NOWCAST

Latest visible satellite imagery shows banded stratocumulus cloud 
advancing over the region behind the cold front that arrived early 
this morning. Deep low pressure continues to wind up well to the 
NE at this time. RADAR is picking up some weak returns within the 
cloud bands but with more noticeable showers well to the N. Winds 
eased for a time with the FROPA this morning but since increased 
near 30 knots at the airport and occasional gusts to 35 knots was 
reported this afternoon. Winds have eased slightly late this afternoon 
but are still in the 22-26 knot range. Model agreement remains good 
tonight in the upper levels as a deepening trough is carried away 
to the E, leaving weak cyclonic flow over the region with broad, 
amplified ridging building over the SE U.S. The ridge flattens some 
and brings zonal flow to the region by late evening. At the surface, 
a tight pressure gradient will keep wind speeds at 20-25 knots into 
the early evening before building high pressure eases winds 15-25 
knots by the overnight. Model guidance remains good to this point 
leaving high confidence in the forecast. Seas will continue to build 
tonight and reach near 15 feet by late night. We will continue to 
keep the Small Craft Warning in place through the evening and overnight.

SHORT TERM FORECAST

On Saturday, zonal flow aloft will be the rule as a weak perturbation 
in the flow develops along the polar jet just upstream from a developing 
ridge within the flow over the E U.S. At the surface, high pressure 
ridging builds just S and W of the region and allows winds to back 
and ease though the day. The pressure gradient increases again from 
the N later in the day as a secondary cold front develops on the 
backside of the long departed low pressure system. A surface wave 
beneath the perturbation in the upper flow develops and enhances 
moisture convergence along the front by late night. By Sunday, the 
Island will continue to be affected by upper flow originating around 
a stalled and deep upper low over Nova Scotia. The flow remains zonal 
to start the day and then becomes more cyclonic by late evening as 
a potent shortwave trough develops on the backside of the main upper 
trough over the NE U.S. Models continue to have trouble handing the 
evolution of this feature and are still not in great agreement with 
the strength and timing of it. Jet streak divergence should be present 
out ahead of the developing shortwave however and at the surface 
will act to develop a frontal boundary on the backside of the long 
departed low pressure system from last night. A wave develops along 
the front but with continued model disagreement. The latest model 
run sees the GFS and ECMWF in good agreement in developing low pressure 
to the N while the CMC GEM develops the low further W and the UKMO 
is slower to develop it and much further E. We will cautiously side 
with the GFS/ECMWF blend but continue to have rather low confidence 
in the forecast. Winds increase to strong and back more WSW with 
showers returning later in the evening. Previous guidance consensus 
brought the low closer to the region and thus a tighter gradient 
but the last few runs have strayed from such a scenario so we will 
take out the threat for gales for the time being. UKMO instability 
indices hint as the chance for thunder still but not as aggressive 
as before. Increased Total Totals values near 48 will allow us to 
word the chance for thunder, especially the marine area. Seas will 
begin Saturday at 10-15 feet then fall through the evening and reach 
7-11 feet late before settling at 6-10 feet on Sunday. We sided with 
the GFS derived wave model for the forecast which is a bit more aggressive 
than the UK wave model. We should need a Small Craft Warning for 
the duration of the period.

LONG TERM FORECAST

By Monday, model guidance continues to disagree with the placement 
of features in the upper levels. The GFS and ECMWF still show some 
alignment with the base of the upper trough digging while sliding 
E of the Island and bringing a return to weak ridging by later in 
the day. The CMC GEM is much slower with such features while the 
UKMO is not as strong with them. Guidance does seem to come back 
in line by late evening with the ridge weakening and a zonal flow 
setup returning. At the surface, large variability continues with 
the movement of aforementioned low pressure. The UKMO is much faster 
than other guidance and slides the associated cold front through 
by early morning and reintroduces high pressure from the NW. The 
GFS and ECMWF place the low in near alignment but the EC tilts the 
front more S thus bringing it through a bit faster than the GFS. 
The CMC GEM is still much too slow with the low and lags it over 
the W Atlantic through much of the day. We will side with the ECMWF 
solution for the time being but hold only moderate confidence in 
the forecast for Monday. By Tuesday, another shortwave trough develops 
and is carried near the region, this time with the ECMWF and UKMO 
in better agreement as compared to the GFS. At the surface, we will 
side with a UKMO/ECMWF blend which sees high pressure quickly slide 
E and allows a cold front to approach from the W through the day. 
This brings return flow back by the afternoon and another increase 
to strong winds by late night with showers arriving ahead of the 
front. UKMO brings a ribbon a 34+ K-index values so we will word 
the chance of thunder in the forecast for late night. Seas will gradually 
fall during the period and may slip under 9 feet by day 5 which will 
finally end the Small Craft warning until the strong winds return 
late Tuesday. We have sided with the WW3 wave model for the period.