Another heavy rain event from a stalled front: How well was it forecast?

The rain and cloud are clearing away from what has become a very wet week for Bermuda.  On four days from March 17th through 20th inclusive, a whopping 4.92 ” (125 mm) of rain fell. A few more drops on the 21st brings March 2019’s rainfall total to 7.19 ” (183 mm). On average, Bermuda sees around 4.50 ” (113 mm) of rain in March.  Furthermore, three consecutive sunless days were recorded making for a gloomy start to the week.

So what was going on?

2019-03-19-00UTC_UWyoming_annotated
Skew T-log P diagram during the period of persistent wet weather at Bermuda. The cool and humid low-level air and northeasterly flow is highlighted in blue, while the warm and humid air with southwesterly flow aloft is highlighted in red. (Source: UWyoming)

A stationary front set-up near the island. This marked the boundary between warm, humid, tropical air to the east and south of the island; and cool dry air to the west and north of the island. This helps to focus where upper-level disturbances can trigger a surface response and therefore tangible weather. As waves of low pressure formed in response to fast-moving upper-level disturbances they tracked along the front bringing alternating wetter and drier periods.

In this case, Bermuda remained on the cold side of the front with strong easterly or northeasterly flow combining with the overall gloomy weather for a chilly feel to the air. However, only 500 m above the surface winds were out of the southwest. Moisture transported from warm side of the front was lifted up and over the shallow layer of cool air and easterly flow where some of it fell as rain.

GFS_rainfall_forecast
GFS forecast rainfall total for the period 06Z 17th March to 06Z 21st March. Data accessed via NOMADS.

Furthermore, it turns out that there was plenty of lead-time to this dreary forecast. A very wet start to the week was in discussion at the Bermuda weather service as much as five days in advance, and made it into the forecast synopsis prior to the weekend. To put some numbers to that, here’s how the rain total for this wet 4-day period from one weather model (GFS) changed with decreasing lead time compared to observations (right). Despite predicting rainfall totals that are roughly half of what was observed in this period, this model consistently indicated the potential for heavy rain for more than four days in advance (i.e. model data starting on or before the 13th March was already suggesting that the 17th -21st March would be very rainy). Numerical model data like this, combined with conceptual models and experience of forecasters were able to predict this event well in advance.

This winter and early spring has been no stranger to heavy rain events with much of the year-to-date rain falling in a handful of events. So far, there have been 8 days with more than 1.00 ” (25.4 mm) of rain. These 8 days account for about 63 % of the year-to-date rainfall, 18.92 ” (480 mm). This is 5.75 ” (146 mm) above average.

Date Rainfall
9th Jan 1.06 ” (27 mm)
29th Jan 2.56 ” (65 mm)
14th Feb 1.26 ” (32 mm)
15th Feb 1.66 ” (42 mm)
6th Mar 1.02 ” (26 mm)
17th Mar 1.06 ” (27 mm)
18th Mar 1.45 ” (37 mm)
20th Mar 1.77 ” (45 mm)
Total (> 1″) 11.84 ” (301 mm)
Current year-to-date 18.92 ” (481 mm)
Normal year-to-date 13.17 ” (335 mm)
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Active Front Across Bermuda

The 29th January saw very active weather as a cold front dragged across the island. The slow advancement of the front led to heavy rain accumulations, while the front was accompanied by thunderstorms that produced small hail. Between the 26th, 27th, and 29th, the official rain total reached 3.90″.

2019-01-29_193033utc_goes-east_geocolor
GOES-East GeoColor satellite imagery showing the band of cloud draped across Bermuda associated with the slow moving cold front. This image is valid for 19:30 UTC on 29 Jan 2019, near the time hail was reported at the airport.

A cold front then made it to Bermuda on the 26th bringing the first round of rain with official rainfall totals reaching 0.67″ on that day. The front became stationary near to the east of the island in the evening of the 26th.

On the 27th, the front lifted westward and then northward across Bermuda bringing another round of rain and showers for an official daily rain total of 0.68″. Low pressure started to organize over the Gulf of Mexico along the tail end of this same front.

Our weather front continued northward (away from Bermuda) on the 28th making for a dry day. Through the 28th, the low pressure organizing over the Gulf of Mexico moved northeastwards into the Atlantic and began to mature.

By the early morning of the 29th, this more powerful low pressure drove our front back across Bermuda. This front brought with it a band of showers that moved slowly across the island. Winds shifted slightly and temperatures dropped as the leading edge of the rain band began to clear the island around 7 AM, indicating frontal passage. With deep-layered southwesterly flow continuing, individual showers in the band were moving relatively quickly from southwest to northeast, while the band itself was making only slow forward motion.  This made for a prolonged period of wet weather.

Hail featured in both The Royal Gazette and Bernews

Showery rain persisted behind the front into the afternoon. Isolated showers and thunderstorms were then able to develop behind the cold front. One such thunderstorm (embedded in the continued deep-layered southwesterly flow) tracked from southwest to northeast  across much of the island dropping small hail. Rain totals on the 29th reached 2.56″ for a daily record when compared to the period 1949-2018.

Florence Passing South of Bermuda

Florence_Satellite
GOES-EAST geocolor satellite imagery of Hurricane Florence just prior to the latest NHC advisory. Source: Slider – CIRA/RAMMB

Hurricane Florence underwent a remarkably well-forecast period of rapid intensification into a major hurricane yesterday evening. As of this morning’s 11am advisory from the National Hurricane Center, Florence is still a major hurricane with maximum sustained winds near 130 mph. Florence is moving west-northwestward, and is expected to continue that motion around deep-layered high pressure centered between Bermuda and Nova Scotia over the next day or so. Florence is expected to remain a very dangerous major hurricane during this time as it tracks through an environment conducive for hurricane growth.

Bermuda

Florence should make its closest point of approach to Bermuda mid-afternoon as it passes about 300 nm south-southwest of the island. Impacts on Bermuda should be fairly limited. Expect elevated and hazardous surf and rip currents along the South Shore from southeasterly swell generated by the major hurricane. Isolated showers are possible and winds could briefly increase to strong (20-33 kts) in the southern marine area. A Small Craft Warning is in effect for elevated surf and strong winds.

Follow the Bermuda Weather Service for the latest official forecast for Bermuda.

East Coast/Mid-Atlantic

Florence is expected to continue west-northwestward today before turning northwestward on Wednesday through its final approach to the US East Coast. Florence is forecast to track over open ocean with sea surface temperatures near 29°C. Furthermore, environmental vertical wind shear is low and humidity sufficiently high. These ingredients suggest that Florence will continue as a major hurricane through its approach to the coastline on Thursday, and expected landfall Friday morning. Changes in the hurricane’s intensity are likely to be related to internal structural changes known as eye-wall replacement cycles. One such cycle is nearing completion this afternoon.

During such a cycle, the eye-wall (region of the hurricane with strongest winds and heaviest rains) is encircled by outer bands that form a secondary eye-wall.  This has the effect of increasing the overall size of the hurricane. If the cycle is uninterrupted, the secondary eye-wall begins to intensify at the expense of the original, resulting in an overall weakening of the hurricane. Eventually, the original eye-wall dissipates and the secondary eye-wall assumes the role of the primary eye-wall, and the larger hurricane resumes strengthening.

Coastal and inland watches have been issued by the National Weather Service for parts of South and North Carolina in response to the wind and storm surge threat.

Florence_Steering
Map of the steering flow appropriate for hurricanes of Florence’s strength. White streamlines indicate the direction of the steering flow, while the filled colors indicate the strength of the flow. Image valid at 1200 UTC 11 Sep 2018, Source: CIMSS

Increasing surf and dangerous rip-currents are expected to commence tonight or tomorrow along the US East Coast.

On Thursday, as Florence nears the coast of North and South Carolina, the hurricane is expected to enter a region of fairly weak steering. Florence should be near the southwestern edge of high pressure centered over the North Atlantic, and near the northeastern edge of high pressure centered over the Northwest Caribbean Sea imposing steering flow in opposite directions across the hurricane resulting in a weak steering regime. Both the track and intensity, and therefore the severity and duration of impacts from the hurricane depend strongly on the structure of the steering flow as Florence approaches the coast, and the timing with which the hurricane approaches this change in steering. While confidence is high that there will be severe impacts somewhere in the Mid-Atlantic region, exactly who gets what is still to be determined.

Residents should prepare for life-threatening storm surge inundation at the coast, freshwater flooding inland, and damaging winds – particularly at the coast, but extending some distance inland. See the National Hurricane Center’s Key Messages for more.

Stay up-to-date on the latest official forecast products from the National Hurricane Center and warning products from your local National Weather Service office.

Hurricane Fabian: 15 Years on

radarslide
Source: Bermuda Weather Service Tropicals Archive

5th September 2003, Hurricane Fabian made a direct hit on Bermuda as a Major Hurricane. Fabian tracked across the Tropical Atlantic over the previous week, before turning toward the island.

When the hurricane reached Bermuda, it brought approximately 16 hours of tropical storm conditions, with winds above hurricane force for about half of that time. The eastern eyewall of the hurricane (the region of strongest winds and heaviest precipitation) passed over the island between 5 and 7pm with peak winds between 100 and 105 kts with peak gusts near 130 kts, mostly on south-facing hilltops. These severe winds resulted in widespread roof, structural, and vegetation damage across the island.

Wind is often the first hazard that comes to mind when ‘hurricane’ is mentioned. However, hazards associated with water are historically more dangerous and destructive where they occur. That was again the case in Bermuda as large battering waves atop a significant storm surge resulted in significant and widespread coastal flooding and erosion. Strong wind and low pressure associated with Fabian resulted in an increase in water level an estimated 8 ft above normal tide levels. It was the tempestuous sea that tragically claimed four Bermudian lives during the Hurricane. The sea compromised key infrastructural components, such as The Causeway, the airport, and sections of South Shore Road.

[Read More on Storm Surge]

We are once again near the peak of the Atlantic Hurricane Season, and the current outlook includes Hurricane Florence in the distance Central Atlantic as a system to monitor for possible impacts this weekend. Bermuda’s national hurricane preparation plan has matured respectably since Fabian’s impact and the impacts from many subsequent tropical storms and hurricanes over the past 15 years. Between the Bermuda Weather Service, the Emergency Measures Organisation, and social media, information about incipient hurricanes and their associated disruption to services can spread through the island rapidly.

This same 15 years of experience with hurricanes is also reflected on the personal level, and many Bermudians consider themselves well-versed in hurricane preparedness. All the same, remembering Fabian is a good reminder to plan and prepare well in advance of the next serious hurricane to avoid the rush for supplies and to minimise personal impacts should a hurricane threaten.

Follow the latest official forecast for Bermuda from the Bermuda Weather Service who are monitoring the progress of Hurricane Florence closely for any potential impacts.

Dry Early May Weather

Early May Weather
Observations from the Bermuda Weather Service. Temperature and dew point on top, wind direction on bottom. To the right of the yellow line, the relative humidity (indicated by how close together the temperature and dew point are) increases as winds settle out of a south-southeasterly (135-180°) direction.

April ended by clearing out a very warm and humid airmass that was accompanied by patches of fog that advected across the island. This left the start of May quite dry with northerly winds and a ridge of high pressure to the north. As the ridge shifted to the east of Bermuda, northerly flow gradually veered southerly bringing in more humid air. High pressure has remained centered generally to the east of Bermuda since the fifth of May maintaining humid south-southeasterly flow.

This setup is fairly common for this time of year: ridges of high pressure settling in near Bermuda bringing persistent settled and mild weather across the western Atlantic. The Bermuda-Azores high typically starts to build in during late-May/early-June to its July peak extent and intensity, prevailing winds come from the south and southwest with a warm and humid the norm.

This high pressure setup is generally effective at keeping rain-making frontal systems at bay. However, it appears the ridge extending across Bermuda might weaken enough to allow a weakening front to bring some showers to the island over Friday. High pressure then re-establishes itself with a return to more settled weather over the weekend. The weak front is unlikely to be able to bring any real change in the airmass and the warm humid weather should continue without much break.

May is typically one of the driest months in Bermuda. Long periods without much rainfall are characteristic of April through June. Water availability can become an issue when several such dry spells occur in one year – particularly so as water demand increases in the warm season.

Quite A Squall

Between 5:15 and 5:30pm on Sunday April 8th, a severe thunderstorm blew across Bermuda. The squall brought heavy rain, severe wind gusts along with thunder and lightning. Gusts greater than 50 kts were observed at Pearl Island (62 kts), the Crescent (58 kts), Bermuda Esso Pier (54 kts), and at the official observing site at the airport where a peak gust of 71 kts was recorded.

The rain and thunderstorms brought 0.56″ (14.2 mm) of rain to the airport.

GRAPH_Pearl Island_2018-04-08-2009-L.png
From top to bottom: 2-minute sustained winds, wind gust, wind direction, air pressure, air temperature, relative humidity, and precipitation total. Around 5:20pm local time, the main squall moved through with peak gust of 62 kts, and a drop in temperature of about 4°C. (Source: Bermuda Weather Service)

 

 

Ship Tracks Around Bermuda

2018-02-23 060220 UTC
GOES-East Geocolor imagery centred on Bermuda at 06:02 UTC, 23 February 2018. This multi-spectral product highlights low clouds (e.g. stratocumulus) as light blue shades. The brighter light blues indicate thicker low cloud.

[Watch animation]

High pressure over the western Atlantic over the last few days has been associated with persistent stratocumulus cloud cover. This typically thin layer of low-altitude cloud is a fairly common feature over the eastern half of ocean basins, and its occurrence around Bermuda is not uncommon – particularly in the cooler months.

Strong high pressure comes with subsidence. Air is compressed and warms as it descends. The descending air, with origins in the mid/upper-atmosphere is also quite dry.  This warm, dry air cannot make it all the way to the surface because it doesn’t have enough momentum to push through the cooler layer near the surface (the boundary layer). This leads to a stable layering of air that manifests as warmer drier air over cooler more humid air.

The boundary layer is often well mixed with an even distribution of heat and moisture (and can become increasingly humid over the ocean). Meanwhile, the warm air above acts like a lid over the cooler boundary layer preventing mixing between the two layers. Moisture trapped in the well mixed layer can form a sheet of stratocumulus cloud given the right conditions.

Stratocumulus_Annotated
Potential temperature (the temperature that dry air would theoretically have if it was brought to the surface without external heating), Specific humidity (the mass of water vapour for every kilogram of air), and Relative Humidity (the ratio between the actual water vapour content and the water vapour content needed for saturation). Profiles measured by weather balloon at 00:00 UTC 23 February 2018 at Bermuda.

Exhaust from ships contains aerosols that act as cloud condensation nuclei. These are airborne particles that water can condense onto to form cloud droplets. The aerosols from the ships tend to form more, and smaller, cloud droplets making the clouds contaminated with ship exhaust appear brighter and thicker to satellite instruments.

The ship tracks in the animation above appear to advance from east to west or west to east, following the path of the ships. Meanwhile, another cloud enhancement appears to advance southeastward, starting from Bermuda. This track follows the wind direction in the boundary layer (unlike the ship tracks). It could be the result of aerosols from BELCO or the Incinerator acting to enhance the cloud brightness through the same mechanism as ship tracks.

High pressure and the settled weather it brings has been a theme for much of February 2018. Few mid-latitude cyclones have impacted the island so far this month. As a result, the cold air has stayed away and temperatures have remained far above normal. The lack of cyclones has also meant that precipitation totals are below normal and no days this month have experienced gale force winds. Sea surface temperatures are also far above normal as a result.

The month is expected to end with changeable weather as high pressure gives way. A cold front then takes the opportunity to push southeastward across Bermuda with some rain and showers followed by cooler air.

Further Reading:
Coakley, J. A., R. L. Bernstein, and P. A. Durkee, 1987: Effect of Ship-stack Effluents on Cloud Reflectivity. Science, 237, 1020-1022.
Hudson, J. G., 2000: Cloud Condensation Nuclei and Ship Tracks. J. Atmos. Sci., 57, 2696-2706.
Wood, R., 2012: Stratocumulus Clouds. Mon. Wea. Rev., 140, 2373-2423.