The disturbed condition in the lower atmosphere near Sri Lanka may develop into a low-pressure area within the next 24 hours, the Department of Meteorology said in a statement, warning that rainfall will increase over the island and the surrounding seas from Wednesday and for several days afterwards.

Showers or thundershowers will occur at times in most parts of the island, with heavily clouded skies in many areas, Hiru News reported at 6.13 a.m.

Falls above 100mm are possible at some places in the Western, Sabaragamuwa, Central, North Western, Northern and Southern provinces and in the Anuradhapura district.

The department asked the public, including fishing and naval communities, to follow the forecasts and advisories it issues on the system, and to take steps to reduce the danger from the temporary strong winds and lightning that can accompany thundershowers.

The system has been building since Tuesday

This is the second advisory on the same disturbance, and comparing the two shows how it has developed.

On Tuesday morning the department said rainfall would increase across the island from that evening because of a low-level atmospheric disturbance near Sri Lanka, the Daily Mirror reported at 11.09 a.m. It forecast showers or thundershowers in many areas after 1.00 p.m., morning showers in the Northern and North Western provinces, and early-morning fog in parts of the Western, Sabaragamuwa, Central and Uva provinces.

Two things changed by Wednesday morning.

The system may now intensify. Tuesday’s advisory described a disturbance. Wednesday’s gives it a 24-hour window in which it could become a low-pressure area — a named stage that the department tracks and issues separate advisories for.

The heavy-rain map has shifted west and south. Tuesday’s list of provinces at risk of more than 100mm was Northern, North Central, Central, Eastern, North Western and Uva. Wednesday’s is Western, Sabaragamuwa, Central, North Western, Northern and Southern, plus the Anuradhapura district specifically.

Only the Central, North Western and Northern provinces appear on both lists. The Eastern and Uva provinces have dropped off; the Western, Sabaragamuwa and Southern provinces have been added. North Central province appears on Tuesday’s list as a whole and is narrowed on Wednesday’s to Anuradhapura district alone.

A reader who saw only Tuesday’s forecast would therefore have the wrong half of the island. The south-west is now the focus.

Context

The warning follows landslide warnings issued for five districts by the National Building Research Organisation on Tuesday. Heavy rain on already-saturated ground is the condition those warnings are issued against.

Not reported

Neither filing gives the time the statement was issued, says where the disturbance is centred, or forecasts what the system would do if it does deepen into a low-pressure area. Neither gives expected wind speeds at sea or says whether a separate advisory has been issued for fishing vessels. The department’s own site, meteo.gov.lk, has not been reachable from this desk, so the original English release could not be read directly.

Sources: Disturbance in the atmosphere near Sri Lanka: rains will increase in the next few days from today — Hiru News, October 21 (Sinhala), Heavy showers of up to 100mm expected in several provinces — Daily Mirror, October 20.