Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 181125
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
625 AM EST Sun Feb 18 2018

.SYNOPSIS...
High pressure briefly builds across our area today, then offshore
tonight. A strong warm front lifts northeast across our area early
Tuesday, followed by a cold front late Wednesday. That cold front
stalls to our south with waves of weak low pressure of moving
northeast along the front Thursday through Saturday. Eventually a
larger low pressure system should move across southeast Canada on
Sunday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
For the 630 AM update, sped up the clearing as satellite imagery
and surface observations show the low clouds are now rapidly
eroding southeastward and should clear the coastal areas within
the hour. Some flurries or a brief snow shower cannot be ruled
out in the southern Poconos the next few hours, however upstream
activity looks to be weakening. The temperatures were adjusted
up a little for some areas to start as a breeze has allowed a
bit more mixing.

Watch for some slippery spots on untreated surfaces early this
morning due to residual moisture and/or slush.

A strong short wave trough with an associated robust short wave will
shift east of the New England coast this morning, with the flow aloft
going more zonal in its wake. As this occurs, a strong ridge centered
in the southwestern Atlantic Ocean extends westward into the Gulf
of Mexico. A sprawling area of high pressure at the surface will
quickly build over our region during today.

Some cold air advection occurs this morning, however this quickly
diminishes this afternoon. As drier air along with increasing
subsidence overspreads our area, the low clouds are rapidly eroding
early this morning. There could then be some stratocumulus leftover
for a time within a northwesterly wind which diminishes this
afternoon as high pressure arrives.

Temperatures could be a bit of a challenge today thanks to the
expansive area of 4+ inches of snow last evening. While temperatures
are expected to rise into the 40s for much of the area this
afternoon, these could be tempered some due to energy going into
melting the snow. We therefore went a little cooler especially north
and west of I-95 but not to much lower given lots of sunshine
expected. To the south and east of here, there is much less snow and
there will be a downslope component of the wind.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
As a significant upper-level trough settles into the West, a ridge
begins to build across the East. Surface high pressure will
therefore shift offshore allowing a return surface flow to start
developing. In addition, the flow aloft backs to more southwesterly
through tonight along with an increase in warm air advection. There
are some weak short waves riding the northern side of the gradually
building ridge, and these along with an increase in warm air
advection could toss some clouds our way during the course of the
night especially late. This may have an impact on temperatures, as
winds will be light to calm at the surface with dew points low
enough to assist in radiational cooling. For now, we opted to go
somewhat colder across the northern areas assuming at least some
snow cover will remain.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
**Record warmth possible Tue and expected Wed**

Hazard potential worthy of monitoring: Dense fog may become a
widespread hazard event in e PA and NJ Monday night. Icing,
possibly an intermittent but prolonged event may be setting up
for the far northern part of our area between Thursday and
Saturday morning.

500 MB: Very warm and strong ridging builds along the east coast
early this week then gradually weakens late in the week as a NEG
NAO develops and may be starting to change the pattern here by
the weekend. A series of short waves will be ejecting enewd out
of the western USA trough late this week into the weekend
setting us up for periods of pcpn.


Temperatures: Please the climate section on all the upcoming
temperature records that will be approached or exceeded and a
reevaluation of PHL Feb monthly projection. Calendar day
averages should be about 3 to 5 degrees above normal Monday,
then 20 to 24 degrees above normal Tuesday, 25 to 30 degrees
above normal Wednesday, cooling to 10 above normal Thursday, 5
above normal Friday and then an uncertain weekend in which
average temperatures should warm to anywhere between 8 and 18
degrees above normal.


Forecast basis...Unless otherwise noted a 50 50 blend of the GFS/NAM
MOS for Monday-Tuesday, the GFS MEXMOS for Tuesday night-Wednesday
and then the 06z/18 WPC D4-8 12 hr elements of max/min temp/pop and
6 hrly sky/wind/td were used Wednesday night-Saturday. We did
apply the colder ECMWF to our fcst temps beginning 06z Thursday
through Saturday morning, which means we are under virtually all
guidance except a bit warmer than the ec 2m temps.

The dailies...

Monday...after a potentially frosty start to the day, rapidly
lowering cloudiness. An instability burst from the southwest
should develop-advect a band or bands of showers into the
western and northern part of our forecast area during the
afternoon. Cloud cover and a light southerly wind limits
afternoon warming. Confidence: Above average.


Monday night...any leftover showers should end around midnight.
Nearly steady or slowly rising temps with stratus/fog-some of
the fog may be dense, especially over the remaining snowmelt I78
region northward. Confidence: above average.


Tuesday...skies will brighten and it will warm during the afternoon
but thinning stratus may limit the warm up in some areas.
Confidence: average with the uncertainty how fast skies partially
clear in the afternoon. Southwest wind should permit the the
status to break up. Confidence: average.

Tuesday night...Stratus may continue in parts of our area or
reform for a few hours late at night but for now, expect the
boundary layer wind to be too strong to permit dense fog
forming. Confidence: average.


Wednesday...Record daily warmth forecast Wednesday and potentially
near the all time February record max.  Record daily warm minimums
appear possible but will depend on the strength of caa near
1159 PM Wednesday. Highs well up in the 70s over SE PA as well
as much of the Delmarva and NJ. Stratus may continue in parts of
our area for a couple of hours during the morning before
thinning to reveal partly to mostly sunny skies. An approaching
cold front from the northwest could bring some showers to the
region northwest of I-95, toward sunset.

Wednesday night into Thursday...A fairly strong cold front with
scattered or isolated rain showers pushes southeast through our
area before stalling somewhere between southern DE and NC. From
this point forward, confidence on details drops below average.
There are several scenarios, and they all are cold. It would
not be impossible to see some sleet Thursday I78 north if a wave
of low pressure spreads its pcpn shield north of I78.

Friday - Saturday...an even bigger forecast problem may be
looming. Presuming sub freezing boundary layer cold air seeps
south into the I78 region, will low pressure in southeast Canada
become dominant and yank the cold air out. I`m tending to think
not, especially with blocking starting to press southwestward. I
may be premature on this but if the Low forecast across se
Canada is alot weaker, then we will be in a situation near the
top of the ridge with positive tilt short waves ejecting out of
the western USA and creating periods of light to mdt pcpn here,
which could be ice I80 north. Right now its a long shot but the
EC has a hint and the GFS is not too far off. We`ll watch the
trends in the modeling consensus the next few days. For now
uncertainty but i think, worthy of monitoring. The two winter
events that occurred this week were briefly hinted at one time
or another a week prior to their occurrence Tuesday and
yesterday. The GGEM was onto this ydy and todays GGEM op member
has lost it. Its February and winter and hopefully this extensive
discussion of a colder late week scenario is not useless.

&&

.AVIATION /11Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...MVFR ceilings at MIV and ACY are expected to quickly
improve to VFR early. Otherwise, VFR. Northwesterly winds around
10 knots this morning with some local gusts to 15-20 knots
through midday, then diminishing by late afternoon and becoming
west-southwest.

Tonight...VFR. Light and variable or light southerly winds.

OUTLOOK...
Monday...VFR early, lowering to MVFR or IFR during the afternoon or
evening in showers and/or stratus. South to southwest wind.
Confidence: above average.

Monday night...Widespread IFR conditions in st/fog with showers or
drizzle possible early. Conditions may deteriorate to LIFR in
areas of dense fog late. Confidence: Above average for IFR,
average for LIFR.


Tuesday...IFR conditions may linger through midday in low clouds/fog.
Conditions should improve to VFR during the afternoon. Southwest
winds may gust around 20 knots in the afternoon. Confidence:
average.

Tuesday night...VFR early, then possibly lowering to MVFR/IFR
as low clouds/fog may develop overnight. Confidence: average.

Wednesday...MVFR/IFR conditions possible early with low clouds/fog.
Conditions improving to VFR during the day. chance of showers
late in the day. Southwest winds may gust around 20-25 knots.
Confidence: Above average.

Wednesday night through Thursday...Chance of showers with
associated restrictions. A wind shift from southwest Wednesday
evening to northwest and then north on Thursday. Confidence:
Below average.

&&

.MARINE...
A short surge in cold air advection should result in increased
mixing for a time this morning. As a result, we will maintain the
Small Craft Advisory through late morning with a round of wind gusts
to around 25 knots. Otherwise, the conditions are expected to be
below Small Craft Advisory criteria as high pressure builds in
through today then offshore tonight.

OUTLOOK...
Monday...Sub Small Craft Advisory south winds. Confidence: Well
above average.

Monday night...Sub Small Craft Advisory conditions expected,
although southwest winds may gust around 20 knots. Confidence:
Above average.

Tuesday-Wednesday...Winds should stay below advisory levels,
but seas may increase to near 5 feet. Confidence: Average.

Wednesday night - Thursday...Winds should stay below SCA levels
with seas persisting in the 3 to 5 foot range. Confidence:
Below average because of a wind shift Wednesday night from
southwest to northwest and then north northeast on Thursday.

&&

.CLIMATE...
Record high temperatures could be challenged on Tuesday,
February 20th and probably will be equaled or exceeded Wednesday
February 21st. These records are listed below.

February 20:

Atlantic City...70 in 1930

Philadelphia....70 in 1939

Wilmington......71 in 1930

Allentown.......68 in 1930

Trenton.........70 in 1930

Georgetown......68 in 2002

Mount Pocono....59 in 1930

Reading.........72 in 1930


February 21:

Atlantic City...74 in 1930

Philadelphia....72 in 1930

Wilmington......70 in 1953

Allentown.......67 in 1953

Trenton.........70 in 1930

Georgetown......71 in 2014

Mount Pocono....60 in 1930

Reading.........71 in 1930


We are not posting the monthly records at this time since our
forecast is 3 to 7 degrees below the monthly record. We`ll check
again Sunday and Monday to see if our forecast edges warmer.

The following are the record warm mins for Wednesday Feb 21.
Barring a sudden immediate sharp cool down behind the cold
front at 1159PM Wednesday, there is a chance of record warm
daily minimum temperatures as well the previously referenced
record highs.

Record high mins for 2/21

ABE 46-1981
ACY 49-1954
PHL 49-2002
ILG 47-2002
RDG 48-1930
TTN 48-2002

Atlantic City rainfall of 5.81 inches currently ranks 3rd
wettest in February history with 6.50" in 2010 the wettest and
#2 is 1958 with 5.98 inches. Wilmington`s rainfall now ranks
#12 and Philly #15. All rankings will increase as the rest of
the month looks a little on wet side.

Yesterday we calculated the February average temp for PHL to
projects 41.0 or 5.3F above normal or 8th warmest on record.
This placement can change anywhere from 3rd to 12th depending on
a 1 degree change in the eventual average. Bottom line...we`re
likely on our way to a top 10 warmest February on record. The
warmest was last year with a 44.2F average. Years 2002 and 2012
flank our currently projected 8th warmest.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 11 AM EST this morning for ANZ430-
     431-450>455.

&&

$$
Synopsis...Drag
Near Term...Gorse
Short Term...Gorse
Long Term...Drag
Aviation...Drag/Gorse
Marine...Drag/Gorse
Climate...



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