Scientific Forecaster Discussion
fxus61 kphi 271118
Area forecast discussion
National Weather Service Mount Holly New Jersey
618 am EST Mon Feb 27 2017
high pressure over the mid altantic coast will move offshore
today. Low pressure will organize and develop over the northern
and Central Plains tonight, and that will lift a warm front
through the mid Atlantic and northeast on Tuesday. That low
will track towards the Great Lakes, then towards eastern Canada
during the mid week period. It will drag a cold front through
the region Wednesday night through Thursday morning. An upper
level trough builds into the northeast for the end of the week.
Weak low pressure passes through the region on Friday, then
high pressure returns for the weekend.
Near term /until 6 PM this evening/...
for the 630 am update, adjusted temperatures down a bit for some
areas as radiational cooling continues early this morning. An
area of mid level clouds is spreading eastward from about the
immediate Philadelphia to Wilmington areas westward. The sky
cover grids were adjusted to reflect a little faster increase
especially in this corridor.
Otherwise, strong surface high pressure will continue to shift
farther offshore during today. This will result in a return flow
occurring with southwesterly winds increasing some through the
day. As we look aloft, a strong westerly/zonal flow occurs which
will carry a short wave eastward into our area toward late
afternoon. This feature may start to shear out some as it moves
eastward given the stronger westerly flow aloft.
There is low-level warm air advection forecast to develop
through the day, however the guidance indicates that this is not
all that robust. Despite this, airmass modification along with
veering low- level flow will result in warmer temperatures this
afternoon (perhaps cooler along the coast given a south-
southwest flow). The combination of the short wave, fast flow
aloft, some warm air and moisture advection will assist in weak
isentropic lift. This should mainly go into cloud production
with clouds lowering down mostly into the mid level range
through the day. However, a band of light showers may develop to
our west with perhaps some sprinkles getting into our western
areas late in the afternoon. As of now, we think the low levels
will remain dry enough and therefore no precipitation is
After a cold start, a decent rebound is expected. Highs were a
blend of MOS and continuity, and we did not go much higher than
this given the expected increase in cloud cover. If the cloud
cover is thinner than anticipated, high temperatures could be a
Short term /6 PM this evening through 6 am Tuesday/...
the upper-level flow is more zonal across the east tonight with
surface high pressure becoming centered farther offshore. A
short wave is forecast to weaken early in the evening as it
shifts offshore. Some weak isentropic lift owing to warm air
advection may produce a few sprinkles early across the western
zones. Given that the low-level airmass continues to look dry,
we maintained a dry forecast at this time.
Our attention then turns more to the southwest and south, as a
warm front will become more defined with time. This is in
response to increasing southwesterly low-level flow that veers
to westerly aloft. The guidance shows a more pronounced warm air
advection surge arriving from the south-southwest late tonight
and more closer to daybreak. Some guidance develops some showers
mainly across the far south/southeast areas near daybreak, on
the nose of the stronger warm air advection and also moisture
advection. The organization of this potential shower activity is
in question, however we will maintain some slight chance to low
chance pops near daybreak for the far south/southeast zones.
Plenty of clouds should linger tonight given the warm air
advection pattern, although some thinning may occur for a time
especially late. Due to the combination of a light southerly
wind, warm air advection and clouds should yield to milder
temperatures. Low temperatures were mainly an even blend of MOS
Long term /Tuesday through Sunday/...
unsettled weather on tap for much of the upcoming week.
Surface high pressure over the western Atlantic continue to
drift out to sea on Tuesday. Meanwhile, low pressure over the
Midwest will lift northeast towards the Great Lakes. A warm
front develops out ahead of that low and lifts north through the
mid-Atlantic and northeast on Tuesday.
Light rain develops throughout the day as some upper level
shortwave energy passes through the region. Warmer air spreads
north through the region, and highs will top off in the upper
60s to around 70 across the Delmarva and southern NJ, in the low
to mid 60s for much of southeast PA and central NJ, and in the 50s
over the Poconos and northern New Jersey.
Surface low pressure over the Great Lakes deepens as 500 mb trough
digs into the Midwest. This will push an 850 mb warm front north
through the mid-Atlantic and into the northeast late Tuesday
night into Wednesday morning. Rain is likely to develop during
this time, and there is the potential for a few thunderstorms to
develop by Wednesday morning. Going into Wednesday afternoon,
strong SW flow develops behind the passage of the warm front,
and temperatures will warm well into the 70s across much of the
County Warning Area. Will side with the warmer NAM temperatures given the strong
Instability increases through the afternoon, with sb cape
values upwards of 500-1000 j/kg, along with a lifted index up to
-3c. Showalter index dropping to around -2c. 0-6 km bulk shear
will increase to 50-60 kt, setting the stage for the potential
for some severe weather Wednesday afternoon/evening.
Prefontal trough develops ahead of the approaching cold front,
and several strong shortwaves will approach ahead of the main
trough digging towards the Ohio Valley. Showers and
thunderstorms will develop in the afternoon, spreading east as
the cold front approaches from the west. Surface cold front
passes through the region and moves offshore by midnight
Wednesday night, and then 500 mb trough with strong shortwaves will
pass through the region by Thursday morning. The bulk of the
showers/thunderstorms will end by midnight, and then additional
showers are possible into daybreak Thursday.
Upper trough then digs into much of the northeast and Great
Lakes to close out the work week. With cold air advection underway, temps
across the region will drop about 20-30 degrees from highs on
Wednesday. West- northwest winds will increase to 15-20 miles per hour with
gusts up to 30 miles per hour.
Another shortwave dives into the base of the trough Thursday
night, and then surface low pressure develops over the Ohio
Valley by Friday morning. That low passes through the region on
Friday. Too soon to determine exact ptype, but depending on when
precip starts, could have a brief period of snow before
changing to rain south of I-80, and precip remains snow north
of I-80, where an inch or so is possible. Highs will be below
normal, ranging from the low 30s across northern zones to the
mid 40s across the Delmarva.
Upper trough moves offshore on Saturday as surface high
pressure builds south of the region. Temps look to remain below
normal again on Saturday, and then temps rebound to above normal
levels on Sunday.
Aviation /11z Monday through Friday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Today...VFR. Increasing clouds with a ceiling developing around
10,000 feet through 18z. Light/variable or light southwest
winds, becoming southwest which increases to around 10 knots
later this morning and afternoon (some local gusts to 15-18
knots possible in the afternoon).
Tonight...VFR ceilings at or above 7000 feet. South-southwest
winds 5 knots or less.
Tuesday...VFR initially, lowering to MVFR in the afternoon as
Tuesday night through Wednesday night...rain Tuesday night, and
then showers with thunderstorms Wednesday and Wednesday night.
Storms may be strong to severe. Areas of fog possible Tuesday
night and Wednesday morning. IFR and lower conditions expected.
SW winds 5-10 kt, increasing to 15-20 knots Wednesday afternoon,
and then winds abruptly shift to the northwest Wednesday night with the
passage of a cold front.
Thursday through Thursday night...VFR. West-northwest winds 15-20 knots with
20- 30 knots gusts. Winds diminish Thursday night.
Friday...sub-VFR conditions possible. Snow possible initially
Friday morning, changing to rain for all but kabe/krdg. Northwest winds
around 15 knots with gusts up to 30 knots.
high pressure shifts more offshore through today allowing for a
southwesterly flow to occur. The airmass will be moderating
today and tonight and despite some increase in the southwesterly
flow, the conditions are expected to remain below Small Craft
Advisory criteria through tonight. Some modest increase in the
southwesterly winds this afternoon and early this evening look
to occur mostly across our northern Atlantic coastal waters
Tuesday...sub-sca conditions. Lowered vsbys possible in rain.
Tuesday night through Wednesday night...Small Craft Advisory conditions expected
with gusty SW winds. Showers and thunderstorms likely,
especially Wednesday afternoon and evening. Winds shift to the
northwest late Wednesday night.
Thursday...northwest winds gusting to 30 knots.
Friday...gale force winds possible.
record high temperatures for Wednesday, March 1st.