The rain and cloud are clearing away from what has become a very wet week for Bermuda. On four days from March 17th through 20th inclusive, a whopping 4.92 ” (125 mm) of rain fell. A few more drops on the 21st brings March 2019’s rainfall total to 7.19 ” (183 mm). On average, Bermuda sees around 4.50 ” (113 mm) of rain in March. Furthermore, three consecutive sunless days were recorded making for a gloomy start to the week.
So what was going on?
A stationary front set-up near the island. This marked the boundary between warm, humid, tropical air to the east and south of the island; and cool dry air to the west and north of the island. This helps to focus where upper-level disturbances can trigger a surface response and therefore tangible weather. As waves of low pressure formed in response to fast-moving upper-level disturbances they tracked along the front bringing alternating wetter and drier periods.
In this case, Bermuda remained on the cold side of the front with strong easterly or northeasterly flow combining with the overall gloomy weather for a chilly feel to the air. However, only 500 m above the surface winds were out of the southwest. Moisture transported from warm side of the front was lifted up and over the shallow layer of cool air and easterly flow where some of it fell as rain.
Furthermore, it turns out that there was plenty of lead-time to this dreary forecast. A very wet start to the week was in discussion at the Bermuda weather service as much as five days in advance, and made it into the forecast synopsis prior to the weekend. To put some numbers to that, here’s how the rain total for this wet 4-day period from one weather model (GFS) changed with decreasing lead time compared to observations (right). Despite predicting rainfall totals that are roughly half of what was observed in this period, this model consistently indicated the potential for heavy rain for more than four days in advance (i.e. model data starting on or before the 13th March was already suggesting that the 17th -21st March would be very rainy). Numerical model data like this, combined with conceptual models and experience of forecasters were able to predict this event well in advance.
This winter and early spring has been no stranger to heavy rain events with much of the year-to-date rain falling in a handful of events. So far, there have been 8 days with more than 1.00 ” (25.4 mm) of rain. These 8 days account for about 63 % of the year-to-date rainfall, 18.92 ” (480 mm). This is 5.75 ” (146 mm) above average.
|9th Jan||1.06 ” (27 mm)|
|29th Jan||2.56 ” (65 mm)|
|14th Feb||1.26 ” (32 mm)|
|15th Feb||1.66 ” (42 mm)|
|6th Mar||1.02 ” (26 mm)|
|17th Mar||1.06 ” (27 mm)|
|18th Mar||1.45 ” (37 mm)|
|20th Mar||1.77 ” (45 mm)|
|Total (> 1″)||11.84 ” (301 mm)|
|Current year-to-date||18.92 ” (481 mm)|
|Normal year-to-date||13.17 ” (335 mm)|