Bermuda Weather Service Forecast Discussion

For Monday, October 23, 2017  08:00 UTC

FORECASTER - Andrea Pedrini 

NOWCAST (Today)
An upper level ridge extends NNE to our west while an upper low 
sits to our SSE throughout. At the surface, high pressure dominates 
with a centre located SE of Nova Scotia. Local ridging serves to 
block a weakening stationary boundary well to our S, where the upper 
low is responsible for the areas of enhanced convection too. These 
are visible from the latest satellite imagery as an area of widespread 
cloudiness spinning cyclonically to our south, where spherics sporadically 
report isolated lightning activity. Through our region largely clear 
skies resulting from the ridging subsidence, are observed and captured 
by the modelled vertical soundings. Some occasional scattered stratocumulus 
develop at the lower levels. The strong inversion observed yesterday 
and last night around 800hPa is likely to weaken as the flow became 
more Ely but very dry air persists aloft delivering good sunny spells 
among some passing fair weather cloud. Local radar shows a weakening 
band of precipitating stratocumulus overhead. Winds have steadily 
built since yesterday evening from light to reach moderate to strong 
and continue to gradually veer E-ESE through the afternoon. Slight-touching-moderate 
seas slowly start to build under the effect of the moderate to strong 
flow, especially outside the reef through the course of the day with 
a Small Craft Warning active for winds. A good agreement between 
all models gives the forecast a high confidence for continued settled 
conditions under the umbrella of the high. Expect mainly VFR conditions 
for the aviation with unrestricted visibility and 110 around 18-20kt 
sustained at TXKF throughout.

SHORT TERM FORECAST (Tuesday-Wednesday)
High pressure continues to dominate over this period while the centre 
of the broad high drifts to our NE by Wednesday. Models consistently 
suggest that mainly fair conditions are maintained until Wednesday 
night, with persistent subsidence and dry air aloft, ensuring largely 
sunny to partly cloudy conditions for most of the period, perhaps 
more on the cloudy side on Wednesday as the moderate to strong flow 
veers SE. A bunch of light and short-lived showers cannot be ruled 
out for the marine area, especially for Wednesday night but the bulk 
of the precipitation is blocked to our S-SW, according to the motion 
of the high, with a stationary convergence yielding scattered showers 
around the S-SW flank of the ridge. Meanwhile, a sharp and deep upper 
trough moves E across the central and E States, prompting the development 
of a complex system of low pressure over the Great Lakes and NE Canada. 
This system trails a slow moving frontal trough E over the E US coast 
into the Atlc seaboard, orientated N-S. The strength of the high 
is the reason why the frontal trough is slow moving and such configuration 
allows for a slow local progression. Seas build moderate to rough 
through Wednesday with a Small Craft Warning remaining in place throughout. 
No additional watches, warnings or advisories are expected at this 
time.

LONG TERM FORECAST (Thursday-Friday)
On Thursday, the frontal trough lying to our W starts approaching, 
pushed E by the upper trough, whit Bermuda sitting in a warm sector. 
The moderate to strong flow becomes generally S while easing light 
to moderate later in the day. Seas continue to be moderate to rough 
until they abate as winds relax. Since the front looks rather slow 
moving, expect increasing showers to become scattered at times and 
skies to turn occasionally overcast due to the advection of moisture 
acting on the vertical stability. In their previous runs, GFS and 
ECMWF went for a rise in pressure for Friday, with a post-frontal 
moderate wind regime from the N quadrants, whereas UKMO insists on 
an area of low pressure lingering in the region towards the weekend. 
Stability indices in fact seem to suggest a chance of thunderstorms 
too, especially for the marine area, just ahead of the frontal trough 
but confidence is low for thunder detail at this stage though. The 
forecast has been tweaked towards a more unsettled solution for the 
period. A Small Craft warning is expected to cease Thursday morning 
and a Thunderstorm Advisory may be considered as conditions evolve 
for a time on Friday. Otherwise, no other watches, warnings are anticipated 
at present.

TROPICAL UPDATE 
As of 0200 AM EDT Mon Oct 23 2017, the NWS National Hurricane Center 
Miami FL is closely monitoring a broad area of low pressure expected 
to form over the southwestern Caribbean Sea during the next few days. 
 Slow development of this system is possible thereafter while it 
moves northwestward to northward.
* Formation chance through 48 hours...low...near 0 percent.
* Formation chance through 5 days...medium...40 percent.