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.

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.

Bermuda Tropical Cyclones

Following from my previous post, I’m looking into where Tropical Cyclones (TCs) that impact Bermuda form, how they track, and when during Hurricane Season they impact the island. Using NOAA’s Historical Hurricane Tracks mapping tool, I found a list of TCs that came within 75 nautical miles of Bermuda. I made sure they were at least tropical storm strength (ie. maximum sustained winds of >34 kts) and got their track coordinates from the National Hurricane Center’s latest HURDAT2 dataset. I did this for all cases fitting the above criteria for seasons between 1950 and 2014.

This turned up 29 cases in 65 years which corresponds to a TC coming within 75 nm of Bermuda every 2.25 years. For this period, the median date of a TC within 75 nm of Bermuda is September 11th, with an inter-quartile range (ie. the range that includes 50% of the cyclones) from August 15th to September 27th. This coincides with the peak in Atlantic Hurricane Season Activity which is around September 10th.

Of the three main genesis regions for all Atlantic TCs, the area between the Bahamas and Bermuda (SWATL), and the Main Development Regions (MDR) are responsible for cyclones that pass near Bermuda. No cyclones that formed in the Caribbean between 1950 and 2014 tracked within 75 nm of Bermuda – however, there have been cases where a cyclone has formed in the Caribbean and tracked near Bermuda prior to 1950. For all Atlantic TCs, the Caribbean is the most frequent genesis location, meanwhile the SWATL and the MDR each had less TC formation than the Caribbean, but had similar numbers between the two. For TCs that passed near Bermuda, about two times more formed in the SWATL than in the MDR.

If we class the SWATL as west of 60W, and the MDR east of 60W, then we see that the cyclones that form in the SWATL are on average weaker than those that form in the MDR by the time they pass within 75 nm of Bermuda. On average, the cyclones that formed in the SWATL passed Bermuda with maximum sustained winds near 53 kts (strong tropical storm strength), while those that formed in the MDR passed Bermuda with maximum sustained winds near 77 kts (hurricane strength).

Genesis Locations
The genesis locations of TCs that passed within 75 nm of Bermuda with maximum sustained winds over 34 kts, between 1950 and 2014. The red and white bulls-eye over the SWATL is where the majority of Bermuda TCs form. A secondary peak in genesis is in the MDR.

The tracks of TCs that pass near Bermuda are generally straightforward: if they form in the SWATL they either get picked up by strong mid-latitude flow and propelled northeastward toward Bermuda, or they meander in the area and dissipate. However, if they formed in the MDR, they track west-northwestward around the Bermuda-Azores high, then recurve to the north into a weakness in the Bermuda-Azores ridge near Bermuda’s longitude. Because TCs that form in the MDR often have days over warm open waters, they tend to be stronger by the time they make it to Bermuda, whereas TCs that form in the SWATL are already very close to the island and so don’t have as long to strengthen before passing. These SWATL cyclones are often also afflicted by dry continental airmasses from North America.

Bermuda Tracks
The track density plot of the 29 tropical cyclones that passed within 75 nm of Bermuda from 1950 to 2014.

With the 2015 Atlantic Hurricane Season now under way, please keep an eye to the tropics through the National Hurricane Center‘s and the Bermuda Weather Service‘s forecast and warning products. With lower that normal sea surface temperatures through the MDR, warmer than normal sea surface temperatures in the SWATL, and an El Nino in place to increase wind shear across the MDR – it is quite possible that the SWATL will be more active than normal. This increases the chances for a TC to pass near Bermuda this season.

Where do Atlantic Tropical Cyclones Form?

The official start to the 2015 Atlantic Hurricane Season is fast approaching on June 1st. With that in mind, a question I want to answer today is: Where do tropical storms and hurricanes form?

To answer this I turned to the National Hurricane Center’s Best Track Data in their HURDAT2 dataset The hurricane seasons for the period 1950-2014, and only tropical and subtropical cyclones with top 1-minute sustained winds greater than 34kts were included. The HURDAT2 dataset contains 6-hourly data for Atlantic Tropical cyclones. Once it was determined that the tropical cyclone had fit the above criteria (1950-2014, at least TS,) its genesis coordinates were recorded (the latitude and longitude of its first entry as a tropical or subtropical cyclone).

These coordinate locations were split up into cyclone groups based on their peak intensity as follows:

– Peaked at tropical storm strength (34-63 kts)
– Peaked at minimal or moderate hurricane strength (64-96 kts)
– Peaked at major hurricane strength (over 96 kts)

Using an R spatial statistics package {spatstat} the spatial density of the genesis locations was calculated and plotted on a map of the Atlantic Basin. The resolution for these plots is roughly 0.664 x 0.291 degrees or about 76 km for the diagonal across each pixel.

Some interesting patterns are immediately evident in the distribution of genesis location density. For ‘All Tropical Cyclones,’ there are two main regions:

1. The open Tropical Atlantic known as the ‘Main Development Region – MDR’ (the area between the West Coast of Africa and the Caribbean)

2. The Southwest Atlantic (the area north of the Bahamas, the northwest Caribbean, and the Gulf of Mexico)

In region 1, the main source of tropical cyclones is likely African Easterly Waves, inverted troughs that move from east to west into the Atlantic associated with disturbances in the West African Monsoon. In region 2, some of these African Easterly Waves don’t encounter an environment favorable for development until they get steered into or North of the Caribbean where they have another chance to develop. Less frequently, midlatitude weather systems (extratropical cyclones) get steered southward into the subtropics and tropics where they encounter relatively weak steering flow and begin to transition into subtropical or tropical cyclones over the warm waters. This is more common north of the Bahamas. A further mechanism for development, particularly in the Northwestern Caribbean and Gulf of Mexico, is related to the monsoon trough over Central America. This trough, present mainly during the summer months, can be displaced northward into the Northwest Caribbean or Gulf of Mexico where, if it persists under favorable conditions, it can organize into a tropical cyclone. The number of cyclones that form in region 2 is similar to that in region 1.

All Tropical Cyclones

For those tropical cyclones that did not intensify to become hurricanes, ‘Just Tropical Storms,’ most form in the Gulf of Mexico or north of the Bahamas. This area takes up most of region 2 from above. This is likely because cyclones that form in the Gulf of Mexico are so close to land that they either make landfall and dissipate, or interact with dry air that disrupts their circulations.

For the cyclones north of the Bahamas, this is also a two-pronged possible explanation; cyclones that form here are typically quickly pulled poleward into the midlatitudes where they encounter wind shear and cooler waters, or they are formed from transitioning extratropical cyclones. The midlatitudes are a hostile place for tropical cyclones because of the strong wind shear associated with the jet stream, the cooler waters further north, and dry air. Dry air and wind shear from the midlatitudes is often nearby transitioning extratropical cyclones and this often slows the transition and limits intensification.

Region 2 is also active through the entire season whereas region 1 is active mainly during the peak of the season. Less than ideal early and late season conditions also likely play a role in limiting the intensity of cyclones here.

Just Tropical Storms

Probably for similar reasons, minimal-moderate hurricanes form most frequently in the Northwest Caribbean as seen on ‘Minimal Hurricanes Only.’ Here, they quickly interact with land limiting their intensification. Meanwhile, on the ‘Major Hurricanes Only’ plot, we see that the most common formation area is off the west coast of Africa. Cyclones that form in this region are notorious for their longevity and intensity and are informally known as “Cape Verde-type” Hurricanes. This region not only becomes most active during the peak of the season when the tropical Atlantic is most favorable for tropical cyclone development, but with a typical track towards the west-northwest, there is no land to interact with for over a thousand miles – plenty of open ocean to intensify over.

Minimal Hurricanes OnlyMajor Hurricanes Only

A more subtle pattern that becomes most evident in the ‘All Hurricanes’ plot are the two maximum of formation frequency west of the Cape Verde islands. This could be due to diurnal variations in convective intensity that help to organize the African Easterly Waves (ie. showers and thunderstorms flare up around the disturbance at around the same time each day – think afternoon thunderstorms.) Since each of these waves move westward at similar speeds, the maximum near the Cape Verde island could be related to the first diurnal maximum in convection, and the maximum in the central tropical Atlantic (further to the west) could be related to the second diurnal maximum. (ie. the disturbance becomes a tropical cyclones after one day over water or after two days.)

All Hurricanes

Wind shear and stronger trade winds in the Eastern Caribbean limit tropical cyclone formation. The wind shear disrupts the organization of deep convection – a key element of tropical cyclones. Similarly, strong trade winds keep the initial disturbance for forming a closed circulation which prevents the tropical cyclone from sustaining the deep convection it needs. Overall, conditions are not as favorable for tropical cyclone formation in the Eastern Caribbean despite the adequately warm waters.

It should be noted that while more major hurricanes are born in the Cape Verde region than anywhere else in the Atlantic, cyclones that form here do not necessarily impact any land and can therefore go unnoticed. The cyclones that form in the Southwestern Atlantic are much more likely to impact land, particularly when they form in the Caribbean and Gulf of Mexico, but they less frequently become major hurricanes. The most intense hurricane on record, Hurricane Wilma (Oct, 2005), formed in the Northwest Caribbean. Bermuda has also been impacted by major hurricanes originating from both of the active regions of the Atlantic. Hurricane Fabian (Aug-Sep, 2003) formed in the Main Development Region, while the Havanna-Bermuda Hurricane formed in the Northwest Caribbean (Oct, 1926.)

Where do landfalling cyclones and cyclones that impact Bermuda typically form? See a follow-up post for the answer to this!

The Best Track Data can be found here.

A Weak Cold Front Passage

20150514-1315z-RGB-EAUS
RGB Satellite image at 10:15am local time showing the cloudiness over Bermuda associated with this morning’s front.

A weak cold front passed between 8:30 and 9am this morning bringing isolated showers followed by patchy rain and drizzle. Westerly winds veered sharply to the north following the passage of the front. Expect slightly cooler and less humid weather over the next few days with persistent north-northeasterly flow.

North to north-northeasterly winds continue this afternoon and strengthen overnight, becoming moderate, as a low develops along the front far to the northeast of the island, and a trough rotates around it, increasing the pressure gradient and, in turn, winds for Friday. As this low slowly drifts away to the northeast and high pressure builds in from the west, this moderate north-northeasterly flow is maintained through Saturday. By Sunday morning, high pressure will have moved in with light northerly winds. However, winds start backing westerly ahead of another weak frontal system, and increase again, moderate to strong, by Sunday night. This next system could bring showers and rain for Monday evening.

For the official forecast and weather information for Bermuda see the Bermuda Weather Service.

Ana Landfalls in South Carolina, Weakens

Tropical Storm Ana

Ana transitioned from a subtropical to a tropical storm on Saturday at around 6am Bermuda time. Tropical Storm Ana then slowly moved northwestward while weakening for a landfall in South Carolina near Myrtle Beach at around 7am Bermuda time this morning.

Tropical Storm force gusts were observed in isolated spots of South and North Carolina, mainly along and near the coast, for much of the morning leading up to and following landfall. Heavy rain continued as the now remnants of Ana drift through North Carolina with drenching rains of over 4″ already being reported near where Ana made landfall. Minor flooding concerns continue because of Ana’s continued slow motion this evening.

Meanwhile, mostly dry weather is expected in Bermuda for the next few days. A weak cold front will enhance the chances for precipitation as it passes on Thursday. Expect winds to slowly veer from southeast to southwest through Tuesday, then sharply veer from southwest to northwest on Thursday with the frontal passage. Generally moderate winds could become strong for a time ahead of Thursday’s front.

Subtropical Storm Ana Forms

The area of troughiness north of the Bahamas earlier this week has drifted northwards and organized a closed area of low pressure, cut off from fronts, and is sustaining organized deep convection. As a result of these structural changes in the low (confirmed via Air Force Hurricane Hunter missions,) the National Hurricane Center has determined that this low has become a Subtropical Storm.

Ana is expected to meander over the Gulf Stream, just offshore of the US east coast, for two to three more days before being picked up and taken northwestward into South or North Carolina by increasing southerly flow ahead of a mid-latitude weather system late in the weekend/early next week. This mid-latitude system is also expected to bring heavy snow to the Wyoming, Colorado, Nebraska and South Dakota – in addition to a “moderate” risk for severe thunderstorms (including hail, damaging winds and tornadoes) in the southern Great Plains as it pushes eastward.

Screen Shot 2015-05-08 at 10.41.53 AM
Counties highlighted blue for Winter Storm Watches from NWS at 10:45am local.
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Storm Prediction Center convective outlook for Friday showing a moderate risk for severe weather in red over Oklahoma and Texas.

Meanwhile, troughiness related to what were the fronts that Ana shed to become subtropical in addition to a cold front approaching from the northeast are bringing chances from showers and thunderstorms this afternoon through early Sunday morning. As a result, the Bermuda Weather Service has issued a Thunderstorm Advisory. The cold front doesn’t quite make it across Bermuda, dissipating on Saturday, so winds remain light to moderate and southeasterly through the beginning of next week thanks to Ana to the distant southwest this weekend and high pressure to the northeast behind that weak front early next week. While the cold front never really made it to Bermuda, slightly less humid air will still filter in on return flow from the southeast associated with that high pressure to the northeast of Bermuda early next week.

Subtropical Storm Ana circled in red, motion indicated by a red arrow. Troughiness near Bermuda circled with a dashed red line, and an approaching cold front highlighted with a blue line.
Subtropical Storm Ana circled in red, motion indicated by a red arrow. Troughiness near Bermuda circled with a dashed red line, and an approaching cold front highlighted with a blue line. Overtop RGB Satellite imagery at 10:45am local time.

For the latest on Ana see updates from the National Hurricane Center and the Bermuda Weather Service – Tropical Products. For the latest official forecasts including watches and warnings for Bermuda, also see the Bermuda Weather Service.