Area Forecast Discussion
Issued by NWS Mt. Holly, NJ
FXUS61 KPHI 231929
Area Forecast Discussion
National Weather Service Mount Holly NJ
329 PM EDT Thu Mar 23 2017
High pressure across the middle Atlantic will move offshore by
Friday, while a warm front moves through the Ohio Valley. This
frontal boundary will become nearly stationary near our region
through Tuesday, as several waves of low pressure move along it. A
cold frontal passage is expected Wednesday, followed by Canadian
high pressure building into our area on Thursday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
It looks very likely now that any precip will hold off until after
sunrise tomorrow morning. All the same, we will be monitoring
temperature trends very carefully overnight as that could have
implications for precipitation type once it does begin Friday
morning. Until the clouds move in, we will have efficient
radiational cooling conditions with light winds and dry boundary
layer conditions. For now, I have gone mostly on the colder side of
guidance for temperatures, with lows ranging from the upper teens to
.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/...
As mentioned earlier, a big factor in how precipitation will begin
will be how quickly clouds move in tonight and how much radiational
cooling we have before then. Further complicating precip type is
that the air will likely remain quite dry at least through the
morning hours until moisture advection begins. Thus, could see some
wetbulb effects further lowering the temperature with and just
before the start of precipitation. All that to say, we have the
highest confidence that the southern Poconos will see a wintry mix
of precipitation. For Carbon and Monroe Counties, will issue a
winter weather advisory through the morning hours. Much less
confident (but still possible) for the next row of counties
including NW NJ, Lehigh Valley, and Berks Co. Delmarva and far
southern NJ are unlikely to get any precip at all.
By early afternoon, expect all locations to be above freezing and
all precipitation to be rain.
.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
An unsettled period of weather is in store this weekend thru the
middle of next week. At the mid-levels across the conus, a generally
zonal northern stream flow will be in place, while several shortwave
disturbances traverse the southern stream. Meanwhile at the surface,
Canadian and sub-tropical high pressure systems will maintain a
frontal boundary in the vicinity of our region until a more
substantial frontal passage on Wednesday.
The primary uncertainty will be the location of the aforementioned
boundary, along with the timing of several areas of low pressure
that ride along it, and accompanying areas of precipitation. Of most
concern is the potential of mixed precipitation Sunday night.
On Saturday, the NAM and EC are most extensive with the overrunning
precipitation associated with the stationary boundary to our north.
The GFS looks more reasonable with the placement of the precip in
relation to the frontal boundary, confining PoPs to the northern
half of our CWA, and our forecast reflects this.
As the frontal boundary moves southward in response to Canadian high
pressure building to the north, scattered showers are possible
across our entire area Saturday night into Sunday. As low pressure
moves through the Ohio Valley into the Great Lakes from Sunday night
into Monday, more substantial overrunning precip is likely during
this time frame.
As mentioned previously, there is some potential for mixed
precipitation Sunday night into Monday morning. A review of plan
view 2-meter and 850 hPa temperatures, 1000 to 500 hPa and 1000 to
850 hPa thicknesses, model p-type, ens plume diagrams, model
soundings, and CIPS Analogs points to freezing rain as the primary
concern. There is a high amount of uncertainty, especially at this
range in the forecast, which translates into low confidence. The
probability for freezing rain is low at this time, therefore it was
not mentioned in the HWO (per our directives), but this will need to
reassessed with future forecast packages. Also, please note that ice
map on our website is for Friday, March 23.
Expect showers to continue Monday night into Wednesday, and another
round of more substantial precip likely centered on Tuesday night
into Wednesday, associated with a cold frontal passage. There is
uncertainty with the timing of this front, particularly given the
time range, but a gradual return to fair weather should occur on
Wednesday and continue into Thursday.
Temperature-wise, at or above normal through the period.
.AVIATION /20Z THURSDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.
VFR conditions will continue through at least 12Z. After 12Z, there
is an increasing risk for precipitation (expected to be all rain at
the TAF sites, but freezing precipitation will be possible N and W
of KABE and KRDG). With any rain, brief MVFR conditions will be
possible. The rain should move out by mid afternoon bringing a
return to VFR conditions.
Light and variable winds may continue through much of the overnight
hours before settling in out of the south by 12Z Friday.
Saturday and Sunday...A period of MVFR is possible in low
clouds, especially northwest of an ACY-MIV line. Otherwise, VFR.
Southwest winds 10 to 15 knots on Saturday will shift to the
east Saturday night into Sunday, increasing to 10-20 knots.
Sunday night thru Tuesday...Periods of MVFR possible in low clouds
and fog. South-southwest winds Sunday night into Monday may shift to
the east-southeast Monday night and Tuesday, generally at or below
Winds and seas should stay below SCA criteria tonight into tomorrow
morning. By mid day tomorrow, winds will increase. Gusts above 25 kt
will be possible especially for the New Jersey Coastal waters
through the afternoon hours.
If confidence increases for elevated winds and seas to linger
into Saturday night, the Small Craft Advisory (SCA) may need to
be extended further into this time frame.
As a front moves south across the waters on Sunday, winds and seas
may increase to SCA levels during the day, mainly across the
northern NJ waters.
Looking ahead to Monday and Tuesday, wave heights may build in
excess of five feet during this period. Therefore, a SCA may be
PA...Winter Weather Advisory from 8 AM to 1 PM EDT Friday for
MARINE...Small Craft Advisory from 10 AM to 8 PM EDT Friday for