Bermuda Weather Service Forecast Discussion

For Saturday, March 06, 2021  21:00 UTC

FORECASTER - Gary Hall 

NOWCAST: This evening through Tonight:
Upper-level low pressure over the Canadian Maritimes extends a broad 
and deepening trough southwestward towards the Gulf states with increasing 
cyclonic flow ahead of the trough towards Bermuda. 300hpa wind speeds 
increase to become 100 knots late tonight while the trough continues 
to sharpen to our west. As a result, increasing cloudiness is seen 
on satellite upstream to our southwest, though the radar is still 
echo free. Weak high pressure with a nearby COL area is in our vicinity 
but this will soon wane and allow a northerly wind component to develop 
that transitions to northeasterly tonight. Meanwhile with the deepening 
upper trough, our atmosphere will become saturated with significant 
overrunning setting up. The frontal boundary to our south will become 
energized with this upper energy thus spawning a series of lows beginning 
tonight and into the remainder of the weekend. Rainfall will begin 
after midnight and become moderate to possibly heavy by Sunday morning 
while the northeasterly wind intensifies to near 20 knots. There 
is a bit of model discrepancy on timing and strength of possible 
20 knots, but a general consensus would favour Sunday morning rather 
than tonight, though a watchful eye shall be maintained. As such, 
no watches, warnings or advisories are in effect through tonight, 
but expect that to change by Sunday morning as the wind continues 
to increase. Seas will abate this evening to around 7’.

Aviation: VFR becomes MVFR late evening and into tonight with possible 
IFR conditions late tonight towards Sunday dawn. A wet runway is 
likely as the rain commences overnight while an increasing northeasterly 
wind will occur ~ 12-18 knots. For more information, please see www.weather.bm/aviation 


SHORT TERM FORECAST: Sunday-Monday:
The first low will pass to our south on Sunday morning with a bout 
of rain, possible drizzle/mist and strong wind. There is decent model 
agreement on splitting up a morning round of moderate to possible 
heavier rain then a break Sunday afternoon/evening as the low exits 
to the northeast. However, all guidance suggests the next low to 
be deeper while cruising up along the frontal boundary positioned 
to our near south. This low will pass Sunday night and into early 
Monday morning which times up well with the upper trough/vorticity 
axis passing through. The 500hpa signature is impressive and points 
to wind increasing to 30 knots by Monday morning while gusts could 
be 35+ knots. The vertical profile loses its’ overrunning as the 
upper trough moves through thus precipitation at this point will 
become showery, though possibly moderate to heavy at times. Ample 
cold air is associated with this feature, though Total Totals do 
not point to any hail, as well as the moisture field not extending 
deep enough into the mid-levels. Model rainfall amounts are quite 
high as the GFS is ~ 70mm through the entire event while the UKMO 
and ECMWF are about half of that. The big story developing will be 
the chilly northeast wind while staying damp on Sunday then blustery 
northwesterly wind and showers while staying rather cold on Monday. 
Seas will be building to rough levels by Monday but the small craft 
warning for wind will already be out by that point. Conditions will 
begin to settle Monday night as high-pressure inches closer from 
the west-northwest.

LONG TERM FORECAST: Tuesday-Wednesday:
High pressure continues to build into the west-northwest with local 
pressure rising to above 1032mb on Tuesday. The airmass will be cold 
with probable stratocumulus ceilings through much of the day. 850hpa 
temperatures may dip to slightly below zero while 1000-500hpa thickness 
will begin to rise heading into Wednesday. A couple of showers could 
squeeze out of the moist unstable marine layer to start on Tuesday, 
but the likelihood of showers becomes near nil thereafter as the 
high centre draws nearer. Sunshine should develop on Wednesday under 
an easing wind though the the GFS is attempting to bring in a convergent 
boundary from the north with the chance for showers whereas the other 
models do not. It may be wise to leave this out until more proof 
can be seen. The seas may linger at rough levels early on Tuesday 
but with the decreasing wind we can expect a steady abatement to 
moderate levels with peaks only near 5-6’ by Wednesday. Given this, 
any small craft warning carried forward from the previous period 
probably won’t last long, if even valid at all.