Bermuda Weather Service Forecast Discussion

For Friday, May 14, 2021  21:00 UTC

FORECASTER - James Dodgson  

NOWCAST (This Evening/Tonight)
Broad upper troughing anchored to the NW is supporting a quasi-stationary 
frontal boundary just to the S of the area. After a cloudy damp start 
to the day, some sunny periods developed. The wind regime started 
light and variable under a col-type pattern. However, winds gradually 
settled into the W, before veering N, then NE, increasing all the 
while towards the strong (20kt) threshold. The 14z Ascat pass nicely 
depicted the wind veer/increase upstream to the N before it set into 
the area. The sfc analyses showed the development of a wave on the 
front to the SW and this will ride up along the front (ENE’wards) 
into tonight and beyond. As for this evening into tonight, cloud 
cover will once again increase from the N as a trough-like feature 
moves over the area. This is already borne out in the GOES-16 satellite 
imagery, and some isolated light showers have also developed upstream. 
These are likely to affect the local area overnight (as per latest 
NWP guidance). Winds will be solidly in the moderate to strong range 
and this will allow moderate seas to build towards the rough threshold 
(9ft) tonight. A small craft warning has already been issued for 
this evening and tonight, primarily for the strong winds.

AVIATION: Largely VFR conditions are expected through the TAF period. 
However, showers overnight could dip the category down to MVFR (low 
chance IFR) primarily for occasional low ceilings. Winds will veer 
from the NE into the ENE later in the period, while gradually increasing 
towards 20kt. At this stage, an Airfield Warning for sustained strong 
winds (25kt+) seems relatively unlikely given latest model guidance. 
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The weekend is dominated by upper level troughing. During Saturday, 
BDA remains on the forward side of the trough axis. The axis then 
moves across the area on Sunday morning (between 09-12z) leaving 
a somewhat cyclonic upper pattern in its wake. The upper trough supports 
the development of a sfc frontal wave that passes to the near S of 
BDA during Saturday evening. Given this is a relatively small feature, 
each model run has fluctuated somewhat. Earlier runs, especially 
the GFS has wind speeds towards 30kt developing Saturday evening. 
However, the last two runs of the GFS have backed off somewhat, more 
in alignment with the somewhat steadier UK and EC solutions. Given 
this latest NWP guidance, there is increasing confidence of the already 
stated wind threshold for Saturday day into Saturday night - 18-24kt, 
occasionally increasing 20-25kt. Despite increasing confidence in 
wind details as the low passes to the near S, forecast precipitation 
is somewhat harder to gauge. The UK has remained bullish with QPFs 
of 0.5” or more, while the GFS has fluctuated considerably. Wording 
has now been toned down a touch in the detailed products, with patchy 
rain and embedded showers expected to develop Saturday evening into 
Sunday morning before easing with the exit of the low. Latest UK/GFS 
wave model guidance on seas is generally high end moderate (8ft), 
abating on Sunday. A small craft warning is in place through at least 
Saturday, likely to be extended into Sunday for strong winds, which 
should ease moderate by later Sunday afternoon. Despite somewhat 
favourable instability indices, the cool dig of NE’ly winds in the 
boundary layer should preclude any thunder risk.

The upper pattern continues cyclonic through the extended range, 
with embedded trough axes passing the area on Monday morning and 
again Tuesday night. Despite this, sfc pressure nevertheless rises 
through the period, as high pressure builds in from the N behind 
the aforementioned exiting low. Moderate NE winds will likely ease 
light to moderate at times and aside from a few showers associated 
with weak trough-like features moving in from the E, it promises 
to be mostly dry with the promise of some pleasant sunny periods. 
Although no significant WAA develops at 850hPa, the easing NE winds 
will allow some uptick in temperatures once again. Moderate seas 
will gradually diminish into the slight to moderate range. Currently, 
no watches/warnings are expected.