Scientific Forecaster Discussion

Return to Local Conditions & Forecast
Area forecast discussion 
National Weather Service Mount Holly New Jersey
311 PM EDT Thursday Apr 17 2014

Synopsis...
a ridge of high pressure will remain across the forecast area
tonight through Friday night. A cold front will cross the region
Friday night. More high pressure will build down from Canada and
remain across the area this weekend and into Monday night. Another
disturbance will affect the region Tuesday into Wednesday.

&&

Near term /until 6 am Friday morning/...
frost advisories issued for some locations around the i95 corridor
for late tonight where the growing season has started.

A GFS/WRF-nmmb blend seemed to work best off the 12z soundings and
modeling initialization. Our forecast impact unfortunately is
coming from non sounding locations and thus adds a layer of
uncertainty to the near and short term forecasts.

The moisture off the ocean, the rap has had a better handle on it
than the plan forecast moisture fields of the GFS and WRF-nmmb.
It was used as a basis for tonight's forecast.

The spreading uncertainty of the cloud cover and wind coupled with
below normal temperatures did not make the frost/freeze decision an easy or
confident one. Please check for further updates later this afternoon
and evening.

Based on the rap's premise we see that mixing is finally stopping
the westward progression of the stratocu ceiling. But, we expect this
process to start itself later this evening and overnight as the
moisture remains trapped underneath the inversion and is forecast to
spread. East of the GFS and WRF-nmmb ridge axis line is a proxy to
where the ceilings have been able to sustain themselves. This axis
shifts west tonight as we are expecting the clouds to go. Based on
increasing dew points and more marginal temperatures we have opted
to convert the freeze watch to a frost advisory. Highest confidence
is in the western part of the headlines with lower confidence in New Jersey.
We did keep the mention of some patchy frost (in the severe weather potential statement too) in
locales farther to the southeast (but not in central/lower delmarva). Being
consistent with the cloud and wind idea, for min temperatures we are higher
than stat guidance east and closer to a stat guidance blend west.

&&

Short term /6 am Friday morning through 6 PM Friday/...
the 925mb level dry air south of Maine and east of Massachusetts is
destined to move westward overnight and be over our County Warning Area on Friday.
Also the surface high is forecast to be nestled pretty much over our
County Warning Area. We are expecting the low cloud ceiling to dissipate during the
morning. This in part will be replaced by high level cloudiness
associated with the weakening cold front approaching from the west.
Given the latest ridge axis position, we removed the low chance of precipitation
near the coast. Away from the coast we leaned toward the higher of
the stat guidance for maximum temperatures and more of a blend closer to and
along the Atlantic.

&&

Long term /Friday night through Thursday/...
a fairly benign pattern for our region during the long term. A weak
upper trough will move through the northeast and middle Atlantic
Friday through Saturday. Another trough will will move from the Great
Lakes Monday night and cross into the Atlantic by early Wednesday.
These two systems are fairly weak and should only bring small chance
for showers to the area Monday night into Tuesday night. Previous forecasts had
some showers in for Friday night also...but we have removed them from
the forecast at this time...since the chance looks less with the meager amts of
lift and limited moisture.

Temperatures will start out cool for Friday with highs only in the
low/middle 50s across the area. Readings will then climb to near normal
levels for Sat through Monday. Temperatures will then be 5 to 10
degrees above normal next Tuesday...before settling back to near
normal next Wednesday. Normal temperatures for this time of year are highs
in the middle 60s and lows in the middle 40s for Philadelphia. Up across
the north in the Poconos an northern New Jersey...the normals are close to 10
degrees less than those for Philadelphia.

&&

Aviation /19z Thursday through Tuesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

The 18z tafs MVFR conditions are forecast to occur at all of the
taf sites tonight and Friday morning, although confidence is
progressively lower the farther west our taf sites are.

For the rest of this afternoon into early this evening a VFR stratocu
ceiling is expected to continue to move west and be battled by some
mixing and dry air. From kphl area terminals eastward we are
expecting a VFR ceiling at times, although kacy might become MVFR.
Farther west no ceiling expected.

East winds will average around 10 kts with gustiness up to around
20 kts from the kphl area terminals eastward.

Rest of evening and overnight, predicting a MVFR ceiling to overspread
all the terminals arriving at kabe and krdg after midnight. Winds
should become lighter from the east.

On Friday morning, the MVFR ceiling is expected to dissipate with a
cirrus deck above it. Synoptic scale winds overall should be
lighter (generally an eastward component) than today and a sea
breeze front may form during the afternoon.

Outlook...
Friday night through Monday night...mostly VFR.
Tuesday...mostly VFR with scattered showers.

&&

Marine...
no changes to the marine headlines. Small Craft Advisory conditions in the lower
Delaware Bay are expected to end this evening. The wind part of the
Small Craft Advisory should also end over the ocean tonight. But, seas will take
longer to get below Small Craft Advisory criteria on the ocean as we are expecting
them to linger through the day on Friday.

Outlook...
Friday night and Sat...seas on the ocean will remain just below Small Craft Advisory
conditions.
Sat night through Sun night...Small Craft Advisory conditions on the ocean.
Monday through Tuesday...mostly sub-Small Craft Advisory conditions.

&&

Phi watches/warnings/advisories...
PA...frost advisory from 1 am to 8 am EDT Friday for paz060-101>104-
106.
New Jersey...frost advisory from 1 am to 8 am EDT Friday for njz012-015-019.
Delaware...none.
Maryland...frost advisory from 1 am to 8 am EDT Friday for mdz008.
Marine...Small Craft Advisory until 6 PM EDT Friday for anz450>455.
Small Craft Advisory until midnight EDT tonight for anz431.

&&

$$
Synopsis...O'Hara
near term...gigi
short term...gigi
long term...O'Hara
aviation...gigi/O'Hara
marine...gigi/O'Hara