Bermuda Weather Service Forecast Discussion

For Wednesday, December 13, 2017  21:00 UTC

FORECASTER - Andrea Pedrini 

NOWCAST (Tonight)
An upper trough axis currently overhead continues NE tonight, while 
at the surface a deep low centred over the Canadian Maritime drives 
a very broad area of strong to gale force winds across the central 
and northern portions of the eastern seaboard. The associated frontal 
trough crossed Bermuda earlier today to presently lie well to our 
E. As the low edges N overnight, the area of strong winds retreats 
N as well. Meanwhile, a new low deepening S of the Great Lakes travels 
E along 40N. Latest satellite and radar imagery shows a vast area 
of scattered to widespread stratocumulus to out W through N with 
isolated, mainly light showers embedded, slowly drifting E. A generally 
zonal flow currently steady strong is expected to gradually ease 
moderate overnight as pressure rises and cooler advection ceases. 
Clouds break but remain patchy at around 3500ft with very isolated 
light showers possibly embedded within the most resilient cloud structures. 
GFS and UKMO are in general good agreement and suggest dry air persists 
aloft, especially at the mid-levels, hindering any significant vertical 
development. A minimum temperature of about 63F is expected towards 
dawn, slightly lower than the average of the period. Seas have been 
analysed around 17ft as of 18Z by the OPC, closely matching wave 
model guidance. Very rough seas persist throughout as well as a Small 
Craft Warning. No additional watches, warnings or advisories are 
VFR to patchy MVFR for aviation with a chance of –SHRA this evening, 
otherwise dry with winds around 270deg at 24kt easing 15-20kt tonight 
and backing towards 240deg Thursday morning.

As the upper troughing flattens out, there remains a steady zonal 
gradient throughout this period, while at the surface the low traveling 
along 40N reaches our distant N Thursday afternoon and steers NNE-NE 
thereafter. The trailing frontal trough moves SE-SSE across the eastern 
seaboard while weakening to linger to our near N on Friday. A warm 
front move across between Thursday morning and afternoon bringing 
increasing cloudiness and isolated showers with mainly fair visibility 
later on. Then, Bermuda sits in a warm sector for the rest of the 
period with a new increase in cloud and chance of showers through 
Friday as the frontal trough draws to our near N. Winds wobble around 
the WSW direction in the moderate to strong speed range throughout, 
according with the motion of the distant low centers. Rough seas 
possibly abate moderate for a time Friday evening, when winds ease 
moderate as well. This is likely the only time window outside Small 
Craft criteria, otherwise the Warning remains in effect. No additional 
watches, warnings or advisories are considered at this time. Model 
guidance is rather consistent throughout with minor difference on 
detail between GFS and UKMO, the latter being slightly more aggressive 
in terms of winds speed. 

LONG TERM FORECAST (Saturday-Sunday)
A new upper trough sharpens over the region on Saturday moving E, 
then clearing on Sunday as an upper ridge builds N to our W. The 
upper trough supports the development of a surface low just off Cape 
Hatteras early Saturday, which rapidly deepens while shooting NE. 
This system impact Bermuda with strong to near gale force SW-WSW 
winds late Friday night and Saturday morning, ahead of a frontal 
trough, the latter forecast to reach the island early in the afternoon 
causing winds to sharply veer NW strong. Mostly cloudy to overcast 
skies are associated with the frontal feature, as well as scattered 
to widespread showers, then gradually easing to become very isolated 
across the marine area overnight. UKMO is slightly more aggressive 
in terms of precipitation and speed of the front, but there is good 
confidence that the passage occurs by late afternoon. Astern, a broad 
high builds over the mid-Atlc US coast on Sunday, extending a ridge 
into the region, causing an abrupt rise in pressure. The flow slowly 
veers N through the course of Sunday, easing from strong to moderate 
by the end on the day. Expect bands of fair weather stratocumulus 
to move across with conditions remaining mainly dry on Sunday. Since 
rough seas are forecast to persist throughout under the aforementioned 
wind forcing, a Small Craft Warning is likely to remain active throughout 
the period. No additional watches, warnings or advisories are anticipated 
at this stage.