Whenever the Mayor finds that falling snow, sleet, or freezing rain will create a condition which makes it necessary that the parking of motor vehicles on snow emergency routes be prohibited, or whenever he or she finds on the basis of a firm forecast of snow, sleet, or freezing rain that the weather conditions so forecasted may create a condition making it necessary that such parking be prohibited, he or she is authorized to announce such prohibition, to become effective at a time specified by him or her. After the effective time of such prohibition no person shall park any vehicle or permit any vehicle to remain parked on a snow emergency route. However, if a fall of snow, sleet, or freezing rain occurs after 11:00 p.m. and prior to 6:00 a.m., and the Mayor has not announced prior to 11:00 p.m. that parking on snow emergency routes is to be prohibited after a specified time, a vehicle parked on a snow emergency route may remain so parked until 7:00 a.m. following such fall. The prohibition of parking announced by the Mayor under the authority of this section shall remain in effect until he or she announces the termination of the snow emergency, in part or in whole, after which the prohibition of parking authorized by this section shall no longer be in effect.