2017 Hurricane Season Begins

peakofseason
The National Hurricane Center’s climatological progression of tropical cyclone activity as measured by Number of Storms per 100 years. The peak of hurricane season can be seen as from mid-August through mid-October. For more official climatology, see here.

The Atlantic Hurricane Season is each year from the 1st June through the 30th November, with tropical cyclone activity peaking in the first half of September. Over the past three seasons, Bermuda has seen disruptive impacts from four October hurricanes; Fay (2014), Gonzalo (2014), Joaquin (2015), and Nicole (2016).

Early indications suggest that this hurricane season will be more active than normal. On 25th May, the National Hurricane Center released their forecast for the upcoming hurricane season. They expect a near normal or above normal season with 11-17 named storms, with 5-9 storms becoming hurricanes, and 2-4 hurricanes becoming major hurricanes. An average season has 12 named storms, of which 6 become hurricanes and 3 of those hurricanes become major hurricanes.

Factors like the phase of the El Niño-Southern Oscillation (ENSO) and Atlantic sea surface temperatures (and many others) in their current and forecast states are used to make educated assessments of whether a season will be more or less active than normal. This year, returning weak El Niño phase may act to limit hurricane activity while warmer than normal Atlantic sea surface temperatures may act to enhance activity: this conflict is one of the reasons that this season’s forecast comes with quite a bit of uncertainty. Should the effects of El Niño not materialize, or should Atlantic sea surface temperatures cool through the season, the final assessment of overall activity could be dramatically different.

On ENSO

Screen Shot 2017-05-31 at 1.34.19 PM
Sea Surface Temperature Anomalies for Aug-Sep-Oct of the 1997 El Niño event. Warmer than normal sea surface temperatures across the central and eastern Pacific are key characteristics of El Niño.
Screen Shot 2017-05-31 at 1.34.58 PM.png
500 hPa Relative Humidity Anomaly for Aug-Sep-Oct of the 1997 El Niño event. Higher than normal mid-tropospheric relative humidity over the central-eastern Pacific is indicative of enhanced deep convection there. Suppressed convection, indicated by abnormally low relative humidity, is seen over South America and the Maritime Continent in the western Pacific.

The enhanced deep convection over the central and eastern Pacific results in vertical wind shear across the Western Atlantic. Wind shear acts to weaken tropical cyclones by keeping their convection disorganized and allowing dry air into their circulation, suppressing further convection.

[Watch wind shear push deep convection (greens) to the south while the low level swirl (grays) of 2006’s tropical storm Chris continues to the west]

The Atlantic hurricane season peaks in September. We see that for inactive hurricane seasons, there is an El Niño signal across the eastern and central Pacific. But for active hurricane seasons, that signal is more reminiscent of La Niña with cooler than normal sea surface temperatures in the central and eastern Pacific. However, for active seasons, there is the additional ingredient of warmer than normal sea surface temperatures across the tropical Atlantic – deep convection (and therefore supporting tropical cyclone development) there.

[Compare SSTs for Active and Inactive seasons]

It is important to remember that seasonal forecasts make no skillful assessment of where the storms form or track. Just one tropical storm or hurricane can make for a bad season, and so it is important to prepare for each and every season by topping-up supplies and having an emergency plan.

It is important to exercise constant vigilance during hurricane season by regularly checking for updates by the Bermuda Weather Service and National Hurricane Center.

Advertisements

Dry Start to Spring

After a very wet January, mainly thanks to a month’s worth of rain falling in one event, Bermuda has seen a succession of progressively drier months to end winter and begin spring.

Spring itself is typically the driest time of year in Bermuda. The driest month is May with 3.01″ of rain (1971-2000 average). In spring, mid-latitude cyclones start to track further away from the island, yet the western Atlantic hasn’t quite become the warm, humid environment that supports isolated summer showers and tropical disturbances. At the same time, the Bermuda-Azores high extends westward across the Atlantic suppressing clouds and rain.

This April was characterized by a blocking ridge of high pressure over Northeast United States and Southeast Canada. This kept most mid-latitude cyclones away as they tracked northward into Canada rather than westward into the Atlantic. Their trailing fronts found dry and stable air in place leading to them fizzling out on approach.

[See the ridge in 500 hPa height anomalies]

April2017Dry
Year-to-Date rainfall for 2017, through 30 April (blue) compared to average for the year (red, dashed) with the range of values each day in the period of record (POR: 1949-2017).

The record wet year-to-date precipitation totals that January’s deluge brought have receded to below average levels as successively drier February and March led to the second driest April since 1949 with only 0.73″ of rain at the airport.

Here’s how April 2017 stacked against some other dry months:

April Any Month
1 Apr 1973 (0.33″) 1 May 1991 (0.28″)
2 *Apr 2017 (0.73″) 2= Apr ’73
& May ’82 (0.33″)
3 Apr 1954 (0.76″) 4 Jan 1950 (0.54″)
4 Apr 1964 (0.79″) 5 May 1993 (0.59″)
5 Apr 2010 (0.84″) 6 May 2011 (0.62″)
6 Apr 1957 (0.94″) 7 Jun 2005 (0.64″)
7 Apr 1955 (1.13″) 8= Jan ’74
& *Apr ’17 (0.73″)

April 2017 didn’t quite make it into the top 5 of any dry month. Prolonged periods without significant rain lead to low water levels in tanks around the island. My unofficial TankRain project seems to capture this decline in tank level quite well. Not much rain is expected through the first weekend of May – mild and dry weather persisting a little longer. See the latest official forecast and observations at the Bermuda Weather Service.

Unusual March Cold

2017-03-03-1445UTC-rgb-l.jpg
GOES-East RGB imagery showing a cold front just east of Bermuda with trailing cloudiness spreading over the island on the 3rd March at 14:45UTC. This front was responsible for bringing yesterday’s very cold weather.

On the 3rd of March a cold front pushed across the Western Atlantic and reached Bermuda in the morning. It brought an unremarkable amount of rain (0.67″ at the Bermuda Weather Service) and a sharp wind shift from west-southwest to north-northwest. This northwesterly flow continued through the weekend bringing in continental polar air originating over Northern Ontario and Quebec. This same airmass was responsible for record cold air across the Northeastern United States.

Cold air filtering over the waters of the Western Atlantic resulted in widespread ocean-effect shower activity. These showers began to reach Bermuda on Saturday night and intensified during the day on Sunday afternoon with some of the heavier showers containing small hail. Rain-cooled air in the downdraughts of these showers helped keep and nudge temperatures down through the day. In fact, a new record low was set for the 5th by early-morning and Bermuda stayed below the previous record for much of the afternoon and overnight, bottoming out at 47.7°F. This passes the previous record for the 5th of 52.0°F set in 1978. Meanwhile, near-gale force winds occasionally reaching gale force around those showers helped make it feel even colder.

temperaturegraph
Temperatures 3rd March through the 5th March meteorological days. We see the sharp drop in temperature on the 3rd associated with showers along the cold front, followed by a slow and steady decrease in temperatures on the 4th. Temperature fluctuations dominate on the 5th as widespread showers move in with rain-cooled downdraughts. Temperature data from Bermuda Weather Service.

How unusual was Sunday’s cold weather?

Yesterday’s high temperature only reached 56.1°F, this is the second coldest March high temperature on record (since 1949), only surpassed by 19th March 1967 when the high was 56°F*. This was also the coldest high temperature of any day since 55.4°F on the 15th January 2000.

The average temperature for the day (as the max + min divided by two)** was 51.9F, making it the coldest March day on record since 1949 beating 52°F in 1979 and 1951. This was the lowest average temperature of any day since 25th January 2003 with a mean temperature of 51.6°F. Incidentally, this mean temperature is also lower than the former record low for the day.

The low temperature of 47.7°F was the 7th coldest March temperature on record. It was the coldest March low since 2001 when it hit 45.6°F on the 7th of March, and the coldest day of any month since a low of 47.1°F on 27th February 2006.

Essentially, yesterday was about as cold as it gets in Bermuda in March – and it has only been a few degrees colder a handful of times on any day in the period of record. Bermuda should see a gradual return to near seasonal averages as high pressure builds in from the west and the northerly flow weakens begins to veer, becoming easterly by Wednesday morning.

Follow the Bermuda Weather Service to track the warm-up over the next few days and for the latest official forecast, observations, and warnings.

Table of ranked low temperatures as measured for the meteorological day 0600UTC to 0600UTC at the airport.

1 11-Mar-1951 45.0°F
2 7-Mar-2001 45.6°F
3 10-Mar-1951 46.0°F
12-Mar-1991 46.0°F
4 19-Mar-1979 47.0°F
5 8-Mar-1999 47.1°F
6 9-Mar-1999 47.3°F
6-Mar-2001 47.3°F
7 5-Mar-2017 47.7°F
8 21-Mar-1979 48.0°F
17-Mar-1981 48.0°F
16-Mar-1988 48.0°F
11-Mar-1991 48.0°F
25-Mar-2009 48.0°F
* Observations made prior to 1995 were made to the nearest 1°F, nearest 0.1°F after 1995.
**Daily averaged temperatures are impacted by the above uncertainty. Rounding to the nearest 1°F yields a mean of 52.0°F, tying the record. Additionally, a recent method change results in a daily averaged temperature of 52.2°F for yesterday which is the second coldest March temperature, but isn’t comparable to the majority of the period of record.

Nicole’s Direct Hit

Hurricane Nicole passed very close to Bermuda midday on Thursday 13 October as a strong category three hurricane. Preliminary center fixes indicate that Nicole passed about 9 nm (10 mi) east of Bermuda at noon on Thursday with maximum sustained winds near 105 kts (120 mph). At this time, Nicole’s eye was about 30 nm (35 mi) wide, and so Bermuda briefly entered the calm of the eye, but not before enduring hours of violent winds and torrential rain that led to isolated structural damage, significant utilities disruptions, and widespread flooding in coastal areas, low-lying areas, and poor-drainage areas.

NicoleReconTrack.png
Visible satellite image of Nicole near time of closest approach. At this time, the entire island was experiencing the relative calm of the eye. An approximate track of Hurricane Hunter flight through Nicole is overlaid in red, starting at A in the bottom right.

After peaking as an extremely dangerous category four hurricane the night before impacts on Bermuda were felt, Nicole began to weaken on its final approach on the island. Vertical wind shear had markedly increased during this time, disrupting the circulation and degrading the convective organization. The solid ring of deep convection around the eye, aka the eyewall, opened up into a semi-circle that became more poorly defined as the hurricane approached.

[See these changes on long radar loops of Nicole’s approach Here]

Traditional hurricane structure and observations from ongoing Hurricane Hunter missions suggest that the strongest winds of Hurricane Nicole missed Bermuda to the east as the island saw effects from the northern and then western parts of the eyewall, not the eastern eyewall where, in this case, the strongest winds would have been found. However, damaging hurricane force winds were observed for several hours, mainly as the northern eye wall of the hurricane crossed the island.

Peak gusts in this part of the hurricane were measured up to 118 kts (136 mph) at an unofficial station at Commissioner’s Point, while the official peak winds measured at the airport reached gusts of 91 kts (105 mph). The peak official 10-minute sustained winds reached 68 kts (78 mph) at the airport. Areas exposed to easterly winds, particularly near hill tops, likely saw the highest winds in Nicole.

Additionally, the Bermuda Weather Service was able to get an estimate of rainfall total at their office. During the hurricane, 6.77″ of rain was caught. This makes 13 October 2016, the wettest October day on record, beating the previous record of 5.24″ set on 29 October 1967. This rain comes on the heels of a very wet week prior to the hurricane and a record wet September as 11.80″ of rain was observed for the month, beating the previous record of 11.15″ in September 1983.

An official and comprehensive post-storm summary will be released by the National Hurricane Center in the coming months.

Isolated Sunday Downpours

20160911-rain
Spatial distribution of unofficial rainfall reports around Bermuda yesterday, over 5″ of rain fell in some spots. Interpolated using data from WeatherUnderground, WeatherLink, and Bermuda Weather Service.

Late-morning, isolated showers popped up in the central parishes on Sunday dumping heavy rain from roughly 10am to 3pm with intermittent periods of thunder. The showers were largely stationary over the central parishes, at times spreading to the east. This left the western parts of the island mostly dry until isolated showers moved in from the west late in the afternoon.

While most places saw less than an inch of rain, where those showers and thunderstorms set up in the central parishes, unofficial rain totals of 2-5″ were reported. Light winds through much of the lower troposphere kept the showers that formed from moving much, and the heating of the island likely helped support the showers initiate.

A surface ridge to the south of Bermuda is maintaining light mostly westerly flow today. Meanwhile, a subtle surface trough embedded in that flow is helping to spark isolated showers and thunderstorms in the area.

Thanksgiving Gales Possible

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.

Fair Holiday Weather

After a full seven days of heavy, thundery and locally squally showers sparked by a dissipating stationary front in the area, the weather changed its pace and cleared out for the holiday. The Bermuda-Azores high is rebuilding a ridge in from the southeast and that is providing a return to fair summertime weather for the coming week. The Bermuda Weather Service measured 7.51″ of rain during that seven day period – more than a month’s worth of rain that brought the dry spell to an abrupt end! Highlights of this event include the 3.63″ of rain that fell on the 25th, near record low temperatures on the 25th and 27th, and the squall on the 27th. This unsettled weather brought July’s rain total to 8.94″ at the Bermuda Weather Service. See BWS for the latest official forecasts.

The Squall

Generally light winds were the theme for most of the 27th. However, just after noonday, a line of showers developed to the west of the island and began moving eastward. Just before the showers began to cross the island from west to east they became thunderstorms. The generally light winds that persisted into the early afternoon suddenly increased to strong with gale or near-gale force gusts as the torrential rain and associated rain-cool downdrafts of the thunderstorms passed overhead. Just as quickly as the weather came on, the rain and wind died off as the thunderstorms exited to the east of the island.

Squall
Temperature, Dew Point, Pressure, Wind, and Rain Rate from my PWS during the squall. Plotted with R. From about 12:30pm to 4:45pm 27 July 2015. Further brief but gusty showers were observed later in the evening.

At my PWS, 1-minute sustained winds increased from less than 5 mph (4 kts) to a peak of 23 mph (20 kts) just after 2:00pm local time. Peak gusts reached 36 mph (31 kts). This coincided with heavy rain with rainfall rates approaching 7 inches per hour and a dip in temperature from 80.3 F to 73.1 F. Further, as evidence of the downdraft, there was a spike in pressure by about 0.5 hPa during the heaviest rains and strongest winds. Similar squally conditions were observed by other PWS mainly in the central and eastern parishes around the island.

Squall : “A sudden increase of wind speed by at least 16 knots, the speed rising to 22 knots or more and lasting for at least one minute. It is often accompanied by showers or thunderstorms.” (BWS Glossary – search ‘squall’) While the conditions were just shy of this definition at my PWS, observations at the airport met the criteria for a squall: winds increased from 5 to 23 kts (an 18 kt increase) between 1:55pm and 2:12pm. The peak gust at the airport from this squall was 35 kts. Similar wind gusts were observed at the airport in other showers during the week’s heavy rains, but the increase in wind speed around the showers wasn’t sufficient to define them as squalls.