Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KPHI 201440
Area Forecast Discussion
National Weather Service Mount Holly NJ
940 AM EST Fri Jan 20 2017
High pressure over the Mid-Atlantic will move offshore early
today. Weakening low pressure to our west will lift northeast
through the region this afternoon and evening. High pressure
returns briefly for Saturday. A strong and complex area of low
pressure will impact the Mid-Atlantic from Sunday through
Tuesday, with dry weather for the remainder of the week.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
-- Changed Discussion --Early morning surface analysis shows a 1018 mb high centered along
the Mid-Atlantic coast. Looking upstream over our area, a weak wave
of low pressure was evident over the Midwest (central IL-IN) with a
warm front extending southeastward from the low through the southern
Appalachians and along the GA-SC coast. Water vapor satellite loop
shows a well-defined upper low spinning somewhere near the Upper
Mississippi and Red River basins. A low-level jet continues to draw
deeper moisture from the Gulf of Mexico ahead of the system,
resulting in an organized band of precipitation over the Southeast
and Midwest States overnight.
Hi-res models are in good agreement with the timing of precipitation
across the area. Light rain is forecast to arrive in our far
southwestern zones (eastern MD shore of the Chesapeake Bay and
southeastern PA) during the late morning, then move northeastward
into the Delaware Valley closer to midday and finally our far
northeastern zones (northwestern NJ and coastal central NJ) during
the early to mid afternoon. The rain doesn`t last that long and
should end quickly from west to east during the late afternoon. The
back edge of the precipitation should already be near the coast and
the I-80 corridor shortly after sunset. The upper shortwave trough
that will provide the lift for precipitation will become
increasingly sheared as it lifts northeastward toward the downstream
ridge and while the the upper low reorganizes farther upstream over
the Great Plains. Accordingly, light rainfall amounts are favored
across our region owing to the weakening dynamics by the time it
reaches our area. QPF ranges from one-tenth to one-quarter inch.
Several days out from this event, there was a concern for freezing
rain in the southern Poconos and far northwestern NJ today. However,
the icing threat has decreased as models have trended warmer with
temperatures. The stratocu that developed across the northeastern PA
and northwestern NJ overnight has kept temperatures up across the
far north and ultimately may be our saving grace. Nonetheless,
cannot rule out a few icy spots along the ridges of Carbon and
Monroe County (mainly at elevations above 1700 ft) late this
afternoon and early this evening where wet-bulb temperatures are
forecast to be near 32F.
Forecast high temperatures range from the mid 30s in the Poconos to
around 50F in the coastal plain of NJ and Delmarva.
-- End Changed Discussion --
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
The bulk of the precipitation is expected to have already moved
north and east of the area into southern NY and off the coast.
Boundary-layer winds are forecast to be light tonight so even with
veering flow, low-level moisture and clouds are expected to remain
over the area for much of the night. The question is if we see any
breaks in the clouds overnight. Dense fog could develop where we see
partial clearing. For now, will advertise patchy coverage with the
stratus already established.
Low temperatures range from the mid 30s across northeastern PA and
northwestern NJ to around 40F along and east of I-95.
.LONG TERM /SATURDAY THROUGH THURSDAY/...
Sat will be dry with wk high pres over the area. The dry conds
will continue into Sat night and erly Sun.
Then, low pres over the srn plains will move newd on Sun. Guid
has come into better agreement wrt a slower onset time. It now
appears that the first part of Sun will remain dry, with precip
holding off until aftn. The further n you are the better chc you
are of staying dry thru the day. But even the srn Delmarva will
stay dry thru the mrng hours based on current guid. Precip will
overspread the area thru the aftn hours.
It looks like there will be a pd of steady, heavy rain Sunday
night, especially over srn areas, though the guid still differs
on where the heaviest rain will fall and when.
The GFS wants to bring in a lull in the precip area wide on
Mon mrng while the ECMWF keeps it wet. The EC has perhaps a lull
later Mon. But it seems there will be a second wave at some
point on Mon, again, exactly where and when is difficult to
pinpoint attm. Yet a third wave looks to move thru Mon night
before the low pulls away from the region and things dry out
durg the second half of Tue.
Temps look to be above nrml durg this whole event and the vast
majority of the region will see plain rain durg the whole time
frame. However, temps could be marginally cold enough in the
Poconos and portions of Sussex County NJ Sun night for some
light amounts of freezing rain and there could be some light
amounts of snow as the cold air wraps around the low Monday
night. Neither of these look to be significant events, but
obviously bear watching.
High pres builds in behind the low for a dry Tue night into Wed,
though there cud be a passing shower in the nw, with a wk cdfnt.
More genly dry wx is expected for Thu.
.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
VFR conditions continue this morning with high clouds streaming
in. Light and variable winds under 5 kt this morning.
Rain will overspread the area from SW to NE between 15-18Z. After a
few hours, the rain will quickly end from W to E between 20-23Z.
CIGs/VSBYs will lower to MVFR after the rain arrives. Think LAMP
guidance is too pessimistic for this afternoon and even into
tonight, showing an extended period of IFR conditions this afternoon
and LIFR tonight. The current thinking is we will eventually get to
IFR and LIFR in spots but it will take longer (maybe not until after
sunset in spots) than indicated by guidance.
Sat thru Sat night...Some MVFR or IFR in low clouds or fog
erly, otherwise VFR. High confidence.
Sun...Conds deteriorating from S to N during the aftn with IFR
conditions psbl. Moderate confidence.
Sun night thru Mon night...IFR and lower conds psbl in RA. E
winds 15-20 KT with 20-30 KT gusts at terminals north and west
of I- 95, and 20-30 KT with 30-45 KT south and east of I-95
psbl on Mon. Decreasing wind Mon night. Moderate confidence.
Tue...IFR/MVFR erly with RA then improving to VFR by eve from s
to n. Moderate confidence.
-- End Changed Discussion --
No hazards are anticipated on the waters today and tonight. Wave
heights in our coastal zones continue to hold steady around 4 ft
with longer period easterly swells. Seas will gradually subside to 2-
3 ft by tonight.
Sat through Sun morning...No marine headlines anticipated
during this time. Seas will genly be around 2 to 3 ft with wind
10 kts or less.
Sun aftn thru Mon night...Easterly flow increases to 15-20 KT
with 25-30 KT gusts starting Sun aftn, and then gale force gusts
of 35-40 KT likely Monday and Monday night. Pds of mdt to hvy
rain with low VSBY expected. Storm force winds possible,
especially over the nrn waters on Monday. Conditions begin to
improve late Monday night, and winds decrease to SCA levels.
However, seas on the ocean should remain above SCA criteria.
Mon night thru Wed...wind decreases below headline criteria,
but ocean seas will remain above SCA criteria. It is possible
that some areas, especially across southern waters, may drop
below SCA seas on Wed.
Due to the persistent and increasing easterly flow around a
coastal low Sunday-Monday, there could be several tidal cycles
of coastal flooding, but this will also be dependent on the
track of the low and will become more evident as we approach
Sunday night and Monday. Latest guidance indicates the potential
for widespread minor coastal flooding, but that will come into
better focus this weekend.