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.
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:
^Actual activity sourced from National Hurricane Center Tropical Cyclone Reports.
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.
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.
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.
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.
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.
A series of non-tropical low pressure systems tracked across the Gulf of Mexico, across Florida and northeastwards past Bermuda bringing swings between warm and humid weather and cool and blustery weather separated by periods of rain and thunderstorms. The last of these lows passed Monday morning and has since merged into a massive low over the Labrador Sea.
This large scale low has been driving strong to gale force west-northwesterly winds across the Western Atlantic since Monday. A series of troughs embedded in that flow have each brought reinforcing cool weather, widespread showers, and renewed winds to the island. This has led to some of the coldest weather for Bermuda so far this season with a low temperature last night in the mid-50s. A further trough embedded in this persistent flow is expected to bring a final reinforcement of cool weather to Bermuda on Thursday afternoon/evening and a Gale Warning is in effect for winds possibly reaching gale force over exposed and elevated areas as that system passes.
High temperatures are typically near 69ºF while lows are typically near 61ºF this time of year. Yesterday and today are some of a handful of below normal temperatures so far this winter.
A transient ridge of high pressure calms winds on Friday. Winds then reverse, coming out of the east Friday night and increasing as the ridge moves away to the north and northeast. A powerful coastal storm developing in the Mid-Atlantic region of the United States then draws a warm front northwards across Bermuda on Saturday morning allowing wet but warmer weather on Saturday before its respective cold front passes Saturday night with renewed cool weather. Strong easterly winds veer to the south on Saturday behind the warm front. Southerly winds slowly veer to the southwest through Saturday and increase to near gale force. The cold front passes Saturday night and southwesterly winds remain near gale force and slowly veer to the west on Sunday.
A deep low passed to the near north of Bermuda on Friday brining gale force southwesterly winds to the island for several hours before veering to the west and northwest and diminishing into the evening. This followed a day of steady, moderate rains that accumulated around an inch islandwide. The low responsible had a minimum central pressures below 990 mb and maximum sustained winds around 55 kts.
Peak gusts officially reached 51 kts (59 mph) at the airport. Other unofficial gusts were measured at 75 kts (86 mph) at Commissioner’s Point WindGuru Spot, 59 kts (68 mph) at MARops via Skylink-Pro, and 37 kts (43 mph) at my PWS. These winds resulted in some isolated power outages and transportation disruption.
Following the Shapiro-Keyser Cyclone evolution model, the low fits the mature “warm seclusion” stage. This is fairly common in ocean cyclones but is more typical further north in the Atlantic.
However, because the low formed so far south and was headed east it is remaining over fairly warm sea surface temperatures. Its thermal structure, plus a detachment from fronts and continued convection around the center of the low could allow a transition to it being sustained by more tropical processes. This would allow the low to become a subtropical or tropical cyclone. Fortunately, it is tracking into the depths of the Central Atlantic and isn’t expected to pose further threat to land for at least five days. The National Hurricane Center is monitoring this low for a 40% chance that it transitions to a subtropical or tropical cyclone in the next 48 hours as it continues to produce storm force winds over the open Atlantic.