Bermuda Weather Service Forecast Discussion

For Monday, October 23, 2017  19:00 UTC

FORECASTER - Jennifer Smith 

NOWCAST (Tonight) 
Satellite imagery and local observations indicate scattered, fair 
weather cumulus clouds across the region. Bermuda remains under the 
influence of a broad surface high to the N which is shifting to the 
NE tonight. Seasonal or just below season temperatures persist through 
the night as the 850mb temps hold around the 11-12C mark. A series 
of troughs pass by along the southern flank of the ridge as evidenced 
by high cloud-top heights and lightning activity, but our protective 
cell of high pressure keeps that convection from reaching the southern 
Bermuda Marine Area. A very meridional upper level pattern is setting 
up with an impressive upstream jet streak signature today. Injection 
of cyclonic shear vorticity will help to dig a deep trough over Great 
Lakes tonight, while the amplified ridge over the North Atlantic 
holds in place through mid-week. Winds are 20 knots from the ESE 
and expected waiver between 18-24 knots overnight, and as such a 
small craft warning is in effect. Seas are presently analysed around 
4-7’according to OPC analysis and surrounding buoy reports and are 
expected to build slightly through the night.

Aviation: VFR…no hazards.  Winds favour runway 12, and expected 
to maintain 090-110 18-22KT thru period.

SHORT TERM FORECAST: (Tuesday & Wednesday)
The meridional pattern continues to amplify through Tuesday and 
Wednesday. A rapidly deepening low pressure system over the Great 
Lakes undergoes bombogenesis, and the associated surface cold front 
ejects from the SE USA. High pressure situated NE of Bermuda continues 
to extend a protective ridge SW to encompass Bermuda, however the 
tight gradient that results yields 25 knots winds for late Tuesday 
through Wednesday. Despite an influx of low-level moisture in the 
strong SE flow; generally fair skies should prevail with a trend 
upward in cloudiness over the course of Wednesday and a noticeable 
increase in humidity. Isolated showers are likely to begin populating 
the SE Marine Area Wednesday evening and spread NW overnight. In 
response to the winds, seas will build rough Tuesday and persist 
through Wednesday. A Small Craft Warning is in effect for both days. 
An airfield may also been needed for a time as sustained wind flirt 
with the 25 knot warning threshold.

LONG TERM FORECAST: (Thursday & Friday)
With the upper trough approaching expect a cirrus shield over the 
top of scattered to, at times broken cumulus cloud coverage. The 
models for the most part agree, though the ECMWF is slightly slower 
and gives the high a bit more credence to block most shower activity 
away to the NW. This means shower activity will begin as isolated, 
but through Thursday increase to scattered in coverage as the front 
is nudged SE to stretch SW-NE across the Island by a narrow swath 
of high pressure along the US Eastern Seaboard. Upper level troughing 
hints at the development of a 1012-1015mb low embedded along the 
frontal wave on Friday. Thermodynamics suggest a favourable environment 
for deep convection and possible thunderstorms Friday, which may 
prompt a Thunderstorm Advisory. There is considerable variability 
in the directional wind guidance Friday and beyond as it largely 
depends on the possible development and positioning of the aforementioned, 
weak surface low. That said a slight decline in speed can be anticipated, 
particularly late in the period as high pressure rebuilds briefly 
from the NW. Showers begin to taper overnight and winds further ease 
heading into Saturday. A Small Craft Warning will be continued for 
both winds and seas Thursday and for seas part of Friday as they 
slowly abate below warning criteria. 

The National Hurricane Center in Miami, FL is monitoring the western 
Caribbean Sea as a broad area of low pressure is expected to form 
during the next few days.  Slow development of this system is possible 
thereafter while it moves northwestward to northward over the northwestern