Bermuda Weather Service Forecast Discussion

For Tuesday, December 01, 2020  20:00 UTC

FORECASTER - Gary Hall 

NOWCAST: Tuesday evening through Tuesday night:
A low-level convergent band was situated over the east-end of the 
Island through the morning with stubborn cloud and light showers 
that finally began to break up this afternoon. Meanwhile, cirrus 
decks are leaking over the top of the upper ridge to the west while 
blanketing Bermuda. Area wind has been steady from the southeast 
in the 15-knot range, give or take a couple of knots. The ridge will 
continue to progress eastward with the apex passing overnight. Specifically, 
the 700hpa ridge axis will pass after 06Z/02L which at that point 
opens the door for impending showers to develop as a cold front approaches 
from the west. Aside from a random shower, it does not appear that 
there will be much by way of pre-frontal activity as this front can 
be classified as a traditional “Active boundary”, meaning most precipitation 
will occur along and slightly behind the front, or on Wednesday morning. 
A gradual wind veer to the south will occur tonight while slightly 
increasing to touch 20 knots at times late tonight as a marginal 
isobaric squeeze takes place immediately ahead of the front. Seas 
are currently near 6’ but will be on the rise overnight…but not to 
small craft warning level just yet. However, the wind increase is 
enough to warrant the issuance of a warning starting late tonight. 


Aviation: VFR no hazards—dry runway. Southeasterly wind ~ 15 knots 
becomes southerly overnight ~ 15 knots. For further aviation information 
please visit: www.weather.bm/aviation
 
Short Term: Wednesday-Thursday:
The active cold front by all accounts will spread showers across 
the region on Wednesday morning then push through by early to mid-afternoon. 
Both the UKMO cloud height product and the MOS show a window of IFR 
conditions and possible mist near mid-day then rapidly becoming VFR 
in the afternoon onwards. There does not seem to be any appreciable 
wind surge ahead of the front and as usual, the UKMO is excellent 
to point out a lull as it passes and veers westerly…5-8 knots probable 
within the boundary and a minimum pressure only falling to 1016mb. 
The gradient level wind never surpasses 20 knots per all output so 
there is virtually no threat of any surprise gusts. Once the front 
clears to the south, expect a sharp veer to the north-northwest in 
the 12-18 knot range, perhaps touching 20 knots for a time into the 
early evening before settling a bit overnight. The upper trough will 
meekly pass through by Wednesday evening with zonal flow to follow 
before short wave ridging moves in on Thursday evening and night. 
The parent low to the front will be over northern Quebec thus Bermuda 
being so far south and a lack of cold air aloft the threat of thunderstorms 
with the front seems very minimal at this time…rather heavier showers 
with possible patchy rain for a time. The low-level lapse rate will 
cool, but probably not to the point of developing cold air stratocumulus 
until sometime on Thursday afternoon or night, and likely not extensive 
and long lasting. A transient high pressure with a centre value ~ 
1025mb will move off the east coast under the short wave ridge and 
be to our north along 34-35 degrees from Thursday night and into 
early the next period. Northerly wind on Wednesday may tend north-northeasterly 
late Thursday as it reacts to the high centre to our north, but aside 
from the ECMWF which has a brief swath of 20 knots, we should safely 
under 20 knots. The current small craft warning runs into tomorrow 
morning for wind only, but the end of the 20 knot wind will coincide 
with the onset of 9’ seas thus expect the small craft warning to 
run into Wednesday evening/night before expiring by Thursday morning 
at the latest. 

Long Term: Friday-Saturday:
All indications are for Friday to be a fair weather day, though 
there are hints of lingering moisture caught along the base of the 
ridge to propagate northward and be in our vicinity at times. As 
such, a variably cloudy day is anticipated, but still mostly dry 
while a gradual warming takes place, including the dew points as 
the high exits to the distant northeast and local wind veers easterly 
by Saturday morning, then southeast to south while increasing Saturday 
night. It is important to note that the models have been struggling 
with the evolution of the low and front developing over the southeastern 
US over the weekend. There is fair confidence for Friday and into 
much of Saturday, but after that everything is extremely muddy. Another 
issue is each model has no consistency either, so not only are there 
vast differences on strength, position and timing from one model 
to the other, each individual model cannot settle on one particular 
outcome from run to run as of yet. So, for now, the forecast will 
maintain continuity by going for an increase in wind on Saturday/Saturday 
night with a rise in the seas, but not committing to much more than 
that until there is better agreement across the board.