diff --git a/nfdrs/driver.go b/nfdrs/driver.go
index a5a133e..48f99cf 100644
--- a/nfdrs/driver.go
+++ b/nfdrs/driver.go
@@ -1,6 +1,7 @@
 package nfdrs
 
 import (
+	"fmt"
 	"math"
 	"time"
 
@@ -10,6 +11,10 @@ import (
 	"alpineworks.io/firewx/simple"
 )
 
+// driverSchemaVersion is the schema version stamped into a DriverState. Increase
+// it when the meaning of a field changes.
+const driverSchemaVersion = 1
+
 // Config holds the site settings for a Driver.
 type Config struct {
 	// FuelModel is the fuel model of the site.
@@ -216,3 +221,119 @@ func (d *Driver) Indices() Indices { return d.indices }
 func (d *Driver) DeadMoistures() (mc1, mc10, mc100, mc1000 firewx.Percent) {
 	return firewx.Percent(d.mc1), firewx.Percent(d.mc10), firewx.Percent(d.mc100), firewx.Percent(d.mc1000)
 }
+
+// DriverState is the full serializable state of a Driver. It holds the state of
+// the four Nelson sticks, the two live fuel moisture models, the drought index,
+// and the driver's own daily accumulators and last outputs.
+//
+// A DriverState marshals to JSON and back with an exact round trip, so a caller
+// can persist a Driver between runs and resume without the long spin-up that the
+// 1000-hour stick needs from a cold start. The caller must construct the Driver
+// with the same Config, then restore the state.
+//
+// The sub-model pointers alias the Driver's live models. Marshal the state
+// before the next call to Update.
+type DriverState struct {
+	SchemaVersion int `json:"schema_version"`
+
+	Stick1    *nelson.Stick    `json:"stick_1hr"`
+	Stick10   *nelson.Stick    `json:"stick_10hr"`
+	Stick100  *nelson.Stick    `json:"stick_100hr"`
+	Stick1000 *nelson.Stick    `json:"stick_1000hr"`
+	Herb      *gsi.Model       `json:"herb"`
+	Woody     *gsi.Model       `json:"woody"`
+	KBDI      simple.KBDIState `json:"kbdi"`
+
+	// Dead fuel moisture and fuel temperature from the most recent update.
+	MC1      float64 `json:"mc_1hr"`
+	MC10     float64 `json:"mc_10hr"`
+	MC100    float64 `json:"mc_100hr"`
+	MC1000   float64 `json:"mc_1000hr"`
+	FuelTemp float64 `json:"fuel_temp_c"`
+
+	// Live fuel moisture and drought from the most recent daily update.
+	GSIValue  float64 `json:"gsi"`
+	MCHerb    float64 `json:"mc_herb"`
+	MCWood    float64 `json:"mc_woody"`
+	KBDIValue float64 `json:"kbdi_value"`
+
+	// Daily accumulators over the current local standard day.
+	HaveDay    bool    `json:"have_day"`
+	DayMinTemp float64 `json:"day_min_temp_f"`
+	DayMaxTemp float64 `json:"day_max_temp_f"`
+	DayMinRH   float64 `json:"day_min_rh"`
+	DayPrecip  float64 `json:"day_precip_in"`
+
+	Last     time.Time `json:"last"`
+	HaveLast bool      `json:"have_last"`
+	Indices  Indices   `json:"indices"`
+}
+
+// State returns the full state of the Driver. Marshal it to JSON to persist the
+// Driver between runs.
+func (d *Driver) State() DriverState {
+	return DriverState{
+		SchemaVersion: driverSchemaVersion,
+		Stick1:        d.stick1,
+		Stick10:       d.stick10,
+		Stick100:      d.stick100,
+		Stick1000:     d.stick1000,
+		Herb:          d.herb,
+		Woody:         d.woody,
+		KBDI:          d.kbdi,
+		MC1:           d.mc1,
+		MC10:          d.mc10,
+		MC100:         d.mc100,
+		MC1000:        d.mc1000,
+		FuelTemp:      float64(d.fuelTemp),
+		GSIValue:      d.gsiValue,
+		MCHerb:        d.mcHerb,
+		MCWood:        d.mcWood,
+		KBDIValue:     d.kbdiValue,
+		HaveDay:       d.haveDay,
+		DayMinTemp:    d.dayMinTemp,
+		DayMaxTemp:    d.dayMaxTemp,
+		DayMinRH:      d.dayMinRH,
+		DayPrecip:     float64(d.dayPrecip),
+		Last:          d.last,
+		HaveLast:      d.haveLast,
+		Indices:       d.indices,
+	}
+}
+
+// SetState restores the Driver from a saved state. The Driver keeps its Config,
+// so construct it with the same Config first. SetState returns an error if the
+// schema version does not match or a sub-model is absent.
+func (d *Driver) SetState(s DriverState) error {
+	if s.SchemaVersion != driverSchemaVersion {
+		return fmt.Errorf("nfdrs: driver state schema version %d, want %d", s.SchemaVersion, driverSchemaVersion)
+	}
+	if s.Stick1 == nil || s.Stick10 == nil || s.Stick100 == nil || s.Stick1000 == nil || s.Herb == nil || s.Woody == nil {
+		return fmt.Errorf("nfdrs: driver state has an absent sub-model")
+	}
+	d.stick1 = s.Stick1
+	d.stick10 = s.Stick10
+	d.stick100 = s.Stick100
+	d.stick1000 = s.Stick1000
+	d.herb = s.Herb
+	d.woody = s.Woody
+	d.kbdi = s.KBDI
+	d.mc1 = s.MC1
+	d.mc10 = s.MC10
+	d.mc100 = s.MC100
+	d.mc1000 = s.MC1000
+	d.fuelTemp = firewx.Celsius(s.FuelTemp)
+	d.gsiValue = s.GSIValue
+	d.mcHerb = s.MCHerb
+	d.mcWood = s.MCWood
+	d.kbdiValue = s.KBDIValue
+	d.haveDay = s.HaveDay
+	d.dayMinTemp = s.DayMinTemp
+	d.dayMaxTemp = s.DayMaxTemp
+	d.dayMinRH = s.DayMinRH
+	d.dayPrecip = firewx.Inches(s.DayPrecip)
+	d.last = s.Last
+	d.haveLast = s.HaveLast
+	d.indices = s.Indices
+	return nil
+}
diff --git a/nfdrs/driver_test.go b/nfdrs/driver_test.go
index 865e818..d7592c2 100644
--- a/nfdrs/driver_test.go
+++ b/nfdrs/driver_test.go
@@ -2,6 +2,7 @@ package nfdrs
 
 import (
 	"encoding/csv"
+	"encoding/json"
 	"math"
 	"os"
 	"strconv"
@@ -111,6 +112,100 @@ func TestDriverRejectsIncompleteObs(t *testing.T) {
 	}
 }
 
+// TestDriverStateRoundTrip checks that a driver marshals to JSON and back with
+// an exact round trip. A restored driver produces identical output for the same
+// further weather, so a caller can persist the driver and resume without the
+// spin-up.
+func TestDriverStateRoundTrip(t *testing.T) {
+	cfg := Config{
+		FuelModel: FuelModelY, Latitude: 47.7, SlopeClass: 1, KBDIThreshold: 800,
+		MeanAnnualPrecip: 40, AnnualHerb: true, RegObsHour: 13, LSTOffset: -8 * time.Hour,
+	}
+	base := time.Date(2026, 7, 1, 0, 0, 0, 0, time.UTC)
+	mkObs := func(i int) firewx.Obs {
+		fi := float64(i)
+		rain := 0.0
+		if i%17 == 0 {
+			rain = 3.0
+		}
+		return firewx.Obs{
+			Time:             base.Add(time.Duration(i) * time.Hour),
+			Temperature:      firewx.Some(firewx.Celsius(15 + 10*math.Sin(fi/6))),
+			RelativeHumidity: firewx.Some(firewx.Percent(40 + 30*math.Cos(fi/5))),
+			SolarRadiation:   firewx.Some(firewx.WattsPerSquareMeter(400 * math.Abs(math.Sin(fi/12)))),
+			Precipitation:    firewx.Some(firewx.Millimeters(rain)),
+			WindSpeed:        firewx.Some(firewx.MetersPerSecond(3)),
+		}
+	}
+
+	// Run the driver across two daily boundaries so the daily state is set.
+	orig := NewDriver(cfg)
+	for i := range 60 {
+		orig.Update(mkObs(i))
+	}
+
+	blob, err := json.Marshal(orig.State())
+	if err != nil {
+		t.Fatal(err)
+	}
+	var st DriverState
+	if err := json.Unmarshal(blob, &st); err != nil {
+		t.Fatal(err)
+	}
+	restored := NewDriver(cfg)
+	if err := restored.SetState(st); err != nil {
+		t.Fatal(err)
+	}
+
+	// A second marshal after the restore is byte-identical.
+	blob2, err := json.Marshal(restored.State())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(blob) != string(blob2) {
+		t.Fatalf("re-marshalled driver state differs from the original")
+	}
+
+	// Drive both with the same further weather. The output must stay identical.
+	for i := 60; i < 96; i++ {
+		o := mkObs(i)
+		orig.Update(o)
+		restored.Update(o)
+		if orig.Indices() != restored.Indices() {
+			t.Fatalf("step %d indices: original %+v, restored %+v", i, orig.Indices(), restored.Indices())
+		}
+		a1, a10, a100, a1000 := orig.DeadMoistures()
+		b1, b10, b100, b1000 := restored.DeadMoistures()
+		if a1 != b1 || a10 != b10 || a100 != b100 || a1000 != b1000 {
+			t.Fatalf("step %d dead moistures differ", i)
+		}
+	}
+}
+
+// TestDriverSetStateErrors checks that SetState rejects a wrong schema version
+// and an absent sub-model.
+func TestDriverSetStateErrors(t *testing.T) {
+	cfg := Config{FuelModel: FuelModelY, SlopeClass: 1, KBDIThreshold: 800, RegObsHour: 13}
+	d := NewDriver(cfg)
+	good := NewDriver(cfg).State()
+
+	wrongVersion := good
+	wrongVersion.SchemaVersion = good.SchemaVersion + 1
+	if d.SetState(wrongVersion) == nil {
+		t.Errorf("expected an error for a wrong schema version")
+	}
+
+	absentModel := good
+	absentModel.Stick10 = nil
+	if d.SetState(absentModel) == nil {
+		t.Errorf("expected an error for an absent sub-model")
+	}
+
+	if err := d.SetState(good); err != nil {
+		t.Errorf("unexpected error for a valid state: %v", err)
+	}
+}
+
 type hourly struct {
 	obs                     firewx.Obs
 	mc10, mc100, mc1000     float64
diff --git a/nfdrs/indices.go b/nfdrs/indices.go
index 835028d..a836b49 100644
--- a/nfdrs/indices.go
+++ b/nfdrs/indices.go
@@ -20,16 +20,16 @@ const (
 type Indices struct {
 	// SpreadComponent is the forward rate of spread of the head fire, related
 	// to feet per minute.
-	SpreadComponent float64
+	SpreadComponent float64 `json:"spread_component"`
 	// EnergyReleaseComponent is the energy release per unit area of the flaming
 	// front.
-	EnergyReleaseComponent float64
+	EnergyReleaseComponent float64 `json:"energy_release_component"`
 	// BurningIndex combines the spread component and the energy release
 	// component.
-	BurningIndex float64
+	BurningIndex float64 `json:"burning_index"`
 	// IgnitionComponent is the chance that a firebrand starts a fire, from 0 to
 	// 100.
-	IgnitionComponent float64
+	IgnitionComponent float64 `json:"ignition_component"`
 }
 
 // Conditions holds the fuel moisture and the weather for an index computation.