Hazardous Conditions: Winter (Nov - Mar)

• Hazardous weather and high seas associated with frontal passages are the primary winter season problem. Frontal passages are frequent as indicated by the average 12 days of rain per month. The predominant air mass is maritime polar. The intensity and speed of movement of frontal systems varies in response to the eastern Atlantic ridge configuration and polar front jet stream strength.

• The most prolonged and severe weather generally occurs when a cut-off low develops to the southwest of Rota. This is typically associated with an Omega type blocking pattern.

• The average winter wind at the airfield is east at 7 kt. However, severe weather will be associated with westerly winds. Over the water sustained westerly winds of 40 to 50 kts will be experienced during passage of strong frontal systems during all months of winter. The westerly winds associated with frontal passages will push vessels off Pier 1; tug assistance may be required to hold positions. Heavy swell will enter the area when west to southwest winds prevail.

• Early morning thunderstorm activity over water will occur during the time between passage of the surface front and that of the 500 mb trough. With slow moving systems thunderstorm activity may occur for 2 to 3 days running. On average, thunderstorms occur 2 days per month.

• Fog occurrence averages 7 days per month.

• January is the coldest month of the year, with a mean daily maximum temperature of 61degF and a mean daily minimum of 43degF. The extreme low temperature of 25degF, however, was recorded in December 1970. Periods of near or below freezing temperatures are rare, and are normally associated with outbreaks of polar/arctic air which reaches the Rota area from over Europe. Favorable conditions for extreme minimum temperatures include:
  • north-northeast wind at 500 mb
  • 500 mb height below 5647m
  • clear skies and light northeast surface winds at night

• While not a concern on most days at Rota, wind chill--the cooling effect of temperature combined with wind--is a factor to be considered on cool, windy days during winter for personnel working in exposed locations.