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Area forecast discussion 
National Weather Service New York New York
222 am EDT Tuesday Oct 21 2014

Synopsis...
low pressure moves in from the west tonight and remains in the
vicinity through the middle to late week period before moving into
the Canadian maritime provinces by Saturday morning. A cold front
approaches from the northwest Saturday...then crosses the tri-
state Saturday night. High pressure then builds into the Middle
Atlantic States early next week.

&&

Near term /until 6 am this morning/...
minor update to adjust the diurnal curve. Cloud cover has slowed
the cooling in outlying areas. No other major changes made.

Upper trough shifts towards US tonight along with a broad area of
low pressure at the surface. Clouds will continue to lower through
the night. Models agree that there will be some isentropic lift
and moisture increasing over the easternmost portion of the tri
state area late at night but with some differences in placement.
There will also be increasing positive vorticity advection...especially west of the Hudson
as the upper trough axis approaches. Will continue with overall
slight chance to chance probability of precipitation focused mainly after midnight
tonight...but will go with the highest chances for the far eastern
and far western areas.

&&

Short term /6 am this morning through 6 PM Wednesday/...
the system shifts east during Tuesday...with a surface low center
starting to develop near the northern middle-Atlantic coast by the end
of the day. Overall lift is not impressive...however with continued
positive vorticity advection and steepening lapse rates in the middle levels...thinking is that
there will be at least scattered showers around. For the
afternoon...there will also be a slight chance of a thunderstorm
with better the steeper lapse rates and elevated cape.

The high temperature forecast is tricky for Tuesday as extended
breaks of partly to mostly sunny conditions could develop. Have gone
with a mav/NAM MOS blend for highs...which were similar.

The low strengthens to our south Tuesday night...with overall better
lift favoring the southwest half of the tri state area. Have gone
with likely probability of precipitation late at night here...with chance probability of precipitation elsewhere.
Elevated cape remains with isolated thunderstorms...generally shifting east as
the night progresses.

&&

Long term /Wednesday night through Monday/...
confidence is high that a cutoff low will track from near the middle
Atlantic coast Wednesday morning to somewhere near Nova Scotia by
Saturday morning. As a result have gone with a model blend through
then.

It also appears there will be sufficient middle level instability over
the entire County Warning Area...and to some extent low level instability mainly
over southern and far eastern zones...to warrant a slight chance of
thunder Wednesday and Wednesday night...in addition to likely probability of precipitation
for rain. Appears all ingredients come together isentropically to
warrant forecasting moderate rainfall from Wednesday afternoon
through Wednesday night...with the moderate rainfall potential
retreating to the northeast during the day on Thursday. As the
cutoff low pulls away there will be a chance of showers Thursday
night...tapering off to a slight chance of showers by Friday
afternoon. For now it appears Friday night should be dry due to
subsiding northwest boundary layer flow. Refer to the hydrology section of
the afd for storm total quantitative precipitation forecast and any potential impacts.

The GFS and European model (ecmwf) agree on the general features to impact the area
Saturday through Monday...but differ on timing...and southern push
of a surface cold front Saturday night. Noting the GFS seems to be
living up to its progressive bias...and the consistency of the European model (ecmwf)
and noting that the CMC supports the European model (ecmwf)...leaned towards the
European model (ecmwf) in this time frame.

Saturday should be dry with a shortwave Ridge Crossing the area.
700-500 hpa trough then crosses the area Saturday night and
Sunday...but with limited low level moisture to work with...it
should pass by dry. A deep layered ridge then builds in then begins
to build into the region into Monday.

For temperatures in the long term:

A blend of mav/met/ecs guidance...with NAM 2-meter temperatures and
a mix down from 975-950 hpa per BUFKIT soundings was used for highs
on Wednesday...with values forecast to be around 5 degrees below
normal. For lows Wednesday night a blend of mav/met guidance with
NAM 2-meter temperatures was used - with readings forecast to be
near normal in NYC and around 5 degrees above normal elsewhere. For
highs on Thursday a blend of mex/men/ekd/ece/ecm/wpc guidance was
used with NAM 2-meter temperatures and a mix down from 950-925 hpa
per BUFKIT soundings - with values expected to be around 5 degrees
below normal.

From Thursday night-Monday a blend of mex/men/ekd/ece/ecm/wpc
guidance was used...with European model (ecmwf) 2-meter temperatures blended in
Sunday and Sunday night...and the blend adjusted upward 1-2
categories to counter bias towards climatology in the guidance that
far out in time - noting forecast 850 temperatures of at least 10c
in all models on Monday. Lows are forecast to run above normal
Thursday night-Sunday night...while highs are forecast to be above
normal...except slightly below normal on Sunday. Highs on Monday
could very well end up 5+ degrees warmer than forecast if current
trends hold.

&&

Aviation /06z Tuesday through Saturday/...
low pressure approaches from the northwest through tonight...then
becomes nearly stationary across New York...Pennsylvania...and into
Virginia Tuesday as another low develops off the coast.

Tricky forecast next 24 hours with a weak pressure gradient for most
of the period leading to light and vrb winds. Tried to pinpoint a
direction...but directions could be all over the place until the
low tracks to the S of the area and a NE flow between 5 and 10 knots
develops this evening.

Conds should be VFR for the majority of the period...but could see
isolated MVFR in any rain that moves directly over any of the
terminals. Speaking of rain...tough to pinpoint when and where but
do not expect a prolonged period of precipitation at any of the
terminals...except kswf. Am probably too early in steady -radz
beginning at 23z tonight...but further adjustments can be made in
subsequent tafs.

Outlook for 06z Wednesday through Saturday...
Tuesday night...MVFR to locally IFR conds developing with scattered light
rain.
Wednesday through Thursday...MVFR or lower conds in rain. Northerly flow 10 to 15
knots g20-25kt.
Friday...improving conds. Northwest winds 15-20 knots with 20-25 knots gusts.
Sat..VFR. Northwest g20kt.

&&

Marine...
SW winds expected to subside tonight with a somewhat weaker pressure
gradient developing over the waters. Winds gradually back towards
the east by late in the day Tuesday...then as a low center develops
to the south Tuesday night...winds back farther towards the NE and
pick up as the low becomes stronger. Winds and seas should however
remain below advisory levels through this period.

Due to uncertainty in the exact track and strength of a coastal low
Wednesday through Friday...there is a degree of uncertainty in the
forecast in that time frame that is greater than normal. For now it
appears there is a high chance of Small Craft Advisory conditions on
the coastal ocean waters from Wednesday through Thursday
night...with a chance of small craft conditions on the remainder of
the waters during this time frame.

Sub-small craft conditions are forecast on all waters
Friday-Saturday - however there is a small chance that small craft
conditions could linger into Friday on the eastern waters.

&&

Hydrology...
dry today...with spotty light rainfall amounts possible tonight.

There is the potential for around an inch of rain from Tuesday
night into Thursday night over the western 1/3 of the
County Warning Area...including NYC...and up to 1 1/3 inches of rain over the
eastern 2/3 of the County Warning Area...with the best potential for the highest
amounts over eastern CT and Long Island.

There is the potential for locally higher amounts in any area
experiencing stronger convection...which is locally possible
Tuesday night through Wednesday night. If this occurs...then there
is the potential for localized minor urban/poor drainage
flooding. Otherwise...with the precipitation falling over 48+
hours...little or no significant hydrologic impact is expected.

&&

Okx watches/warnings/advisories...
CT...none.
New York...none.
New Jersey...none.
Marine...none.

&&

$$

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