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.
December 2015 ended as the warmest on record at the airport where records have been kept since 1949. Further, since 2000, December 2015 also featured the warmest average sea surface temperatures measured by the Bermuda Weather Service. Defining the mean monthly temperature as the mean of all observations (as has been done since 1996), December had a mean monthly temperature of 70.0ºF (21.1ºC). However, if we define the mean as half the sum of the daily high temperature and the daily low temperature (as has been done since 1949), December had a mean monthly temperature of 69.8ºF (21.0ºC), tying the record set in 1998. Sea surface temperatures were the highest on record for the month, averaging 71.4ºF (21.9ºC) for the month, breaking the previous record of 69.4ºF (20.8ºC) set in 2012 for the period 2000-2015. The below tables and figures summarize this information.
1 Highest Daily Hi Temperature
2 Highest Daily Lo Temperatures
70.3ºF (21.3ºC) and 70.5ºF (21.4ºC)
18th and 22nd
2 Highest Daily Mean Temperatures
72.6ºF (22.6ºC) and 72.1ºF (22.1ºC)
18th and 23rd
Highest Monthly Mean Temperature
70.0ºF (21.1ºC) or 69.8ºF (21.0ºC)
Previous record was 69.8ºF (21.0ºC) in 1998
Highest Monthly Lo Temperature
Previous record was 65.7ºF (18.7ºC) in 1956
Highest Monthly Mean Sea Surface Temperature
Previous record was 69.4ºF(20.8ºC) in 2012
The record warmth wasn’t confined to Bermuda’s corner of the Atlantic; much of eastern North America experienced their warmest December on record. Further, much of Europe and parts of Russia were also near or at their record warmest Decembers.
An notably wet start to December has continued today thanks to an area of low pressure south-southwest of Bermuda. As of December 9th, the Bermuda Weather Service has measured 3.28″ of rain for the month-to-date. In the 1949-1999 climatology, December sees 4.50″ of rain on average.However, the wettest December had 11.73″ in 1960 – a long way to go before breaking a record there.
A shield of steady rain spread across the island overnight as the low approached and lifted a lingering stationary front towards the island as a warm front.
The surface low is set in an environment of favorable upper divergence between the left exit region and right entrance region of jet streaks in a fairly active subtropical jet. This positioning suggests that some further deepening of the low is possible as it approaches, then passes to the south of Bermuda early tonight. As it passes, expect easterly winds to back sharply, becoming northerly and increasing from moderate to strong – possibly briefly to near gale force. Steady rain is expected to continue until late tonight when the low clears away to the east. Heavier embedded showers are also possible, and rain totals for the day could exceed an inch.
[Jet streaks are locally stronger winds within the jet stream. The jet stream is a narrow region of strong winds typically >7 km above ground, near the top of the troposphere. The jet stream often separates cold air poleward and warm air equatorward.]
A strong cold front crossed the island with extensive and slow moving shower activity on Monday. The very active front brought between 0.50″ and 0.75″ of rain to the island on Monday – much needed rains as November has been relatively dry so far. Behind the front, today strong north-northeasterly winds are ushering in cooler and less humid weather, with some lingering post-frontal isolated passing showers.
Strong north-northeasterly flow is expected to continue through Wednesday. This flow then veers overnight Wednesday into Thursday, becoming more northeasterly. This is in response to either a wave or an area of low pressure forming along Monday’s cold front which, by Wednesday night, has stalled to the south through east of Bermuda. This wave then pushes westward towards Bermuda, crossing the island on Thursday night. There is potential for heavy rain, thunder, and gales to develop in the northeasterly flow ahead of the wave or area of low pressure. Such an area of adverse weather could impact the island on Thursday.
The official forecast (from the Bermuda Weather Service) for the second half of this week should therefore be monitored closely for updates if or as this system develops.
Kate strengthened into a Hurricane and passed about 250miles northwest of Bermuda in the predawn hours of Wednesday morning. A broken band of showers slowly moved across the island during the day along with strong southwesterly winds with gusts to gale force, particularly in higher elevations with exposure to winds from that direction. By sunset, a weak cold front associated with the non-tropical low that steered Kate northeastward crossed the island with another brief period of shower activity. Kate has since transitioned into a post-tropical cyclone and is expected to bring strong and gusty winds and heavy rain to the UK in the next three to four days as a non-tropical cyclone.
Northwesterly winds have now settled in and are strong at times. Less humid and slightly cooler weather has developed in the flow behind the cold front. A brief warm-up on Friday will be followed by another cold front by Friday evening with much cooler and much less humid air. Expect a narrow band of brief showers to blow through Friday evening with winds shifting from southwest to west. Well below normal temperatures settle in for the weekend, again with strong westerly winds veering northwesterly – some post-frontal shower activity could develop as much cooler continental air moves over the still warm waters of the western Atlantic.
Afternoon high temperatures will likely struggle to reach 70F on Sunday following Friday’s front. Northwesterly flow begins to veer to the north then northeast and diminish from strong to moderate by Monday keeping cooler than normal weather in place. High pressure slides in from the west on Monday so, while remaining cool, more settled weather develops.
Tropical Storm Kate formed early yesterday morning. The storm tracked north of the Bahamas Islands, clearing those islands by late Monday night, turning northward. Kate continued to recurve and is accelerating northeastward and strengthening. The National Hurricane Center’s forecast as of 11am expects Kate to pass north of Bermuda Wednesday morning as a minimal hurricane. This is a relatively high confidence forecast, taking Kate more than 150nm north of Bermuda. Because Kate is a small storm, small differences in the actual track will therefore have little bearing on the impacts in Bermuda.
Kate has strengthened and developed a more robust circulation than originally expected. This will delay it being absorbed by a non-tropical low until after Kate passes Bermuda.
Expect southerly winds to increase from moderate to strong, then veer to the southwest overnight. Shower or possibly thunderstorm activity associated with rain bands far removed from the center of Kate could bring gusts to tropical storm force (>34kts or >39mph) on Wednesday morning. Winds could briefly peak at sustained tropical storm strength in the northern marine area and in elevated areas of the island with exposure to southwesterly winds, particularly around any squallier showers or thunderstorms.
Following Kate’s passage, a cold front rolls through Wednesday night and winds veer to the west, ushering in cooler and less humid weather. Kate serves as a reminder that Hurricane Season runs from June 1st to November 30th for a reason.