Bermuda Weather Service Forecast Discussion

For Tuesday, March 31, 2020  07:00 UTC

FORECASTER - Gary Hall 

NOWCAST:  This morning through tonight
A weak cold front passed overnight bringing mainly MVFR with brief 
IFR ceilings down to ~ 700’. Spotty light precipitation was seen 
on radar, but as a whole it never really amounted to much. The wind 
has begun to slowly veer to the west-northwest while peaking around 
18-20 knots, though all guidance points to diminishing speed today 
as we enter into the influence of weak high pressure to our north 
and west. Satellite shows the scattered to broken cirrus currently 
being reported on observations while RH cloud base charts depict 
a lifting of the bases today, which makes sense as we see a gradual 
drying trend under the veering wind flow. Forecast vertical profiles 
also show this drying trend through today, though it will stay periodically 
cloudy, we can also expect sunny spells to develop especially this 
afternoon. Little change is anticipated in sea state today with peak 
wave heights around 7’. Today’s high will be a touch lower than yesterday 
due to post-frontal cooling, though expected to crack the 70F degree 
mark. No watches, warnings or advisories through this period.

Aviation: Patchy MVFR early this morning will give way to predominant 
VFR conditions thereafter. Wind from the west-northwest in the 15-18 
knot range decrease to 10-12 knots this afternoon, then 5 knots or 
less for a time through this evening before settling southeasterly 
~ 10 knots late overnight.

Short Term: Wednesday-Thursday
This period, and next are going to be very active as a deepening 
low pressure system is forecast to pass to our northwest on Wednesday 
afternoon. However, the 00Z model run trended towards the low passing 
further away to our northwest than previous runs thus resulting in 
in lower wind speeds, perhaps not quite reaching gale force, though 
higher gusts still expected to occur. As AB mentioned in the previous 
discussion, vertical traces suggest stratiform precipitation developing 
Wednesday morning as the warm front passes through, but since the 
low center is expected to be further west, it appears less likely 
for heavy precipitation, rather light rain with low ceilings then 
improving by late morning as we enter the warm sector. We will not 
stay in the warm sector that long as all guidance indicates the low 
will swing the cold front through Wednesday evening. Given this, 
it may be prudent to hold off on a gale warning as of now. Numerical 
data hovers around 28-32 knots with the southwesterly wind in the 
warm sector, then veering westerly Thursday evening behind the cold 
front. Boundary layer wind speeds are also lower than previous runs 
so it may be wise to drop the 55 knots down to 50 knots, or let us 
see one more model run before deciding on that. Cold air advection 
will follow as we head towards Thursday while the low level gradient 
remains very tight. Any chance of deep convection should be confined 
to Wednesday evening with the arrival of the cold front and obvious 
mechanical forcing as the cold air field clashes with the cooler 
air behind it. There is a noteworthy vorticity lobe supporting the 
cold front so while the window for thunderstorms may be brief, there 
seems to be a good chance for it to occur. There may be a brief lull 
in wind speed immediately following the front on Wednesday evening, 
but don’t expect much of a drop, if any on Thursday under this west-northwesterly 
flow setting up on the back side of the low. This low is going to 
pester us for days ahead as models stall it, and cut it off from 
the main frontal system. Potential gales are possible going forward. 
Wind driven seas will rapidly build on Wednesday as rough levels 
are expected by Wednesday afternoon and very rough seas are forecast 
by Thursday. A Small Craft Warning is already in effect for Wednesday 
morning due to wind, then seas thereafter will continue to justify 
it longer. Will this transition to a gale warning for Wednesday afternoon/evening? 
Well, as said above, we may want to see more data as to commit or 
not just yet. A Thunderstorm Advisory is highly likely from Wednesday 
afternoon up until the cold front arrives Wednesday evening. 

Long Term: Friday-Saturday
This period is not going to be overly committal, though it is fair 
to say there is decent model agreement on how each resolves the deep 
low to our north. Very rough seas will possibly reach high, or 20-22’ 
by Saturday due to the fact the low is forecast to not only stay 
very strong and deep, but retrograde back closer to Bermuda and be 
to our near north through this period. This would mean very blustery 
northwesterly wind and much cooler temperatures. In fact, most guidance 
points to the strongest wind occurring Friday night into the first 
half of Saturday while surpassing speeds of what we expect on Wednesday 
with the cold front. The low will eventually move away to the east 
later on Saturday as upper ridging builds in to our north and west 
thus our wind will veer northerly, yet still strong and gusty into 
Saturday night. Forecast vertical profiles depict ample low level 
instability so occasional showers are likely for both days this period 
perhaps peaking Friday night. MOS maintains highs in the mid to upper 
60’s while lows will dip down to around 60, perhaps lower in a moderate 
or heavy shower on either day. A Small Craft Warning will be a sure 
bet for both wind and seas on both days, while a possible gale may 
be needed at the peak of the event Friday into early Saturday.