Yes, it is permissible to pay zakat ahead of its due date once you already own the niṣāb, and this is valid according to the majority of scholars, including the Ḥanafī, Shāfiʿī, and Ḥanbalī schools. The evidence is that the Prophet ﷺ accepted his uncle al-ʿAbbās's zakat two years in advance (narrated by Abū Dāwūd and al-Tirmidhī). This is a concession rather than the norm — the sunnah is to pay when the ḥawl completes — but paying early is allowed when there is benefit, such as an urgent famine or to be certain of discharging the duty. At year's end you should still verify that what you prepaid covered your actual zakatable wealth and pay any shortfall.
Evidence — the daleel
Sunan Abi Dawud (hadith of al-ʿAbbas paying zakat in advance)Sunan al-Tirmidhi (hadith of al-ʿAbbas paying zakat in advance)
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.