Top 5 Driest October

October, typically the wettest month of the year for Bermuda, is ending with only 1.08″ (27.4 mm) of rain. This is 5.17″ (131.3 mm) below the 1981-2010 average for the month and is one of the driest Octobers on record. Below is a ranked table of October monthly rainfall data from NCDC* back to 1949.

Rank Year Rainfall Total
1 2019 1.08″ (27.4 mm)
2 1954 1.35″ (34.29 mm)
3 1997 1.64″ (41.7 mm)
4 1949 1.75″ (44.5 mm)
5 1975 2.28″ (57.9 mm)

So what’s going on?

This year, much of the deep tropical moisture remained confined to the Caribbean Sea and tropical SW Atlantic. Passing mid-latitude systems were only briefly able to bring transient plumes of this moisture across the island before either dissipating or moving on. Behind these systems, vast expanses of drier and more stable air is left in their wake. This can be seen in loop of Total Precipitable Water below.

201910.gif
a) Precipitable Water (PWAT) in mm where deep tropical air has higher PWAT. b) Total accumulated rainfall for the month from Bermuda Weather Service.

Typically in October slow-moving fronts can interact with tropical systems to deliver longer-duration and heavier precipitation events. This year, those types of systems remained deep in the tropics only interacting with the mid-latitudes until they were west of Bermuda.

A commonality of the 500-hPa anomalies associated with these top driest months is anomalous ridging over the northeastern United States or southeastern Canada. This anomalous ridging drives anomalous northerly flow into the western Atlantic (i.e. across Bermuda).  This anomalous northerly flow implies anomalous advection of cooler, drier air across the region. 2019’s pattern seems to be a variant of this general pattern with ridging extending northeastward across eastern Canada and Greenland. See below:

500hPaDryOctoberPattern
Monthly 500-hPa geopotential height anomalies from the 1981-2010 mean for the top five driest Octobers. Top row is 2019 (left), 1954 (middle), and 1997 (right). Bottom row is 1949 (left), 1975 (middle) and the mean anomaly pattern for 1954, 1997, 1949, and 1975 (right). NCEP/NCAR Reanalysis images provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/. All plots are on the same scale: -80 to 80m.

The human impact…

We can measure the impacts of this dry spell by estimating the direct impacts of reduced rainwater catchment and therefore increased demand for trucked and piped water. Looking at the rainwater supply vs. typical household demand, (e.g. our Tank Water model) we see that tank levels steadily declined through October. The rainwater supply exceeded typical daily demand on only a few days this month.

Below we show the model estimated tank volume for 2019 compared to the climatology of tank volume for our period of record 1949-2018. For January through the end of March, plentiful rain brought tank levels much higher than normal – exceeding the 90th percentile for this dataset. However, an unusually dry April followed by May which is usually the driest month of the year led to a dramatic decline to below normal capacity for the start of summer.

MeanVol
The model climatology (heavy blue line and shading) compared to the actual tank volume for 2019 through the end of October. Each red diamond represents supplemental water from 2 truck-loads being added to the tank.

It is worth noting that the typical modern Bermuda household runs an annual deficit in water supply. In the above model, each time trucked water is required, 2 truck-loads are supplied. On average, the typical home requires 12 loads of trucked water to supplement rainwater harvesting based on this model. In the model, the majority of supplemental water demand comes in Bermuda’s driest month May. October and November typically have low demand following the wetter early autumn months (see below).

TotalLoads
The total number of trucked loads each month for the period of record 1949-2018.

Fortunately, this dry spell is set to come to an end. Periods of rain and heavy, possibly thundery showers are expected ahead of and along a slow-moving frontal system. This should deliver some much needed tank rain. See the Bermuda Weather Service for the latest official forecast for Bermuda.

* Note that this dataset is unofficial and might not match official BWS figures, particularly early in the dataset.

Jerry Post-Tropical

As of the 6AM advisory from the National Hurricane Centre, Jerry has transitioned into a post-tropical cyclone. A tropical storm warning remains in effect for Bermuda as Jerry is still expected to bring tropical storm conditions to island this evening – a delay compared to earlier forecasts.

AL10_adv32_annotated
GOES-East Shortwave Infrared (3.9 μm) imagery with Post-Tropical Cyclone Jerry highlighted in a red box, and Bermuda blue. This imagery is at 0900 UTC 25 Sep 2019, the same time as the latest NHC advisory. This imagery displays ‘brightness temperature’ where lower temperatures are more cyan, and higher temperatures are greys. Note that the cyan colors are absent immediately around Jerry. This indicates a lack of cold, high cloud tops and therefore a lack of deep convection. The cyclone is characterised by a low-level swirl of shallow clouds with lower, warmer cloud tops (darker greys and black colours).

Jerry became completely devoid of deep convection yesterday as it succumbed to dry air and vertical wind shear. The storm was not able to take advantage of the brief period of lighter vertical wind shear and favourable upper-level dynamics as dry air prevented development. The storm has been unable to regenerate significant convection since then and has therefore transitioned into a post-tropical cyclone. Without deep convection, Jerry was steered further to the west and left caught in weaker low-level steering flow which delayed its approach to the island.

Jerry is now tracking east-northeastward, generally toward Bermuda. This motion is expected to continue until the centre of Jerry passes near or over Bermuda before midnight tonight. It is likely that Jerry will remain void of deep convection and continue to slowly weaken on approach as the surface low fills and decays.

bwswtnt25
The latest forecast track from the National Hurricane Centre via the Bermuda Weather Service Tropical Update Bulletin. On this track, Jerry is expected to pass less than 25 nautical miles (or directly over) Bermuda at 11PM Wednesday (today). See BWS’s Tropical Update Bulletins for the latest.

However, Jerry is still expected to bring squally showers starting this morning, with a period of tropical storm conditions (sustained winds ≥39 mph) this evening just before the storm-centre passes the island. Expect peak winds to be out of the south through southwest. With weakened infrastructure following Hurricane Humberto, these winds will likely result in scattered power outages.

Karen a ‘Potential Threat’

Tropical Storm Karen is currently tracking northward away from Puerto Rico. In two to three days, Karen is expected to be in weak steering flow south of Bermuda and is expected to be within 400 nautical miles of Bermuda in the next 72 hours which makes it a ‘Potential Threat’. No direct impacts are currently expected from Karen.  The current track forecast is at unusually low-confidence due to possible interaction with both a mid-latitude system and the remnants of Jerry, in addition to an environment marginally conducive for sustaining a tropical cyclone.

Continue to regularly check for the latest forecast updates from the Bermuda Weather Service and the National Hurricane Centre.

Jerry Threatening

Not a week after Hurricane Humberto’s strike, Tropical Storm Jerry is a threat to Bermuda with a Tropical Storm Warning in effect. Tropical Storm conditions are expected in Bermuda as Jerry nears and passes the island sometime between Tuesday evening and much of Wednesday morning. However, impactful weather is expected well in advance of the closest approach of the storm, with the outermost rainbands moving in as early as late this afternoon with strong easterly winds.

bwswtnt25
The latest forecast track from the National Hurricane Centre via the Bermuda Weather Service. On this track, Jerry is expected to pass 54 nautical miles away from Bermuda at 6AM Wednesday. See BWS’s Tropical Update Bulletins for the latest.

Today through Tuesday Morning

Forecast: Moderate to strong easterly winds veer southeasterly this evening and overnight. Occasional showers or periods of rain. Then strong southeasterly winds with gusty, possibly thundery showers and periods of rain Tuesday morning.

The midday update from the National Hurricane Centre finds Jerry 345 miles southwest of Bermuda, moving slowly north-northwestward at 7 mph. Jerry currently has maximum sustained winds near 65 mph and a minimum central pressure near 991 hPa.

Jerry remains in an environment dominated by strong vertical wind shear. The upper-level flow is from the southwest, while the low-level flow is from the southeast. This change in wind direction with height is forcing all of the inclement weather to remain over the eastern half of the storm, limiting organisation and therefore preventing strengthening despite Jerry being over high sea surface temperatures of 28-29°C.

Jerry is generally moving slowly north-northwestward into a break in the sub-tropical ridge. This general motion is expected to continue over the next 24 hours.

On a technical note, a cluster of deep convection near the center of Jerry appears to be keeping the cyclone vertically coupled. As a result, Jerry is steered by a vector-average of the southeasterly low-level flow and southwesterly upper-level flow. The east-west components of the flow at these two levels roughly cancel, leaving a net southerly flow across the cyclone and a storm motion toward the north. Whenever the convection wanes, the low-level circulation becomes decoupled, follows the low-level flow, and drifts more northwestward. Whether Jerry can maintain the deep convection will determine how far to the west the storm drifts. This will have a knock-on effect for the timing (i.e. a drift further west could delay impacts) and potentially the severity of impacts to Bermuda (i.e. a more vertically-coupled cyclone could be stronger). Convection is currently waning over the centre.

GeoColor_Jerry201909231500UTC_annotated
GOES-East GeoColor false color imagery with Tropical Storm Jerry and Bermuda highlighted. This imagery is at 1500 UTC 23 Sep 2019, the same time as the latest NHC advisory. Note that the centre of Jerry is displaced to the west of the deepest convection.

Tuesday Morning through Wednesday

Forecast: Strong southeasterly winds veering southerly and increasing to tropical storm force (> 38 mph) by Tuesday evening. Showers, some thundery, become more frequent with tornado potential (always possible with nearby Tropical Cyclones). Winds veer westerly as Jerry passes in the pre-dawn hours of Wednesday morning. At its peak, expect strong tropical storm conditions with hurricane force (>73 mph) gusts in elevated and exposed locations. Conditions then quickly improve on Wednesday with tropical storm conditions subsiding by late morning.

The strong vertical wind shear is expected to diminish somewhat as the upper-level trough cuts off forming an upper-level low which backs away to the west in the 18-36 hour timeframe. As this occurs, the upper-level winds across Jerry decrease and the flow pattern becomes more diffluent. This could result in some short-term strengthening, but it is on the whole unclear whether Jerry will be able to take advantage of this short window of more favourable conditions. The most likely outcome is that Jerry maintains its intensity but restructures, and this is what the National Hurricane Center calls for.

After 36 hours, the deep layered flow shifts out of the west in response to a deepening trough over Atlantic Canada, steering Jerry eastward then accelerating northeastward. The centre of Jerry is expected to pass less than 100 nautical miles away from Bermuda in the pre-dawn hours of Wednesday morning.

Continue to regularly check for the latest official forecast information from the National Hurricane Centre and the Bermuda Weather Service.

Humberto a ‘Threat’ to Bermuda

AL08_adv16_annotation
GOES-East GeoColor false color imagery of the Southwestern Atlantic with Humberto and Bermuda highlighted. This imagery is at 1500 UTC 16 Sep 2019, the same time as the latest NHC advisory.

As expected, Humberto has strengthened into a hurricane overnight Sunday. As of the latest advisory, Humberto had maximum sustained winds near 75 kts and a minimum central pressure near 978 hPa. The National Hurricane Centre’s latest forecast track for Humberto now indicates that the centre of the hurricane could pass 79 nautical miles (91 miles) away from the island on Thursday morning, and therefore impacts from Humberto are expected to commence in under 72 hours.

As a result, the Bermuda Weather Service now lists Humberto as a ‘threat‘ to the island, where the BWS’s Tropical Cyclone Threat listings are as follows (in order of increasing threat level):

No threat A tropical storm or hurricane has passed its closest point of approach to Bermuda and is moving away under the influence of a well defined steering flow.
Potential Threat The centre of a tropical storm or hurricane is expected to pass within 400 nm of Bermuda within 72 hours.
Threat Effects from a tropical storm or hurricane are possible within 72 hours and/or the centre of a tropical storm or hurricane is expected to pass within 100 nautical miles of Bermuda within 72 hours.

Tropical storm conditions are just about locked in for overnight Wednesday into Thursday as Humberto passes the island, and a tropical storm watch or hurricane watch is likely to be issued this evening. Watches are issued when adverse conditions from a tropical storm or hurricane are possible within the next 48 hours. They are upgraded to warnings if adverse conditions are expected within the next 36 hours.

However, the exact impacts are not yet so certain. The latest timeline from the BWS has showers with gusty winds moving in Wednesday morning. Showers become squally as tropical storm conditions (sustained winds 34-49 kts) spread across the island through Wednesday afternoon and evening with winds out of the south.

As with any tropical storm or hurricane, tornadoes are possible – primarily in these squally outer bands.

Southerly winds veer southwesterly then westerly, and strong tropical storm conditions (sustained winds 50-63 kts) develop overnight Wednesday into Thursday. Humberto makes its closest approach to Bermuda early Thursday morning (between 6 and 9 AM). During that time, winds peak and veer further, becoming northwesterly. Expect damaging winds (mainly vegetation damage, isolated property damage) and dangerous surf. Some minor coastal flooding is possible.

165105_5day_cone_no_line_and_wind
NHC forecast track from the BWS Tropical Update Bulletin today at noon. At this time, Humberto was expected to make its closest point of approach at 7AM on the 19th Sep – just 79 nautical miles (91 miles) away to the north-northwest.

Humberto is expected to be a serious hurricane at its closest point of approach. Today and tomorrow, high sea surface temperatures ~29°C support intensification from its current category 1 strength to category 3. Increasing vertical wind shear and encroaching drier air could begin to degrade the hurricane’s core and begin a weakening trend on Wednesday into Thursday. However, favourable upper-level dynamics and continued high sea surface temperatures could offset some of these weakening effects. Altogether, the NHC currently expects a slowly weakening category 2 or 3 hurricane passing the island.

[Saffir-Simpson Hurricane Wind Scale]

A track error of around 50 nautical miles, plus an intensity error of just 10 kts could bring major hurricane conditions to Bermuda Thursday morning. (Typical 72 hour track error is 102 nautical miles, and typical 72 hour intensity error is 20 kts.)

[Typical Track and Intensity Errors]

Make preparations for this worst  case scenario now and tomorrow before conditions deteriorate Wednesday morning. Continue to regularly monitor updates from the Bermuda Weather Service and the National Hurricane Centre.

Tropical Storm Humberto

AL08_adv08_annotation
GOES-East GeoColor false color imagery of the Southwestern Atlantic with Humberto, the Bahamas and Bermuda highlighted. This imagery is at 1500 UTC 14 Sep 2019, about the same time as the latest NHC advisory.

Tropical Storm Humberto formed just east of the Bahamas islands last night. As of the midday advisory from the National Hurricane Centre, Humberto has maximum sustained winds near 50 mph and a central pressure near 1005 hPa. Humberto is narrowly keeping the worst weather offshore of the islands of the northwestern Bahamas which were so severely hit by Hurricane Dorian at the beginning of the month. However, squally weather is still expected and tropical storm warnings are in effect for the northwestern Bahamas.

Confidence increasing for Bermuda impacts from Humberto

a. Now through Monday

Humberto is currently in a region of weak steering flow and it is thus stationary just east of the northwestern Bahamas. An upper-level low to the west of Humberto is imparting some vertical wind shear over Humberto and preventing significant organisation. This upper-level low is drifting away to the west, so vertical wind shear is expected to decrease today. This decrease in vertical wind shear in combination with Humberto embedded in a humid environment over sufficiently high sea surface temperatures is allowing steady strengthening today.

Tonight and tomorrow, southerly steering flow starts to impinge on Humberto as high pressure to the northeast of the storm begins to weaken and drift southeastward. As a result, Humberto is expected to track northwestward and then northward this evening and Sunday. The environment is expected to continue to become more favourable for Humberto and the storm is expected to become a Hurricane on Sunday.

By Monday, a deepening trough over Atlantic Canada begins to impart westerly flow over Humberto. The storm then begins to track toward the east, continuing to strengthen.

145105_5day_cone_no_line_and_wind

b. Tuesday through Thursday

On Tuesday, Humberto is expected to accelerate eastward and then northeastward around the southern edge of a trough positioned over Atlantic Canada. This takes a hurricane-strength Humberto near Bermuda overnight on Wednesday into the pre-dawn hours of Thursday.  The environment on approach to Bermuda is expected to remain favourable for strengthening. It is possible that Humberto could be a major hurricane on approach to Bermuda.

While confidence is increasing for hazardous surf, rain, and wind from Humberto as early as Tuesday night in Bermuda, these impacts are not yet locked-in. In Bermuda, now is the time to start going over your hurricane preparedness plan and make a point of regularly checking for updates on Humberto from the Bermuda Weather Service and National Hurricane Centre

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)

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.