Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
401 PM EDT Thursday Jul 10 2014
a stationary frontal boundary will remain over the Delaware-Maryland-Virginia tonight
before slowly pushing south of the area Friday. High pressure builds
in from the northeast Friday into Saturday. Return flow develops
late in the weekend and early next week as high pressure moves
offshore and an strong upper low digs into the upper Midwest. A cold
front will approach the eastern Seaboard Tuesday before moving
offshore by Wednesday.
Near term /until 6 am Friday morning/...
a Flash Flood Watch has been issued for our Delaware-Maryland-Virginia and southern
New Jersey counties where it has rained heavily during the day
today, and the potential for additional heavy rain exists.
The frontal boundary has stalled across southern New Jersey and
central Delaware-Maryland-Virginia today. Precipitable water values remain high with values 1.75-2
inches across the area. Meanwhile, the middle-level trough continues to
approach the area from the west and several vorticity impulses will
continue to slide across the area through this evening. All of this
combined will help additional showers and thunderstorms develop. It
has already rained across portions of the watch area today, and
where it has rained, it's been very heavy with up to 3-4 inches in
some areas in an hour or two. With the potential for additional
heavy rainfall across these areas, we have issued a Flash Flood
Watch through this evening.
There are scattered showers and thunderstorms moving through central
and northeastern Pennsylvania, through these have been moving and
will likely be more diurnal in nature and should dissipate around
sunset; so no watch further north.
We expect much of the precipitation to begin to diminish later this
evening and toward midnight as insatiability wanes and precipitable water values
begin to lower. Therefore the watch ends at local midnight. However,
with the front remaining nearby and the middle-level trough still to
our west, there will still be the potential for a few showers and
thunderstorms into Friday morning.
Short term /6 am Friday morning through 6 PM Friday/...
the pesky frontal boundary is expected to push back offshore on
Friday, and stall across the ocean waters to our east. Precipitable water values
continue to lower through the day, but the middle-level through will be
moving overhead through the day. There will remain the chance for
isolated-scattered showers and thunderstorms across the area, with
the best potential across southern New Jersey and southern Delaware-Maryland-Virginia.
Highs on Friday will depend on how much sunshine we get, with cloudy
areas likely being closer to the cooler met guidance and sunnier
areas closer to the warmer mav guidance. For now, we took a blend of
the mav/met/mosguide for daytime highs.
Long term /Friday night through Thursday/...
surface high pressure centered just east of New England will provide
nice weather through Saturday. Friday night will feature clear skies
and cool temperatures for this time of year, and an easterly flow
will keep highs in the 80s on Saturday. Conditions will begin to
deteriorate somewhat on Sunday as a frontal boundary approaches from
the west. Clouds will be on the increase and showers and
thunderstorm activity will become increasingly likely from west to
east as the day wears on. These clouds should keep the high
temperatures on Sunday one or two degrees cooler than Saturday.
An anomalously strong northern stream closed low is forecast to dive southward
from near Hudson Bay into the upper Midwest early next week, setting
up a rather unusual pattern for this time of year. The middle-Atlantic
region will be situated in an increasingly diffluent flow pattern
aloft on the southeastern periphery of the amplifying upper trough
and on the western periphery of the western Atlantic ridge. The heat
and humidity will return Monday and Tuesday during this time as
strengthening southwesterly flow ahead of the low becomes
established over the eastern Continental U.S.. maximum temperatures in the lower 90s
appear likely Monday and Tuesday. Leaned toward the slower European model (ecmwf)
solution regarding the arrival of the frontal passage on Tuesday. Raised maximum
temperatures a few degrees from 12z wpc guidance as a slower solution would
keep the pre-frontal warm airmass that is characterized by 850 mb
temperatures near 20c over the area into Tuesday. Maximum temperatures in the lower
90s over the coastal middle-Atlantic region for one more day.
Monday and Tuesday both look to be a rather stormy period for the
area. The focus for convection appears to be a pre-frontal/Lee
trough on Monday and the main cold front for Tuesday. Favorable
dynamics in the right entrance region of an unseasonably strong
upper-level jet streak and ahead of the approaching longwave trough
also looks to contribute to synoptic lift over the area on Tuesday.
There are still plenty of details that need to be worked out this
far out, but the potential is there for severe thunderstorm events
both days. Storm Prediction Center has depicted the area in a day 6 slight risk for
Cold frontal passage likely Tuesday night or early Wednesday. Much cooler
and drier conditions are forecast to return behind the cold front
by Wednesday. The 12z European model (ecmwf) indicates 800 mb temperatures dropping into the
single digits behind the front.
Aviation /20z Thursday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
We are forecasting VFR conditions for most areas through the taf
period. The exception is miv/Acy where there have been MVFR and even
IFR ceilings/visibilities associated with showers and thunderstorms and cloud
cover from the frontal boundary off the coast. There could be some
patch fog overnight tonight as well, especially for ridge/miv and
possibly Acy. The rest of the taf sites we do not anticipate much
fog potential. There remains the possibility of showers and
thunderstorms late this afternoon into this evening, especially for
southern New Jersey and Delaware-Maryland-Virginia. We have kept any rain showers/thunderstorms and rain group
out of the tafs except at miv/Acy. There are a some
showers/thunderstorms moving across central and northeastern
Pennsylvania, but we expect these to mostly dissipate as they
approach our area. If they maintain themselves, we will amend as
Winds will generally be out of the east to northeast today, before
going light and variable overnight, then settling back to a
northeast direction again on Friday.
The NAM shows the possibility of MVFR/IFR ceilings developing overnight
for Acy/miv, so we will need to monitor this in case this comes to
Friday night through Saturday night...potential for IFR ceilings with
marine stratus to impact terminals, particularly Acy/miv, Friday and
Saturday nights with onshore flow. Otherwise VFR expected with dry
Sunday and Monday...chance for rain showers/thunderstorms and rain both days. Better chance
may be Monday with a well-defined surface trough developing over the
eastern Seaboard. Local/brief restrictions possible.
Tuesday...better chance for rain showers/thunderstorms and rain Tuesday as a cold front
sub-advisory conditions are expected through tonight and Friday with
a frontal boundary draped across the waters. There remains a chance
for showers and thunderstorms through friday; expect higher winds
and waves near thunderstorms.
Friday night through and Saturday night...hipres builds in from the
northeast states to start the weekend. Onshore flow will be light.
Sunday through Tuesday...winds veer, becoming southwesterly on Sunday.
Southwest winds strengthen Monday but should remain below Small Craft Advisory
levels. Gusty southerly winds ahead of a cold front on Tuesday.
Marginal Small Craft Advisory possible. Chance for rain showers/thunderstorms and rain late Sunday. Better
chance on Monday and Tuesday.
New Jersey...Flash Flood Watch until midnight EDT tonight for njz016-
Delaware...Flash Flood Watch until midnight EDT tonight for dez001>004.
Maryland...Flash Flood Watch until midnight EDT tonight for mdz008-012-