Scientific Forecaster Discussion
fxus61 kphi 231806
Area forecast discussion
National Weather Service Mount Holly New Jersey
106 PM EST Tue Jan 23 2018
strong low pressure will track northeastward from the eastern Great
Lakes to just north of Maine tonight. It will whip a strong cold
front across the mid Atlantic states early this afternoon. Strong
high pressure will spread east during the balance of the week,
before moving out to sea Saturday. A cold front and low pressure
then arrives from the west on Sunday and proceeds offshore next
Near term /until 6 PM this evening/...
1245 PM update...quick changes taking place as a prefrontal line
of showers moves through and as of this writing was situated
across New Jersey near the I-95 corridor. This will bring brief heavy
rain and gusty winds as it continues to push east but the good
news is it's fast moving and will be off the coast by mid
afternoon. West of here some clearing is actually taking place
and we do not expect any further heavy rain or strong winds. As
far as the gridded forecast, reduced pops and sky cover for this
afternoon west of I-95 to reflect this trend. Despite the rain
moving out, still some Hydro concerns and this is detailed in
the Hydro section below.
10 am update...cold front pushing into western PA with rounds
of showers moving through the forecast area well ahead of this
feature. The southerly flow ahead of the front is bringing very
mild temps for this time of year with 50s to low 60s across much
of the region. Moving through the latter part of the morning
into the early afternoon, concern will continue to be locally
heavy rainfall with these showers that could cause roadway,
urban and poor drainage flooding issues. For further details on
potential Hydro issues, please see the Hydro section below. In
addition, winds are very strong just off the surface with 50 to
60 knots in the lowest few thousand feet. As a result, any
heavier showers or embedded thunderstorms could still mix
locally damaging winds down to the surface. Main period of
concern is through the first part of the afternoon after which
time front and associated showers clear the area to the east. In
terms of the gridded forecast, some adjustments were made to
temp, pop, and quantitative precipitation forecast grids based on latest obs and trends.
As a surface low moves through the Great Lakes region this
morning, large-scale ascent will increase markedly downstream
across much of the eastern U.S. Today, aided by a combination of
differential cyclonic vorticity advection downstream of a potent
vort Max in the Midwest, considerable low-level warm advection
aided by a 50+ kt 925-mb jet, and left-front quadrant upper-
level divergence as a cyclonic jet streak noses into the
northeast. Several areas of showers have developed this morning
(with some embedded storms in the eastern Ohio Valley and
adjacent areas), including from central PA south-southeastward
into lower Delmarva. Hi-res model output suggests this
precipitation will grow rapidly during the next few hours as it
races northeastward into the northern mid-Atlantic. Showers are
already beginning to reach Berks County and far southern Sussex
County (de) at this time.
Model quantitative precipitation forecast has trended upward overnight, with the Poconos and
Lehigh Valley solidly in the 0.75-2.00 inch range from virtually
all of the hi-res output. The 00z GFS looks dry everywhere and
generally discounted given its outlier ("outliar") look. Quantitative precipitation forecast
looks generally under an inch southeast of the fall line, though
there is some indication of a second maximum in portions of
Delmarva and south/east New Jersey that will need to be watched. For
now, increased total quantitative precipitation forecast southeast of the fall line to generally
a third to three-quarters of an inch. Given the showery nature
of the precipitation, some spots will be relatively dry, with
localized totals possibly higher than the area-averaged storm
totals in the grids.
With the relatively high totals in the Poconos, Lehigh Valley,
and northern New Jersey, may see some isolated Hydro issues
(unlikely much more than nuisance in nature). Additionally,
particularly strong cells may produce locally heavy rain and
poor-drainage/urban flooding in isolated spots.
The convective environment is at least somewhat favorable for
isolated damaging wind gusts and even a brief/weak tornado.
Shear is generally off the charts today, with srh in the warm
sector generally 300+ j/kg and deep-layer shear 60+ kts.
Instability looks rather marginal, however, and it is not
entirely clear that much (if any) surface-based instability can
develop, particularly if widespread showers occur. Shear may be
so strong that updrafts fail to acquire much vertical depth as
well, with corresponding downdrafts unable to generate
considerable downward momentum transport (further impeded by
meager low-level lapse rates). Nevertheless, given the output of
the 00z NAM nest and several recent hrrr simulations, it will
not take much instability to generate reasonably strong
convective cells, and with very strong winds off the surface,
the potential is there for an isolated severe gust or two.
Of note, several wind gusts 35-50+ kts reached the surface with
a convective line to our west (west/central PA, WV, and eastern
oh) overnight. That certainly suggests any convective cells
require close monitoring today for strong-to-severe wind gust
potential. Additionally and not unusual for these convective
setups (given the time of year, for starters), convection may
contain little or no lightning.
Surface trough should move through most of the area by 21z, with
precipitation shunted eastward by this point. Skies will rapidly
Temperatures will be warm today in advance of the potent upper
trough, with strong mixing and warm advection aiding in much
above average temperatures today. However, there is some
question regarding how high temperatures will go given the
widespread precipitation expected. Kept things close to
consensus for now, but the temp forecast is low confidence.
Finally, with south/southwest winds increasing this afternoon in
the warm sector and strong winds aloft, expect some gusts 25-35
mph generally along/southeast of I-95.
Short term /6 PM this evening through 6 am Wednesday/...
conditions dry out tonight as the synoptic cold front moves
through. Skies should be clearing out, and temperatures will be
much colder than those observed early this morning. Kept
forecast close to statistical MOS blend, with winds likely
strong enough to mitigate more substantial radiational cooling.
This leads to lows forecast to be in the middle 20s in the
Poconos to the middle 30s in the urban corridor, Delmarva, and
the New Jersey beaches.
Long term /Wednesday through Tuesday/...
500mb: another short wave moves a cross the mid Atlantic coast
Wednesday night followed by substantial ridging Friday and Saturday,
then another trough develops to the East Coast next Sunday and
Temperature: calendar day averages should range between 3 and 7
degrees above normal Wednesday, near normal Thursday and Friday,
on Saturday near 10 degrees above normal, Sunday probably 12 to
18 degrees above normal, cooling a bit to 5 to 10 degrees above
Forecast basis: Wednesday-Thursday is based on a blended 00z/22
GFS/NAM MOS, Thursday night-Friday was based on the 00z/22 GFS
mex MOS and then d4-8 was based on the 0441z wpc guidance.
Wednesday...considerable cloudiness after early sunshine. A
slightest chance for a few flurries in the Poconos. Brisk with
a northwest wind gusty 20-30 miles per hour. Clearing at night. Max temps
Wednesday generally 2 above normal with the Wednesday night mins
Thursday...mostly sunny and brisk. Northwest gusty 20-25 miles per hour.
Max temps 2 to 7f below normal. The Thu night mins about 2f
below normal. Confidence: above average.
Friday...mostly sunny. Wind becoming light southwest. Max temps
near normal. Confidence: above average.
Saturday...increasing clouds and milder. Southwest gust 20 to
30 miles per hour. Chance of showers at night. Max temps 10 to 15f above
normal. Sat night mins. Probably 15 to 20 degrees above
normal! Confidence: above average.
Sunday...showers with a coldfront and developing low pressure on
the front. This part of the fcst differs considerably than what
i'd anticipated earlier and it is a gefs combo with the ggem
and European model (ecmwf) both pretty confidence on a wet 12 to 24 hours much
more so than the 00z/23 GFS op. So have followed wpc guidance
probability of precipitation. Max temp 10 to 15 above normal. Gusty southerly flow with
a wind shift to west probable at night. Confidence on overall
scenario: above average but below average on details including
timing cold front passage.
Monday...cooler but daytime Max temps still probably 5f above
normal. Showers should b E ending or gone. Confidence: average.
Aviation /18z Tuesday through Sunday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...mainly VFR this afternoon for PA sites and kilg as
showers have pushed east. For kacy/kmiv, still dealing with
significant restrictions as of early afternoon but this will
clear out by 21z.
Tonight...VFR with west to northwest winds 5 to 15
kts. Confidence above average.
Wednesday...VFR sct-bkn aoa 3500 ft clearing at night. West
northwest wind gust 25 to 30 kt.
Thursday...VFR. West to northwest wind gust 20-25 kt.
Friday...VFR. Light wind becoming southwest.
Saturday...VFR gusty southwest wind 20-30 kt. Chance of MVFR conds
later at night in showers.
Small Craft Advisory conditions are expected on Delaware Bay
today and on the Atlantic waters today and tonight. There may be
some marginal gale-force gusts on the Atlantic waters today as
strong southerly flow increases this morning. However, the
bigger threat will be isolated/sporadic gale-force gusts with
showers/storms that move through the waters during the day. A
few special marine warnings may be required, given that it will
not take much for showers to transport strong winds aloft to the
Showers should move east of the nearshore waters tonight, and
advisory conditions should cease on Delaware Bay. However,
winds/seas will remain elevated (switching to west/northwest
this evening) through the night on the Atlantic waters.
Wednesday...Small Craft Advisory extended Atlantic waters and will probably be
needed for the de Bay waters.
Wednesday night through Thursday...a west northwest wind could
gust around 25 knots and the Small Craft Advisory may need to
be extended in time.
Thursday night through Friday...no marine headlines are anticipated.
Saturday... a southwest wind Small Craft Advisory may be needed.
rainfall...rain ending by mid afternoon with storms totals of mainly
0.50 to 1.00 inches with locally higher amounts.
Forecast points...with up to two inches of rain across portions
of Schuylkill County, rises were quick this morning at the
Landingville USGS gauge. Landingville has crested and water is
currently working down the Schuylkill. The crest forecast has
been updated for Berne. Caution/action stage will be approached.
Although the Schuylkill crest has been bumped up, no mainstem
flooding is expected.
Snow...any remaining snow on the ground, which is near zero across
much of the hsa, is hydrologically insignificant.
River ice...we continue to receive reports of solid ice cover on the
Delaware River, but conditions are not as widespread as last week.
We know of solid ice near Trenton from about the Route 1 bridge
south down to about borderntown, or near the Head of the Tide. We
also know of ice further north near and in the Delaware Water Gap.
With the warm temperatures the last few days combined with today's
precipitation and rising water levels, melting and fracturing ice
will occur. Restrictions in flow or ice jams are possible as ice
breaks up and begins to move.
Since ice jams can not be predicted with certainty, the best
approach is awareness and to take notice of day to day changes on a
river or stream of concern.
monthly departures should drop about 2 degrees from this mornings
(jan 23) cf6 values that are seen on our Mount Holly web site.
The overall monthly average will probably end up somewhere
around 1 to 2 degrees below normal for January.
Marine...Small Craft Advisory until 6 PM EST Wednesday for anz450>455.
Small Craft Advisory until 6 PM EST this evening for anz430-