Scientific Forecaster Discussion
fxus61 kphi 240924
Area forecast discussion
National Weather Service Mount Holly New Jersey
424 am EST Sat Feb 24 2018
a warm front will move slowly northward tonight
into Sunday. A low pressure system will cross to the north of our
area with its attendant cold front moving through Sunday night. High
pressure will build into the region through midweek. A backdoor cold
front will drop down through the region on Wednesday, followed by an
area of low pressure crossing the area Thursday and perhaps
intensifying along the coast early Friday.
Near term /until 6 PM this evening/...
several changes were made to the forecast today. Fog is getting
dense in Chester County and the Maryland Eastern Shore. Have
issued a dense fog advisory through 8 am for these areas, and
there is at least some chance the advisory may need to be
carried eastward into Delaware given short-term model forecasts
and recent trends in dov/Ged surface observations. Visibility
should improve by mid to late morning, but patchy fog and even
some drizzle look to remain through much of the day across the
This leads to concern number two, which is the forecast highs today.
Inherited forecast simply looks too warm for the given pattern.
Despite potential for rising cloud bases (albeit gradually) during
the day, the overcast will very likely stick around. Though it has
been a warm February, it is still February -- and with my
pessimistic sky forecast today, I am awfully skeptical temperatures
will reach the upper 50s in portions of southeast PA/S New Jersey. Generally, I
lowered temperatures 3-5 degrees across the area, and I am wondering
if this is enough, especially if this morning's observations agree
with the colder NAM. Forecast temps may require more fine-tuning in
future updates, but I felt that starting the trend downward was
Several midlevel perturbations move northeastward into the northern
mid-Atlantic today, and with favorable positioning of the northern-
stream 250-mb jet Max (right-entrance region dynamics in play)
combined with low-level isentropic ascent along/north of a
baroclinic zone stretching zonally across the region, periodic bouts
of rain look to occur in the County Warning Area beginning this afternoon,
especially north of the Mason-Dixon line. There is still some
variability in timing of the precipitation, so I kept hourly pops in
the 50-80 percent range for the afternoon until there is more solid
agreement amongst the hi-res models. Nevertheless, rain is a pretty
good bet in PA/New Jersey this afternoon with diminishing chances in
southern MD/de. Quantitative precipitation forecast is generally a quarter inch or less through 00z
Sunday, though amounts may be a little higher than that in the
Poconos and Lehigh Valley if the more aggressive models are
Short term /6 PM this evening through 6 am Sunday/...
one round of rain should be diminishing and moving away from the
region during the evening hours, but conditions will likely once
again be favorable for low clouds, fog, and drizzle during this
time. Trended pops downward during the evening hours, but maintained
categorical pops after midnight as the next round of rain is
expected to move in. Kept mention of fog through the night, but
generally kept drizzle confined to the evening hours, as steadier
rain is more likely after midnight, especially north of the Mason-
This next round of rain will occur as yet another vort Max
approaches the region in midlevel southwesterly flow between 06z and
12z Sunday. Isentropic ascent poleward of the quasi-stationary
baroclinic zone will allow for precipitation to break out again
(with help from the steady 250-mb jet Max to our north). The
enhanced ascent provided by the stronger vort Max, the slowly
poleward-moving warm front, and the considerable upper-level
divergence aided by the jet Max will likely lead to more formidable
rainfall totals during this period. Generally, amounts should be
approaching a half inch to an inch north of the Mason-Dixon line by
night's end with lighter amounts to the south. Fairly decent
agreement among hi-res models on qpf, so confidence is slightly
Temperatures will not fall too much as the warm front makes its move
(slowly) northward, overcast conditions continue, and warm advection
curtails nocturnal cooling. Forecast lows are in the mid to upper
30s in the Poconos and northwest New Jersey to the mid to upper 40s
southeast of the fall line. Will need to keep an eye on temperatures
in the Poconos, as models tend to have a warm bias in this area
north of warm fronts, and even a couple degrees cooler could mean at
least a chance for some isolated slick spots. For now, chances are
too low for mention.
Long term /Sunday through Friday/...
a progressive mid-level flow pattern will provide changeable weather
for the Sunday through Friday time period, with normal to slightly
above normal temperatures through most of the period.
A warm front associated with a low pressure system moving NE through
the Great Lakes region will lift north Sunday morning before its
associated cold front pushes east during the afternoon and evening
hours. The passage of this cold front will bring an end to any
lingering rain and rain showers. As precipitation from this system
ends Sunday night,rainfall totals are still expected to range
between 1 and 2 inches. This will cause ponding of water on roadways
and in poor drainage areas, and rises on small stream, but no
significant stream and river flooding is anticipated at this time,
especially given the longer duration of the event. Please see Hydro
High pressure from the Tennessee River valley is then expected to
build east, bringing dry weather to the region from late Sunday
night through most of Wednesday. Cooler temperatures are expected
Monday into Tuesday with a northwesterly flow (but still above
normal), followed by warmer temperatures on Wednesday as a
southwesterly flow once again establishes itself over the area.
Another cold front is expected to cross the area Wednesday night but
with limited moisture, precipitation amounts will be light. A
potentially more significant rain storm could move in later Thursday
and Friday as low pressure develops near the mid-Atlantic coast.
Some light snow is even possible on Friday in the far north with a
northwest flow as the system moves NE away from the area.
Aviation /10z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
This morning...IFR to LIFR conditions in very low clouds and fog,
with the densest fog southwest of philly. Cigs should slowly rise
after daybreak but may remain IFR for much of the morning. Timing
the improvement in cigs/vsbys is very uncertain, but current
thinking is that this may be slower to occur than previous forecasts
indicated. Winds generally southwest under 10 kts, becoming more
variable after daybreak. High confidence through daybreak, but low
This afternoon...some improvement to MVFR or even VFR possible,
especially at Acy/miv, but periods of rain will likely move in from
the west this afternoon. Sub-VFR vsbys/cigs in the steadier rain are
likely. Winds becoming northeast under 10 kts. Moderate confidence.
Tonight...prolonged sub-VFR conditions likely as fog/drizzle are
expected this evening followed by steadier rain after midnight.
Winds generally east or northeast under 10 kts. Moderate confidence.
Sunday...IFR conditions expected with limited improvement
through the day. Confidence is low that we will return to VFR during
the day. East to southeast winds will become southwest to west by
the afternoon. Moderate confidence.
Sunday night...improving to VFR through the night. Rain showers will
taper off through Sunday evening and conditions are expected to
improve. High confidence.
Monday through Tuesday...VFR conditions expected. West to
northwest winds becoming gusty to around 20 knots on Monday.
Wednesday...VFR conditions expected. West to southwest winds,
generally around 10 knots or less, becoming more south to
southeast late. High confidence.
generally sub-advisory winds/seas expected through this evening.
However, winds will begin to increase on the northern/central New Jersey
coastal waters late tonight as east to northeast winds begin to
crank up in response to an approaching storm system. Have issued a
Small Craft Advisory for these areas beginning at 3 am Sunday.
Another concern is visibility restrictions this morning. Dense fog
is occurring off the Maryland coast, and surface visibilities
are now deteriorating in Delaware and in coastal southern New
Jersey. As a result, a dense fog advisory was issued for
Delaware Bay and the Atlantic coastal waters of southern New
Jersey and Delaware through 10 am.
Visibility restrictions may continue most of the day, though
visibilities may improve somewhat, and rain chances will
increase this afternoon and tonight, especially off the New Jersey
Saturday night...sub-advisory conditions are expected on the
Sunday and Sunday night...seas are expected to build and exceed
5 feet on the ocean on Sunday. Winds also pick up through the
day Sunday with gusts of 25 to 30 knots possible. Conditions will
improve late Sunday night. A Small Craft Advisory has been
Monday through Wednesday...sub-advisory conditions are expected
on the area waters.
periods of rain through the weekend could result in minor and
poor drainage flooding. The heaviest rain is expected Saturday
night and Sunday, when up to additional inch of rain will fall.
By the time the rain ends Monday morning, the region will have
received between 1 and 2 inches of rain since Thursday, with the
highest amounts falling over the northern third of the forecast
area. By Sunday night and Monday, there will be rises on areas
rivers and streams, with some possibly approaching bankful.
Of note will be how much rain falls over the Susquehanna, as
there is the potential for inconvenience flooding below the
Conowingo dam on Monday.
**top 3 warmest February on record and top 10 wettest February
February projected climate ranking as of noon today based on
our mid shift fcst through the 28th and mins this morning; and
for rainfall, amounts through noon today. Ridge and ttn not
included due to too much missing data.
Abe #3 warmest 38.2 or 7.5f warmer than the 30.7 norm.
Acy #1 warmest 43.0 or 7.7f warmer than the 35.3 norm. Tied
with last year. #2 40.6 in 1954
Ilg #3 warmest 41.6 or 6.6f warmer than the 35.0 norm.
Phl #3 warmest 42.0 or 6.3f warmer than the 35.7 norm.
Water equivalent monthly pcpn
Phl ranked #10 with 5.28" (0.21 as of noon daily)
ilg ranked #8 with 5.49" (0.49 as of noon daily)
Abe ranked #16 with 4.29" (0.07 as of noon daily)
Acy ranked #2 with 6.12" (0.28" as of noon daily)
wettest is 6.50-2010. Our expectation is that Acy will exceed the
previous all time record rainfall for February, by the end of the
PA...dense fog advisory until 8 am EST this morning for paz101-102.
Maryland...dense fog advisory until 8 am EST this morning for mdz008-012-
Marine...Small Craft Advisory from 7 am Sunday to 7 am EST Monday for
Dense fog advisory until 10 am EST this morning for anz430-431-
Small Craft Advisory from 4 am Sunday to 7 am EST Monday for