Bermuda Weather Service Forecast Discussion
For Tuesday, January 26, 2021
22:00 UTC
FORECASTER - James Dodgson
NOWCAST (This Evening/Tonight)
The precipitation associated with a frontal boundary has mostly
stayed just to the N for much of today. However, there are now signs
of precipitation developing S of the boundary in concert with model
projections. Some sferic activity has even been evident on the boundary
itself, but this should mostly fizzle and not impact the immediate
Bermuda area. The 12z weather balloon/skew-t data generally matched
model guidance well, with just a little more moisture evident in
lower levels when matched with the NWP. As expected, winds increased
into the 20-30kt range with occasional gusts towards 40kt in exposure
and offshore. This wind increase was nicely represented upstream
of Bermuda in the 1430z Ascat data. Looking ahead to this evening,
the NWP appears fairly bullish on developing patchy rain and showers,
and given current evidence of the radar, this seems reasonable. Therefore,
expect a wet end to the day, with some drying expected overnight.
Both temperatures and dew points have recovered today, and light
to moderate rainfall could help some patchy mist to develop (as air
temperature is cooled towards the dew point), given the already high
humidity levels. Winds will ease back this evening, say 18-24kt,
but will continue to be gusty at times, especially around showers.
Although some slight easing of seas can be expected, they will remain
in the moderate to rough category. A small craft warning remains
in force for both winds/seas, and at this point a thunderstorm advisory
seems unlikely.
AVIATION: Winds will generally be out of the W, maintaining moderate
to strong strength, but remaining below AF Warning criteria (25kt).
Cigs and vis in moderate rain/showers could push the category down
from VFR to MVFR or even briefly IFR. Although thunderstorms have
not explicitly been mentioned, there is a chance of a distant CB
and this is now catered for in the short term.
For current airfield information please visit: http://www.weather.bm/Aviation
SHORT TERM FORECAST (Wednesday-Thursday)
Wednesday sees an upper zonal to slightly ridged pattern, with the
core of the polar jet a couple of 100 miles to the N. This is short-lived
though, as Thursday sees a sharpening upper trough approach from
the W, moving overhead later Thursday into the early hours Friday.
The polar jet max also moves across BDA late Thursday, with 300mb
winds approaching 150kt. This all serves for a very active weather
pattern, likely the most significant of the winter so far. At the
sfc, BDA is essentially squeezed between high pressure to the S and
low pressure to the N on Wednesday, making for a breezy day (moderate
to strong winds), as well as plenty of cloud and patchy rain/showers
in concert with a nearby frontal boundary. Then, during Thursday
an area of low pressure moves off the US Carolinas and deepens very
quickly thanks to the active upper dynamic (upper diffluent trough).
All the NWP points to strong ‘explosive cyclogenesis’ somewhere in
the range of a 30-40mb drop in central pressure between 00z Thursday
and 00z Friday. The centre of this low is expected to pass around
100nm to the N of BDA later Thursday evening. In terms of local weather,
Thursday starts off mild in the warm sector, with plenty of cloud
and patchy rain/showers. Later in the afternoon a cold front associated
with the low sweeps through the area bringing more organised and
heavier shower activity with the chance of thunder. All the while,
SW winds increase rapidly, reaching gale to storm force by late afternoon/early
evening. The winds veer NW with FROPA and only gradually ease overnight
into Friday. Spot values in the NWP briefly have up to around 50kt
sustained, with gusts approaching 70kt (hurricane force). The 12z
UK and GFS run also suggest a nasty ‘string-jet’ feature just passing
to the near N. This will have to be monitored closely in case it
moves into the marine area and island, which would mean even stronger
winds. Seas also build rapidly in response to the deepening low,
reaching towards 30ft later Thursday. A small craft warning is upgraded
to a gale warning by late Thursday morning. This is then upgraded
to a storm warning for late Thursday pm through evening time. A thunderstorm
advisory may also need to be considered for any deep convection on
the forward side of the cold front later Thursday.
LONG TERM FORECAST (Friday-Saturday)
The axis of the upper trough is expected to move through on Friday
morning, although the upper pattern will remain cyclonic well into
Saturday, before hints of upper ridging finally emerge from the W.
At the sfc, the isobaric pattern remains tight through Friday, with
gales likely to continue through the day. Only on Saturday do the
winds finally begin to fall comfortably below gale force, although
still remaining strong for a time. With regards local weather, Friday
will see considerable CAA develop in the wake of the front. 850mb
temperatures fall to between around -2 and -4C (down towards mid-20s
F) and that translates to sfc temperatures struggling to reach 16C/60F.
Indeed, with such a cold unstable airmass in place (as per forecast
vertical profiles), there is the chance of thunder (Total totals
index is 50+) and even the chance of small hail (occasional cloud
tops -20c or lower with SALR and air temperature profile difference
supporting small hail), which could temporarily bring air temperatures
down towards 10C/50F or lower. By Saturday, the airmass finally begins
to warm out aloft, allowing the sharp showers to moderate somewhat,
with negligible risk of hail by this stage. During Friday, showers
will very likely be accompanied by gusty/squally winds. Sea heights
begin to ease during Friday, becoming very rough to high. By Saturday
they diminish further, easing rough to very rough. A gale warning
will likely continue through Friday, to then be downgraded to small
craft on Saturday. A thunderstorm advisory will need to be considered
for Friday.