Bermuda Weather Service Forecast Discussion

For Thursday, December 03, 2020  20:00 UTC

FORECASTER - Gary Hall 

NOWCAST: This evening through Tonight:
Two sperate convergent bands of cumulus with generally isolated 
showers propagated through today while moving in a north to south 
fashion. Most activity seen on radar at the present time is to the 
south while both the satellite and radar show a clear slot upstream 
to the immediate north. However, cool air stratocumulus to the distant 
north may advect over the Island overnight with another spotty shower 
or two, but certainly nothing significant. Shortwave ridging with 
northwesterly flow aloft builds in overnight with 500/300hpa heights 
nearing 582DM/946DM respectively.  At the surface, a high pressure 
centre will move off the east coast underneath the shortwave to be 
to our near northwest late tonight while travelling eastward along 
34-35 degrees north latitude. As such, our wind will continue to 
be a steady diet of moderate northerlies while tending slightly to 
the north-northeast near Friday dawn and slightly easing. Significant 
wave heights shall be near 6-7 through tonight. No watches, warnings 
or advisories are anticipated, though we were close to small craft 
warning wind today, the threat of that lessens overnight.

Aviation: VFR with intermittent ceilings ~ 3-5K overnight and a 
spotty quick shower possible. Wind along the runway, which should 
remain dry for much, or all of the night will be from 360-020 degrees 
~ 10-12 knots. For more information please see www.weather.bm/aviation 


SHORT TERM FORECAST: Friday-Saturday:
High pressure ridging holds firm on Friday into Saturday morning 
then begins to breakdown as troughing digs to our west from Saturday 
afternoon into Saturday night. Another dry day is expected on Friday 
under veering northeasterly wind in the 10-15 knot range, perhaps 
easing to 8-12 knots overnight while further veering to the east-southeast 
as the surface high transits to be positioned to the northeast relative 
to Bermuda. There is little doubt that Saturday will be changing 
a lot as the upper trough supports an inactive frontal boundary set 
to pass Saturday night. All of the models have trended with a sooner 
passage than the previous runs with the bulk of the precipitation 
occurring Saturday night and clearing to the south and east towards 
Sunday morning. There are also signs of pre-frontal showers that 
may develop as early as Saturday afternoon and increasing in coverage 
during Saturday evening. It is also probable that we see the showers 
turn to patchy rain along the frontal boundary with a slight chance 
of thunder. There is notable divergence aloft above the front as 
it nears the Island while key thermodynamic indices have also trended 
less stable. The wind will continue on a southerly to southwesterly 
veer while increasing to 20-25 knots, with higher gusts possible 
near showers as the gradient level wind touches gale force strength 
which may be thrust down to the surface in any convection. Rainfall 
totals are not impressive as all global models point to ~ 5mm. Due 
to the nature of the frontal structure which resembles an inactive 
boundary rather than an active boundary, most precipitation will 
occur ahead and along the front then waning behind it as we veer 
westerly towards Sunday dawn while easing slightly. The seas will 
build to 9 Saturday night, though a small craft warning will have 
gone into effect on Saturday evening due to the increasing wind.

LONG TERM FORECAST: Sunday-Monday:
A rather strong jet maxima passes to our near north on Sunday while 
a deeper shortwave trough digs to our southwest on Monday which will 
cause a rapidly deepening surface low pressure system to form and 
spread inclement weather over Bermuda by Monday night. This is a 
rather complex scenario that the models have been struggling to agree 
on for the past few days, though are trending towards a bit better 
agreement with the overall synoptic evolution, though strength and 
timing differences still exist. The weather associated with the Saturday 
night front will clear out early on Sunday morning leading to a partly 
sunny, yet breezy day from the west. Rapidly increasing southwesterly 
wind will occur Monday evening and night with some suggestion of 
a potential gale late Monday night into Tuesday morning. The UKMO 
and ECMWF are the most aggressive at this point while the GEM and 
GFS are weaker, yet still wet. Warm sector rainfall with possible 
thunderstorms look to be a fair bet Monday night with rainfall totals 
ranging from 15mm (GFS), 35mm (UKMO) and a whopping 90mm with the 
ECMWF. While the finer details are still low in confidence, the overall 
trend is becoming better in line that we can expect a wet and windy 
Monday night into Tuesday. Seas will build under the strengthening 
wind to rough levels Monday night and possibly very rough heading 
into Tuesday. Expect a small craft warning possibly rising to gale 
warning for Monday night while a thunderstorm advisory is also possible.