Bermuda Weather Service Forecast Discussion

For Thursday, September 16, 2021  20:00 UTC

FORECASTER - Troy Anderson 

Nowcast (Thursday Night)
The day saw an increase and thickening of cloud cover as the disturbance 
to the southwest began to move northward. Locally, high pressure 
began to erode as the rather unorganized disturbance approached. 
A few early showers began to near the area under the cirrus outflow 
extending from the southwest. Mainly moderate southeasterly winds 
continued through the day with low-end moderate seas. The latest 
GOES 16 visible satellite imagery shows Bermuda near the outer extent 
of the forming circulation within its northeastern quadrant. Extensive 
amounts of upper cirrus continues to spill into the area with thickening 
mid-to-lower level cloud on the horizon. It should be noted that 
the developing low pressure continues to remain asymmetric with majority 
of its convection and banding on the northeast side. The centre continues 
to appear rather barren from significant convection. The latest RADAR 
imagery shows a few sporadic showers in the vicinity and around the 
marine area with more prominent banding of rain and showers in the 
distant southwest and west approximately 150 km mostly out. 

In the upper level analysis, Bermuda sits under the influence of 
upper ridging with an upper trough retreating in the nearby west. 
The positioning of the trough keeps the developing low pressure area 
under its forward side and continues to encourage development ahead 
of its axis. Synoptically, Bermuda sits under the southwestern flank 
of the broad high pressure with ridging extending from the northeast. 
Global models have done quite well with the synoptic pattern for 
the nowcast period. Expect the potential development as this disturbance 
moves northward and its centre moves to our west between the Carolinas 
and Bermuda. As most of the convective activity extends out from 
the centre into to the northeast, Bermuda continues to remain in 
the path of the advancing outer banding. Scattered showers and rainfall 
develop and approach from the southwest and west increasing in frequency 
and spatial coverage through the night. The potential for precipitation 
increases as the centre of the low pulls further north and the outer 
bands begin to approach the area thus the chance of overnight showers 
exists. Wind data has aligned fairly well and maintains the light 
to moderate southeasterly regime. With respect to the sea state, 
the latest OPC analysis continues to hold 3-4 foot seas in our area. 
The UK and GFS models hold low-end moderate seas through the overnight 
period. Temperature data indicated by the NWP suite and the GFS MOS 
guidance bring minimums towards 77F, and with the moderate wind regime 
and extensive spilling cloud cover, this will likely be the case, 
perhaps +/ 1 degree. The leading bands of the system do carry the 
threat of embedded thunderstorms and heavier showers though indices 
remain fairly benign overnight. Confidence remains moderate to high 
for the nowcast period. No watches or warnings are anticipated during 
the overnight period.

Aviation: Mainly VFR conditions start the night as moderate winds 
near 9-13 knots continue at TXKF from 140 degrees. There remains 
the chance for developing showers from the southwest overnight with 
MVFR conditions and ceilings. For more information please see www.weather.bm/aviation. 


Short-Term (Friday- Saturday)
 For Friday and Saturday, the developing low pressure will move 
west of the island between the South Carolina coast and Bermuda. 
On Friday, the extensive cloud cover will continue and thicken as 
showers and rain encroach from the west and southwest. Global models 
bring the leading bands forward creeping towards Bermuda. This pattern 
will see the growing potential for showers through the day towards 
the evening. Wind data continues to indicate the maintenance of moderate 
winds veering from the southeast to south. Models tend to diverge 
in intensities but bring an increase to the wind regime by Saturday 
morning from 14-22 knots with the potential for stronger gusts nearing 
30 knots in and around showers. Expect the developing low to drag 
a band across the area on Saturday leading to showers and pronounced 
rainfall. Heavier precipitation is expected on Saturday morning and 
afternoon before beginning to taper off as the band moves away. At 
this time, models begin to start a transition towards baroclinicity 
with a front beginning to develop behind the initial band. A Small 
Craft Warning has been made valid from Saturday morning for winds 
flirting and above the 20-knot threshold. The UK and GFS increase 
seas towards 4-7 feet but maintain states below the 9-foot threshold. 
Concerning convective development, indices increase and become fairly 
conducive on Saturday especially with the upper support, increase 
of moisture, and increasing instability with the band overhead. The 
chance of thunder has been maintained within the forecast and a Thunderstorm 
Advisory remains on the table. NHC has assigned the developing low 
pressure a 70% chance of development in both the 2- and 5-day periods 
A blend of solutions has been applied to the forecast as try to handle 
the position the developing system. Moderate confidence exists for 
the short term period. 

Long-Term Period (Sunday- Monday) 
On Sunday, global solutions continue to hint at a dry wedge developing 
during the morning before the approach of a secondary band of moisture. 
This may see a brief dry period during the morning before the development 
of scattered to widespread showers. During this time, the centre 
of the low will move to the distant north near Nova Scotia and the 
Canadian maritime while a boundary will extend southward and approach 
from the west. Analysis of this band reveals the characteristics 
of a front-genesizing boundary. Expect the passing of this secondary 
boundary on Sunday before clearing to the south early on Monday. 
Moderate to strong west-southwesterly winds ease during the afternoon 
becoming moderate and veering  northerly by Monday. Global models 
indicate the potential for the development of cold air stratocumulus 
clouds with a few showers in the area before both drying and clearing. 
The valid Small Craft Warning will likely end on Sunday afternoon. 
The UK and GFS data holds and then slowly decreases seas 4-6 feet. 
Confidence remains moderate to low for the long term. Wind data keeps 
expected local intensities well below the tropical storm strength, 
but the proximity of this system  continues to warrant the monitoring 
of this system as it moves northward. A tropical storm watch/warning 
is currently not anticipated.

Tropical Weather Outlook
Please refer to the NHC Tropical Weather Outlook for the 
latest tropical information in the Atlantic Region   https://www.nhc.noaa.gov/gtwo.php?basin=atlc&fdays=5