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FXUS61 KPHI 231528

Area Forecast Discussion
National Weather Service Mount Holly NJ
1028 AM EST Tue Jan 23 2018

Strong low pressure will track northeastward from the eastern Great 
Lakes to just north of Maine tonight. It will whip a strong cold 
front across the mid Atlantic states early this afternoon. Strong 
high pressure will spread east during the balance of the week, 
before moving out to sea Saturday. A cold front and low pressure 
then arrives from the west on Sunday and proceeds offshore next 

10 am update...Cold front pushing into western PA with rounds 
of showers moving through the forecast area well ahead of this 
feature. The southerly flow ahead of the front is bringing very
mild temps for this time of year with 50s to low 60s across much
of the region. Moving through the latter part of the morning
into the early afternoon, concern will continue to be locally
heavy rainfall with these showers that could cause roadway,
urban and poor drainage flooding issues. For further details on
potential hydro issues, please see the hydro section below. In 
addition, winds are very strong just off the surface with 50 to 
60 knots in the lowest few thousand feet. As a result, any 
heavier showers or embedded thunderstorms could still mix 
locally damaging winds down to the surface. Main period of 
concern is through the first part of the afternoon after which 
time front and associated showers clear the area to the east. In
terms of the gridded forecast, some adjustments were made to 
temp, POP, and QPF grids based on latest obs and trends. 

Previous discussion...

As a surface low moves through the Great Lakes region this
morning, large-scale ascent will increase markedly downstream
across much of the eastern U.S. today, aided by a combination of
differential cyclonic vorticity advection downstream of a potent
vort max in the Midwest, considerable low-level warm advection
aided by a 50+ kt 925-mb jet, and left-front quadrant upper-
level divergence as a cyclonic jet streak noses into the
Northeast. Several areas of showers have developed this morning
(with some embedded storms in the eastern Ohio Valley and
adjacent areas), including from central PA south-southeastward
into lower Delmarva. Hi-res model output suggests this
precipitation will grow rapidly during the next few hours as it
races northeastward into the northern Mid-Atlantic. Showers are
already beginning to reach Berks County and far southern Sussex
County (DE) at this time.

Model QPF has trended upward overnight, with the Poconos and
Lehigh Valley solidly in the 0.75-2.00 inch range from virtually
all of the hi-res output. The 00Z GFS looks dry everywhere and
generally discounted given its outlier ("outliar") look. QPF
looks generally under an inch southeast of the Fall Line, though
there is some indication of a second maximum in portions of 
Delmarva and south/east NJ that will need to be watched. For
now, increased total QPF southeast of the Fall Line to generally
a third to three-quarters of an inch. Given the showery nature
of the precipitation, some spots will be relatively dry, with
localized totals possibly higher than the area-averaged storm
totals in the grids.

With the relatively high totals in the Poconos, Lehigh Valley,
and northern New Jersey, may see some isolated hydro issues
(unlikely much more than nuisance in nature). Additionally,
particularly strong cells may produce locally heavy rain and
poor-drainage/urban flooding in isolated spots.

The convective environment is at least somewhat favorable for
isolated damaging wind gusts and even a brief/weak tornado.
Shear is generally off the charts today, with SRH in the warm
sector generally 300+ J/kg and deep-layer shear 60+ kts.
Instability looks rather marginal, however, and it is not
entirely clear that much (if any) surface-based instability can
develop, particularly if widespread showers occur. Shear may be
so strong that updrafts fail to acquire much vertical depth as
well, with corresponding downdrafts unable to generate
considerable downward momentum transport (further impeded by
meager low-level lapse rates). Nevertheless, given the output of
the 00Z NAM Nest and several recent HRRR simulations, it will
not take much instability to generate reasonably strong
convective cells, and with very strong winds off the surface,
the potential is there for an isolated severe gust or two.

Of note, several wind gusts 35-50+ kts reached the surface with
a convective line to our west (west/central PA, WV, and eastern
OH) overnight. That certainly suggests any convective cells 
require close monitoring today for strong-to-severe wind gust 
potential. Additionally and not unusual for these convective 
setups (given the time of year, for starters), convection may 
contain little or no lightning.

Surface trough should move through most of the area by 21Z, with
precipitation shunted eastward by this point. Skies will rapidly
improve thereafter.

Temperatures will be warm today in advance of the potent upper
trough, with strong mixing and warm advection aiding in much
above average temperatures today. However, there is some
question regarding how high temperatures will go given the
widespread precipitation expected. Kept things close to
consensus for now, but the temp forecast is low confidence.
Finally, with south/southwest winds increasing this afternoon in
the warm sector and strong winds aloft, expect some gusts 25-35
mph generally along/southeast of I-95.


Conditions dry out tonight as the synoptic cold front moves
through. Skies should be clearing out, and temperatures will be
much colder than those observed early this morning. Kept
forecast close to statistical MOS blend, with winds likely
strong enough to mitigate more substantial radiational cooling.
This leads to lows forecast to be in the middle 20s in the
Poconos to the middle 30s in the urban corridor, Delmarva, and 
the NJ beaches.


500MB: Another short wave moves a cross the mid Atlantic coast 
Wednesday night followed by substantial ridging Friday and Saturday, 
then another trough develops to the east coast next Sunday and 

Temperature: Calendar day averages should range between 3 and 7
degrees above normal Wednesday, near normal Thursday and Friday,
on Saturday near 10 degrees above normal, Sunday probably 12 to
18 degrees above normal, cooling a bit to 5 to 10 degrees above
next Monday.

Forecast basis: Wednesday-Thursday is based on a blended 00z/22 
GFS/NAM MOS, Thursday night-Friday was based on the 00z/22 GFS 
MEX MOS and then D4-8 was based on the 0441z WPC guidance. 

Wednesday...Considerable cloudiness after early sunshine. A
slightest chance for a few flurries in the Poconos. Brisk with 
a northwest wind gusty 20-30 MPH. Clearing at night. Max temps 
Wednesday generally 2 above normal with the Wednesday night mins
similar. Confidence: 
Above average.

Thursday...Mostly sunny and brisk. Northwest gusty 20-25 MPH. 
Max temps 2 to 7F below normal. The Thu night mins about 2F 
below normal. Confidence: Above average.

Friday...Mostly sunny. Wind becoming light southwest. Max temps
near normal. Confidence: Above average.

Saturday...Increasing clouds and milder. Southwest gust 20 to 
30 MPH. Chance of showers at night. Max temps 10 to 15F above
normal. Sat night mins. probably 15 to 20 degrees above
normal! Confidence: Above average.

Sunday...Showers with a coldfront and developing low pressure on
the front. This part of the fcst differs considerably than what
I'd anticipated earlier and it is a GEFS combo with the GGEM 
and ECMWF both pretty confidence on a wet 12 to 24 hours much 
more so than the 00z/23 GFS op. So have followed WPC guidance 
POPS. Max temp 10 to 15 above normal. Gusty southerly flow with 
a wind shift to west probable at night. Confidence on overall 
scenario: Above average but below average on details including 
timing CFP.

Monday...cooler but daytime max temps still probably 5F above 
normal. Showers should b e ending or gone. Confidence: average.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

Today...Generally sub-VFR through 20Z with rapid improvement 
west to east thereafter. Showers likely through the morning and
isolated storms possible, including some occurring at 10Z near
the Philly terminals. May need to add thunder in subsequent amendments
once timing of embedded convection within the broader shield of
rain/showers becomes clearer. Winds will become southwest 
during the day, possibly becoming gusty at times, especially at
ILG/MIV/ACY. Additionally, showers are capable of producing 
strong/erratic gusts and localized convective turbulence. LLWS 
likely through early afternoon. Overall confidence is average.

Tonight...VFR with west to northwest winds 5 to 15 kts. 
Confidence above average.

Wednesday...VFR sct-bkn aoa 3500 ft clearing at night. West 
northwest wind gust 25 to 30 kt. 

Thursday...VFR. West to northwest wind gust 20-25 kt. 

Friday...VFR.  Light wind becoming southwest. 

Saturday...VFR gusty southwest wind 20-30 kt.  Chance of MVFR conds 
later at night in showers.


Small craft advisory conditions are expected on Delaware Bay
today and on the Atlantic waters today and tonight. There may be
some marginal gale-force gusts on the Atlantic waters today as
strong southerly flow increases this morning. However, the
bigger threat will be isolated/sporadic gale-force gusts with
showers/storms that move through the waters during the day. A
few special marine warnings may be required, given that it will
not take much for showers to transport strong winds aloft to the

Showers should move east of the nearshore waters tonight, and
advisory conditions should cease on Delaware Bay. However,
winds/seas will remain elevated (switching to west/northwest
this evening) through the night on the Atlantic waters.

Wednesday...SCA extended Atlantic waters and will probably be 
needed for the De Bay waters. 

Wednesday night through Thursday...A west northwest wind could
gust around 25 knots and the Small Craft Advisory may need to 
be extended in time.

Thursday night through Friday...No marine headlines are anticipated.

Saturday... A southwest wind small craft advisory may be needed.


Rainfall...mainly 0.50 to 1.00 inch of rainfall can be 
expected into the afternoon. Some higher amounts can be expected 
where there is a convective contribution.

Snow...any remaining snow on the ground, which is near zero across 
much of the HSA, is hydrologically insignificant. 

River ice...we continue to receive reports of solid ice cover on the 
Delaware River, but conditions are not as widespread as last week. 
We know of solid ice near Trenton from about the Route 1 Bridge 
south down to about Borderntown, or near the head of the tide. We 
also know of ice further north near and in the Delaware Water Gap.

With the warm temperatures the last few days combined with today's 
precipitation and rising water levels, melting and fracturing ice 
will occur. Restrictions in flow or ice jams are possible as ice 
breaks up and begins to move. 

Since ice jams can not be predicted with certainty, the best 
approach is awareness and to take notice of day to day changes on a 
river or stream of concern.

Monthly departures should drop about 2 degrees from this mornings
(Jan 23) CF6 values that are seen on our Mount Holly web site. 
The overall monthly average will probably end up somewhere 
around 1 to 2 degrees below normal for January.


MARINE...Small Craft Advisory until 6 PM EST Wednesday for ANZ450>455.
     Small Craft Advisory until 6 PM EST this evening for ANZ430-


Near Term...CMS/Fitzsimmons
Short Term...CMS

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