Scientific Forecaster Discussion
fxus61 kphi 231415
Area forecast discussion
National Weather Service Mount Holly New Jersey
1015 am EDT Mon Oct 23 2017
strong high pressure is south of Nova Scotia today. A cold front
will move slowly through our region Tuesday afternoon and night. Low
pressure is expected to develop along the front and pass over
coastal New England Wednesday. High pressure follows for Thursday
and Friday. Another cold front should cross our region this coming
weekend as low pressure develops northward along it.
Near term /until 6 PM this evening/...
a gradual clearing trend is working north across the area this
morning. Despite this, skies will remain mostly cloudy north of
the fall line. Fog was denser than anticipated this morning. We
issued a Special Weather Statement through 1030 am to cover this
hazard. The narre did not have a good handle on the fog to the
south and east of I-95, and was slow to catch up.
Based on 12z soundings and current trends, have boosted temps a
couple of degrees across the entire region. Low-level moisture
is also advecting into the region a bit faster than forecast, so
adjusted dewpoints upward. Also maintained slight chance pops
across the southern Poconos, as moist southeast flow ascends
the higher terrain, with some light showers possible.
The remainder of the forecast remains on track.
today will be transitional from the spectacular past few days to
what awaits Tuesday. In between, today, we will have an increase in
clouds across the area with perhaps a few stray showers up north
this afternoon. The low clouds across the area early this morning
will likely mix out as an increasing southeast flow develops across the
area. There is still plenty of drier air at the mid-levels, so
support for organized precip is not there. A couple showers probably
near the higher elevations across the north/west could form later on.
Temperatures today will be mild, but with the increasing clouds,
readings will not reach the highs that were over the area Sunday.
Mostly low/mid 70s are expected. Dew points will be on an upward
trend too, trending up through the 60s by afternoon. Winds will
increase from the southeast at 10 to 15 mph by afternoon.
Short term /6 PM this evening through 6 am Tuesday/...
a slow moving cold front and a wave of low pressure moving along it
will affect the weather tonight across the area. Pops will increase
overnight with the steady precip reaching the wrn areas first,
probably after midnight, then spreading towards the Lehigh and
Delaware valleys by dawn. Tstms are possible with the activity and
gusty winds are also possible with the strengthening low level
winds. A few gusts across the srn Poconos may be over 40 mph
tonight, but probably the stronger winds will arrive later Tue.
Probably not enough for a Wind Advisory attm, but something to
consider later today. Temperatures will remain very mild for late
October with mostly 60s over the area.
Long term /Tuesday through Sunday/...
hazards: Tuesday as in the severe weather potential statement...poor drainage street flooding
from periodic heavy showers (see wpc qpf), and potential for
damaging thunderstorms (see Storm Prediction Center products) if there are organized
short lines in the hslc unidirectional southerly flow environment.
Tuesday night...dense fog potential E pa? Not in the grids attm
since there was a lot to consider.
Thursday night ... potential I-78 region and The Pine
Barrens of New Jersey. Not yet in the severe weather potential statement.
Looking just beyond day 7...frost freeze potential Halloween
morning along and northwest of I-95 (not Philadelphia urban
500 mb: a -2sd trough in the Great Lakes-Ohio Valley Tuesday weakens
negatively tilted across the mid Atlantic states Thursday. Heights
rise Fri leading to surface temp warming, ahead of another strong
-2sd trough developing down into the Great Lakes and Ohio Valley
Temperatures: calendar day averages near 15f above normal Tuesday,
around 5f above normal Wednesday, near normal Thursday and
Friday, then about 5f above normal Saturday and uncertainty
Sunday whether its normal or above.
Forecast basis: unless otherwise noted a 50 50 blend of the
00z/23 GFS/NAM MOS was used for Tue-Wed, the 00z/23 GFS mexmos
Wed night and Thu, then the 05z/23 wpc gridded elements d4-8
including 12 hr pops-Max/min temps, and 6 hrly wind/sky/dewpoint.
Tue and Tue evening...hslc qlcs potential svr storms within the
bands of southerly gusty wind showery rains, some of which will
be heavy in the 1.8" precipitable water within a deep generally unidirectional
flow (storm motion around 200 degs at 40-45kt). Subtle sfc waves
in diffluent thickness ahead of slow moving cold front may
allow potential Maddox synoptic scale flash flood situation but not in
the forecast attm. Note the 00z/23 NAM fcst a narrow sliver of
5-7" rainfall over west New Jersey 09z Tue-06z Wed. This may happen but
think it will be a little further east if it does.
Humid with dewpoints 65-70 much of the area for awhile Tuesday.
Minimum temps about 20f above normal and potentially record
breaking at a number of locations including Acy phl and long
shot Abe...pls see the cli section. We will probably have
to wait until 1 am Wednesday to know tuesdays calendar day low.
Tue night...showery rains end from west to east except maybe along
the Atlantic coasts. Wind shifts to light west but precipitable water remains high
and so too considerable cloud cover. But, if there is clearing in E
PA/E Maryland...patchy dense fog could form in the still moist boundary
Wednesday...the cold front and its associated cloud cover and rain
will continue to move slowly eastward as a final wave of low
pressure develops along the boundary and moves up into coastal
New England. Precipitable water still modeled 1.1" along the I-95 corridor
12z Wed. Becoming partly sunny with a west to southwest wind.
Wednesday night...chance of showers late as the negative tilted but
weakening trough axis approaches from the west. The associated
clouds should minimize the potential for frost.
Thursday...the axis of the mid level long wave trough is forecast to
pass overhead Thursday morning and it should finally kick the
surface system well to our east. Coolest daytime high temperatures
of this work week. Residual sprinkles possible in the morning-
Thursday night...frost-freeze potential I-78 under mostly clear
skies and decoupling winds
Friday...sunny and becoming milder during the afternoon.
This weekend... didn't spend alot of energy here. Lots of
variability but suffice to say with a high amplitude trough to
our west... it will rain, but timing is uncertain so we used
straight wpc probability of precipitation. Best estimate is that it rains Saturday
night into Sunday. Confidence on details below average.
Aviation /14z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Fog has lifted across all taf sites as of 14z, but will have low
clouds to contend with. Acy and miv are already VFR and this trend
will move north into ilg, phl, pne, and ttn through noon. Further
northwest at Abe and rdg, MVFR ceilings will persist into at
least the early afternoon. Southeast around 10 knots regionwide.
More low clouds and rain arrive tonight as low pressure and a
slow moving front track towards the region. Winds will increase
tonight to 15-20 knots mostly from the southeast.
Tuesday through Tuesday evening...occasional MVFR and IFR conditions
in showers. Scattered thunderstorms and heavy rain likely, especially
New Jersey and Delaware. South wind gusts around 25 to 35 knots in the afternoon.
Gusts in a tstm could reach 45 kt. Low level winds just above
the sfc near 1500 feet southerly near 50 kt Tuesday.
Tuesday night...conditions improve from west to east at least
temporarily as showers end and wind becomes light west. This
may set up the potential for IFR St/fog late at night?
Wednesday...VFR. West to southwest wind.
Wednesday night...VFR, chance of a short period of MVFR showers
late. Wind shift northwest.
Thursday...VFR. Chance of morning sprinkles.
sub-Small Craft Advisory conditions today and into the evening, then Small Craft Advisory flag will
fly with increasing winds and seas developing ahead of a cold front.
We will make a change to the start time, moving it up by a few hours
to better align with our neighboring offices. Today, the weather
will be fair with only some patchy fog this morning. Tonight,
showers will overspread the waters late and a tstm is possible
towards dawn. Winds will gust over 25 knots after midnight. Seas
will increase to 5 to 7 ft on the ocean by dawn.
Tuesday...a Small Craft Advisory for southerly wind gusts near 30
knots. Localized gale force gusts are possible on Tuesday. Waves on
our ocean waters will build to 7 to 10 feet.
Tuesday evening...a Small Craft Advisory extension may be
needed for southerly wind gusts near 30 knots in the evening
then the wind decreases as it shifts to west. Waves on our ocean
waters are forecast to be 7 to 10 feet early, then slowly
Late Tuesday night through Thursday night...a Small Craft Advisory
for hazardous seas will be needed on our ocean waters for wave
heights of 5 to 6 feet, even as the wind becomes west to northwest
around 10 to 20 knots.
Friday...no marine headlines are anticipated.
hydro: storm total rainfall Tue-Tue night, generally 1-2" with
in excess of 2 inches most favored over New Jersey. Leaf- clogged drains
will enhance the possibility of ponding of water on roadways.
Note: 00z/23 NAM has 6-7" of rain near I-95 between 09z/24 and
06z/25. We are not forecasting that... but in the small chance
it would occur, then the recent dry spell will not be able to
prevent small pockets of flash flooding. We will monitor NAM
high resolution model trends the next 24 hours but for now...we
are conservative but not discounting the potential.
vulnerable record high minimums Tuesday the 24th. As of this 330 am
forecast...we are forecasting records here. However, if the cold
front speeds up and a little clearing ensues then the min temp would
probably fall just short. Right now the mid shift forecaster has
odds favoring records. Elsewhere for now...no other records anticipated.
We'll add locations if we forecast warmer in future forecasts.
Allentown 58 1975
Atlantic City 63 2001
Philadelphia 63 1900
44091 buoy drifted away from its mooring and has since been
recovered. Its return to service date is still unknown, though
we'll try to have an answer Monday afternoon.
Weather observations at kvay should be incomplete through Monday
Marine...Small Craft Advisory from 10 PM this evening to 6 PM EDT
Tuesday for anz430-431-450>455.