Scientific Forecaster Discussion
Area forecast discussion
National Weather Service New York New York
128 am EDT Tuesday Mar 31 2015
a cold front moves offshore tonight...followed by a quick moving
low passing to the south on Tuesday. High pressure will build
across the area on Wednesday ahead of a frontal system moving into
the plains. The frontal system will then progress slowly across
the area from late Thursday into early Saturday. High pressure
will return later this weekend.
Near term /until 6 am this morning/...
initial cold front moved offshore earlier this evening.
However...a secondary cold front is currently making its way
across the region. This front in combination with a middle level
shortwave has allowed isolated/scattered showers to move into parts of NE
New Jersey...NYC metropolitan and southern. These rain showers should diminish
by 06z. Thereafter expect clearing skies...and diminishing west to
west-northwest flow. Lows will be in the upper 20s inland and in the Long
Island Pine barrens...30-35 most elsewhere...and middle/upper 30s
in vicinity of NYC.
Short term /6 am this morning through 6 PM Wednesday/...
Alberta clipper low now moving from the northern Mississippi Valley
toward the upper Great Lakes will continue southeast...reaching the northern
middle Atlantic by Tuesday afternoon. Expect Tuesday morning to be dry with
increasing clouds...then precipitation should overspread the area in the
afternoon. Have likely/Cat pop for nearly the entire County Warning Area...except
for southern CT as there is some uncertainty as to how far north
the associated precipitation shield will extend since nwp guidance shows
best warm air advection and 500 mb forcing passing mainly over NYC metropolitan/Long Island
and points south.
Precipitation during the daylight hours should be mainly rain for NYC
metropolitan and Long Island...but inland there is some uncertainty as to
precipitation type. At this time expect an elevation-dependent snowfall...with a
cold rain or a rain/snow mix in the lower elevations...and mostly
snow in the highest elevations. P-type should trend more toward
snow throughout before ending as the boundary layer cools via
wet-bulb cooling and an increasing colder northerly flow. At this time
expect little to no accumulation in NYC metropolitan...no more than a
coating most elsewhere and up to an inch in the highest
elevations. The worst-case scenario would be a coating of
accumulation in NYC...up to an inch most elsewhere...and 1-2
inches in the higher elevations.
Long term /Wednesday night through Monday/...
a progressive...yet somewhat amplified upper flow across the lower
48...will be featured during the period. This will result in
several swings in the temperature regime as we go from below normal at the
start of the period...to above normal ahead of an approaching
frontal system at the end of the week...followed by yet another shot
of unseasonably cold air at the end of next weekend. There are also
some timing issues...in particular with the frontal system at the
end of the week. The GFS is more aggressive with a rainfall with a
frontal wave...while the European model (ecmwf) has that wave displaced farther north
and west. Even so...both the 12z GFS and European model (ecmwf) dry conditions out
behind the cold frontal passage on Sat. The differences arise due to
the timing of the Pacific shortwave energy. The specifics of such can
not be worked out this early in the forecast process. Preference was
to take a middle ground approach with the timing of the late
Aviation /06z Tuesday through Saturday/...
low pressure approaches from the west today and passes S of Long Island
VFR until this afternoon when ceilings lower and precipitation overspreads the region.
The precipitation starts as a mix of rasn or all rain...then changes to
snow into the evening.
LIFR possible in the 22-04z time period when potential will be
highest for 1/2sm snow.
Improvement from west to east tonight.
Winds veer through this afternoon...then vrb direction becomes NE as the
low approaches the metropolitan. The NE flow then veers to the northwest as the
low passes east of the region.
Outlook for 06z Wednesday through Saturday...
Tuesday night...VFR with northwest flow 10-15kt.
Thursday night-Friday night...MVFR in rain showers.
Saturday...sub VFR possible in showers early...then VFR late.
Small Craft Advisory continues on the ocean waters...eastern sound...and Peconic
and gardiners bays. Ocean seas 4-6 feet and may increase another feet
by daybreak. Gusts out east after frontal passage should reach Small Craft Advisory levels.
Gusts to 25 knots look to be only brief/occasional in nature this evening
on the western sound and South Shore bays.
Small Craft Advisory conds could last on the ocean through Tuesday night...and also
return to the eastern waters Tuesday night...as low pressure passes to the
S and northerly flow increases in its wake. Then expect sub-small
craft level conditions Wednesday through Thursday as a weak pressure gradient
remains over the region. Wind gusts and waves increase to small
craft levels ahead of the next system approaching the ocean
waters Thursday night into Saturday.
quantitative precipitation forecast of 1/10 to 1/4 inch expected Tuesday afternoon and evening.
There is the potential for widespread rainfall at the
end of the week with a frontal wave along a slow moving frontal
Rivers and streams are expected to remain below bank full...but
increased river/stream flow across the lower Hudson Valley and
southern Connecticut will continue ice breakup and movement into
this weekend. Isolated ice jams are possible...which if they
occur...could result in localized flooding. Since ice jam flooding
is unpredictable and sudden...interests along ice covered rivers
should monitor National Weather Service forecasts for the latest information.
For details on specific area rivers and lakes...including
observed and forecast river stages and lake elevations...please
visit the advanced hydrologic prediction service /ahps/ graphs on
our web site.
Marine...Small Craft Advisory until noon EDT today for anz330-340-350-