Sumatra squalls are lines of thunderstorms that develop over Sumatra and the Malacca Straits between March and November each year during the southwest monsoon season. They typically move east towards Singapore and peninsular Malaysia in the early morning hours, bringing sudden strong gusty winds and heavy rain lasting 1-2 hours. Maximum wind gusts of 93 km/h have been recorded with these squalls. They form at night due to conditions like the diurnal wind variation, katabatic winds from nearby highlands, adiabatic cooling, high humidity and convection over the Straits of Malacca.

1. SUMATRA SQUALLS
SCOPE:• Overview
• Description
• Appearance
• Formation
• Locale
• Movement
• HAZARDS

2. SUMATRAS

3. OVERVIEW
“Sumatras” are lines of thunderstorms which usually occur
between March and November each year. (SW monsoon)
These squalls (lines of thunderstorms) develop at night
over Sumatra or the Malacca Straits and move east
towards Singapore and Peninsular Malaysia usually
during the pre-dawn and early morning.
They are often characterised by sudden onset of strong
gusty surface winds and heavy rain lasting from 1 to 2
hours as they move southwards along the west coast of
the Peninsular until it reaches Singapore in the early
morning.
Maximum gusts of up to 26 metres per second (93 km/h)
have been recorded during the passage of a Sumatra
squall.

4. CONDITIONS REQUIRED -TS

5. LIFTING AGENT/TRIGGER
HOW MANY HERE?
HOW ABOUT NIGHT?

6. SOUTH WEST MONSOON
Between April – September
(Inter monsoon – mid March/mid October)
Nights in the Straits of Malacca.
Conditions are ripe for thunderstorms. How?

7. MULTI CELLS
Although a thunderstorm can consist of just one ordinary cell that
transitions through its life cycle and dissipates without additional new
cell formation, thunderstorms often form in clusters with numerous cells
in various stages of development merging together. Unlike ordinary
single cells, cluster storms can last for several hours producing large
hail, damaging winds, flash flooding, and isolated tornadoes.

8. SQUALL LINES
Sometimes thunderstorms will form in a line which can extend
laterally for hundreds of miles. These "squall lines" can persist for
many hours and produce damaging winds and hail. The rain
cooled air or "gust front" spreading out from underneath the squall
line acts as a mini cold front, continually lifting warm moist air to
fuel the storms.

9. LOCAL PHENOMENAS

10. ...NITE IN THE STRAITS
• DIURNAL VARIATION OF SURFACE
WIND
• KATABATIC WIND FROM SUMATRAN
& MALAYSIAN HIGHLANDS
• ADIABATIC COOLING
• CONVECTION & CONVERGENCE
• HI – RELATIVE HUMIDITY

11. Rapid formation of Squall lines

12. …SO WHEN NITE FALLS

13. HAZARDS
•
•
•
•
(Thunderstorms) HAIL
DAMAGING WIND
Wind shear at low levels.
Heavy precipitation that can last for a few hours.
Degrades aircraft performance (& nav systems)
• Lightning