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Nerd mode: every forecast, readouts, filters and range & bearing for weather enthusiasts

Storm · Forecasts split · Forecast to formNorth America

A new low could form near Sacramento on Tuesday night, then pass near San Francisco

San Francisco — Likely to pass 60–100 mi NE of San Francisco Wed night; 5 in 10 forecasts bring it within 100 mi.

Updated Mon., Sept. 28, 5:47 p.m. PDT · 111 forecast tracks from NOAA, ECMWF and Environment Canada: high

This storm is no longer in our active list. This is its last forecast, from the Mon 28 Sep 12Z–18Z runs.

San Francisco — Likely to pass 60–100 mi NE of San Francisco Wed night; 5 in 10 forecasts bring it within 100 mi.

Quick answers

As of the Mon 28 Sep 12Z–18Z model runs · updated Mon., Sept. 28, 5:47 p.m. PDT

Where will the low over California form?
Forecasts that develop it have the centre forming near Sacramento, over California on Tuesday night. The forecasts split: 6 in 10 move more slowly, while 3 in 10 move faster, while 2 in 10 in-between track.
When will the low over California be closest to San Francisco?
Most likely Wednesday, Sept. 30, around 11 p.m. PDT. Across the forecasts the comes between Tue 11 p.m. and Fri 2 a.m. PDT. Most forecasts pass the centre 60–100 miles northeast of San Francisco.
What is the chance it passes within 100 miles of San Francisco?
54%: 5 in 10 forecasts bring the storm’s centre within 100 miles of San Francisco. Elsewhere: Sacramento 56%, Los Angeles 14%, San Diego 10%. Rain, wind and coastal flooding can reach well beyond 100 miles from the centre.
How much rain could San Francisco get while the low over California passes?
From Mon 5 p.m. (when the forecast’s rain totals start) to Fri 5 a.m. PDT, the end of the window when its centre is closest to San Francisco, the middle of NOAA’s 31 GEFS forecasts brings almost no rain or melted snow to San Francisco; fewer than 1 in 10 bring 0.5 in. By Tue 11 p.m., when that window opens, the middle forecast has almost none yet. That is all the rain in that time, not only the low over California’s, from one model (GEFS) on a coarse grid; local official forecasts can differ. San Francisco: rain and wind this week →
How sure is the forecast?
Path certainty: high. 111 of 185 show the storm, and they broadly on its early path.
Do the AI and physics models agree?
Broadly, with some differences. 6 in 10 AI- (AIFS ENS, AIGEFS) move more slowly, against 5 in 10 physics-ensemble forecasts (ECMWF ENS). By Thursday night the AI ensembles’ middle track is about 220 miles southeast of the physics ensembles’. Both take it to about 1,004 at its deepest.
Will the low over California become a bomb cyclone?
Unlikely. None of the 185 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 near Sacramento, over California on Tuesday night, at about 1,005 .

How sure. 111 of 185 show the storm, and they broadly on its early path.1

How strong. Forecasts have it filling steadily, to about 1,009 by Thursday. None of them turn it into a rapidly deepening .

What’s uncertain

Whether it forms. 111 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. Six in 10 forecasts move more slowly; the other three in 10 move faster. By Thursday evening the two paths are about 30 miles apart.

How close it comes. For San Francisco, most forecasts put the between 60 and 100 miles away, most likely Wednesday 11 p.m.

Later on. The forecasts fan out after Thursday. By Thursday night, the is about 600 miles 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 100 miles. Shaded columns are night; times are in your time zone.

Rain and wind this week in Sacramento, San Francisco, Los Angeles and San Diego.

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

ModelTracks
NOAA
0 of 3118Z
arrived 2 h 43 min ago
ECMWF
44 of 5112Z
arrived 5 h 18 min ago
ECMWF
43 of 5118Z
arrived 1 h 33 min ago
NOAA
24 of 3112Z
arrived 7 h ago
Environment Canada
0 of 2112Z
arrived 8 h 48 min ago

Also tracking

  1. Southern Ocean low south of New ZealandVery deep low near 921 mb now, in the Southern Hemisphere; all 185 forecasts show it
  2. Southern Ocean lowVery deep low in the Southern Hemisphere, near 936 mb now and filling; all 185 forecasts show it
  3. Southern Ocean lowVery deep low bottoming out near 953 mb Thu morning, in the Southern Hemisphere; all 185 forecasts show it
  4. Amundsen Sea lowVery deep low bottoming out near 957 mb Tue night, in the Southern Hemisphere; all 185 forecasts show it
  5. Hurricane PoloCould pass near San Diego Wed morning; 1 in 10 forecasts bring it within 100 mi
  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 100 miles of a spot at any time in the next 9 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-2026-0263.

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 11 p.m. PDT