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.