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.
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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.