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Office: CTP

FXUS61 KCTP 150017
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
717 PM EST Thu Dec 14 2017

.SYNOPSIS...
A weak ridge of high pressure will slide across the region tonight
with cold, but mainly dry conditions.

Another clipper will move north of the region overnight Friday
and Saturday morning followed by high pressure and a brief
moderating trend in the temperature Sunday.

A series of cold fronts will move across the area late Sunday
into Wednesday. Windy and colder temperatures will follow.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Radar showing a few light lake effect snow showers/flurries
across the Warren/Mckean counties early this evening. However,
boundary layer flow is progged to back to the southwest shortly,
shifting remaining lake effect activity north of the border.

Otherwise, fair and chilly conditions overnight, as surface high
drifts over the state. Some increasing high clouds expected
overnight associated with waa aloft ahead of approaching
shortwave. However, light winds and snow cover should allow
readings to drop into the single digits north to upper teens
south.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
After a cold start temperatures will recover only to highs near 20
in the north to around freezing in the extreme southeast. Clouds
and snow showers will be most likely late in the day over the
Allegheny Plateau, as a shortwave turns the flow more NWLY and
brings a chance of Lake Effect back into the areas normally
affected this time of year. Developing low off the Mid Atlantic
coast could potentially graze the extreme southeast corner of
the forecast area during the afternoon, so have included low
POPs down there for a period of light snow.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
The period starts out with below normal 500 hPa heights and
below normal 850 hPa temperatures. The heights and temperatures
rise to above normal values over the northeastern United States
Sunday as a strong ridge retrogrades to our south. The really
warm air should stay to our southeast. The heights and
temperatures settle back toward normal Tuesday and Wednesday at
500 hPa and 850 hPa respectively.

Temperatures may rebound Thursday ahead of another frontal
system. Overall with the strong ridge to our southeast the
fronts do not get too far south.

Saturday we should still be in the cold air with some LES based
snow activity in the northwest. As the high retreats to our east
temperatures should begin to rebound and the snow showers in the
northwest diminish.

Sunday into Monday a strong shortwave to our north will help
bring in warmer air. We may be cold air dammed near the surface
so the +1 to -2C air at 850 hPa may not translate to a warm
boundary layer. The warm advection should bring snow and snow to
rain in the south but QPF amounts are low and EFS based POPS are
not very high in central and southern areas.

As this northern stream wave zips by to our north it should drag
cold air back into the region Tuesday into Wednesday. The
northwest flow triggers snow in relatively high POPS in NW PA
due to LES. This cold surge should be relatively fleeting and
high pressure and a potential surge of warmer air in the
NAEFS/GEFS. This suggests relatively warmer on Thursday.

The Warm advection and approaching cold front will likely
produce increased potential for snow in NW Thursday into Friday.
Farther south any precipitation would likely be mixed or rain.

A strong frontal boundary will likely push another surge of
cold air into the region Friday into Saturday.

At this time and recent GEFS/CMCE and combined NAEFS runs keep
the strong ridge suppressed to our south. This keeps the warm
air from getting here. This keeps us relatively dry with all
the significant QPF events suppressed to our south.

There is considerable spread in the ensemble members. And we
have seen run-to-run variations in the solutions with the strong
southwest Atlantic ridge. The NAEFS deeps reforming vortex over
east-central Canada and maintains the sharp ridge over
northwestern North America. This seems to keep us in or on the
edge of cold air intrusions. Thus so far single model runs of
big warm ups appear to disappear in the ensembles.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
* Cloud/Ceiling info will be missing at KBFD until new part can
  be installed *

All Central Pennsylvania airfields now reporting VFR
conditions, although appears there are still some patchy MVFR
ceilings as depicted on satellite across the northern mountains.

Overnight will be clear to partly cloudy and cold with some
MVFR ceilings expected to move back into the higher elevation
terminals of the west and north. The chance of snow showers will
be on the increase Friday, mainly over the north.


Outlook...

Fri...MVFR with snow showers north and west mountains, mainly
VFR elsewhere.

Sat...Scattered snow showers and local restrictions NW/Laurels.

Sun-Mon...Mainly VFR.

Tue...mainly VFR, but possible restrictions in the western and
northern mountains in snow showers.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...La Corte
NEAR TERM...Fitzgerald
SHORT TERM...Fitzgerald/La Corte
LONG TERM...Grumm
AVIATION...Jung



Office: PHI FXUS61 KPHI 142027 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 327 PM EST Thu Dec 14 2017 .SYNOPSIS... High pressure will build in across the area tonight through Friday and then weaken. A low pressure system will move up the Middle Atlantic coast Friday afternoon and evening. High pressure will return for most of the weekend. A weak low pressure system will move through Sunday night into Monday with another low expected for Tuesday. High pressure will build in for the middle part of next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... High pressure builds into the region tonight. This will allow winds to become light, especially late this evening and overnight as the pressure gradient weakens. High clouds associated with an upper jet streak will spread into the area this evening and overnight before eroding from south to north toward early morning. We currently do not anticipate this cirrus deck to have a significant impact on radiational cooling. Accordingly, forecast low temperatures are on the cooler side of the guidance with lows in the teens area wide (slight cooler in the Poconos and slightly warmer in the cities. As far as how we get there: temperatures should drop quickly after dark despite the continued NW winds 5-15 mph given the tail end of the stronger post-frontal CAA surge is projected to occur 21Z-00Z this evening, then continue to fall but at a slower clip late this evening and overnight. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... The next shortwave disturbance is forecast to dig around the backside of the large scale trough across the Midwest/Great Lakes region on Friday. There looks to be another weaker shortwave farther south that will be tracking eastward into the central Appalachians. The latest 12Z models trended toward slightly more pronounced with (at least some degree of) of phasing between these two systems as they approach the Mid-Atlantic region in the afternoon. This is evident in the upper-level jet streak that strengthens to 180-190 kt at 250 mb) just to our south by the afternoon as well as coastal cyclogensis that occurs near the NC/VA border midday. Large-scale lift in the left-exit region of the upper-level jet streak and in WAA/southwesterly flow ahead of the trough will likely lead to the development of light precipitation during the afternoon over coastal plain and possibly back into the I-95 corridor. Given the trends seen in today's guidance, PoPs were increased to chance for the I-95 corridor and to likely for Delmarva and southern NJ by late afternoon. Although temperatures in the coastal plain may be in the mid to upper 30s early in the afternoon, evaporative cooling should bring them down to freezing as precip arrives. Accordingly, ptype looks to be primarily snow for this event with the possibility of a brief rain/snow mix at the onset near the coast. The progressive pattern makes this a short-duration event, limiting the risk for more substantial snowfall accumulations. Forecast amounts that went out with the afternoon forecast package are 1" or less for Delmarva and southern New Jersey. There is some support for mesoscale banding (through snow bands would be much more transitory than what we saw with the December 8-9 event) that would lead to locally higher amounts with frontogenesis potentially occurring on the northwestern side of the developing surface low. For the I-95 corridor, kept snowfall accumulations out of the forecast for now with them predicted to be on the north/western fringe of the organized lift and precipitation shield. We may need to expand light accumulations a bit farther back to the north/west if trends continue this way. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... The offshore low (from the short term) will continue to pull away Friday night and pops will decrease through the night. Any accumulations of snow across srn NJ or coastal Delmarva will be light after 00Z Saturday. High pressure will begin to build in by morning. Low temperatures Saturday morning will bottom out in the in the teens or low 20s far north and mid/upper 20s other areas. The weekend looks to be mostly fair at this point with high pressure affecting the area. Temperatures will be a little below normal for mid December, but not the the harsh chill like we had the past few days. Clouds will increase later Sunday as a weak system begins to arrive from the south/west. This system will be rather disorganized and is not even shown producing qpf on the 12Z GFS model. Nonetheless, since the other models are showing some precip, we have put some chc pops are in the fcst for these areas. Mostly rain S/E with some snow psbl N/W. Low confid overall. Monday through Tuesday night will have mostly fair weather for the area. Under upper ridging, temperatures will rise back to above normal values with highs Monday and Tuesday in the 50s over Delmarva and metro Philadelphia and mostly mid/upper 40s for the Lehigh Valley and central NJ areas. It's possible that a few showers may occur, but confid is rather limited and thus any pops in the grids attm are in the slgt chc range. Chance pops will be found Tuesday across the N/W areas with a front affecting those areas. Wednesday and beyond will be mostly fair as another high pressure areas moves in. Most of the operational 12Z models have this period as dry. Temperatures should be back near normal values. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of this afternoon and tonight...VFR. W-NW winds 10-15 kt with gusts to 20 kt will begin to subside toward sunset. We should lose the gusts everywhere between about 23-01Z. Winds will decrease to 5- 10 kt this evening and 5 kt or less by late tonight. Friday...VFR and light/variable winds in the morning. Confidence is increasing in light snow developing along and especially south/east of I-95 terminals. Would envision SN with vsby/ceiling restrictions being mentioned first in the 00Z TAF for MIV and ACY sometime after 18Z, where the potential for snow appears to be greatest. A light SW winds around 5 kt should develop. Outlook... Fri night...VFR conditions returning to S/E terminals with snow ending during the late evening. VFR elsewhere. Sat thru Sunday...Mostly VFR. Sunday night thru Tuesday...Mostly VFR. Few showers psbl. && .MARINE... A SCA remains in effect thru 6 PM for the DE Bay waters and thru 10 PM for the coastal NJ waters. NW wind gusts were near 25 kt over most of the waters this mid afternoon. Seas remain elevated in the 4- 7 ft range for the coastal waters this afternoon, but are expected to diminish in accordance with the winds this evening. Winds and seas below SCA criteria overnight tonight and Friday. Snow will likely develop over the DE Bay and the coastal waters of southern NJ and DE Friday afternoon. Outlook... Fri night/Saturday...SCA conditions developing on the ocean behind the departing low. Mostly sub-SCA for Delaware Bay. Sat night thru Monday...Sub-SCA expected. Showers psbl Sun night. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 10 PM EST this evening for ANZ450>455. Small Craft Advisory until 6 PM EST this evening for ANZ430- 431. && $$ Synopsis...O'Hara Near Term...Klein Short Term...Klein Long Term...O'Hara Aviation...Klein/O'Hara Marine...Klein/O'Hara
Office: PBZ FXUS61 KPBZ 150015 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 715 PM EST Thu Dec 14 2017 .SYNOPSIS... Cold conditions will give way to temperature moderation after Friday and early Saturday snow showers. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... The eve update featured the removal of the "flurry" mention and cover adjustments given the ongoing clearing for many areas with backing flow and subsidence. Cloud cover, in the form of cirrus, is expected to increase again later tonight as the next shortwave draws closer. Given the evening clearing and lower than projected dewpoints, have tweaked the overnight lows further downward. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY/... A shortwave trough will move sewd across the Great Lakes region on Fri, approaching the Upper Ohio Valley region late in the day. Persistent clouds and an increasing chance for snow showers can be expected amid increasing low-level instability and weak synoptic lift. If any instability can be realized, the convective thermal profile is favorable for efficient snow production, although mixing layer wind would have to better align for sustained accumulation. So, although QPF is low, snow ratios might be quite high and could lead to non- trivial snow accumulation, especially in north of the I-80 corridor. Snow chances will peak Fri evening and begin to wane late Fri night as nwly flow resumes and the upper shortwave trough exits ewd. Any lingering showers will translate nwd as wind backs to wswly. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Residual snow showers may linger into Sat, but will decrease in coverage as warmer air invades the region and heights rise. Flow will become zonal by Sun in the Great Lakes region as a shortwave trough strengthens in the swrn CONUS. Ahead of this system, warm-air advection will help restore temperature to near-seasonal values during the weekend and into next week. An impulse ejected from the trough in the swrn CONUS may bring light rain to the region on Sun afternoon-evening, but it will be a decaying system by the time it approaches the Great Lakes. A better chance for precipitation comes mid-week as the swrn CONUS trough phases into a nrn stream disturbance and drives a cold front through the region. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... General VFR is anticipated overnight despite lingering high- based stratocu. A drop back to MVFR is possible during the day Friday as the next disturbance approaches. Snow is expected at FKL and possibly at DUJ Friday afternoon with visibility restrictions. .Outlook... Next chance for restrictions is late Friday into Saturday. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$