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Area forecast discussion 
National Weather Service Mount Holly New Jersey
746 PM EST Monday Mar 2 2015

Synopsis...
high pressure will build over our area tonight, then shift off the
middle Atlantic coast on Tuesday. A warm front is expected to lift
through our area on Tuesday evening. A cold front is anticipated to
arrive Wednesday afternoon, then the front should settle slowly to
the southeast Wednesday night and Thursday as low pressure rides
along it. Another Arctic high is forecast to build across our region
for Friday into Saturday. A warm front may begin to approach from
the southwest on Sunday.

&&

Near term /until 6 am Tuesday morning/...
630 PM estf: slightly less evening cirrus, temperatures adjusted slightly
but the 330 PM forecast continues.

A clear night with decoupling radiational cooling developing over the
Countryside. Some cirrus toward dawn. Diminishing wind to light
northwest by midnight.

2m temperatures suggest zero to 5 above by morning in the Countryside
near and north of i80 and will probably lower the min temperatures a bit in
the Countryside around for the 930 PM estf update.

&&

Short term /6 am Tuesday morning through 6 PM Tuesday/...
**winter weather advisory already posted for late tuesday**

The question: do we need to expand this a tier of counties further
southeast in the 330 am forecast. No change will be made at 930 PM.

Am favoring the NAM thermal profile and this looks like it has a
bunch of sleet at the start in PA/New Jersey south of i78, then fzra,
while along and north of i78 snow to sleet to freezing rain. In Maryland/Delaware it
will be freezing rain probably changing to rain.

From the 330 PM discussion: the start of a rather unsettled time
frame begins later Tuesday. This is courtesy of an upper-level
trough amplifying across the Midwest with energy also ejecting out
of The Four Corners region into the Southern Plains. The Midwest
energy drives low pressure mostly north of the Great Lakes during
Tuesday. This feature however will be pulling a cold front toward
our area late in the day. As these features move eastward,
strengthening warm air advection and lift occurs downstream.

The flow is forecast to increase quite a bit above the surface by
the end of the day as a southerly low-level jet arrives. This will
drive strong warm air advection aloft and also result in a ribbon of strong lift.
The forecast soundings show warming aloft much faster than at the
surface across much of the area. This results in a strong inversion,
which will tend to hold in the colder air at the surface. We are
expecting some moderation in the low-level airmass due to the
southerly flow developing, however this should be light initially
and incoming precipitation combined with low dew points should
result in an initial drop in temperatures for most if not all of the
area. This leads to a forecast challenge regarding icing potential.

An examination of the forecast soundings is pointing toward more
sleet in general. This is due to the robust warm air advection allowing for a deeper
warm layer above a decent cold layer. The initial rather dry low-
level airmass though should result in some evaporational cooling,
therefore we are thinking some snow to start before transitioning
to some sleet from south to north by early evening. The overall
start time across the area appears to be after 18z. This will
likely have an impact on the evening commute across a good portion
of the County Warning Area.

Given the above /even though amounts by early evening should be on
the lighter side/, factoring in the evening commute impact and a
transition to freezing rain beyond the daylight hours leads US to
issuing a Winter Weather Advisory. The advisory was placed where
confidence was highest for mixed precipitation, however some
expansion may be needed if freezing rain looks more likely.

As for high temperatures, we mostly used an even blend of the
GFS/NAM MOS but did adjust the hourly temperatures down some by late
afternoon to account for some evaporative cooling.

&&

Long term /Tuesday night through Monday/...
**a more potent winter weather event shaping up for 03z Thursday-19z thu**

This will need to be monitored for much longer and possibly greater
public impact than the Tuesday evening event.

Complex forecast shaping up during the Tuesday night through Thursday time frame
with two chances of winter precipitation. For Tuesday night...overrunning will
lead to snow changing to sleet and then freezing rain. Confidence
is greatest for freezing rain along and northwest of the I-95
corridor where we are hoisting a Winter Weather Advisory. There is
still some uncertainty in the forecast...and this advisory may be
need to be adjusted southeast.

A cold front will move through on Wednesday...and this will set the region
up for another overrunning event Wednesday night and Thursday.

Thermal profiles and quantitative precipitation forecast are still highly uncertain at this
point...but a rain to sleet to snow situation seems more likely.
Where the heaviest snow sleet accumulation axis sets up is also
uncertain...but several inches of combined snow and sleet are
expected. Some model thermal profiles suggest a possible extended
period of sleet.

This system is projected to move offshore by late Thursday or early
Friday...with the next cold front approaching over the weekend.

&&

Aviation /01z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.

Tonight...VFR with cirrus ceilings possible after 09z. Northwest winds
diminishing to 10 knots or less this evening, then locally becoming
light and variable overnight.

Tuesday...icing should be a deicing problem for aircraft late
Tuesday afternoon at many taf locations but also in flight at or below
roughly 6000 feet as an above freezing layer arrives at 6-7000 feet
late in the day while modeled thermal profiles maintain temperatures
well below freezing below 5000 feet late in the day.

VFR ceilings lower to MVFR/IFR in light fzraplsn late in the afternoon
from west to east. 00z/3 tafs attempt to define that a period of
light snow would primarily occur at kabe/kttn with pl and freezing rain at
all other taf sites; maybe mostly pl at krdg through 00z/4. Light
wind becoming southeast at 5-8 knots. A southerly low- level jet
/2000 feet/ near 40 knots should arrive late in the afternoon.

Outlook...
Tuesday night...mainly IFR conditions with snow/pl quickly changing to
freezing rain and then rain from south to north in the evening. Freezing rain may hold on until
after midnight at rdg/Abe. Low level wind shear will be the other problem due to a
strong southwesterly low-level jet that is forecast to develop overhead.
Low level wind shear is in the phl taf when a wind of 210-220 at 50 knots is modeled
by the NAM around 03z/4. Low level wind shear most predominant over S New Jersey where
stronger SW winds are forecast near 2000 feet.

Wednesday...occasional MVFR and IFR conditions in periods of rain.

Wednesday night and Thursday...MVFR and IFR conditions in rain. The
rain is expected to mix with and then change to snow from northwest
to southeast. There is still lower confidence on timing of the
changeover but Abe/ridge look to change over first Wednesday night and
then the I-95 terminals either late Wednesday night or Thursday
morning. IFR/LIFR if heavier snow develops. Gusty northwest winds develop.

Thursday night through Saturday...mainly VFR.

&&

Marine...
strong high pressure will build over the area later tonight, however
a lingering pressure gradient will keep the winds up some for awhile
longer. We are seeing some diminishing already especially across the
southern waters. As a result, we trimmed back the end time of the
Small Craft Advisory to 00z for Delaware Bay and the far southern
coastal waters, then 03z elsewhere. The seas have been running a bit
lower as well, therefore adjusted them down a bit quicker which is
closer to the latest guidance. As the boundary layer cools tonight,
some freezing rain is expected however this should be light as the
winds subside. A southerly flow develops on Tuesday ahead of the
next system, and while a low-level jet will be arriving later in the
day it appears the winds at the surface will remain below advisory
levels during the daylight hours on Tuesday.

Outlook...
Tuesday night and Wednesday...Small Craft Advisory will likely be needed with a
strengthening southwesterly flow. Soundings continue to show very strong
winds just above the surface but the warmer air moving atop the
cold waters does not provide a favorable setup for mixing down
these winds to the surface. Seas in our coastal zones increase to
4-6 feet.

Wednesday night and Thursday...Small Craft Advisory likely with gusty northwest winds and at
least light freezing spray.

Thursday night through Saturday...no marine headlines are
anticipated.

&&

Hydrology...
the next round of precipitation is forecast to move into our area on
Tuesday afternoon and it will continue into Wednesday and Thursday.

This midweek event appears to start as a wintry mix before changing
over to rain as temperatures rise well above freezing on Wednesday.
The quantitative precipitation forecast amounts for Tuesday into Wednesday favor the 0.25 to 0.75
inch range.

The combination of the brief warm-up on Wednesday, locally
moderate to heavy rainfall, melting snow, a frozen ground, and
above normal ice in creeks, streams, and rivers could lead to
increased runoff and possible flooding.

But where the depth of the snowpack is greatest (in NE PA and north nj),
rapid snowmelt is not expected since temperatures will not rise as sharply
as in areas to the south. In addition, current model solutions have
the greatest amounts of rainfall across the coastal plain where
there is much less liquid water equivalent in the snowpack.

Looking at the mmefs solutions, ensemble quantitative precipitation forecast is running higher than
the operational. Running with the 0.25 to 0.75 inch over a snowpack
or not, we feel nuisance poor drainage and low lying flooding is the
most likely outcome. Modeled solutions suggest 1.50 inches of
rainfall along with about an inch of snow smelt could get some
smaller streams and creeks to leave their banks.

Still, increased flow on area creeks, streams, and rivers could also
start to move ice, and in a worst case scenario, create restrictions
or jams.

Continue to monitor the forecast. Keep in mind that snowmelt and
warmth by themselves typically do not cause flooding. The amount
of rain that we receive will be the determining factor.

The rain is forecast to change back to snow on Wednesday night.

&&

Climate...
second coldest February on record for Allentown, PA...

Average temperature year
1 16.5 1934
2 18.9 2015*
3 20.4 1979
4 21.3 1936
5 22.0 1963
6 22.3 1978
7 22.4 1923
8 23.9 2007
9 24.3 1967
10 24.6 1958

Por 1922 to present

Also, this 18.9 average is the 3rd coldest month on record dating
back to 1922 for Allentown with Jan 1977 18.6 ranking number 2.

January 2015 had an average temperature of 25.9, 7 degrees warmer
than February!





Seventh coldest February on record for Philadelphia, PA...

Average temperature year
1 22.2 1934
2 23.0 1979
3 24.3 1885
4 24.6 1978
5 25.4 1895
6 25.7 1875
7 25.8 2015*
8 26.3 1905
9 26.4 1963
10 27.1 1907

Por 1872 to present

The February average at philadelphia's 5.1 degrees colder than
janaury's 30.9 average.



Fourth coldest February on record for Atlantic City, New Jersey...

Average temperature year
1 21.6 1979
2 23.6 1934
3 23.8 1978
4 24.4 2015*
5 25.5 1875
6 25.7 1905
7 25.8 1885
8 26.0 1895
9 26.8 1968
10 26.9 1904

Por 1874 to present

For Atlantic City this tied for the 7th coldest month on record.

The February average was 5.8 degrees colder than the average of
30.2 in January.





To our knowledge this is the fifth coldest February on record for
Wilmington, Delaware...

Average temperature year
1 20.2 1934
2 22.1 1979
3 22.7 1978
4 24.6 1936
5 24.8 2015*
6 27.2 1895
7 27.3 1958
8 27.5 1963
9 27.8 1947
10 28.1 2007

*new

The February average was 5.5 degrees colder at Wilmington than the
30.3 of January 2015.

The por 1894 to present but is missing a number of years of data.

&&

Phi watches/warnings/advisories...
PA...Winter Weather Advisory from 3 PM Tuesday to 6 am EST
Wednesday for paz054-055-060>062.
Winter Weather Advisory from 3 PM Tuesday to 1 am EST
Wednesday for paz070-071-101>106.
New Jersey...Winter Weather Advisory from 3 PM Tuesday to 6 am EST
Wednesday for njz001-007>009.
Winter Weather Advisory from 3 PM Tuesday to 1 am EST
Wednesday for njz010-012-015>019.
Delaware...Winter Weather Advisory from 3 PM Tuesday to 1 am EST
Wednesday for dez001.
Maryland...Winter Weather Advisory from 3 PM Tuesday to 1 am EST
Wednesday for mdz008-012.
Marine...Small Craft Advisory until 10 PM EST this evening for
anz450>453.

&&

$$
Synopsis...franck
near term...drag/gorse
short term...drag/gorse
long term...franck
aviation...drag/Klein
marine...drag/Klein
hydrology...
climate...747p

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