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

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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.

Other PWS

There are quite a few personal weather stations in Bermuda that broadcast online. Particularly during daytime, some of these stations record temperatures that are likely too high. Regardless, these should all be considered unofficial. The official observations for Bermuda can be found at the Bermuda Weather Service, and are archived on their Climate page. Here is a list of PWS and other weather stations in Bermuda as of 26 July 2015:

Name Site Notes
Chaingate Hill, Devonshire Wunderground Temp/RH/Wind/Press/Precip
Moore’s Lane, Pembroke Davis WeatherLink Temp/RH/Wind/Press/Precip
Strathmore, Paget Davis WeatherLink Temp/RH/Wind/Press/Precip
Cedarvale South VP9NI, Devonshire Davis WeatherLink Temp/RH/Wind/Press/Precip
Devon Heights/Elnoc, Devonshire Wunderground/Davis WeatherLink Temp/RH/Wind/Press
Nine Man Cave, Smith’s Davis WeatherLink Temp/RH/Wind/Press/Precip
Gilbert Hill, Smith’s Wunderground/Davis WeatherLink Temp/RH/Wind/Press/Precip
McGall’s Bay/The Aviary, Smith’s Wunderground/Davis WeatherLink Temp/RH/Wind/Press/Precip
Knapton Hill/Vraiment, Smith’s Wunderground/Davis WeatherLink Temp/RH/Wind/Press/Precip
Milton Cottage, Pembroke Davis WeatherLink Temp/RH/Wind/Press/Precip
Port’s Island, Warwick Davis WeatherLink Temp/RH/Wind/Press/Precip
St. David’s, St. George’s (Peter’s PWS) Wunderground/Davis WeatherLink Temp/RH/Wind/Press/Precip
Spanish Point, Pembroke Wunderground Temp/RH/Press
Civil Air Terminal?/Bermuda Esso Pier, St. George’s Wunderground/NDBC/Tides&Currents(NOAA) Temp/Wind/Press/SeaTemp/SeaLevel
Tucker’s Town, Hamilton Wunderground/PWS Weather Temp/RH/Wind/Press/Precip
Magnolia Hall, Hamilton Wunderground Temp/RH/Wind/Press/Precip
Trimmingham Hill, Paget Wunderground Temp/RH/Wind/Press/Precip
Hinson’s Island, Warwick Wunderground/Other Temp/Wind/Press/Precip
Laurel Lane, Sandy’s Wunderground Temp/RH/Press
Heydon Drive, Sandy’s Wunderground Temp/RH/Press
Commissioner’s House Dockyard, Sandy’s WindGuru Temp/Wind
Ocean View, Southampton Wunderground Temp/RH/Wind/Press/Precip
Bermuda Institute, Southampton Weather Bug Temp/Wind/Press
The Mid Ocean Club, Hamilton Weather Bug Temp/RH/Wind/Press
St. David’s Lighthouse (AWOS), St. George’s Other (requires Java) Temp/RH/Wind

This post will be adapted into a page in the future – stay tuned!

Fair Weather for the Holiday

A relatively dry airmass over the island for Thursday and Friday is expected to slowly erode over the weekend and be replaced by a very humid airmass from the west and southwest. An approaching cold front from the northwest on Sunday loses its frontal characteristics and continues its approach as a trough. The trough then taps into some of that very humid air and convergence along and out ahead of that trough is expected to bring showers Sunday night into Monday. Isolated chances for thunder are also possible in the area.

20150612 Ascent
The Temperature and Dew Point (both in degrees C) profile from the morning of June 12th. Where the Temperature (red) and Dew Point (green) are further apart, the humidity is low. Aside from near the surface/bottom of this plot, the temperature and dew point are far enough apart for only a few clouds to form. The vertical axis is height above the surface in meters. The black line represents the temperature profile a parcel of air would have if it were lifted from the surface to 250mb or ~18,000 m.

However, this dry airmass led to mostly clear skies overnight Thursday into Friday. That, plus light winds courtesy of a ridge of high pressure just about overhead, allowed a decent radiative cooling event. During the day, solar radiation comes in (warming the surface) and terrestrial radiation goes out (cooling the surface); where the solar radiation exceeds the terrestrial radiation, there is net warming. At night, however, there is no solar radiation and so there is often net cooling. The calm winds keep the air near the surface from mixing with the warmer air above – enhancing this cooling process. Clouds act to both scatter terrestrial radiation back to the surface, and emit their own radiation down to the surface – a lack of clouds removes this extra process.

Land cools much faster than the ocean, Bermuda is surrounded by the ocean. So here, coastal areas have the added influence of the often warmer waters to moderate the nighttime cooling. Further, the island’s hills can shelter small areas from this effect and allow the cooling process to go unmoderated. This can lead to temperatures near the coast being over 5 F higher than temperatures less than 1/8mile away in a valley. This is what happened last night:

Temperature trace with time from the airport (exposed and coastal - black) and my PWS (sheltered and inland - orange). Temperatures were near 5F different overnight, yet similar the next day because of the clear night with light/calm winds.
Temperature trace with time from the airport (exposed and coastal – black) and my PWS (sheltered and inland – orange). Temperatures were near 5F different overnight, yet similar the next day because of the clear night with light/calm winds.

For the latest official forecast for Bermuda, see the Bermuda Weather Service.