Morning Briefing — Saturday 25 April 2026

Synoptic overview

Map legend

Station plot

 TT  ω  PPP
 VV  ∘  ww /
TdTd N  CL

TT	Temperature °C
PPP	Pressure, last 3 digits (278 = 1027.8 hPa)
ω	Pressure tendency (rising/falling)
VV	Visibility (km)
∘	Cloud cover (fill = amount)
ww	Present weather code
TdTd	Dewpoint °C
N, CL	Cloud amount & type

Wind barbs

Short line = 5 kt (~2.5 m/s)
Long line = 10 kt (~5 m/s)
Triangle flag = 50 kt (~25 m/s)
Barb points where wind blows from

Fronts

—▲— Warm front
—▲— Cold front
—▲— Occluded front

Areas

M = Low pressure K = High pressure
💧 Rain ❄ Snow 💧❄ Sleet
≡ Fog zone

Source: Estonian Environment Agency

SIGWX chart unavailable. View on lennuilm.ee

Charts: lennuilm.ee (Estonian Aviation Weather)

Sounding — Tallinn-Harku 00Z

Sounding unavailable for this date.

Skew-T legend

Lines on the diagram
— Temperature (observed, black solid)
- - Dewpoint (observed, black dashed)
— Dry adiabats (red, slope right) — unsaturated parcel cooling rate
— Moist adiabats (green, curve right) — saturated parcel cooling rate
- - Mixing ratio (green dashed) — constant humidity lines

Reading for thermals
• Where temp & dewpoint lines are close → clouds likely at that level
• Steep temp line (follows dry adiabat) → unstable, good thermals
• Temp inversion (line tilts right with height) → thermal ceiling
• Wind barbs on the right show wind at each level
• Y-axis: pressure (hPa), X-axis: temperature (°C, skewed)

Text indices (right-hand column on the diagram)

SLAT / SLON / SELV — station latitude (°), longitude (°), elevation (m)

SHOW — Showalter Index (°C). 850→500 hPa parcel lift. <0 unstable, <−3 strong storms.

LIFT — Lifted Index (°C). 500 hPa T minus surface parcel T at 500 hPa. <0 unstable.

LFTV — Lifted Index using virtual temperature (adjusts for moisture).

SWET — SWEAT Index. Severe-weather composite (wind shear + moisture + instability). >300 severe.

KINX — K Index (°C). Thunderstorm potential from mid-level moisture + lapse rate. >30 likely.

CTOT / VTOT / TOTL — Cross / Vertical / Total Totals (°C). TOTL = CTOT + VTOT. TOTL>50 strong storms.

CAPE — Convective Available Potential Energy (J/kg). Fuel for thermals/storms. >1000 strong.

CAPV — CAPE with virtual temperature correction (more accurate for moist parcels).

CINS — Convective Inhibition (J/kg). Energy needed to break the cap. 0 = no cap.

CINV — CIN with virtual temperature correction.

LCLP / LCLT — Lifting Condensation Level pressure (mb) and temperature (K). Cloud base.

LCLE — Equivalent potential temperature at the LCL (K).

LFCT / LFCV — Level of Free Convection pressure (mb). Above this a parcel rises on its own.

EQLV / EQTV — Equilibrium Level pressure (mb). Top of the CAPE layer, cloud top for storms.

BRCH / BRCV — Bulk Richardson Number. CAPE divided by low-level wind shear. 10–45 = supercell range.

MLTH — Mean-Layer potential temperature (K) of the lowest ~500 m.

MLMR — Mean-Layer mixing ratio (g/kg) of the lowest ~500 m. Low-level moisture.

THCK — 1000–500 hPa thickness (m). <5400 cold airmass, >5700 warm.

PWAT — Precipitable Water (mm). Total water column if condensed.

−9999 = not computed (e.g. no LFC when CAPE is zero). Indices ending in V use virtual temperature, which accounts for water vapour's lower density and is slightly more accurate in moist air.

Source: University of Wyoming, station 26038

Contrails — Tallinn-Harku

VerdictPERSISTENT

Peak 03 UTC @ 300 hPa-50 °C · RHi 100% · W 22 m/s

300 hPa (~9.2 km)

-50

88%

-50

100%

-50

100%

-51

96%

-51

84%

-50

60%

-48

32%

-48

10%

-47

10%

-47

10%

-46

12%

-46

12%

-46

10%

-44

250 hPa (~10.4 km)

-55

47%

-54

49%

-54

40%

-52

27%

-51

16%

-50

11%

-48

-47

-46

-45

-44

-43

-42

noneshortpersistentspreading

Persistent contrails likely — may spread into thin cirrus.

Each cell: UTC hour, temperature (°C), RHi (%). Formation needs T < −40 °C (Schmidt-Appleman); persistence needs RHi ≥ 100 % (ice-supersaturation). Spreading = ≥3 consecutive hours at or above ice saturation. At 300/250 hPa (always < −23 °C), Open-Meteo's ECMWF IFS reports RH already over ice.

Source: Open-Meteo forecast API, pressure-level variables. Window: civil twilight.

Airmass classification (2026-04-21)

SSC typeDM

DescriptionDry Moderate

Source: SSC v3.0 (Kent State University), station TLL

Current conditions — Tallinn-Harku

Temperature5.5 °C

Relative humidity70 %

Pressure996.5 mbar

WindS 4.8 m/s

PhenomenaLight shower

Obs time (UTC)2026-04-25 05:00:52

Source: Ilmateenistus (Estonian Weather Service)

Kuusiku XC forecast

VerdictNO-GO

BLH max (08–15 UTC)1020 m

Temperature (13 UTC)3.5 °C

Precipitation (08–15 UTC)9.5 mm

Cloud cover mean100%

CAPE max50 J/kg

Wind (13 UTC)W 2.5 m/s

✗ Precip: 9.5 mm (max 0.5 mm)

● BLH borderline: 1020 m (target >1200)

● Wind: W 2.5 m/s

✗ Gusts too strong: 10.1 m/s (max 9.0) — NO-GO

● Temp: 3.5 C (target >12)

● Cloud: 100% (prefer <55%)

● Soil: 0.31 (moderate)

Source: Open-Meteo forecast API (59.0°N, 24.8°E). Thresholds calibrated on 53 top-100 Rapla XC flights.

Ground conditions

Kuusiku (flying site)

Soil moisture0.31 m³/m³ — moderate (Niiske)

Soil temperature3.8 °C

Tallinn (reality check)

Soil moisture0.32 m³/m³ — moderate (Niiske)

Soil temperature5.7 °C

Tallinn shown as reference — compare with what you see outside. Thresholds: DRY < 0.22, WET > 0.32 m³/m³ (calibrated on 114 Rapla flight dates, ERA5-Land 0–7cm). Forecast uses 0–1cm layer — values are preliminary.

Source: Open-Meteo forecast soil moisture model data (not station observations).

Metric reference

Lapse rate	Temperature decrease per km altitude. >6 °C/km = active thermals; >9.8 = superadiabatic (turbulent).
Thermal ceiling	Height where lapse drops below moist adiabatic — approximate top of usable lift.
Inversion	Layer where temperature increases with height, blocking thermals. Common overnight; often breaks by midday.
Lifted Index (LI)	Stability of a lifted surface parcel at 500 hPa. Negative = unstable; < −3 = strong thermals; < −6 = storm risk.
K-index	Thunderstorm potential from temperature and humidity profile. >25 = showers likely; >35 = widespread storms.
CAPE	Convective Available Potential Energy (J/kg). Higher = stronger updrafts. >1500 = overdevelopment risk. Not computed by Wyoming for Tallinn-Harku — shown in Kuusiku section from forecast model.
CIN	Convective Inhibition (J/kg). Energy barrier thermals must overcome. > −50 = weak cap (easy trigger); < −200 = strong cap.
Thermal Index (TI)	Forecast noon surface temp minus 850 hPa sounding temp. Positive = thermals likely; higher = stronger.
BLH	Boundary Layer Height — mixing depth where thermals operate. >1400 m for good XC; >2000 m for excellent.
Soil moisture	Volumetric water content of topsoil (m³/m³). Dry soil (<0.22, Kuiv) heats air efficiently — stronger thermals via Bowen ratio. Wet soil (>0.32, Märg) spends solar energy on evaporation — weaker thermals. Effect strongest on cool/marginal days.

00Z sounding reflects pre-dawn atmosphere (~23:30 UTC previous day). Surface inversions typically break by mid-morning. Kuusiku forecast uses 13 UTC (16:00 EEST / 15:00 EET) for peak thermal metrics — this hour was calibrated against the top-100 Rapla XC flight dates (2018–2025).

Disclaimer: Content generated using AI under guidance of an amateur paraglider pilot. Always check actual weather conditions before flying. No responsibility is assumed for decisions made based on this data.

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