Bermuda Weather Service Forecast Discussion

For Sunday, December 16, 2018  09:00 UTC

FORECASTER - Andrea Pedrini 

NOWCAST (Today)
An upper cut-off low currently over North Carolina and Virginia 
moves NE offshore New England through the course of the day, deepening 
a sharp upper trough into the US eastern seaboard. The upper trough 
becomes blunter tonight as the trough axis passes over Bermuda. The 
upper low and trough support a complex series of surface lows, all 
transiting to our N and a frontal trough across the US eastern seaboard. 
The frontal trough lies NE-SW and advances E trough today to arrive 
in Bermuda tonight after midnight. Ahead of it, moderate to strong 
SSW winds prevail, making for warmer and moister conditions. Latest 
satellite and local radar imagery shows some isolated, non-precipitating 
cumulus cloud travelling N, driven by the gradient between the Bermuda-Azores 
high to our E and the frontal trough to our W. Aloft, veils of cirrus 
of variable thickness are pushed by the upper level trough. Spherics 
report scattered thunderstorm development along the frontal boundary. 
Therefore expect some hazy sunshine today before clouds thicken through 
the evening and overnight as shower activity approaches from the 
W-NW. A few isolated showers cannot be ruled out before the main 
band of showers arrives around or after midnight. As the front crosses 
Bermuda, conditions become less favourable for thunderstorm development, 
making a Thunderstorm Advisory somewhat unlikely at this stage. The 
best chance for it is early tonight, meaning midnight to 3am local. 
Frontal showers may give some gustiness for a time tonight to gale 
or strong gale force before winds sharply veer W around 18-20kt. 
A presently active Small Craft remains in effect throughout. 06Z 
OPC analysis had Bermuda in 7ft seas with little to no change expected 
until tonight, when seas build rough. S swells will make for about 
half of the components of the significant wave height into tomorrow.

MVFR expected until around midnight local with unrestricted visibility 
and 210deg 15-20kt winds. Around midnight local and afterward, MVFR 
to IFR conditions are associated with SHRA affecting TXKF. A brief 
Airfield Warning for Surface Winds and Gusts is likely for a time 
between 9 and 6Z tonight and 230-250deg winds may builds 25kt at 
times with possible gusts to 35-40kt. An Airfield Warning for Thunderstorm 
is less likely at this time. 

SHORT TERM FORECAST (Monday-Tuesday)
As the upper low move towards the Canadian Maritimes on Monday, 
it gets absorbed by a new deepening upper trough tilting positively 
from the Hudson Bay into New England. This causes the surface lows 
to rationalize into a main deepening low S of Nova Scotia which drives 
the frontal trough SE away from Bermuda early on Monday with a post-frontal 
showery trough acrossing the area later in the day. Expect generally 
W moderate to strong winds on Monday and patchy to broken stratocumulus 
delivering some sunny spells during day light hours and a few passing 
showers mainly in the evening. Temperatures and relative humidity 
decrease slightly under the cooler and drier post-frontal advection. 
On Tuesday, a new upper low forms over Nova Scotia out of the new 
upper trough, deepening a new trough axis into the region and carrying 
into the US eastern seaboard a large pool of cold air. A new cold 
front develops to our N early on Tuesday. The instability associated 
with this is forecast to miss Bermuda to the NE while a broad surge 
in winds is expected to our N. Sunnier early on Tuesday until the 
cold advection brings in broken stratocumulus giving a few sprinkles 
around Tuesday evening/night and strong winds veering NW. Then the 
flow relaxes as pressure begins to recover Tuesday night into Wednesday 
as a high amplifies its ridge in from the WNW. Meanwhile, the upper 
trough and low rapidly clear E as an upper ridge builds N over the 
E States. Due to the large fetch of stronger winds to our N on Tuesday, 
generally rough seas are expected to build very rough for a time 
Tuesday evening/overnight before beginning to abate. A Small Craft 
Warning will therefore remain in effect throughout the period with 
no additional watches, warnings or advisories considered at this 
time. 

LONG TERM FORECAST (Wednesday-Thursday)
Upper troughing clearing E leaves room for an upper ridge to drift 
from E States towards Bermuda through the period. This allows a broad 
surface high to move from the US mid-Atlc coast overhead by Wednesday 
night and then continue E through Thursday as a broad area of low 
pressure develops over the central and SE States. Fine conditions 
prevail throughout the period with temps in the low 60ís on Wednesday, 
beginning to recover on Thursday as the light NW winds settle light 
to moderate SE after becoming variable in nature Wednesday night 
into Thursday. Patchy to broken stratocumulus prevail on both days 
among dry conditions given by the subsiding airmass. Rough seas on 
Wednesday are caused by large NW long period swells associated gradually 
abate to moderate on Thursday as per the absence of a driver. The 
Small Craft Warning is likely to terminate Wednesday evening/overnight.