October, typically the wettest month of the year for Bermuda, is ending with only 1.08″ (27.4 mm) of rain. This is 5.17″ (131.3 mm) below the 1981-2010 average for the month and is one of the driest Octobers on record. Below is a ranked table of October monthly rainfall data from NCDC* back to 1949.
|1||2019||1.08″ (27.4 mm)|
|2||1954||1.35″ (34.29 mm)|
|3||1997||1.64″ (41.7 mm)|
|4||1949||1.75″ (44.5 mm)|
|5||1975||2.28″ (57.9 mm)|
So what’s going on?
This year, much of the deep tropical moisture remained confined to the Caribbean Sea and tropical SW Atlantic. Passing mid-latitude systems were only briefly able to bring transient plumes of this moisture across the island before either dissipating or moving on. Behind these systems, vast expanses of drier and more stable air is left in their wake. This can be seen in loop of Total Precipitable Water below.
Typically in October slow-moving fronts can interact with tropical systems to deliver longer-duration and heavier precipitation events. This year, those types of systems remained deep in the tropics only interacting with the mid-latitudes until they were west of Bermuda.
A commonality of the 500-hPa anomalies associated with these top driest months is anomalous ridging over the northeastern United States or southeastern Canada. This anomalous ridging drives anomalous northerly flow into the western Atlantic (i.e. across Bermuda). This anomalous northerly flow implies anomalous advection of cooler, drier air across the region. 2019’s pattern seems to be a variant of this general pattern with ridging extending northeastward across eastern Canada and Greenland. See below:
The human impact…
We can measure the impacts of this dry spell by estimating the direct impacts of reduced rainwater catchment and therefore increased demand for trucked and piped water. Looking at the rainwater supply vs. typical household demand, (e.g. our Tank Water model) we see that tank levels steadily declined through October. The rainwater supply exceeded typical daily demand on only a few days this month.
Below we show the model estimated tank volume for 2019 compared to the climatology of tank volume for our period of record 1949-2018. For January through the end of March, plentiful rain brought tank levels much higher than normal – exceeding the 90th percentile for this dataset. However, an unusually dry April followed by May which is usually the driest month of the year led to a dramatic decline to below normal capacity for the start of summer.
It is worth noting that the typical modern Bermuda household runs an annual deficit in water supply. In the above model, each time trucked water is required, 2 truck-loads are supplied. On average, the typical home requires 12 loads of trucked water to supplement rainwater harvesting based on this model. In the model, the majority of supplemental water demand comes in Bermuda’s driest month May. October and November typically have low demand following the wetter early autumn months (see below).
Fortunately, this dry spell is set to come to an end. Periods of rain and heavy, possibly thundery showers are expected ahead of and along a slow-moving frontal system. This should deliver some much needed tank rain. See the Bermuda Weather Service for the latest official forecast for Bermuda.
* Note that this dataset is unofficial and might not match official BWS figures, particularly early in the dataset.