 3 h 34 min ago
Latest run
ECMWF
49 of 5112Z
arrived 5 h 37 min ago
Latest run
NOAA
31 of 3112Z
arrived 5 h 17 min ago
Latest run
Environment Canada
21 of 2112Z
arrived 7 h 5 min ago
Latest run

Also tracking

  1. Southern Ocean lowVery deep low in the Southern Hemisphere, near 936 mb now and filling; all 185 forecasts show it
  2. Amundsen Sea lowVery deep low bottoming out near 958 mb Tue night, in the Southern Hemisphere; all 185 forecasts show it
  3. Southern Ocean lowVery deep low bottoming out near 953 mb Thu morning, in the Southern Hemisphere; nearly all forecasts show it
  4. Southern Ocean low south of New ZealandVery deep low near 922 mb now, in the Southern Hemisphere; nearly all forecasts show it
  5. Hurricane PoloCould pass near San Diego Wed morning; 1 in 10 forecasts bring it within 160 km
  6. See all 30 storms →

Notes

  1. Forecast tracks from the 18Z model run: GEFS (31), ECMWF ENS (51), AIFS ENS (51), AIGEFS (31), GEPS (21). A single run is one high-resolution forecast; ensemble forecasts are the same model run many times with small changes.
  2. The cone encloses where the centre is expected to be about two times in three at each hour, from the spread of the forecasts, widened by 20 percent; not yet calibrated against past storms. It shows uncertainty in the track, not the size of the storm.
  3. Shading shows the share of forecasts that bring the centre within 160 kilometres of a spot at any time in the next 10 days. Rain, wind and surge can reach much farther.
  4. Times are in your time zone. Arrival windows run from the earliest to the latest closest approach across forecasts that bring the centre within about 300 miles (500 km).
  5. NOAA/NCEP.
  6. Data Source: Environment and Climate Change Canada.
  7. This service is based on data and products of the European Centre for Medium-Range Weather Forecasts (ECMWF). Source: www.ecmwf.int. Licence: CC BY 4.0.
  8. Cyclone tracks computed by Stormlines from the model fields.
  9. Reading these maps: , or the illustrated guide to spaghetti models. The week’s rain, wind and temperature everywhere: the Stormlines globe.
  10. How often these forecasts turn out right: the season’s forecast check, AI vs physics.
  11. Stormlines storm ID: L-2026-0177.

Stormlines draws every storm the world’s ensemble forecasts can see: tracks from NOAA (GEFS and the AI ensemble AIGEFS), ECMWF (ENS and the AI ensemble AIFS) and Environment Canada (GEPS), updated as each model run lands. Coastlines and place names from Natural Earth; sea areas from IHO Sea Areas, version 3 (Flanders Marine Institute, marineregions.org, CC BY 4.0; modified: simplified, some islands removed, names edited).

Not an official forecast. For warnings and decisions, follow your national weather service.

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Stormlines Tracks: NOAA, ECMWF, Environment Canada

Full forecast
Through Fri 9 p.m. GMT-3