Bihar faces annual monsoon floods from Himalayan rivers. The Kosi, Gandak, Bagmati, Kamla-Balan and Mahananda rivers carry massive Himalayan snowmelt and Nepali monsoon runoff into Bihar's flat plains — triggering floods that displace millions every year. VayuMet's cumulative rainfall maps covering Nepal and Bihar together are critical for flood precursor monitoring.
| District / City | Day 1–2 (mm) | Day 3–4 (mm) | Day 5–7 (mm) | Confidence |
|---|---|---|---|---|
| Patna | 2–10 | 8–25 | 15–45 | Moderate |
| Gaya | 2–8 | 5–20 | 12–35 | Moderate |
| Bhagalpur | 5–15 | 10–30 | 18–50 | Moderate |
| Muzaffarpur | 5–15 | 10–28 | 18–50 | Moderate |
| Darbhanga | 5–15 | 10–30 | 18–50 | Moderate |
| Purnea | 8–22 | 12–35 | 22–60 | Moderate |
| Araria | 8–25 | 15–40 | 25–65 | Moderate |
| West Champaran (Bettiah) | 5–18 | 10–30 | 18–50 | Moderate |
Note: North Bihar (Darbhanga, Madhubani, Araria) receives more rainfall than south Bihar (Gaya, Rohtas). GFS is generally reliable over Bihar's flat Gangetic plains.
North Bihar borders Nepal's Terai and receives the monsoon first, around 10–12 June. It receives 1,100–1,300 mm annually from both the Bay of Bengal branch and orographic enhancement from the Himalayan foothills. Despite the El Niño suppression, north Bihar's proximity to the Himalayan moisture source provides some resilience — 2026 outlook is below-normal but less severe than central India.
Patna and the Gangetic plains of central Bihar receive onset around 13–16 June and average 900–1,000 mm annually. Bay of Bengal depressions tracking northwest are the main rainfall drivers. El Niño years reduce the frequency of these depressions — 2026 is forecast below-normal for central Bihar.
South Bihar lies on the Chota Nagpur plateau fringe and receives less rainfall than the north — 700–900 mm annually. It is also more El Niño sensitive, carrying elevated drought risk in 2026. Gaya and Nawada are particularly vulnerable — inadequate monsoon rainfall here stresses the farming community significantly.