Scientific Forecaster Discussion
Area forecast discussion
National Weather Service New York New York
411 PM EDT sun Apr 26 2015
low pressure over the Canadian Maritimes and another low moving off
the middle Atlantic coast today will consolidate into a broad area of
low pressure over the western Atlantic Monday into Tuesday. This
system will meander south of Nova Scotia on Tuesday then depart
to the east as high pressure builds in from the west.
Meanwhile...low pressure over the Gulf Coast states will track to
the middle Atlantic coast by Thursday and continue northeast into the
Atlantic through the end of the week. A few upper level
disturbances may impact the area next weekend.
Near term /until 6 am Monday morning/...
going into tonight...there could be an isolated shower in response
to positive vorticity advection associated with the middle level trough
axis moving across. The main upper level jet stream stays south of
the region. At the surface...a trough axis will be moving across
this evening...with winds becoming more northwest behind it. The chances
will be very low for this shower activity due to the dry air near
the surface. Temperature and dewpoint spreads will be around 20 degrees
going into the early evening so the showers may just dissipate
upon tracking farther southward. Otherwise...mainly a dry
forecast with decreasing clouds late behind the trough axis.
Winds do not become all that light as a pressure gradient remains
between the weakening low across the Canadian Maritimes and high
pressure moving into the northwest part of the Great Lakes. This
will mitigate radiational cooling in addition to the clouds. Lows
were a blend of mav/met/gmos yielding a range from the middle 30s to
Short term /6 am Monday morning through Monday night/...
for Monday and Monday night...the middle levels still convey periodic
shortwaves as diagnosed by positive vorticity advection which rotate
around an upper level low. The upper level low pushes south of Nova
Scotia through the period. The main upper level jet stream continues
to stay south of the region. At the surface...a trough approaches
Monday and moves across Monday night. A few lows offshore...one of
which is associated with the trough will merge...with the models
showing a stronger deeper low well offshore by early Tuesday.
In terms of weather...scattered showers will be in store during the
day and evening hours...slowly decreasing Monday night from west to
east. The showers will be periodic and by the output of mesoscale
model reflectivity fields...coverage at times could be more sparse.
Therefore...did not lower temperatures too much and kept using a
blend of gmos/met/mav guidance. Maximum temperatures Monday are forecast
to range mainly in the lower 60s. Also...models show some weak
instability Monday afternoon into early evening...so there could be
a thunderstorm. For the forecast...kept the thunderstorms to slight
chance probability of precipitation.
There will be a persistent southeast to northwest pressure gradient
remaining...as winds will stay up and increase in the 950-800mb
layer Monday night. The combination of this and abundant clouds will
keep low forecast on the warmer side Monday night...in the lower
Long term /Tuesday through Sunday/...
a blocky upper level pattern will persist into the second half of
the week with the flow flattening across the western and central US
by next weekend. The large persistent cutoff low pressure system
over the northeast and southern Canadian Maritimes will remain S of
Nova Scotia on Tuesday as spokes of vorticity move around it. There
is uncertainty in the timing of these shortwaves tracking within
the cyclonic flow...and have kept schc probability of precipitation over extreme eastern
Long Island and southeast CT early Tuesday morning as most of the model
guidance indicates that the vorticity maximum moves through late Monday
night/Tuesday morning. The GFS brings it through during the afternoon...but
it is alone in this solution.
The cutoff finally moves far enough away on Wednesday to preclude any
additional precipitation chances as upper level ridging builds in. Dry conds
and partly cloudy skies are expected.
There is better agreement with the upper level pattern Wednesday into Thursday
as a northern stream shortwave merges with a southern stream cutoff low pressure
system. However...the GFS remains faster moving the upper ridge axis
east of the area...despite the blocking from the aforementioned
departing cutoff low. Think this is too quick and like the ec's solution
better which has also had more run to run continuity over the past
few days than the GFS. Since 00z last night...the GFS has been
hinting at another cutoff low diving out of southeast Canada and phasing
with southern stream energy with coastal low pressure tracking up the
eastern Seaboard. The ec has consistently held off on cutting the 500 mb
flow off...keeping the flow more progressive and allowing the
southern stream energy to pass well S of the area. Have added low
chance probability of precipitation to the forecast Thursday night through Friday night to account for
the GFS trends...but this is a low confidence forecast.
A few upper level shortwaves may move across the region next
weekend...although it looks like only light quantitative precipitation forecast would impact the
Temperatures will be near normal levels through the period.
Aviation /20z Sunday through Friday/...
a weak trough across the NYC terminals this after/evening will gradually
weaken this evening. High pressure then re-establishes itself
tonight. Another trough swings through the region on Monday.
Broken instability this aft/eve. Isolated rain showers activity early this
evening...mainly CT terminals.
Seabreeze along the CT/NYC/Li coast will slowly work north through the
rest of the after into early evening. With gusty northwest flow to north and west of
surface trough...seabreeze having trouble approaching klga/kewr.
Seabreeze may waver along the southeast periphery of kewr into early
evening...and make a late push at klga. Have adjusted tafs
Winds weaken and become northwest this evening as high pressure
strengthens...with strato-cumulus gradually scattering overnight.
Broken 040-060 strato-cumulus development expected Monday with scattered
shower development in the aft/eve. Brief MVFR possible with rain showers
in the afternoon. Low probability for isolated thunderstorms and rain...mainly kgon. Northwest
winds 10 to 15 knots with gusts to 20 knots Monday afternoon. Winds
generally right of 310 true.
Outlook for 18z Monday through Friday...
late Monday-Monday night...possible MVFR with showers.
Tuesday-Wednesday night...VFR. Isolated rain showers possible Tuesday.
Thursday...VFR. Low probability for -shra.
Friday...low probability for coastal storm.
for the near and short term through Monday night...there
will be a persistent...reinforcing pressure gradient from the
southeast to northwest. The gradient is weak tonight but increases
Monday and especially Monday night with a deeper low forming well
offshore and high pressure gradually sliding southeast into the
Sub Small Craft Advisory conditions are forecast through Monday but Small Craft Advisory will become
likely Monday night across the ocean. This is from the presence
of showers to help bring down momentum in the evening and then the
winds further increase late in the 950-800mb layer. The MOS
guidance also has an increasing trend with sustained winds from
the previous run. For other waters...there could be a 25 knots gust
Monday night but confidence was not as high in these locations and
occurrences for these higher gusts will probably be quite brief.
Small Craft Advisory gusts may linger into Tuesday morning...however there are
differences between the guidance on when the winds aloft
subside...so don't have the confidence to continue the advsy into
the 4th period. Sub-advsy conds then persist through Thursday.
Depending on the track of a coastal storm at the end of the
week....sca flags may be needed once again.
no significant precipitation is not expected through Wednesday.
Marine...Small Craft Advisory from 6 PM Monday to 6 am EDT Tuesday for