North Atlantic Marine Weather Discussion

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AGNT40 KWNM 061430
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1030 AM EDT Sat Jun 6 2020

.FORECAST DISCUSSION...Major features/winds/seas/significant 
.weather for the North Atlantic Ocean W of 50W from 30N to 50N.


Early arriving 12z regional guidance does not offer any
compelling reasons to alter the ongoing forecast in this
intermediate update this morning. See the previous discussion 
below for grid preference and forecast reasoning. Next update 
coming this afternoon once the full suite of 12z global guidance 
and available ensembles have been analyzed. 

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PREVIOUS DISCUSSION...

The latest NCEP surface analysis indicates a low pressure trough
near the Mid Atlc coast this morning, and the GOES infrared
imagery along with the lightning density product shows an area of
showers and tstms ahead of the trough as it acts as a focus for
convection in the marginally stable conditions over SW NT1 and
far NW NT2. The latest SREF guidance indicates that the
convective activity will slowly increase today as the trough
moves offshore, and moves into an area more favorable for
convection. This seems likely as the global models indicate the
winds increase ahead of the boundary. Mixing will be enhanced
later today as the right entrance region of an upper level jet
streak moves over nrn NT2 later today into tonight, while a low
level jet strengthens ahead of the trough. As such, the previous
weather grids will be modified to reflect the expected increase
in tstm activity, and those planning to navigate through this
area should exercise caution as winds and seas will be locally
higher than the surrounding conditions. As far as the sustained
winds are concerned, the Ascat wind retrievals from 03Z late last
night indicated up to 25 kt in the NT2 waters, and current
observations show about the same intensity. The 00Z GFS/ECMWF 
model winds are initialized well when compared to the data, and 
the guidance indicates winds will increase later today over the 
W Atlc as low pressure over SE Canada approaches the area while 
pulling a cold front towards the coast. The models have come into
decent agreement with the H5 vorticity pattern, and most
solutions indicate multiple vort maxes moving into the upper 
trough later today into Sun. There has been some inconsistencies
between the models on the surface forcing, though the GFS and 
ECMWF are now in better agreement when compared to yesterday. The
models all generally show gales developing ahead of the front
tonight into Sun, though most solutions show the stronger
sustained winds developing just E of the offshore waters. 
However, the GFS continues to show a weak signal for gales in the
offshore waters later tonight into Sun, but confidence is fairly
low since the GFS has been trending weaker and is stronger than 
the rest of the models. As such, am planning on continuing with 
no warning headlines in the next forecast package associated 
with this system in the short range. However, can not rule out 
an occasional gale force gusts, as the convection allowing models
indicate a fairly decent chance for a second round of tstms to 
develop especially in the unstable conditions in and near the 
warmer SSTs of the Gulf Stream. For the winds, am planning on
starting out with the 00Z GFS in the next forecast, but will opt
for the first sigma winds in unstable areas to account for the
deeper mixed layer.

In the medium range, the 00Z models indicate low pressure will 
develop along the front to the E of Bermuda Mon, before the low 
moves back to the W Tue into Wed. The GFS has been a bit faster 
with taking the low back to the W than the rest of the models, 
and has not changed much on the timing and track in the 00Z 
run. The slower ECMWF/UKMET seem a bit more reasonable
considering the upper low is cut off from the main upper flow 
without much steering to drive it quickly to the W. Although 
there is some support for the ECMWF solution, there is a fair 
amount of spread in the guidance which is not surprising 
considering the range of the forecast and the weak steering.
However, have a slight preference for the 05/12Z ECMWF solution
from Mon night onward as it is near the consensus of the 00Z
UKMET/GEM solutions. The 00Z ECMWF is also near the 00Z model
consensus, but it trended slower and just E of the other models.
There is also better support from the 00Z GEFS members for the 
12Z ECMWF, so planning on transitioning towards it starting Mon 
night. 

.SEAS...The 00Z wave models are initialized well in the W Atlc
this morning, as wave heights are generally 7 ft or less in the
offshore waters. The models agree fairly well over the short
range, before diverging Tue and beyond as a result of the
different handling by the associated atmospheric models of the 
aforementioned low approaching NT2 from the E. In order to match
up with the wind grids, am planning on using an even blend of the
00Z Wavewatch and ECMWF WAM into Mon night. Will then transition
to the 05/12Z ECMWF WAM thereafter.

.EXTRATROPICAL STORM SURGE GUIDANCE...No significant positive
surge events are expected over the next several days. Consult 
the latest forecast information from coastal National Weather 
Service offices.


.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Collins/Kells. Ocean Prediction Center.