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.

Saharan Dust South of Bermuda


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]


Karl now a Potential Threat

For official Bermuda Weather Information, see: Bermuda Weather Service
For the latest Hurricane Information, see: National Hurricane Center.

Tropical Storm Karl is now expected to pass within 400 nm of Bermuda within the next 72 hours as of the 12pm advisory. This latest advisory from the National Hurricane Center takes the center of Karl about 260 nm to the south of Bermuda in the next 72 hours. Karl is forecast to pass closer to Bermuda between 72 and 120 hours from this advisory and adverse weather is possible from Karl.

At this time, Bermuda can expect dangerous surf and sea conditions over the weekend. Further, strong to possibly tropical storm force winds are to be expected beginning late on Friday through Saturday. The final track of Karl will determine the strength and direction of winds and whether or not any significant rains will fall.

Karl is expected to track northeastward past Bermuda. The latest NHC track  takes Karl east of Bermuda and the strongest winds would be felt from the northeast as a result (left panel). Conversely, should Karl pass to the west of the island, the strongest winds could be felt out of the southwest.

Karl has been steered to the west in the flow south of the Bermuda-Azores ridge of high pressure over the last few days. This steering is expected to continue for the next day or so. Karl then turns more to the northwest as the storm approaches a weakness in the ridge that is moving in from the west of Bermuda. By Friday, Karl begins to turn northwards, moving into this weakness in the ridge. Karl then becomes entrained in deep-layered southwesterly flow that accelerates the storm to the northeast.

Dry air surrounding the cyclone and persistent strong vertical wind shear have kept Karl weak and disorganized thus far. However, Karl is entering a more moist environment with higher sea surface temperatures. These factors should support more vigorous convection around the storm. Further, vertical wind shear is expected to weaken at the same time. This should allow deep convection to organize. Both of these factors support a strengthening storm in the coming days and Karl could be at or nearing Hurricane strength as it passes Bermuda.

Hurricane reconnaissance aircraft (ie. hurricane hunters) are scheduled to begin regular investigations into Karl this afternoon and this will give us a better idea of Karl’s current strength and structure, and how favorable the environment is.

As always, follow the official weather sources for the latest information.

Rain in Sight

Monthly Actual vs. Normal year to date precipitation total for the airport. Normals based off of 1979-2000 climate period as is the climate records from the Bermuda Weather Service. Where there is brown, there is a deficit; where there is blue, there is a surplus of precipitation.
Monthly Actual vs. Normal year to date precipitation total for the airport as of July 9th. Normals based off of 1979-2000 climate period as is the climate records from the Bermuda Weather Service. Where there is brown, there is a deficit; where there is blue, there is a surplus of precipitation.

After a wet start to the year, mostly in February, Bermuda has fallen behind in precipitation. The Bermuda-Azores high has held a persistent ridge across the western Atlantic for the second half of Spring. As of July 9th, the airport was 3.81″ behind the normal year-to-date total precipitation. The last several days have featured a few hit or miss, passing isolated showers – no tank rain. However, an approaching cold front may change that. Keep in mind that a month’s worth of precipitation is 3-5 inches, and a week’s worth is around an inch.

A cell of high pressure to the southwest of Bermuda briefly allowed a period of light northerly winds over the last few days as a weak cold front dissipated in the area. Isolated areas of convergence and less stable air in the area led to isolated showers and thunderstorms for the middle of this week. Winds have since backed to the west and increased to moderate.

This weekend, another weak front is expected to become stationary in the area rather than dissipate. By early-morning Saturday, the cold front to the near north should enhance local convergence enough to produce isolated showers, a band of showers with a chance for thunder is possible Saturday afternoon along or just ahead of the front itself – depending on how well the boundary holds together as it approaches. The front then becomes stationary near the island, oriented from west to east as a wave of low pressure develops offshore of Virginia on Saturday evening. This keeps a chance for isolated showers and a risk for thunder in the area through Sunday.

The general expectation is for around 0.50″ of rain to fall from showers on Saturday and Sunday, but over an inch of rain is possible depending on the exact track and location of the heavier showers and thunderstorms ahead of and along the front as it approaches and becomes stationary. Since it has been very dry lately, this rain is very welcome.

Winds shouldn’t become an issue around this front. Expect today’s moderate westerly winds to continue into Saturday morning, veering northwesterly Saturday afternoon, and becoming light behind the front Saturday night. There is a slight chance for winds to become gusty in and around showers or thunderstorms, mainly ahead of the front. Winds remain variable at times on Sunday, but mostly light.

Monitor the progress of this weather with the Bermuda Weather Service and keep an eye out for possible Small Craft Warning and/or Thunderstorm Advisory.

The Tropics

The tropical Atlantic continues a quiet stretch after mid-June’s Tropical Storm Bill – no tropical cyclone formation is expected in the next five days. Meanwhile, the entire Pacific ocean has become very active; earlier this week there were three typhoons simultaneously in the West Pacific, a tropical storm in the central pacific and four other tropical disturbances with potential for tropical cyclone development.

Japan’s Himawari 8 satellite, the newest suite of sensors operationally observing the Earth caught (from left to right) Typhoons Linfa, Chan-hom, and Nangka on July 8th at 530UTC in its full disc visible band. This geostationary satellite has the highest spatial and temporal resolution of the current operational meteorological satellites in orbit. New York Times Article. NOAA is expected to launch a similar satellite next March – GOES R.

Part of this activity can be attributed to an unusually strong phase of the Madden-Julian Oscillation (MJO), another part to unusually warm equatorial Central and Eastern Pacific waters associated with El Niño, and yet another to the climatological upswing in West Pacific tropical cyclone activity this time of year. The Western Pacific is the most active region in the world for tropical cyclones and so it is not uncommon to have multiple intense cyclones at the same time. The MJO is a wave that circles the globe from west to east, and makes one revolution roughly every 30-60 days. It consists of a region of enhanced convective activity and a more inactive region. The amplitude of this enhanced and reduced convective pattern is typically highest over the Indian Ocean and West Pacific. Tropical cyclones are known to form more readily in the enhanced convective phase of the MJO. The warmer than normal equatorial Central and Eastern Pacific waters associated with El Niño is also known to aid in tropical cyclone development in those regions.

Linfa has since dissipated over southeastern China where it made a landfall as a category one equivalent typhoon. Chan-hom is approaching eastern China for a possible landfall near Shanghai and later likely impacting the Korean peninsula and Japan as a much weaker cyclone. Chan-hom became a typhoon near Guam, Rota and Saipan bringing torrential rains there, then passed between Okinawa and Miyakojima as a category four equivalent typhoon within near hurricane conditions on Okinawa. Nangka has weakened from a brief run at Super typhoon status yesterday thanks to a concentric eye-wall cycle and some increased wind shear. Nangka is expected to continue westward for the next few days before being steered northward – possibly threatening Japan.

Unusual Late June Warmth

20150620-20150625 Heat Index
Temperatures and Heat Index as observed at the Bermuda Weather Service. Dips in the temperature and heat index near the right side of the plot are related to isolated rain showers briefly lowering the temperatures. The area highlighted in orange includes heat index values in the “Extreme Caution” category.

Persistent southwesterly and west-southwesterly flow around the western side of the Bermuda-Azores High in combination with much warmer than normal sea surface temperatures in the Southwest Atlantic has led to a string of above normal temperatures and high humidity. The most recent forecast from the Bermuda Weather Service keeps this persistent flow in place through the end of June so there is little to suggest any significant changes to the temperatures or humidity during that period.

The normal daily maximum temperature this time of year is around 83F and maximum temperatures have been running near 86F for the past several days. Record maximum temperatures are near 88F and the highest recorded June temperature since 1949 is 90F at the airport. However, the temperatures are not the entire story – high humidity has come along with those higher than normal temperatures. The Dew Point temperature – a measure of the humidity – has also been consistently high. During this string of higher than normal temperatures, the dew point has been around 75F. Dew point temperatures above 70F are generally considered uncomfortable to oppressive.

The combination of high humidity and high temperatures has led to heat index values between 95F and 100F. The following advice or qualifications are attributed to the corresponding heat index values by the US National Weather Service: 90-105F “Extreme Caution”, 105-125F “Danger”, and >125F “Extreme Danger.” It should be noted that the heat index does not include the apparent cooling effects of wind or the apparent heating effects of sun. Further, inland and sheltered areas around the island could experience higher temperatures than at the airport where official observations are taken making this heat index value even higher.

The only inklings of a cool off would be found in any isolated shower activity over the next few days as a weak cold front approaches from the northwest. The approach of this front is expected to enhance surface convergence in the area which is expected to lead to isolated showers before the front dissipates to our north and lifts away to the northeast signalling the return to dry but humid southwesterly flow.

For Bermuda’s latest official weather information see the Bermuda Weather Service.

Analog Forecast for Hurricane Season 2015

After an interestingly successful analog forecast for Hurricane Season 2014, I will post a similar forecast for Hurricane Season 2015 where I try to use the same methods. Last March, I forecast that “tropical cyclone activity should be near or below normal”. The two sets of analogs used, ‘warm winter’ and ‘wet winter’, and an average of the hurricane activity for those two sets of analogs were provided:

Analog Forecast Actual
Named Storms 9-10 8
Hurricanes 5-6 6
Major Hurricanes 2 2
ACE 74.4-78.6 66.7

The analog years for that forecast included Hurricane Arlene (1963), Hurricane Faye (1975), and Hurricane Dean (1989) that had impacts on Bermuda. They accounted for 4/14 analog years as 1989 appeared in both the ‘warm’ set and the ‘wet’. While it is impossible to say whether this method of seasonal forecasting has any skill from just the one hurricane season’s forecast, I am compelled to try again this year to see if we get similar levels of accuracy.

500mb Height Anomaly for Jan and February 2015. Very low heights over Eastern North America, and very high heights over East-Central North Atlantic as compared to 1981-2010 climatology.
500mb Height Anomaly for Jan and February 2015. Very low heights over Eastern North America, and very high heights over East-Central North Atlantic as compared to 1981-2010 climatology.

The mean air temperature in Bermuda for January and February was 65.1°F (18.4°C) which is near normal as January was slightly warmer than normal and February slightly cooler. The total precipitation, however, was much greater than normal with 13.20″ accumulating for the two month, most of which fell in February which was the second wettest February on record at the Bermuda Weather Service, and one of three Februaries with more than 9″ of rain since 1949.

Continuing the method from last year, 7 years with the closest mean Jan-Feb temperature and 7 with the closest precipitation total will constitute the two analogs. Here they will be known as the “Temperature” analog and the “Precipitation” analog, rather than “warm” and “wet”.

Jan - Feb Temp Analog

Temperature Analog Set (Top): 1976, 1957, 1986, 1995, 2012, 1983, 2008.

Jan - Feb Precip Analogs

Precipitation Analog Set (Bottom): 1972, 1977, 1969, 1993, 1998, 1973, 1963.

Interestingly, neither set of analogs has a similar 500mb height anomaly pattern for January and February to 2015. The temperature analog captures the idea of higher heights over the East-Central North Atlantic, and lower heights to the north of that, over Greenland. However, it does not indicate lower than normal heights over Eastern North America or even come close to the magnitude of the anomalies anywhere. Further, the 500mb height anomaly pattern in the precipitation analog set is almost opposite that for 2015 with lower than normal heights across much of the North Atlantic and higher than normal heights over Greenland. This shows that these analogs are not necessarily representative of the prevailing atmospheric set up across the North Atlantic, which is not all that unexpected as the analog years are chosen based on conditions at one point. I expect this forecast to demonstrate less skill than last year’s as a result.

Using the same reasoning from last year’s post, I will look at the hurricane season in two parts, the first half of the season June-July-August, and the second half August-September-October.

JJA Temp Analog MSLPAJJA Precip Analog MSLPA Looking at the first half of Hurricane season and mean sea-level pressure anomalies for the Temperature Analog (Top) and Precipitation Analog (Bottom), we see that common to both analogs are lower than normal pressures across the Caribbean, Southwest Atlantic, and West Africa. The big glaring difference, however, lies near the Azores where in the Temperature Analogs, there is much lower than normal pressure and in the precipitation analog there is much higher than normal pressure. This may be a North Atlantic Oscillation (NAO) signal; when the NAO index is negative the Bermuda-Azores high is weaker than normal, and the opposite is true generally speaking. The sign of the NAO mainly impacts the trade winds in the Central Atlantic (negative NAO: weaker trade winds, positive NAO: stronger trade winds). According to Climatology, most of the tropical cyclone genesis locations are in the Gulf of Mexico, Caribbean, and Southwest Atlantic for the first half of the season, not in the Central Atlantic where trade winds become a factor.

[What is the North Atlantic Oscillation?]

In Summary: the lower than normal pressures in the Southwest Atlantic (including the Gulf of Mexico and the Caribbean) in both analog sets suggest that there will be greater than normal chances for tropical cyclone activity in that region. Lower than normal pressures over West Africa suggest that an active monsoon pattern will set up over than region and allow for enhanced disturbance activity in the Central and Eastern Tropical Atlantic, particularly in the Precipitation Analogs. Should the NAO be in a persistent negative phase, enhanced activity could extend eastward into the Central Atlantic with weaker than normal trade winds allowing disturbances to organize. This could result in an early start to the peak part of Hurricane season. Conversely, should the NAO be persistently positive during this period, stronger than normal trade winds across the Central Atlantic could result in a later start to the peak part of the season.

ASO Temp Analog MSLPAASO Precip Analog MSLPAAgain, Temperature Analogs (Top), Precipitation Analogs (Bottom). Looking ahead to the second half of the Atlantic Hurricane Season, including the peak in activity (climatologically around September 10th). Much of the pressure anomaly in the Western Atlantic vanishes. Near normal pressures across the Western Atlantic, Caribbean and Gulf of Mexico in both sets of Analogs. In the Precipitation Analog, we see lower than normal pressures over Central America indicating a more active Central American Monsoon pattern. Monsoon circulations from Central America are known to spin up into tropical cyclones over the Western Caribbean and Gulf of Mexico if given enough time over water. However, monsoon circulations could limit total numbers of named storms by taking so long to develop and absorbing approaching disturbances before they develop. Further, we see this persistent signal in pressures near the Azores. The Temperature analogs continue to indicate lower than normal pressures here while the Precipitation analogs suggest higher than normal pressures.

In Summary: Near normal chances for tropical cyclone activity seems likely across the Southwest Atlantic, and while they may be enhanced by an active monsoon pattern (as indicated in the Precipitation Analogs), generally, near normal chances for tropical cyclone activity extends into the Gulf of Mexico and Caribbean. The West African Monsoon continues more active than normal as indicated by lower than normal pressures there in both analogs. However, the same question of the Bermuda-Azores high and NAO phase arises as in the first part of the season.

Overall: Expect a Near ‘Normal’ Season.

Slightly above normal activity seems to be the theme in the Southwest Atlantic for the early part of the season, followed by near normal activity there during the second part (main phase) of the season with eyes on what the Central American Monsoon does. Between the two analogs, one pushing for stronger than normal trade winds, and the other for weaker than normal trade winds, it seems fair to expect near normal trade winds. That combined with a more active than normal West African monsoon suggests a slightly more active than normal Central Atlantic for the entire season. The big questions are: How strong will the Bermuda-Azores high be? and How strong will the trade winds be as a result?. With an apparently more active than normal West African Monsoon, there should be plenty of disturbances moving from east to west into the main development region of the Central Tropical Atlantic and the frequency with which they develop could be limited or enhanced depending on the strength of the trade winds. A weaker than normal Bermuda-Azores high might also allow cyclones that form in the Central Atlantic to recurve further out to sea rather than tracking into the Caribbean or West Atlantic where they could impact land.

According to 1981-2010 climatology, an average season has 12 names storms, 6 hurricanes, and 3 major hurricanes, with a season total ACE ear 93.5.

Temperature Analog
(1976, 1957, 1986, 1995, 2012, 1983, 2008)
Precipitation Analog
(1972, 1977, 1969, 1993, 1998, 1973, 1963)
Named Storms  12  10
Hurricanes  6  6
Major Hurricanes  2  2
ACE  104.0  84.7

These analog forecasts therefore show a near normal Hurricane Season for 2015. It is important to note that little can be said about the exact track of individual storms this far in advance, and it is important to remember that this forecast is based off of analogs from one spot (Bermuda) and therefore might not represent conditions across the Atlantic as a whole. This forecast also does not account for some important physical conditions that drive/impact tropical cyclone activity such as regional Atlantic sea surface temperature or ocean heat content and the impacts from whatever phase of the El Nino Southern Oscillation we are in during the height of the season.

Again, as mentioned at the beginning, these analog years were chosen because similar temperature and precipitation statistics were observed in Bermuda in Jan-Feb. However, neither set of analogs produced similar patterns of 500mb height anomalies for Jan-Feb. So this analog forecast is likely low confidence at best. Further, there were huge differences in activity in the years within both analogs, including some of the top ten most active seasons and the least active seasons. The commonality was they both featured seasons with hurricane impacts on Bermuda.

Residents in coastal areas should take the time before the season to prepare as regardless of the number of tropical cyclones that form. It only takes one cyclone to make for a bad season. It is therefore important to follow products on individual storms from the National Hurricane Center and from a local meteorological service such as the Bermuda Weather Service.

Stationary Front brings tank rain

Aug 10 14 1645UTC A
RGB Satellite imagery shows the convective activity along the stationary front near Bermuda this afternoon.

Hurricane Bertha merged with a stationary front along the east coast of the United States. As Bertha became extra-tropical in this merger, it passed to the north of Bermuda and a second area of low pressure formed to the south of Bertha. The extra-tropical remnants of Bertha and the secondary low pressure dragged the stationary front toward Bermuda as a cold front. This brought isolated breezy weather on Wednesday with isolated showers overnight. Heavy rains with thunder then moved in on Thursday. The cold front then stalled near Bermuda as both Bertha and the secondary low moved into the central north Atlantic. With the stationary front nearby isolated showers also remained in the area from Thursday night through Friday. They became more scattered on Saturday as an upper level disturbance approached from the west and the stationary front became more active. The same upper level disturbance pulled the boundary northwards enough for heavy showers to become widespread near Bermuda on this morning. Rain associated with the complex of heavy showers and thunderstorms to the near south of Bermuda has persisted into the early afternoon today (Sunday).

Bermuda Weather Service Radar image at 1:13pm this afternoon showing widespread steady rain over Bermuda with heavy showers and thunderstorms to the south and east of the island.
Bermuda Weather Service Radar image at 1:13pm this afternoon showing widespread steady rain over Bermuda with heavy showers and thunderstorms mainly to the south of the island.

Rain cooled air so far today has kept Bermuda in the low to mid 70s all day – very unusual as record lows are typically between 68F and 72F this time of year. This rain comes as Bermuda had been recovering from a dry start to Summer with below normal precipitation from early June into early July, with May typically being fairly dry. Some unofficial rainfall totals between Wednesday and Saturday are below. These are from personal weather stations found at Wunderground. An additional 1.00-2.50″ fell today with isolated amounts nearing 3.00″.

Location Wednesday Thursday Friday Saturday
Moore’s Lane, Pembroke 0.04″ 1.47″ 0.00″ 0.24″
Trimmingham Hill, Paget 0.02″ 2.52″ N/A N/A
Chaingate Hill, Devonshire 0.01″ 1.44″ 0.00″ 0.31″
Gilbert Hill, Smith’s 0.01″ 1.47″ 0.00″ 0.49″
Knapton Hill, Smith’s 0.02″ 1.72″ 0.00″ 0.51″

This stationary front remains nearby into Monday, but a wave of low pressure moving off the US east coast will pull the boundary northward across Bermuda with showers and potential for thunder Monday into Tuesday. Winds will also increase moderate to strong at times as this area of low pressure approaches and moves by Monday through Wednesday. Winds should begin easterly on Monday, then veer more southerly and southwesterly by Tuesday and Tuesday night as the boundary moves to the north of Bermuda.

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