Bermuda Weather Service Forecast Discussion

For Saturday, September 25, 2021  09:00 UTC

FORECASTER - Alex Young 

Bermuda Weather Service Forecast Discussion
Saturday 24th September, 2021, 0900UTC
Forecaster- Alex Young

NOWCAST: (Today and tonight)
The night was mostly clear to partly cloudy with no showers observed 
over the island nor the marine area. Latest satellite shows the same 
picture with partly cloudy skies over the region while cloudier skies 
remain to our north associated with Sub-Tropical Storm Teresa. Latest 
radar imagery shows isolated showers over the northern marine area 
while north of the marine area a band of showers associated with 
the tail-end of Teresa continues to creep to the south. Transient 
upper ridging is seen over the region as a well-defined upper-level 
low remains to our northeast and vertically stacked with STS Teresa 
as evident on WV imagery. Latest surface analysis shows STS Teresa 
to our north with a slack flow and transient ridging between Teresa 
and an approaching slow moving frontal boundary from the west. This 
has initialized really well with global models and there is high 
confidence in the synoptic set up for today. Global models are in 
good agreement that the transient upper ridge will remain dominant 
today while a broadscale upper trough approaches from the northwest. 
In addition, global models agree that a surface high will form to 
our southwest which will maintain dry and sunny weather today and 
partly cloudy skies tonight. However, there is still the low chance 
of isolated showers creeping into the northern marine area as STS 
Teresa remains somewhat nearby. Winds overnight at the airport and 
NMB, gradually backed from the northwest to the southwest while remaining 
around 6-12 knots.  Currently, winds have shifted a bit more west-southwesterly 
and remain near 6-12 knots. This has also initialized well and confidence 
remains high. Expect winds to veer westerly by this afternoon with 
little change in intensity.  Latest OPC has seas near 6ft which is 
in agreement with model initialization. Seas are forecast to slowly 
abate as the day progresses.

Warning Strategy: No watches or warnings are in effect for today 
and tonight.

AVIATION: Runway 30 will be the active runway for the day and night 
as a near westerly wind persists. No aviation hazards are anticipated. 
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SHORT TERM FORECAST (Sunday through Monday)
Broad upper troughing moves into the area on Sunday and the trough 
digs further to the south on Monday while stalling over the region. 
This provides upper level enhancement for a slow moving frontal boundary 
that is well forecast by the global models to begin affecting the 
island during the pre-dawn hours of Monday. As result, expect Sunday 
to begin sunny and dry before increasing cloud rolls in overnight 
as the front approaches. Monday will see periods of wet weather and 
the chance of thunderstorms as the front passes and stalls over or 
to our near east. Showers and thunder may ease at times but current 
guidance continues to suggest ‘tank rain’ and localized flooding 
remains a possibility. Winds tap more southwesterly on Sunday before 
sharply veering northwest and then northeast on Monday as the front 
passes . Seas ease slight to moderate under the weakening wind regime.

Warning Strategy: A thunderstorm advisory will likely be needed 
for Monday.

LONG TERM FORECAST (Tuesday through Wednesday)
The upper trough remains over the region further enhancing showery 
activity at the surface. Global models remain in good agreement that 
the stalled front maintains periods of showers and per instability 
indices thunderstorms over the region especially on Tuesday where 
further localized flooding is likely. The chance of thunder have 
been kept off the forecast for now but will likely be included in 
the forecasts by tomorrow once the trend remains the same. Models 
hint of the front exiting the region with drier weather possibly 
returning by Wednesday, though confidence is low on this solution. 
Expect a light to moderate wind to persist over the period with variances 
in direction at times. Seas will tend more slight towards Wednesday. 
Hurricane Sam will continue to move closer to our region of concern 
and we continue to closely monitor its movement.

Warning Strategy: A thunderstorm advisory may be needed for Tuesday.

TROPICAL: Please refer to the NHC Tropical Weather Outlook for the 
latest tropical information in the Atlantic Region