Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Mount Holly New Jersey
350 PM EDT Sat Jul 26 2014
a warm front will lift northward toward the area tonight, before
lifting across the area on Sunday as an area of low pressure moves
into southeastern Canada. Another low pressure system will lift into
New England Monday, pulling a cold front across the area. High
pressure will return to the East Coast for the middle and end of the
Near term /until 6 am Sunday morning/...
the GFS initialization at 500mb looked better than the WRF-nmmb
while the WRF-nmmb looked better at 850mb and 925mb. Of note the GFS
has been trending downward with its thermal mass fields the last
couple of days as we get closer to f00.
As far as model short wave verification based on 24hr forecasts its the
ECMWF, the GFS @ 18hrs, then can ggem and WRF-nmmb. This is both a
volatile and anomalous regime for late July and we are going to run
with the Assumption until the trough clears that short waves will
outperform. 12z upper air analysis showed a pocket of 19c dew points at
850mb/925mb west of Chicago. This cant be denied overnight and
convection is firing over northern Indiana.
Meanwhile after the along the coast short wave clears, warm air advection is
forecast to continue overnight. Take that pocket of very high
equivalent Theta-E and match it with a favorable jet regime and warm air advection,
see no reason that this feature cant keep on producing precipitation
overnight. While probability of precipitation have been dropped from this evening as the
first short wave passes off the coast. They are increased again
overnight with the greatest emphasis north.
Big dew point differences across our County Warning Area mainly based on where the
rain has fallen more heavily. With an attendant southerly flow
continuing through the night, we believe the higher stat guidance mins
will verify the best, especially southeast half of County Warning Area.
Short term /6 am Sunday morning through 6 PM Sunday/...
slight risk for severe thunderstorms continues for Sunday as our run
of dry weekend weather really takes a hit. Now comes the forecast
problems of short wave strength and their effect on the sensible
weather. Here our confidence is less than average based on this
The next short wave is initiating an mesoscale convective system in eastern Nebraska as
we type. This is the one that the GFS takes to the nth degree on
Sunday morning and makes following its mass fields for the rest of
Sunday impossible. The general idea appears correct. But does this
short wave arrive too soon before instability can increase
adequately for severe and stabilize US "somewhat"? Does the
convection Muddy the convective waters for the real deal coming
out of eastern canada? Does the predecessor short wave linger too
long during the early morning? The forecast bulk shear is well within
range of severe at 30-40kts. The question becomes about air mass
recovery or purity around these short waves. We dont have the
answer yet. The sunnier Sunday is, the truly stronger the
convection will become. Regardless, a more than adequate wind
field is in place (anomalously strong for late july) and we will
continue the enhanced wording in the zones/grids starting during
the afternoon. Can ggem and UKMET timing would give our County Warning Area a
higher chance for severe. GFS/European model (ecmwf) timing at least through the
daytime hours would be less. Probability of precipitation were left as likely with an
increase from southwest to northeast as the afternoon and early
evening progress. Dont want to forgo the heavy rain threat, but
with a faster overall wind field would have to see training to
have any flash flood threat.
Because of uncertainty as to cloud cover and a downward trend to the
GFS thermal fields, we tended to lean toward the lower GFS MOS for
maximum temperatures. Pretty close to continuity.
Long term /Sunday night through Saturday/...
a wet start to the long term, followed by a prolonged dry period.
On Sunday night, we expect an initial surface trough to move across
the area during the evening and push offshore overnight as an area
of surface low pressure moves through pennsylvanian into New York.
Meanwhile, several short wave/vorticity impulses will slide across
the area within the southwest flow aloft as the middle-level low moves
out of the Great Lakes and into western Pennsylvania. This will keep
a high likelihood of showers and thunderstorms across the area. Precipitable water
values are forecast to be close to 1.5-1.75 inches early in the
night, so a period of heavy rain is possible. However, as the night
moves on, the precipitable water values drop, so heavy rain will become less likely.
On Monday, this low pressure will continue to move into New England,
eventually pulling a cold front across the area and off the East
Coast. Precipitable water values may rebound close to 1.5 inches, but overall, it
looks drier than the day leading up. Still, with the middle-level
trough swinging close by or overhead, there will remain the chance
for at least scattered showers/thunderstorms, especially up north
closer to the center of the trough. There may be some lingering
showers Monday evening as the front continues to push offshore and
the middle-level trough lifts out, but any precipitation is expected to
end by midnight.
After this wet period, we then enter into a dry period for much of
the remainder of the week. Precipitable water values drop to an inch or lower for
Tuesday and Wednesday, but may return to 1-1.25 inches for Thursday
and Friday. However, with high pressure building across the region,
and no significant short wave/vorticity impulses through much of the
week, we are not expecting any precipitation. It would not be
surprising if some isolated showers developed Thursday into Friday
with daytime instability, but chances are low enough at this time
that we did not include them for Thursday and Friday only has a
slight chance at this time.
As we move into next weekend, we get more solid return of moisture
across the area. We also have a better chance of vorticity impulses
affecting our area within the southwest flow aloft as the middle-level
trough deepens to our west.
Aviation /19z Saturday through Thursday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
18z tafs showed more of an impact by showers and thunderstorms on the
For the rest of the afternoon the upper air disturbance thunderstorm threat
appears gone for all the terminals with the possible exception of
kacy. Trend is downward elsewhere also. Otherwise, VFR conditions
with possibly a cumulus based ceiling at times. We are not detecting a sea
breeze front today, although winds should back more toward the south
as the afternoon progresses.
This evening a lull in precipitation chances with VFR conditions prevailing
and lighter south winds.
Overnight another chance for showers (too low confidence for
thunder inclusion) for the terminals with MVFR conditions
occurring. Even if the showers do not materialize, it is likely
that visibility restrictions will bring MVFR conditions to most
terminals and airports.
Following the passage of the shower threat, prevailing VFR
conditions are predicted to return to all terminals by the middle
of Sunday morning. Winds should be slightly stronger than today
and slightly more from the west with speeds averaging 10 to 15
More organized convection is likely either Sunday afternoon or
evening. While confidence is moderate about occurrence, it is
lower as to timing. Please look for further updates in future
Sunday night...showers and thunderstorms likely, which could reduce
ceilings/visibilities to MVFR or IFR.
Monday...a chance of showers and thunderstorms, which could reduce
ceilings/visibilities to MVFR or IFR...otherwise, VFR expected. Gusty SW-west
winds 20-25 knots.
Monday night-Thursday...mainly VFR.
no Small Craft Advisory headlines in marine products. The trend for
wave watch guidance has been downward from recent runs through
Sunday day. The attendant forecast low level jet is weaker. Coupled with warmer
air over cooler water does not appear likely that either winds or
seas (on the ocean) will make criteria through the day. The greater
concern is more short fused in nature with special marine warning
type strong/severe thunderstorms.
Sunday night-Monday...it is possible that seas may build close to 5
feet, which may require a Small Craft Advisory. Winds may increase
to 20-25 knots as well.
Monday night...Small Craft Advisory conditions possible early Monday
night, then subside and remain below advisory levels through the