Isolated Downpours Last Night

A stationary front, lingering in the area since the 22nd has cleared to the east and southeast this morning. Winds shifted from the southwest to the northeast through the day yesterday with the main front pushing across the island with little fan-fare. In the evening, a line of showers developed along the island extending northeast and southwest from the island. A region of convergence between northeasterly and east-northeasterly flow helped spark these showers with isolated downpours that persisted for over an hour in spots.

20160525-Rain
Distribution of rainfall from yesterday based on observations from the Bermuda Weather Service, Wunderground, and WeatherLink. This probably underestimates rain totals in Sandy’s parish as radar indicated heavier showers forming there first and being more persistent in that parish. You can really see the isolated nature of the downpours with <0.25″ across much of the eastern parishes, and over an inch in the western parishes.

Meanwhile, an area of low pressure is developing at the tail end of that lingering stationary front north of the Turks and Caicos Islands. The National Hurricane Center is monitoring this area for possible tropical cyclone formation. Their latest updates at 10am indicate a 70% chance for the formation of a tropical or subtropical cyclone in the next 5 days. However, direct impacts to Bermuda from this system appear unlikely at this stage.

Follow the Bermuda Weather Service and the National Hurricane Center for the latest official information.

Heavy Rains Summary

A well anticipated multi-day heavy rain event last weekend ended with a May’s worth of rain nearly everywhere. A band of deep, tropical moisture with total precipitable water amounts near 2 inches aligned over the island from Thursday night through Saturday afternoon. A cold front, turned stationary front remained draped nearby with associated small scale areas of low pressure developing along the front leading to isolated and brief periods of enhanced winds. The front and lows led to persistent shower and thunderstorm activity through much of the weekend resulting in some event disruption and nuisance flooding.

Officially, 3.82″ of rain was measured at the Bermuda Weather Service as a result of this rain storm. This agrees well with island-wide reports generally near 4 inches, including my PWS which measured 3.95″. In an average May, the island sees roughly 3 inches of rain.

20160505-20150507-Rain
Distribution of rain across Bermuda showing a widespread heavy rain event. An islandwide average of 3.86″ fell. Each circle represents one observation. The shading is likely less reliable in areas of few observations. Observations from Bermuda Weather Service Facebook, Wunderground, and Davis WeatherLink.

Heavy Rain Event Looms

A blocked upper level flow pattern is currently developing. This will allow an upper trough to become stationary over the Eastern United States. Meanwhile, an associated cold front slides across the Western Atlantic today, becoming stationary near the island Thursday night. Convergent surface flow in an environment of deep layered moisture ahead of and along the stalling front will help to focus and maintain a region of shower and possibly thunderstorm activity near Bermuda.

High pressure remains in control Wednesday as it begins a retreat to the east. Pressure gradient between the high pressure to the east and advancing low pressure to the west lead to strengthening southerly winds on Thursday. Thursday night sees an initial band of showers ahead of the cold front. The cold front becomes roughly stationary nearby with waves of shower activity persisting through the day on Friday and into Saturday morning. The front then resumes its progression to the east, clearing shower activity as it exits the area.

Model guidance supports rainfall totals for the ~48 hour period Thursday night through Saturday morning of 2-4″. Depending on where the most persistent organized shower activity sets up, over 6″ of rain is possible in this period. Any persistent downpours contributing to these totals could lead to flooding, particularly around high tide and in low-lying areas.

Pay attention to the Bermuda Weather Service for the official forecast for Bermuda.

Analog Forecast: Hurricane Season 2016

The Atlantic has already produced it’s first hurricane-strength tropical cyclone for 2016 in January. However, the bulk of tropical cyclone activity comes during hurricane season which is defined by the National Hurricane Center as the period between 1 June and 30 November each year. For the past two years, I have provided an analog forecast for the upcoming hurricane season based on January and February weather in Bermuda.

This year, I hope to take a closer look and try to better quantify the correlation between Jan-Feb temperature/precipitation in Bermuda with the following hurricane season’s activity.

First, let’s take a look back at the last two year’s forecasts:

Year Named Storms (Actual) Hurricanes (Actual) Major Hurricanes (Actual) ACE (Actual)
2014 9-10 (8) 5-6 (6) 2 (2) 74.4-78.6 (66.7)
2015 10-12 (11) 6 (4) 2 (2) 84.7-104.0 (65.3)

^Actual activity sourced from National Hurricane Center Tropical Cyclone Reports.

hurricanestats

Comparing the mean Jan-Feb temperatures from 1949-2015 to the number of named storms (NS), a measure of total seasonal activity, there is only a weak correlation. Similarly, there is no significant relationship between Jan-Feb total rain and NS. Using a linear model to combine the both temperature and rainfall to predict NS, there is only a weak predictive relationship that fails to describe the majority of the season-to-season variability in NS.

It appears the 2014 and 2015 forecasts were so close to reality because they happened to end with near normal activity. Neither, temperature, precipitation nor the combined model were able to produce a forecast far from the average NS and described less than 5% of the variability in NS. Further, the weak relationship between temperatures and NS is probably only a reflection of the impact of sea surface temperatures on NS. It seems that warmer and wetter winters in Bermuda tend to precede less active seasons while cooler and drier winters precede more active seasons. However, other phenomena are likely controlling the larger variability superimposed on this tendency.

So as expected, this “point analog” forecasting method really doesn’t have any skill. Like the adage goes, a broken clock is right twice a day. For what it’s worth, using this method would suggest 2016 ends with 9 named storms, 5 hurricanes, and 1 major hurricane for an ACE of 72. This is a less active than normal season, where a normal season in this dataset has 11.5 named storms, 6.2 hurricanes, and 2.6 major hurricanes with an ACE of 100.6.

Heavy Rain and Strong Storms

March 29th saw a slow moving cold front push through Bermuda from northwest to southeast. Lines of thunderstorms associated with the front tracked across the island dropping very heavy rain in a short period leading to some morning flooding issues. Lightning strikes associated with those thunderstorms resulted in isolated electricity, and communications interruptions on Tuesday. Further still, one of the thunderstorms produced a waterspout off the west end of the island that was caught on film – a fairly rare occurrence with only 5 days per year seeing either a funnel cloud or waterspout/tornado from the airport for the period 2000-2015, mainly in August or September.

ConvectivePhenomena2000-2015
Using METAR/SPECI observations and monthly climate reports from the Bermuda Weather Service this summarizes the average number of each convective phenomena by month. Annually, one day per year sees hail, 39 days see thunder, and 5 days see a funnel cloud or waterspout/tornado. Data for the period 2000-2015 inclusive.

This heavy rain event followed a string of daily record high temperatures with temperatures in the mid-70s. Highs of 75.9ºF, 76.5ºF, and 76.5ºF broke records on the 26th (previously 75.8ºF set in 1999), 27th (previously 75.2ºF set in 1999) and 29th (previously 75.0ºF set in 1989) respectively. This warmth came with unseasonably high humidity as dew point temperatures held steady near 70ºF or oppressive levels through much of that period. This warmth and moisture likely played a role in fueling the thunderstorms observed on the 29th.

Experimenting with R-code and unofficial reports from Wunderground, I’ve put together this graphical depiction of the distribution of rainfall across the island. Apparently, the heaviest rains fell in the west end where over four inches of rain was reported in spots while parts of the east end saw less than an inch. Unfortunately, there are no rain reports on Wunderground from Hamilton or Sandy’s Parishes. Gaps in report coverage limits how accurately this map represents how much rain actually fell, so I’ve indicated where each report was made.

This heavy rain event follows another event with islandwide totals of over an inch of rain just two days earlier. The combination of these events has made March 2016 unusually wet, and has officially (at the airport) brought 2016’s year to date values roughly a month’s worth of rain above average.

20160329-Rain
Distribution of rain across Bermuda shows higher storm totals to the southwest (unofficially more than four inches) and lower totals to the northeast where 0.92″ were officially measured at the Bermuda Weather Service (included in this plot.) The shaded areas are likely less reliable where there are fewer reports.

Mid-Week’s Heavy Rains

The 17th of February saw a very wet and active frontal system move across the island. With steady rain and embedded heavier showers and isolated thunderstorms. The widespread rains generally totaled more than two inches across the island, with the central parishes seeing the greatest totals. The 2.11″ of rain that fell at the Bermuda Weather Service was a daily record total for the 17th, beating the previous record of 1.76″ set in 1987. The wettest February day at the airport saw 3.80″ of rain on 21 February 1981.

This rain came on the heels of some of the coldest air of the season. The frontal system responsible for all this rain also ushered in a cold air mass in it’s wake with strong winds and below normal temperatures with lows around 55ºF on the 19th.

A quieter pattern is in store for the next five days with generally light to moderate winds and temperatures recovering to the low 70s, follow the Bermuda Weather Service for the latest official weather.

Below are Wunderground personal weather station reports for the meteorological day on the 17th. These are unofficial except where noted.

Location Rain Total
Devonshire, Devonshire 3.11″
South Road, Devonshire 2.77″
Chaingate Hill, Devonshire 2.67″
Wilderness, Smith’s 2.50″
McGall’s Bay, Smith’s 2.37″
Knapton Hill, Smith’s 2.36″
Tucker’s Town, St. George’s 2.20″
Bermuda Weather Service, St. George’s (official) 2.11″
St. David’s, St. George’s 1.78″
Cardinal, Southampton 1.77″
Hinson’s Island, Warwick 1.37″

Heavy Rains Tomorrow

EaUS 16 Feb 2016 2045UTC
GOES East RGB Satellite imagery showing cloud band associated with the approaching frontal system between the US East Coast and Bermuda.

A cold front and associated pre-frontal trough to the west of Bermuda is expected to reach the island early tomorrow with heavy rain, showers, and possible thunder.

Strong southerly flow ahead of the frontal system has allowed temperatures to climb into the low-70s and dew points to recover into the mid-60s after a cold and blustery weekend. Further warmth and moisture should continue to stream into place tonight. Showers with heavy rainfall rates are becoming increasingly likely, beginning early tomorrow morning. Over an inch of storm total rainfall is possible, mostly falling tomorrow. This could lead to some isolated flooding in low-lying areas, especially if this heavy rain occurs around the time of high tide.

Follow the Bermuda Weather Service for the latest official forecasts.