Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
945 am EDT sun Mar 29 2015
a high pressure system crossing the central Appalachians will
sink southeast and pass off the Carolina coastal waters this
afternoon. A cold front will move through the region on Monday.
On its heals will be a fast moving low pressure system which
should sweep through the area late Tuesday. High pressure
will build into the region for Wednesday and move offshore on
Thursday. Another cold front is forecast to move through the
forecast area early on Saturday.
Near term /until 6 PM this evening/...
not much change to the grids this morning. Adjusted the hourly
temperatures up a bit faster than expected for the next couple of
hours. The ample amount of sunshine and west-northwest winds will
help the temperatures rebound nicely but as stated before, still
several degrees below average.
Otherwise, high pressure at the surface will gradually build into
our area, although its center should tend to settle to our south
this afternoon. This high is what is responsible for the very cold
airmass in place, although some moderation is expected this
The surface flow will also start to shift to the west and
southwest this afternoon, however turning much lighter than
yesterday. Overall, a dry day is expected and even though
afternoon temperatures will be higher than yesterday it will still
be several degrees below where we should be for this time of the
year. We used a NAM/GFS MOS blend for the high temperatures,
although leaned closest to the warmer GFS MOS.
Short term /6 PM this evening through 6 am Monday/...
a fast flow regime will place our next storm system on our doorstep
toward daybreak Monday. The evening will start with surface high
pressure sliding to our south and southeast, as an upper-level ridge
also shifts offshore. This will allow the flow to back some more and
become south-southwest. A short wave trough moving across the Great
Lakes should quickly be approaching our area late.
The model guidance shows an increasing low-level jet arriving
overnight from the southwest. This will help to drive warm air advection aloft and
bring in some deepening moisture. The forecast soundings indicate an
increase in middle to high level clouds will then thicken and lower
late. While the cold front remains to our west late, the associated
warm air advection and low-level jet should provide enough low to middle level ascent
to produce some showers. These should be arriving toward daybreak.
The overall airmass in the lower levels may still be a bit to dry,
thus any precipitation looks to be light. In addition, some added
evaporative cooling may also allow for some snow showers especially
across the north. We are expecting the temperatures to bottom out
during the first half of the night, then be steady or even rise some
toward morning due to the southerly flow, increased clouds and warm air advection.
The probability of precipitation increase into slight chance to the lower range of chance well
after midnight from west to east.
As for the low temperatures, we used a NAM/GFS MOS blend with some
downward adjustments for the typically colder spots. We should be
able to drop fairly quickly to start the evening given light winds,
not much clouds and low dew points. We did show a gradual increase
then well after midnight.
Long term /Monday through Saturday/...
GFS had a very good initialization at 500mb and 850 vs the WRF-
nmmb. Both models initialized equally well at 925mb. Looks like
they caught up to the air mass, not as much cold bias. The deep/dt
trend again, just like yesterday was slower and more amplified.
Thus this leans away from the faster envelope of solutions in the
long term. The midweek clipper still showing about a 120 mile wide
width in track differences with fairly large thermal and ptype
implications across our County Warning Area. While we will call on an oldie but
goodie that the NAM/WRF has the track of the clipper too deep into
the cold air, both the sref and gefs lean its way and definitely
do not support the op European model (ecmwf) or UKMET. Model diags on the European model (ecmwf)
show just as much fgen forcing, adequate jet and middle level qvec
convergence as the GFS so not feeling warm and cuddly about its
shredded solution. With the attendant short wave coming into
deeper radiosonde observation coverage in Canada with the next sounding run, we did
not make too many changes from our previous solution.
Further down the Road, the GFS is slowly working its way toward
the European model (ecmwf) solution of a slower cold front passage at the end of the week. The
gefs is slower than the op and here we have gone along with
continuity of a solution closer to the European model (ecmwf).
On Monday morning, we kept low probability of precipitation throughout for a couple of hours as the
cold front approaches our area. Given that the warm air advection push is done and the
boundary layer is continuing to warm, ptype becomes Rainier. We did
keep low chance of rain showers into the afternoon north as a pair
of short waves have yet to move through. The forecast soundings would
support deeper mixture for showers. Going along with the theme of
a slower evolution, we maintained predicted maximum temperatures. Stat
guidance looks reasonable (unless the cold air advection is even slower). It will
become windier as the day progresses, gusts might exceed 30 miles per hour.
With a general fast pace, winds should decrease overnight on Monday and
should permit some radiational cooling in outlying areas particularly.
We do not believe cloudiness will increase in time to stop this. Min
temperatures are a stat guidance blend.
Then on Tuesday and Tuesday night. Its pretty Universal that the models
are thermally cold for mainly snow over the higher terrain locations
of our County Warning Area. The differences though remain that some op models do not
bring measurable precipitation into this area. On the other end of the spectrum,
the WRF-nmmb is quite vigorous with the northern solution qpf,
but way higher than the sref mean. Given the forecast precipitable water is only around .5
suspect that when the 03z sref is run that the op WRF-nmm
is going to be either the wettest or one of the wettest of the members.
At the other end of the spectrum, the European model (ecmwf) is way suppressed and dry
north. Where there is modeling consensus (central) we increased probability of precipitation
to likely. Elsewhere we kept chance probability of precipitation. Once we get away from the
higher terrain, with a strong, warming wrly flow it looks
difficult to maintain much snow except for possibly at the start
or tail end of the event. Not confident about maximum temperatures at all at
this juncture given track differences. This has the look of large
In spite of impact differences, all the models agree that this
event should be done overnight Tuesday with most favoring an exit
during the evening. Then sunshine should return on Wednesday as
convective temperatures are predicted to be out of reach. There was a
thermal downtick for Wednesday, but air mass modification by
afternoon would get US close to continuity for maximum temperatures. So only
a slight downward tweak.
Last week we had a one day wonder of warmer weather. This week looks
like we are going to have a two day wonder of warmer weather. (Maybe
next week we can try for The Hat trick?). maximum temperatures on both Thursday and Friday
are predicted to be warmer than normal. Thursday should be the drier
of the two days with just low chances of showers as a weak warm front
moves through. Friday, especially the afternoon and evening, have the
greater chance of showers occurring. As our predecessors noted, it is
unstable aloft and since both the GFS and European model (ecmwf) agree on this, we added
isolated thunder. Then cooler and breezier weather should return
for the weekend.
Aviation /13z Sunday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy, and surrounding areas.
Today...VFR. Northwest winds 10-15 knots with local gusts to 20
knots early, becoming west 5-10 knots by late afternoon. The winds
should then go more southwesterly near early evening.
Tonight...VFR overall, however clouds increase and lower ahead of
the next system. A few snow or rain showers may be nearing areas
west of kphl toward daybreak. Southwest winds less than 10 knots.
Monday...MVFR ceilings and visibilities possible in rain showers and shsn
primarily in the morning, otherwise VFR expected. Cold front
will bring an abrupt shift of gusty northwesterly winds.
Monday night...VFR with diminishing winds.
Tuesday...MVFR ceilings and visibilities with mainly rain
possible centered on the afternoon and evening. Snow possible
higher terrain airports.
Wednesday through Thursday...mostly VFR conditions. Southwest
winds on Thursday should become gusty.
high pressure builds in across the area today before settling to our
south and east tonight. This will result in the winds diminishing
through the day. As the flow lessens, a shift to the west and then
southwest is expected by late afternoon. The advisory will remain
in effect for our southern waters for now as seas are right around
5 feet still. For tonight, a light southerly flow will then
increase late as a low-level jet arrives ahead of our next system.
The forecast soundings show that enough mixing is maintained to
allow 25-30 knots to reach the surface. There is warm air advection occurring,
however it may not be strong enough to affect the mixing.
Therefore, a new Small Craft Advisory may be needed starting late
tonight. The southerly flow will also build the seas once again
Monday and Monday night...the cold front should sweep through
Monday, bringing an abrupt shift of westerly and northwesterly
winds. Gusts near or above 25 knots will be possible during the
day. Seas will be slow to subside on the ocean, probably taking
until Monday evening.
Tuesday through Wednesday...winds are expected to stay below Small Craft Advisory
criteria through this period. However, if the passing low pressure
system is stronger than forecast, winds above 25 knots are possible
primarily Tuesday night. Seas near or above 5 feet are possible on
the coastal waters through this time, but there is still
Wednesday night and Thursday...increasing southerly and
southwesterly flow will likely result in another round of
Small Craft Advisory conditions on Thursday.
Marine...Small Craft Advisory until noon EDT today for anz431-452>455.