Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 kphi 210852 
afdphi

Area forecast discussion
National Weather Service Mount Holly New Jersey
352 am EST Tue Nov 21 2017

Synopsis...
a cold front will cross the area Tuesday night and early Wednesday,
followed by high pressure into Saturday. Another cold front is
expected to cross the area Saturday night, with high pressure
anticipated for the first part of next week.
&&Outlook...

Wednesday...lull in Small Craft Advisory conditions possible Wednesday morning.
Otherwise, winds shift to the northwest and increase to 15-20 knots with 25-30
knots gusts.

Thursday through Friday...sub-sca conditions expected.

Saturday and Sunday...Small Craft Advisory conditions are possible with the passage
of a strong cold front.



Near term /until 6 PM this evening/...
digging vort Max in the upper Midwest will continue southeastward
through the Great Lakes region today. Downstream, midlevel ridging
will amplify in response to low-level warm-air advection expanding
the atmospheric column. Predecessor perturbations in the flow will
progress through the east today, though these should have generally
minimal impacts on sensible weather other than increasing high
clouds, especially this afternoon. As the surface ridge to our south
continues offshore today, southwesterly flow will continue, getting
a boost from diabatic heating and a strong surface low moving
through Ontario (aiding in an increased surface pressure gradient).
As such, expect breezy conditions again today, especially
along/southeast of the urban corridor, as areas to the northwest
will be in somewhat of a surface col by afternoon.

Statistical guidance shows good agreement, but I suspect there is a
bit of a cool bias with temperatures today given the warming
profiles and the amplified ridging. Hard to go against good
agreement among the guidance, but I did nudge up temperatures a
degree or two in spots from the inherited forecast. It is quite
possible that philly reaches 60 today, with overall Max temperatures
about 2-7 degrees above seasonal averages.
&&

Short term /6 PM this evening through 6 am Wednesday/...
a perturbation over the southeast will lift northeastward during the
day, likely in vicinity of the Carolinas by 21z. Precipitation
should break out near this vort Max, with a surface wave likely
developing during the day near or off the coast of Florida. This
southern-stream system will be affected strongly by the northern-
stream vort Max, with some phasing likely to occur during the
overnight period, and rapid development/northeastward progression of
the surface low near/off the Carolina coast. However, despite the
overall excellent agreement in the large-scale pattern, the NAM/GFS
depictions of the southern-stream low leave some question Marks. The
GFS looks a little on the fast/progressive side, which is a typical
bias it exhibits in the short term. Comparison to the rgem suggests
it might be on the order of 3 hours too fast, and this difference
may be enough to keep the GFS too dry tonight. It will be critical
to watch the evolution of the southern-stream system today (and the
precipitation/upscale feedback processes) to determine if the GFS is
keeping US too dry tonight.

Meanwhile, the NAM looks very similar to the rgem in the overall
evolution of the system tonight, but the NAM is simply drier than
the rgem (another bias observed on multiple occasions this season).
Based on this (and the GFS bias), I weighted guidance that was
slightly slower/wetter (e.G., The NAM nest and the WRF-arw, along
with the rgem) somewhat higher. Also included some of the 00z European model (ecmwf)
with the quantitative precipitation forecast grids tonight, despite its coarser resolution, as it
seems to be doing a good job this fall depicting the cutoff of the
poleward side of the precipitation somewhat better than the NAM/GFS
(likely because of its general slower/deeper depictions of these
phasing systems).

The northern-stream upper-level jet streak will be in a favorable
position by night's end for rather deep ascent, but by this point,
the stronger lift will probably be offshore. Nevertheless, mesoscale
banding of the precipitation is clearly evident in most of the model
simulations tonight, and this does occur as far west as I-95 in most
of these simulations. This makes the quantitative precipitation forecast a challenge, since these
setups can be underdone in the models (both with precipitation rates
and the slow progression of these phenomena compared to the speed of
deep-layer flow). This was another reason I upped the quantitative precipitation forecast from the
previous forecast, but tempered to some degree versus the wettest
guidance (sref/ecmwf).

With the above in mind, brought pops up to chance generally urban
corridor southeastward and to likely southeast of a Dover, de, to
Sandy Hook, NJ, line. The timing of the precipitation looks to be
mainly after midnight, but there is some uncertainty here given the
discrepancies mentioned earlier. Amounts generally in the tenth to
quarter inch range along/southeast of I-95 to potentially a half
inch near the coast, but confidence in these values is low because
of the uncertainty with the exact track of the precipitation. Also,
the cutoff of the precipitation will likely be sharper than I have
depicted in the grids (an inevitable result of precipitation
forecast uncertainty). Bottom line: large errors in the pops/quantitative precipitation forecast are
possible tonight given the remaining model discrepancies.

Temperatures were generally a blend of met/mav/ecs MOS, though I did
nudge temperatures up a little, in general given the increased
clouds and remaining low-level south/southwest fetch. Looks like
temperatures will be too warm for non-liquid, non-freezing
precipitation anywhere in our County Warning Area through daybreak.

One final note: given the strengthening dynamics aloft, strong lift
will aid in midlevel cooling possibly enough to generate some
marginal instability. Not totally out of the question to see a
couple lightning strikes, though the best chances are offshore.
&&

Long term /Wednesday through Monday/...
the extended period starts off with a cold front near or along the
coast Wednesday morning, moving east. Chillier air, normal for this
time of year, will move in behind this front. With ridging aloft
over the western U.S. And troughiness in the east, a seasonably cool
but mostly dry pattern is expected through most of the long term
period, with a chance of precipitation later Saturday and Sunday as
another cold front and associate deep upper level trough crosses the
mid-Atlantic region.

The fropa early Wednesday will trigger a few showers north/west of phl. A
sfc low pressure is also forecast to develop off the NC/SC coasts
with moisture spreading north ahead of it along the Delaware/New Jersey coastal
areas. The highest pops continue over southern New Jersey and Delmarva,
counter to climo. Overnight temps should remain warm enough so that
all precip will be in liquid form. However a few showers may linger
past sunrise, and with cold advection behind the front a few snow
showers or flurries are possible in the Poconos Wednesday morning.

The weather for Wed afternoon through Friday night will be dominated
by high pressure moving across the mid-Atlantic region. Temperatures
will be a few degrees below normal on Thursday but should return to
near normal by Friday as the high shifts offshore and a more swly
flow develops. A cold front may push south in to New York state and New
England on Friday but probably will not affect our area to the south,

Another fast-moving frontal system is forecast to push through our
area on Saturday. In part because of its fast movement it will not
be able to pick up much moisture. The current forecast continues
chance pops, higher north/west of phl, but even this may be overdone.
Again, temps on Saturday appear to be warm enough to avoid any
frozen precip. However, the airmass behind this front seems to be
colder than the one mid-week, with strong low level cold advection
beginning Saturday night and continuing through the rest of the
weekend. This pattern of northwest flow will favor lake effect snow with
some streamers possibly reaching the Poconos and vicinity.

&&

Aviation /09z Tuesday through Saturday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.

Today...VFR with southwest winds 5 to 15 kts with occasional gusts
to 20 kts or so, especially from the I-95 corridor southeastward.
High confidence.

Tonight...rapidly increasing clouds, likely to sub-VFR near the
coasts (kmiv and kacy most likely to see this). Showers should
spread from southwest to northeast through the area after 06z
Wednesday, mainly confined to the urban corridor southeastward. Sub-
VFR vsbys are likely with the heavier showers, but confidence on how
far inland the precipitation reaches is very low. Winds generally
southwest around or below 10 kts, with some veering to westerly
expected by daybreak. Medium confidence with winds; low confidence
with cigs/vsbys.

Outlook...

Wednesday...rain possible south and east of I-95 early Wednesday
morning with MVFR or lower conditions...then becoming VFR. SW
winds 5-10 knots become northwest Wednesday afternoon and increase to
10-20 knots.

Wednesday night and Thursday...VFR. Northwest winds gradually diminishing.
A storm system is currently expected to remain offshore. Forecast
confidence: medium to high.

Friday...VFR. Northwest winds 5-10 knots.

Saturday and Saturday night...VFR early...then MVFR or lower
conditions possible in rain showers later in the day and at night.
SW winds 10-15 knots.
&&

Marine...
Small Craft Advisory in effect for Delaware Bay and the Delaware
Atlantic coastal waters through today and for the New Jersey coastal
waters through this evening as strong southwest winds
continue/increase today. Winds should diminish overnight, especially
over Delaware Bay and the adjacent Atlantic waters, though
confidence is not particularly high given the potentially brief
duration of these lighter winds.

Showers should spread northeastward overnight (generally after
midnight), with localized restrictions possible. Expect erratic
gusts and locally very choppy seas in vicinity to showers. A
lightning strike or two is not out of the question, though the
probability is too low for inclusion in the forecast at this point.

Outlook...

Wednesday...lull in Small Craft Advisory conditions possible Wednesday morning.
Otherwise, winds shift to the northwest and increase to 15-20 knots with 25-30
knots gusts.

Thursday through Friday...sub-sca conditions expected.

Saturday and Sunday...Small Craft Advisory conditions are possible with the passage
of a strong cold front.



&&

Phi watches/warnings/advisories...
PA...none.
New Jersey...none.
Delaware...none.
Maryland...none.
Marine...Small Craft Advisory until 1 am EST Wednesday for anz450>453.
Small Craft Advisory until 7 PM EST this evening for anz454-
455.
Small Craft Advisory until 7 PM EST this evening for anz430-
431.

&&

$$
Synopsis...miketta

National Weather Service Glossary of Abbreviations