The Met Office forecast a showery Monday but then widespread sunshine from Tuesday as highs climb to 17C in midweek and 20C next weekend in Scotland. Average September highs are just 16C.
21C is possible in Scotland, said The Weather Outlook, predicting “Indian Summer-style” conditions.
The Met Office forecast mainly settled conditions with “warm sunshine” for up to two weeks, until the end of September.
Met Office forecaster Helen Roberts said: “A cold front is passing to bring cooler conditions to Scotland – but highs climb through the week, with 17C in midweek and touching 20C possible at end of the week.
“There will be widespread sunny spells from Tuesday, with high pressure and a lot of tropical air being drawn up.
“It's a definite change after an unsettled early September.
“And the week from September 23 also looks like remaining fairly settled with high pressure, although the North-West may see some more unsettled weather at times.”
Met Office forecaster Sarah Kent said: “Spells of warm sunshine are likely to continue. It is likely settled conditions will prevail until the end of September.”
The Weather Outlook forecaster Brian Gaze said: “21C is possible later in the coming week in Scotland as a thrust of subtropical air brings Indian Summer-style conditions, with warm sunshine.” Ex-BBC and Met Office forecaster John Hammond of weathertrending said: “Subtropical southerlies bring the prospect of fine, warm weather.
“The mid-20s may be seen, more typical of midsummer, before a downward trend in temperatures towards September 29.”