Tropical Storm Andrea (2025): A Retrospective Analysis of a Short-Lived Central Atlantic Cyclone
Issued by the Office of Senior Meteorological Analysis – Global Tropical Cyclone Review
Origin and Pre-Genesis Environment
Andrea’s roots were entirely non-tropical. Around 0000 UTC 22 June 2025, a weak area of low pressure developed along a decaying stationary front approximately 400 nautical miles east-southeast of Bermuda. The environment at that time featured:
- Moderately warm sea surface temperatures (SSTs) near 26–27°C.
- Low to moderate vertical wind shear (10–15 kt) — initially unfavorable for rapid organization.
- A broad, diffuse mid-level trough aloft providing weak upper-level divergence.
Over the next 24–30 hours, deep convection gradually aggregated near the circulation center. Satellite imagery from GOES-19 revealed episodes of disorganized thunderstorm activity, but the system lacked the persistent, deep convective core needed for immediate classification as a tropical cyclone.
The National Hurricane Center (NHC) first mentioned this disturbance in the Tropical Weather Outlook (TWO) approximately 30 hours before real-time genesis was declared. In post-analysis, however, the best track shifted the formation time earlier by 30 hours — a notable discrepancy highlighting the difficulty of capturing marginal tropical transitions.
Genesis and Intensity Evolution
Formation (0600 UTC 23 June)
By 0600 UTC 23 June, the low-pressure center had become sufficiently well-defined, and deep convection had achieved enough organization to warrant upgrade to a tropical depression. The system was then located about 425 nautical miles east of Bermuda (near 31.5°N, 52.0°W). Maximum sustained winds were estimated at 30 kt (35 mph) — just one notch below tropical storm strength.
Peak Intensity (1200 UTC 23 June – 1200 UTC 24 June)
Andrea reached tropical storm status by 1200 UTC 23 June, when a series of ASCAT scatterometer passes (from both MetOp-B and MetOp-C) consistently retrieved wind vectors of 34–36 kt. The NHC set the peak intensity at 35 kt (40 mph; 18 m/s) from 1200 UTC 23 June through 1200 UTC 24 June — a full 24-hour plateau.
The minimum central pressure of 1014 hPa was not directly observed but derived using the Knaff–Zehr–Courtney pressure-wind relationship. This value is notably high for a tropical storm. The explanation lies in the surrounding environmental pressure field: Andrea was a small system embedded in a region of anomalously high surface pressure (1030–1032 hPa), so the central pressure gradient was relatively weak despite the presence of 35-kt winds.
Structural Characteristics
At peak, Andrea was a compact cyclone. Satellite imagery (Fig. 1 in NHC’s TCR) showed a small but distinct central dense overcast (CDO) with a hint of an eye-like feature in microwave passes. However, the system’s overall convective structure remained asymmetric due to persistent, albeit moderate, westerly shear. Data from the NASA TROPICS constellation and NOAA’s Advanced Microwave Sounding Unit (AMSU) confirmed a tight radius of maximum winds — approximately 30–40 nautical miles.
Dissipation
After 1200 UTC 24 June, Andrea encountered a sharp increase in vertical wind shear (exceeding 25 kt) as an upper-level trough approached from the northwest. The deep convection rapidly sheared away from the low-level center. By 0000 UTC 25 June, the system had degenerated into a remnant low, and by 0600 UTC 25 June, the circulation had opened into a trough. No tropical storm force winds were observed after 1200 UTC 24 June.
Forecast and Warning Verification Challenges
Andrea’s life cycle posed several forecast challenges, as noted in the NHC’s Tropical Cyclone Report:
| Challenge Factor | Description |
|---|---|
| Small size | Andrea’s tight circulation (< 100 n mi in diameter) made it difficult to resolve in coarse operational models. |
| Non-tropical origins | The transition from a frontal low was gradual and lacked the explosive convective bursting typical of tropical cyclogenesis. |
| Short duration | Only 24 hours of tropical storm intensity — a narrow window for forecast adjustments. |
| Late real-time classification | The post-analysis best track moved genesis earlier by 30 hours, indicating that satellite interpretation missed the critical phase of organized convection. |
Despite these hurdles, no coastal watches or warnings were required. The system’s far-ocean track eliminated any threat to populated areas.
Key Metrics Summary
| Parameter | Value |
|---|---|
| Storm name | Andrea (AL012025) |
| Basin | North Atlantic |
| Period of tropical cyclone | 23–24 June 2025 |
| Peak wind (1-min sustained) | 35 kt (40 mph / 18 m/s / 64 km/h) |
| Minimum central pressure | 1014 hPa |
| Peak intensity classification | Tropical Storm |
| Landfall | None |
| Direct fatalities | 0 |
| Damages | None reported |
| NHC best track reference | NHC Tropical Cyclone Report (12 Aug 2025) |
Meteorological Significance
Andrea’s brief existence underscores three meteorological themes:
- The marginal nature of tropical transition. Andrea flirted with tropical depression intensity for nearly 36 hours before finally organizing. Its development required a precise balance of shear, SST, and upper-level divergence — a balance that lasted barely a day.
- The value of scatterometer data. Without ASCAT passes, Andrea’s true wind speed might have been underestimated. The 35-kt plateau was only confirmed through multiple overpasses of the small system.
- The influence of environmental pressure on central pressure estimation. A 1014 hPa tropical storm is an anomaly; the pressure-wind relationship must account for the ambient high-pressure environment, otherwise it risks overestimating the system’s true intensity.
Final Assessment
Tropical Storm Andrea (2025) was a pedestrian, short-lived cyclone that neither threatened life nor property. Yet its genesis from a decaying front, its 24-hour plateau at 35 kt, and its compact structure provide rich material for researchers studying the lower end of the tropical cyclone intensity spectrum. The NHC’s post-analysis correction — moving genesis 30 hours earlier than real-time — serves as a reminder that even in the satellite era, borderline tropical systems test the limits of operational detection.
Andrea: small, short, and subtle. A storm that barely was, but was exactly what it needed to be.
Data sourced from: National Hurricane Center Tropical Cyclone Report AL012025 (Cangialosi, 2025); satellite imagery from NOAA/NESDIS/STAR; ASCAT from EUMETSAT/NOAA.