Bermuda Weather Service Forecast Discussion

For Saturday, July 04, 2020  19:00 UTC

FORECASTER - Mark Guishard 

NOWCAST (Saturday Night)
Today saw the begrudging retreat of shower activity to the north, 
which lingered and nearly caused a TAF amendment for TCU going to 
CB. This pseudo warm front feature finally pulled north during afternoon, 
allowing for some sunny breaks. The 12z ascent showed a moist and 
unstable airmass, so it is unsurprising that the potential for a 
rumble or two remained. However, this was unrealized and we came 
under the influence of a more stable airmass ahead of the main action 
expected tonight.
The cut-off upper low to the northeast will continue its retreat 
northeastwards as if fills and becomes a weaker shortwave upper trough 
in the background broad cyclonic flow aloft. Tonight, another upper 
low will dig southwards upstream  to replace the last one. This shortwave 
upper low west of us is well initialized by all the models, and will 
gradually fill and become an open wave as it skirts by us to the 
north, but not before invigorating a surface low to our SW, which 
has become TD5, and is likely to become TS Edouard this evening.
The low pressure about 300nm to the SW of Bermuda was designated 
Tropical Depression #5 this afternoon by NHC, and is expected to 
strengthen to tropical storm intensity within the next 12 hours. 
The system is forecast to move northeast, approaching Bermuda and 
bringing convective precip, (including thunderstorms) and the likelihood 
of gales. In our conversation with NHC this morning, a low (5-10%) 
chance of sustained tropical storm force winds for Bermuda was attributed. 
However, this low chance does not negate the possibility of sustained 
34 knot winds temporarily occurring around showers and thunderstorms. 
With this uncertainty prevalent, a Gale Warning was issued for Sunday 
early morning and later adjusted to bring it earlier to late night. 
This will temporarily induce rough seas and low ceilings. 
Timings for tropical storm (i.e. gale) force winds indicate their 
potential in the marine area anytime after midnight, based on the 
12 noon advisory. TD 5 itself look ragged and disorganized on the 
satellite representation, but with some good convective bursts that 
will no doubt help it to organize asit moved into an area of slightly 
reduced shear. The deep layer mean evident on the CIMSS website indicates 
confident steering flow direction, which supports the tight clustering 
of forecast model tracks. However, the model size and intensity is 
poorly constrained by model agreement.  In other words, there are 
very few models that have TD5 producing gales by the time it moves 
by its closest point of approach. With this in mind, there is poor 
forecast confidence in the verification of tropical storm force winds. 
Nonetheless, in the interest of public safety, a Gale Warning has 
been issued in case of un-forecast development. 
For the structure of TD5, the UK model seems to be consistently 
outperforming  the other models (at least in terms of agreement with 
the NHC analysis), having picked up on the low in the first place 
yesterday, and producing a wind swath that swings through Bermuda 
a few hours later than the NHC forecast (hence the earlier assertion 
that this would be an early Sunday event rather than a late Saturday 
Warnings: Small Craft Warning this evening, Gale Warning late tonight, 
Thunderstorm Advisory tonight and tomorrow morning.
Aviation: Mainly MVFR early evening, but deteriorating to IFR to 
occasionally LIFR conditions with low ceilings and SHRA/TSRA. AF 
Warning expected for thunderstorms and strong surface winds. 
SHORT TERM (Sunday and Monday)
Running with a slightly (few hours) delayed the UK model seems to 
be as reasonable approach as any for the current forecast, and as 
such, Sunday morning will see periods of showers with embedded thunderstorms 
moving through the area and clearing away to the east by noon, with 
winds veering southwest and dropping to moderate. Transient ridging 
seems to build for Monday, but a band of precip (‘front?’)  trailing 
back from what will become Tropical Storm Eduoard by that point lingers 
south of the island and will likely allow for a few showers locally. 
LONG TERM (Tuesday and Wednesday)
The weak cyclonic flow aloft continues through the outlook, and 
 the next weak surface low pressure will approach from the west as 
the next in a procession of waves along the pseudo front, enabling 
a wet Tuesday before weak ridging once again builds in for Wednesday. 

NHC Tropical Outlook:
Tropical Weather Outlook
NWS National Hurricane Center Miami FL
200 PM EDT Sat Jul 4 2020

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on newly-formed 

Tropical Depression Five, located a couple of hundred miles 
west-southwest of Bermuda.

Tropical cyclone formation is not expected during the next 5 days.

Public Advisories on Tropical Depression Five are issued under WMO 

header WTNT35 KNHC and under AWIPS header MIATCPAT5.
Forecast/Advisories on Tropical Depression Five are issued under 

WMO header WTNT25 KNHC and under AWIPS header MIATCMAT5.

Forecaster Zelinsky