Bermuda Weather Service Forecast Discussion

For Friday, August 18, 2017  20:00 UTC

FORECASTER - Michelle Pitcher 

FORECAST DISCUSSION	Michelle Pitcher	Friday, August 18, 2017
NOWCAST (Friday)
A semi-stationary front has arrived from the north and clouds with 
a few showers will persist through tonight.  A chance of thunder 
increases to a risk in the public by midnight.  A risk of thunder 
currently remains in the marine area through tonight.  The leading 
edge of the front has dissipated some as it arrive over Bermuda but 
there remains a thin line of showers that is slowly making its way 
into the area from the west through northwest.  Moderate to strong 
winds were not trending lighter as an earlier model run indicated 
and a small craft warning has been extended into this evening.  The 
latest model run holds the stronger winds over Bermuda into this 
evening and then decreases them after midnight.  There is a small 
chance that the small craft warning will need to be extended into 
late tonight.  A thunderstorm advisory is valid for tonight.  Models 
are in good agreement with each other as well as current conditions. 
 The latest NOAA OPC Wind/Wave Analysis has seas 3-4 feet in our 
area.
SHORT TERM (Saturday & Sunday)
Weak upper level high pressure remains over the area through the 
weekend while an upper cut off low to the east slowly moves closer 
to Bermuda.  Saturday morning, a surface ridge axis remains to the 
near south as today’s stationary boundary remnants move away to the 
northeast.  A few morning showers with a risk of early morning thunder 
will give way to mainly fair conditions by noon.  The GFS remains 
very aggressive with the precipitation accumulation while the UK 
model has decreased the expected amount.  As the latest model run 
of the UK matches well with the current satellite imagery and expected 
precipitation, the UK model was followed more closely for Saturday’s 
forecast.  Models agree a bit better on the winds and westerly winds 
are expected to decrease from 12-18 knots to 10-15 knots by Saturday 
evening.  A couple of showers will likely be in the area on Sunday 
and winds continue to decrease, veering west-northwesterly 8-12 knots 
Sunday evening.  Seas are expected to be 3-4 feet outside the reef 
for Saturday and then decrease slightly 2-4 feet on Sunday.  A thunderstorm 
advisory may be needed for early Saturday morning.  No other watches 
or warnings are anticipated for these days at this time.
LONG TERM (Monday & Tuesday)
The upper cut off low, now centered to the southeast, becomes the 
dominant influence in our area as it moves to the near southeast 
during these days.  Monday and Tuesday remain rather broad as model 
agreement is low for both days.  Winds will generally be 8-12 knots, 
10-15 knots at times, during Monday as they veer roughly to the north-northwest. 
 A few showers are hinted at mainly in the UK for late Monday night. 
 Tuesday sees the surface high pressure that tried to build from 
the north weaken as an inverted trough and/or weak low pressure center 
moves to the near southeast.  It is way too early to speculate on 
the nature or position or strength of the weak low at this time. 
 Winds are expected to remain moderate and veer from the north to 
the northeast throughout the day.  Models agree upon some limited 
shower activity for Tuesday.  No watches or warnings are anticipated 
for these days at this time.
TROPICAL OUTLOOK
NWS National Hurricane Center Miami FL   200 PM EDT Fri Aug 18 2017
For the North Atlantic...Caribbean Sea and the Gulf of Mexico:  
The National Hurricane Center is issuing advisories on Tropical Storm 
Harvey, located just west of the Windward Islands.
	Shower and thunderstorm activity associated with an area of low 
pressure located about 600 miles east-northeast of the Leeward Islands 
continues to show signs of organization.  However, upper-level winds 
are becoming less favorable, and the chances for a tropical cyclone 
to form are diminishing.  The low is expected to move west-northwestward 
at about 20 mph during the next few days, and interests in the northern 
Leeward Islands should monitor the progress of this disturbance. 
 * Formation chance through 48 hours...medium...60 percent.  * Formation 
chance through 5 days...medium...60 percent.
	A tropical wave is located over the far eastern Atlantic Ocean 
several hundred miles west of the Cabo Verde Islands.  Environmental 
conditions are forecast to become more favorable for some development 
early next week while the system moves west-northwestward to northwestward 
at about 20 mph. * Formation chance through 48 hours...low...10 percent. 
 * Formation chance through 5 days...medium...40 percent.
Forecaster Beven