Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KPHI 280211
Area Forecast Discussion
National Weather Service Mount Holly NJ
911 PM EST Sun Nov 27 2016
High pressure centered over the Mid-Atlantic region through
tonight will move offshore Monday. Low pressure strengthens over
the Northern Plains on Monday and then slowly drifts eastward
across the Great Lakes region midweek. Several waves of low
pressure system will develop ahead of this low and will impact our
area Tuesday and Wednesday. A cold front moves through on Thursday.
High pressure then builds in from the west Friday through next
.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
-- Changed Discussion --No significant changes to the forecast tonight. Mainly just hourly
temperature/dewpoint adjustments. Now that skies are clearing out,
temperatures are dropping faster in some areas compared to others,
so we`ve tried to adjust for this.
Temperatures will drop down into the mid/upper 20s to mid 30s
across the forecast area.
-- End Changed Discussion --
.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
High pressure will slide off the Carolina coast early Monday and
continue to push offshore through the day. As the high shifts to
the east, the flow will become more southerly across the area.
Skies will generally be cloudless at the start of the day but
cloudcover will increase, especially by late afternoon and into
the early evening in advance of an approaching low pressure
Temperatures will gradually warm and we should highs reach into
the upper 40s to mid 50s, with some low to mid 40s across
portions of Carbon and Monroe counties.
.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Low pressure that is organizing over the Front Range of the Rocky
Mountains today is forecast to deepen into a 975 mb surface low
over the Northern Plains on Monday. The low will gradually fill
out and drift eastward into the upper Midwest midweek. Even though
the center of the low will be distant from our area, our region
will see impacts from this system in terms of wet weather. As
discussed in previous AFDs, rainfall midweek looks to come in two
distinctive waves and are tied to secondary low pressure
development along the system`s frontal boundaries. Models have
come into better agreement with the onset and ending timing of
each round of rain, though there are still some details that need
to be worked out.
The first round of precipitation should arrive in conjunction with
a warm front from the west late Monday night into Tuesday morning.
Southerly winds associated 65 kt low-level jet streak will enhance
isentropic lift and moisture flux convergence across the warm
front. Models also indicate some additional lift in the mid-levels
from a leading shortwave trough aloft that gets ejected ahead of
the primary synoptic trough over the central CONUS. A well-defined
atmospheric river (AR) with a moisture connection from the
tropical Pacific and Gulf of Mexico looks to advance poleward up
the eastern seaboard. PWATs within this AR are forecast to climb
to 1.25-1.5 inches, which is +2-3 SD for this time of year.
Accordingly, the rain could be moderate to locally heavy during
the day Tuesday. The rain then tapers off from northwest to
southeast during the late afternoon and evening on Tuesday.
The second round of rainfall looks to move in from southwest to
northeast during the day on Wednesday as a cold front approaches the
Appalachians. Some of the guidance shows a secondary wave of low
pressure forming along the system`s triple point that tracks
northeastward through the mid-Atlantic states Wednesday night. This
feature could enhance rainfall across the area Wednesday afternoon
and night, especially if the low is able to deepen quickly prior to
it`s arrival. However, models seem to be struggling with how much
the secondary low strengthens as evident in the ensemble spread and
run to run inconsistencies. PWATs are forecast to peak above 1.5
inches, which is +3-4 SD and approaching the max observed values on
record for OKX RAOB. See the hydro section for details on expected
rainfall amounts and potential hydro impacts.
Most of the forecast area becomes situated in the warm sector of the
system, resulting in unseasonably warm conditions Tuesday and
Wednesday. The warm front may get hung up across the far north (near
the I-80 corridor). High temperatures both days in the cool sector
on the northern side of the boundary would only be in the 40s and
50s as opposed to the 60s farther south. There is a legitimate shot
at 70 degrees on Wednesday, particularly for the Delmarva.
The rain likely ends from southwest to northeast early on Thursday
after a cold front moves through. Guidance has trended faster with
the fropa and exit of precipitation compared to yesterday`s runs. It
is looking more optimistic that the daytime hours on Thursday will
be dry across the majority of the CWA. PoPs were adjusted
Gusty W-NW winds will develop behind the cold front Thursday
afternoon. Breezy conditions will likely persist into the weekend as
the pressure gradient remains tight between low pressure to our
north and northeast and high pressure to our west and south. Temps
will trend below normal by Friday through the weekend. Highs
Saturday and Sunday may only be in the 40s area wide. This part of
the long-term period should dry with the exception of lake effect
snow showers/flurries in the higher terrain of NW PA and far NW NJ.
.AVIATION /02Z MONDAY THROUGH FRIDAY/...
-- Changed Discussion --The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
Tonight...VFR conditions will continue. Mid level clouds are
expected to continue to clear out this evening, with only some
scattered high level clouds overspreading the area overnight.
Westerly winds of 5-10 knots are expected to become light and
Monday...VFR conditions expected. High level clouds are expected
to thicken and possibly lower some through the day. Light and
variable winds during the morning will increase to 5-10 knots
during the day and vary from south-southeast to south-southwest.
Monday night through Tuesday...Rain expected to overspread the area
from west to east late Monday night and early Tuesday morning. MVFR
restrictions likely to develop early Tuesday once the steadier rain
arrives. Periods of IFR possible during the daytime where the
heavier rain sets up. LLWS is a concern early Tuesday prior to the
arrival of a warm front. In the lowest 2 kft above the surface,
forecast sounds show winds speeds increasing from 5-10 kt at the
surface to 50-55 kt at 2 kft AGL while the wind direction veers with
height about 50-60 degrees (from SEly at the surface to S-SW at the
top of the shear layer).
Tuesday night...A lull in precip is expected. However, MVFR or IFR
restrictions may persist with low clouds hanging around. Any
breaks in the cloud cover would allow for fog to form.
Wednesday and Wednesday night...Precip moves back into the area on
Wednesday. Rain looks to be heaviest and most widespread Wednesday
afternoon and night. Flight restrictions likely.
Thursday...Improving to VFR early in the day as rain tapers off.
Westerly winds gusting to 25 kt are expected in wake of cold fropa.
Thursday night through Friday...VFR and breezy W-NW winds continue.
-- End Changed Discussion --
-- Changed Discussion --West to northwest winds around 10 to 15 knots will continue this
evening. While the pressure gradient does tighten up just a bit
this evening, wind gusts look like they will max out around 20-23
knots and therefore we will not be issuing any headlines for
tonight. Seas on the ocean will be around 3 to 4 feet and around 1
to 3 feet in Delaware Bay.
Monday night and Tuesday...Sly winds increase Monday night. Winds
sustained 20-25 kt kt will only gust a few knots higher on Tuesday
with weak lapse rates residing over the cooler waters. Seas will
also respond, increasing to 5-7 ft in our coastal waters and 2-4 ft
in the DE Bay by the afternoon. With high confidence of SCA criteria
being met, a late 3rd period-4th period SCA was issued for all of
our waters starting at 4 AM Tuesday.
Tuesday night and Wednesday...Sly winds decrease and should drop
below SCA levels during the evening. Wave heights in our coastal
waters will take longer to subside to below 5 ft so the SCA will
likely need to be extended into Tuesday night and possibly into
Wednesday (for seas).
Wednesday night through Friday...Sly winds and seas increase once
again ahead of a cold front Wednesday night. Winds turn sharply out
of the W-NW on Thursday in wake of fropa. A SCA will likely be
needed for the period. A rapid pressure surge behind the front may
produce gusts to near gale force on Thursday.
-- End Changed Discussion --
QPF guidance from WPC and operational NWP models were in good
agreement regarding the placement of the axis of heaviest rainfall
from this midweek event setting up somewhere in the CWA (consensus
has the axis along the Delaware Valley) although there are some
differences in QPF amounts. Storm total rainfall will most likely
average 1-2 inches but isolated higher amounts of 3+ inches are
certainly possible if the heaviest rain from both waves fall over
the same area. Conversely, some areas may receive less than an
inch of rain; there is ensemble clustering in the QPF fields for a
local min near the Delmarva coast.
We are not expecting main stem flooding or even smaller basin
flooding since these rainfall amounts will be spread out over a
48-hr period and the antecedent conditions are dry. Poor drainage
flooding is always a possibility if the localized 3+ inch rainfall
amounts materialize but impacts from this type of flooding are
MARINE...Small Craft Advisory from 4 AM to 6 PM EST Tuesday for ANZ430-