We read the hourly rain forecast for your location and look only at the hours that are still ahead of you today. Rain that already fell this morning cannot change what you need this afternoon, so it is ignored.
Then we collapse those hours into one answer. Meaningful rain that is reasonably likely, or two rainy hours in a row, is a yes. An isolated or weak shower signal is a maybe. Everything else is a no.
+Do I need an umbrella today?
You do if meaningful rain is reasonably likely during the hours you still have left today. This page checks the hourly forecast for your location and answers yes, no or maybe.
+Why does it ignore rain earlier in the day?
Because you cannot get wet in the past. Daily rain probability counts the whole day, which is misleading at 2pm if the shower happened at 7am.
+What does maybe mean?
That the forecast shows a real but weak or isolated shower risk — roughly a one-in-three chance. A small umbrella is worth it if you mind getting damp.
+Is 40% chance of rain enough for an umbrella?
Usually only if it persists. A single 40% hour is a maybe here, but two 40% hours in a row means rain is likely to arrive somewhere in that stretch, so it becomes a yes.