Bermuda Weather Service Forecast Discussion

For Thursday, November 21, 2019  20:00 UTC

FORECASTER - Gary Hall 

NOWCAST (Thursday evening through Thursday night):
Scattered to broken stratocumulus is evident on satellite over much 
of the western Atlantic in the cold air advection field. There are 
also very brief light showers associated, which is typical under 
this scenario. In general, the wind is northwesterly ~15-20 knots 
with an occasional gust to 30 knots when a shower passes nearby. 
There is a slow veer to the north-northwest with time which is also 
indicative of the surface high inching closer to the west-southwest 
while the upper level trough axis is about through with ridging to 
follow. However, this does not mean an end to the stratotcumulus 
as cold air advection with a strong marine inversion will be seen 
through tonight, then slowly fade away over the weekend. Given all 
this, expect little change in the nowcast from what we have been 
experiencing all morning and afternoon. All model guidance has the 
peak of cold air advection tonight with 850hpa temperatures falling 
to near 4-5C while the MOS has an overnight low of 17C/63F. The seas 
are running rough at present, or near 10-11 and will also peak at 
near 12 late tonight, especially in the north and western marine 
quadrants. As such, a Small Craft Warning is currently in effect 
and will remain in effect through this period.

AVIATION: Primarily VFR with only brief MVFR ceilings within fast 
moving showery cells. Wind will be constant from 320-330 near 15-20 
knots, with isolated gusts to 28-30 knots around showers, while slightly 
veering to direction 340 late tonight. For current airfield information 
please visit: http://www.weather.bm/Aviation

SHORT TERM FORECAST (Friday-Saturday): 
There are no significant challenges facing us on Friday as high 
pressure continues to move eastward to be near 30N73W by evening 
while the centre peaks at 1021mb. This will cause the wind to remain 
in a north-northwesterly fashion, though weaken through the day to 
settle near 5-10 knots. As the high continues to move east, it will 
eventually be to our south on Saturday which will allow for the wind 
to back westerly while increasing once again, though only to moderate 
levels. Meanwhile, the upper flow will become zonal over us while 
a surface low centre will be to our distant north and drag a weak 
cold front into our vicinity Saturday night. Given the lack of upper 
support and no cold air push behind it the only question remaining 
will be whether it makes it to us, or just on top of us before retreating 
northward heading into Sunday. Up until then, vertical profiles suggest 
a weaker marine inversion on Friday afternoon, thus the stratocumulus 
is expected to thin out as the low level temperature advection neutralizes, 
then slowly warms into Saturday ahead of the weak front which may 
deliver a brief, light shower or two Saturday night.  As a whole 
there is good agreement with this frontal placement Saturday night, 
though the GFS has backed off a bit by keeping it to our near north 
while the ECMWF and UKMO bring it to us, then quickly send it back 
northward by Sunday morning. Either way it is rather insignificant 
as only minor differences will be felt in both wind and cloud cover. 
Rough seas on Friday will abate to less than 9 (moderate) by early 
Saturday morning which will lead to the Small Craft Warning ending. 
The surface high will continue to trek eastward and be to our southeast 
by late Sunday night, thus driving the front northward while a more 
significant front is now poised to strike Sunday night.

LONG TERM FORECAST: (Sunday-Monday): 
All guidance points to a deepening low pressure system centred over 
the greater New York City area with a trailing cold front extending 
southward over the Gulf Stream on Sunday morning. Once again, the 
upper flow remains quite zonal through this period so most of the 
noteworthy features are confined to the lower levels. Southwesterly 
wind will increase along with the seas through Sunday afternoon and 
evening then reach maximum strength late Sunday night right up until 
the front arrives. Previous model runs had the UKMO being the fastest 
with a near 24/06Z passage and the ECMWF and GFS some 3-6 hours behind 
it. But now it appears all 3 global models have come into good agreement 
with a passage sometime between 24/06Z and 24/09Z. Give that this 
is strictly a warm air advection scenario we will be unlikely to 
see any significant gusts until the showers arrive shortly before 
the front. However, the gradient briefly tightens during this time 
frame with hints of 30 knots sustained then rapidly veering west-northwesterly 
and dropping off behind the front by Monday morning. Thermodynamics 
hint at the possibility of deep convection just ahead and along the 
front while QPFs range from 10CM with the GFS and double that with 
the UKMO and ECMWF. As is often the case a lot will depend on where 
any deeper convection lines up as to how much we get, but indications 
are for some heavier showers to form along this boundary as it passes. 
Weak high pressure will move in to our near northwest on Monday, 
thus ensuring the front stays south of Bermuda with a gradually north 
veering wind in the 8-12 knot range, perhaps a bit higher at times. 
Nevertheless, temperatures will rebound this period under the developing 
southwesterly flow on Sunday, then cool off on Monday, though only 
slightly. Moderate seas of near 7 Sunday morning will soon build 
to rough once again by Sunday evening, then abate on Monday in the 
post frontal air mass and settle moderate once again by late Monday 
night, or Tuesday morning at the latest. Aside from a Small Craft 
Warning, a Thunderstorm Advisory is also possible for a few hours 
Sunday night with the approaching front.  

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
100 PM EST Thu Nov 21 2019

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

The National Hurricane Center is issuing advisories on Tropical 
Storm Sebastien, located several hundred miles north-northeast of 
the northern Leeward Islands.

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