Bermuda Weather Service Forecast Discussion

For Wednesday, May 22, 2024  08:00 UTC

FORECASTER - Gary Hall 

NOWCAST: Wednesday morning through Wednesday night:
The stagnant and summer-like pattern continues, though not due to 
the traditional Bermuda-Azores high feature we often see this time 
of year. In the upper levels, persistent troughing is seen to our 
west through southwest, though shows signs of elongating and partially 
filling to our north tonight. This translates to cyclonic flow aloft 
while the surface is currently dominated by an area COL as weak high 
pressure is seen to the north while low pressure troughing exists 
to our west and south. However, the high to our north was responsible 
for a mostly dry day and night as local pressure rose from 1010mb 
to 1017mb in the previous 24 hours. The wind has been calm at times 
overnight and will remain variable in direction at 5 knots or less 
through much of today, though the COL appears to be transitioning 
to a more southerly dominated wind flow tonight, though remaining 
light. However, all guidance points to this developing southerly 
fetch to bring moisture in from the south with showers, perhaps light 
rain at times from this evening through tonight. Granted, the troughing 
is weak, but with a warm and moist airmass it doesn’t take much to 
create low ceilings within weak low-level convergence as is seen 
to occur tonight. QPFs are not impressive, though with decreasing 
stability there could be isolated moderate showers and even a slight 
chance of deep convection overnight which warrants a watchful eye. 
Given the slack wind flow across the region, the seas will be slight 
thus there is zero threat of a small craft warning, obviously.

Aviation: VFR today with MVFR ceilings becoming predominant this 
evening with the possibility of IFR periods through the night, especially 
within developing showers after sunset. Either runway could be in 
use today considering the TXKF wind will be variable in direction 
and less than 5 knots, so it is likely ATC opts for the favoured 
runway 12, or pilot choice. For more information please visit: www.weather.bm/aviation
 
SHORT TERM FORECAST: Thursday-Friday:
While zonal flow develops to our north, upper troughing lingers 
to our near west with the base extending southward through Hispaniola 
on Thursday, then gradually fills while transiting overhead on Friday 
only to finally flatten out and give way to weak ridging upstream. 
This translates to a continued weak southerly flow at the surface, 
with a persistent moist and humid fetch that results in cloudier 
skies over both days with the best chances for showers likely to 
be Thursday night into Friday. However, the GFS and ECMWF seem to 
be more optimistic than the UKMO, at least initially. The fly in 
the ointment come late Friday night is a developing low-pressure 
centre to our south, that no doubt will have tropical origins and 
could take on tropical characteristics going forward, though no output 
gives it any wind strength to speak of at this time. With high pressure 
attempting to nudge in from our west on Friday, there is a chance 
that the bulk of the showers stay south of the Island, though there 
are likely to be a few rogue showers that leak in from the south, 
though it doesn’t look like a washout by any means. Confidence wanes 
during this period, particularly due to the fact the pattern is so 
diffuse and fickle that subtle troughing features within weak high-pressure 
circulations will often result in nowcasting, which has been the 
case over the past several days. It is worth noting that the models 
have overall been too pessimistic lately, perhaps struggling with 
the slack wind pattern and the often-seen transitionary process from 
spring to summer. Smooth to slight seas will occur over both days. 
The only potential advisory during this period is a possibility of 
a thunderstorm advisory given the unstable airmass, though significant 
triggers are not very apparent at this time. 

LONG TERM FORECAST: Saturday-Sunday:
Confidence is very low for the weekend due largely to the broad 
low-pressure centre moving into our area from the south. All guidance 
has it but differs in size and more importantly the track. The main 
reason for track differences is the UKMO is more aggressive by bringing 
in surface ridging ahead of a cold front approaching from the distant 
northwest. This would drive the low further east which would mean 
a much drier outlook for the weekend. While the ECMWF has a considerably 
weaker low than the GFS and UKMO, the ECMWF shares in the thinking 
with the UKMO to drive it east with a northerly wind fetch keeping 
Bermuda mainly dry. The GFS gives little to no credence to the ridging 
to our north and west, therefore lunges the low overhead with a very 
wet Saturday before improving on Sunday. Given everything, the forecast 
will be tailored to the UKMO/ECMWF outlook and opt for an optimistic 
forecast until evidence is seen otherwise. This developing northerly 
wind, albeit light should take a bite out of the humidity which is 
always a welcome relief. However, since the UKO and ECMWF attempt 
to being the front in from the north on Sunday, we could see isolated 
showers develop Sunday night even as the front mostly fragments and 
dissolves as it draws near. Smooth to slight seas persist, though 
could touch moderate at times if the low materializes into something 
stronger than expected, though highly doubtful at this time.