Torrential January Rains


20170105-rain
Observations around Bermuda indicate a widespread 4-6″ of rain fell with isolated areas seeing >6″. Reports are sourced from Wunderground, WeatherLink, and Bermuda Weather Service.**Note that shaded areas on this map with fewer reports are less reliable.**

A deep long-wave upper level trough over the Eastern United States slowly edged eastwards on Thursday morning, this allowed a cold front to slowly advance towards and across Bermuda. Deep-layered flow out of the tropics allowed significant moisture transport across Bermuda. The frontal system, supported by upper level dynamics, was able to make use of that moisture in the form of an active band of heavy showers and thunderstorms that slowly progressed across the island, with a trailing region of light-moderate rains.

Rain totals for the meteorological day (0600 UTC to 0600 UTC) at the Bermuda Weather Service far exceeded several records, with 5.34″ of rain. This led to widespread flooding of low-lying and poor-drainage areas (see: Royal Gazette, Bernews). Furthermore, this rain total is roughly the amount of rain that Bermuda typically gets for the entire month of January. The 1981-2010 average January rainfall is 5.43″, or 5.30″ for 1971-2000 climate period.

Table: Records broken with yesterday’s rain, with reference to single, meteorological day records for the period 1949-present at Bermuda Weather Service.
Type of Record Previous Record
Record Wettest for the date 5 Jan 1.54″ (5 Jan 1994)
Record Wettest Jan Day 3.99″ (11 Jan 1986)
Record Wettest Winter (Dec-Jan-Feb) Day 3.99″ (11 Jan 1986)
5th Wettest Day (1.) 7.77″ (1 Jun 1996)
(2.) 6.77″ (13 Oct 2016 – Hurricane Nicole)
(3.) 6.21″ (31 Aug 1982)
(4.) 5.52″ (14 Jul 1980)
(5.) 5.34″ (5 Jan 2017)
(6.) 5.24″ (29 Oct 1967)

It is particularly impressive that this event made it into the top-5 wettest meteorological days because it occurred in winter. Rain events in winter typically produce lower totals for two main reasons, compared to summer events:

  1. these are typically associated with frontal systems that generally pass quickly
  2. there’s typically not as much atmospheric moisture available in winter

This event was associated with a frontal system, but it was progressing very slowly, and there was unusually high amount of atmospheric moisture available for rain because of the deep-layered flow from the tropics ahead of the system.

Model guidance performed well at picking up on the potential for a heavy rain event on the 5th since the end of December. This was reflected in the Bermuda Weather Service forecasts and forecaster’s discussion several days before the event.

More heavy rain is in the forecast

Yesterday’s front has progressed to the East of Bermuda, clearing its rainy weather with it. This is allowing much drier weather to settle in. However, an area of low pressure and frontal system, organizing over the lower Mississippi Valley today, will result in a similar frontal set-up to yesterday’s system over Bermuda at the end of the weekend.

Over the weekend, the low will push off the US East coast and track northeastwards around a long-wave trough over Eastern North America. As it does this, deep-layered southerly flow out of the tropics resumes ahead of a trailing cold front. This front will slowly progress eastwards toward Bermuda, passing on Sunday.

Model guidance is once again suggesting potential for a heavy rain event associated with this system. This is mentioned in the Bermuda Weather Service forecast and forecaster’s discussion. With soil freshly saturated, additional flooding in low-lying and poor-drainage areas is possible on Sunday.

A significant cool-down is then expected to start the work week. Temperatures are likely to struggle to reach 60°F on Monday as a continental polar airmass is drawn off of North America from the northwest across Bermuda.

See the Bermuda Weather Service for the latest official forecasts, warnings, and observations for Bermuda.

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