Emily, A Potential Threat?

TDEmilyTUB6am20170801-0900UTC
The Bermuda Weather Service Tropical Update Bulletin for this morning’s 6am update: Closest point of approach to Bermuda within 72 hrs (3 days) is forecast to be 236 nm to the NNW, 3 pm Thu, Aug 3, 2017. However, this system may move closer to Bermuda after this time period.

Yesterday morning, Tropical Storm Emily quickly formed in the Gulf of Mexico, just west of Tampa, Florida. The tropical storm made landfall south of Tampa a few hours later with maximum sustained winds near 40 kts. Through Monday evening and overnight, Emily tracked across the Florida peninsula, weakening to a tropical depression and emerged into the southwestern Atlantic early this morning. Tropical Depression Emily is a “Potential Threat” to Bermuda.

What does a “Potential Threat” mean?

PotentialThreatGlossary

This means that the center of a tropical cyclone (e.g. TD Emily) is expected to pass within 400 nautical miles of Bermuda in the next 72 hours. As of the 6am local time update from the National Hurricane Center and Bermuda Weather Service, Emily is expected to pass under 250 nautical miles away from the island on Thursday. It is important to note that these threat designations do not suggest that the tropical cyclone in question will result in adverse weather as it passes.

[See the BWS Glossary]

Another Emily?

Yes. The National Hurricane Center has six lists of names, and a new list is started for each hurricane season. The lists are then used in rotation, so every six years the same list of names is used again. A name is retired from the list is a storm is ‘so deadly or costly that the future use of its name on a different storm would be inappropriate for reasons of sensitivity’.  During the annual meeting by the World Meteorological Organization committee, representatives can suggest the retirement of a name and vote on the replacement name.

‘Emily’ has been a name on the Atlantic name lists since ‘Eloise’ was retired after the 1975 Hurricane Season. There have now been 7 tropical storms or hurricanes with the name ‘Emily’.

[More on Hurricane Names from the NHC]

What can Bermuda expect from Emily 2017?

Short answer: not much. As Emily passes on Thursday, the Bermuda Weather Service expects moderate winds (10-15 kts), slight seas (1-2 feet, inside the reef), and only isolated or scattered showers with a risk for thunder to dampen Emancipation Day, the first day of Cup Match.

EastUS_Annotated
GOES-13 infrared imagery at 1045 UTC (7:45am local time) showing tropical depression Emily (red circle), a stationary front (red and blue line), and Bermuda (cyan circle).

Emily is not expected to change much in strength, possibly re-strengthening to a weak tropical storm at most. Strong vertical wind shear over Emily is allowing deep layered dry air to be entrained from the west. This is limiting the depression’s ability to maintain convection, organize, and strengthen despite being over the very warm waters of the Gulf Stream.

Showers and thunderstorms ahead of the stationary front to the west have already been the theme for Bermuda. Wind gusts reached gale force in and around some of those showers (official peak gust reached 35 kts at the airport yesterday, about the same as wind gusts currently around Emily). While showers and thunderstorms are expected to continue through Friday, high pressure building in from the east may allow dry (and even sunny) spells in between showers.

As Emily interacts with the front over the next 36-48 hours, it is expected to transition to a post-tropical cyclone. Emily is forecast to track northeastwards, along the stationary front over the next five days. The flow ahead of Emily is expected to gradually lift the stationary front away from Bermuda starting late today through her closest approach on Thursday. This allows high pressure to build in from the east, slowly reducing the coverage of shower activity and allowing winds to slacken. Emily then passes Bermuda and the flow behind her swings the front back towards Bermuda, bringing increased chances for showers on Friday morning.

Follow the Bermuda Weather Service and National Hurricane Center for any changes to their forecast.

Saharan Dust South of Bermuda

Annotated_12UTC_GOES16_TrueColor_SaharanDust

Preliminary, Non-operational GOES-16 imagery from RAMMB/CIRA shows a plume of dust several hundred miles south of Bermuda this morning. Over the Sahara Desert across Northern Africa, dust is suspended by strong winds. It’s then transported westwards across the Atlantic around the southern periphery of the Bermuda-Azores high.

The dust is accompanied by a warm dry layer of air above the surface known as the Saharan Air Layer (SAL). This makes the lower atmosphere stable, trapping clouds and moisture in a shallow layer below. It has been shown that this hinders the development of tropical cyclones in the Atlantic as they entrain this dry and stable air and subsequently struggle to maintain deep convection.

Some dust from this plume is expected to reach Bermuda in the coming days. It is not uncommon for decaying dust plumes to reach Bermuda during the summer months. This typically results in hazy skies and sometimes bits of dust can be washed out in any rain showers.

[See NOAA’s Hurricane Research Division for more on SAL]

 

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.

Uncommon May Gale

Gales developed on Friday as a small low pressure passed just under 80 km to the north. This low was one of a series of small-scale low pressure systems that tracked from west to east along a quasi-stationary front near Bermuda last week.

Ahead of Friday’s low, that quasi-stationary front lifted northward across the island with patchy light rain showers on Thursday night. This was accompanied by an increase in temperatures and humidity as southwesterly winds strengthened early Friday morning. Southwesterly winds briefly increased to gale force (sustained >34 kts) with unofficial gusts over 45 kts reported at automated weather observing sites. Winds then dropped off slightly, veering through to northerly as the front moved across Bermuda from west.

GRAPH_PearlIsland_2017-05-12-1259-L
Graph of automated unofficial observations from Pearl Island sensors showing falling pressure and southwesterly winds increasing to gale force before 10am. Then winds drop off, veer to the north and temperature falls between 10am and 11am as the cold front passes. Northerly winds return to gale force after 11am as pressure rises. A similar wind trace was observed at Crescent and in official observations, however, the minimum in winds at Pearl Island was much more dramatic.

Cooler air then blew in as winds veered to the north on renewed gales behind the front. Storm force (>48 kts) gusts were recorded around the island, with a peak official gust of 49 kts at the airport, the second highest official gust observed in May since 1949*. High winds resulted in diverted flights (Royal Gazette/Bernews), delayed start to Relay for Life (Bernews), and several boat incidents (Bernews).

2017-05-12-1157_250km_sri
Bermuda Weather Service Radar at 8:57am local time as the small scale low was passing north of Bermuda. Much of the rainfall is confined to an east-west front about 80 km north of Bermuda, and southwest-northeast oriented cold front about 40 km south of Bermuda.

Very little rain fell from this system. Much of the shower activity was confined to the north of the island (along the front near the centre of the low), and south of the island (along the front). Pressure fell to 1000.7 hPa as the low passed the island, the lowest pressure observed in May since 2007**.

* Only 6th May 1978 had a higher gust at 50 kts, but the record is incomplete prior to 1996.
** This statistic is not recorded prior to August 2006.

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.

Hurricane Nicole Post Storm Report Released

nicole-terra-cpa
NASA Terra/MODIS True Color satellite imagery of Hurricane Nicole near the time of closest point of approach 13 Oct 2016.

After each season, the National Hurricane Center prepares ‘post-storm reports’ on each tropical cyclone that formed in the North Atlantic and Eastern Pacific basins. These reports include data collected in real-time that may not have been available during operational analysis, and therefore can sometimes lead to revision of track or intensity. This was the case for 2014’s Hurricane Fay.

nicole-txkf-2-min-gusts-kts
Time trace of wind gusts (red ‘x’) and wind directions (green ‘triangles’) from the airport reproduced from the NHC report. We see the sharp decrease in wind gusts and sharp wind shift as the calm eye of Nicole crossed.

Today, the National Hurricane Center released its report on 2016’s Hurricane Nicole. Their analysis on observations from Bermuda suggest that widespread category one conditions occurred on the island with isolated areas seeing category two conditions. Aircraft reconnaissance measurements near the time of closest point of approach to Bermuda indicate that Nicole was still a category three hurricane with maximum sustained winds near 105 kts (120 mph). The island received impacts from the left-front quadrant of Nicole’s eyewall, missing these strongest winds (located in the right-front quadrant) and thus this was classified as a strike.

[strike, direct-hit, indirect-hitlandfall – Impact Terminology Defined]

Hurricanes are centers of extreme low pressure. Winds spiral inwards, towards centers of low pressure and in the northern hemisphere, this manifests as a counter-clockwise circulation.  Observations from Bermuda indicated that winds backed (turned counter-clockwise with time) from an easterly direction to a northerly, and then northwesterly direction suggesting that the center of circulation remained to east of the island and therefore did not make landfall, despite Bermuda entering the calm eye of the hurricane.

nicole-bws-radar
Bermuda SRI Radar imagery showing steady and heavy rain rates persisting over Bermuda (red ‘+’) for hours before the drier eye and southwestern quadrant passed over the island. Reproduced from the NHC report.

Nicole’s impacts on Bermuda were also remarkable in that the hurricane made for one of the top-5 wettest meteorological days on record at the airport, and the Bermuda Weather Service was able to release a weather balloon in the eye that measured the highest precipitable water here since 1973 at 2.93″.

[What is Precipitable Water?]

Notable for an erratic early track and meteorological evolution (including two periods of rapid intensification), Hurricane Nicole will go down in the record books as the fourth early-October hurricane impact on Bermuda in three years. Once Nicole passed Bermuda, the cyclone underwent a complex transition into a powerful extratropical cyclone in the North Atlantic where it continued to produce storm force winds for several days.

Current Weather:

GRAPH_The%20Crescent_2017-02-16-1208-L.png
Wind measurements at the Crescent, in the northern marine area via Bermuda Weather Service. Note peak in winds just before 9am 16 Feb 2017(sustained, to/gusts, middle).

Bermuda is now well into its ‘winter’ season where changeable weather associated with gales is commonplace. Yesterday’s cold front brought a brief warm and humid spell with southwesterly gales and thunderstorms that brought heavy rain to the island.

Locally severe wind gusts, mainly confined to the marine area, were also observed. The Crescent Buoy, in the northern marine area, measured a peak thunderstorm gust of 54 kts. Winds on island appear to have remained below severe levels (i.e. < 50 kts) with a peak gust of 43 kts measured at the airport prior to any thunderstorm activity.

Follow the Bermuda Weather Service for the latest official forecast, warnings, and observations for Bermuda.