Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 201610
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1110 AM EST Sat Jan 20 2018

.SYNOPSIS...
High pressure centered over the Southeastern States will keep it
dry today but moisture will creep into the western mountains
tonight. Temperatures will rise above normal today and stay
there through mid-week. A cold front will move through the area
later Monday night and early Tuesday.

&&

.NEAR TERM /UNTIL 8 PM THIS EVENING/...
Temps rising very nicely this morning as we mix and tap the high
theta air just aloft. Winds are gusty and will only slacken a
little through the day as the big Quebec low moves a little
farther away.

Prev...
Fast zonal flow aloft, occurring well to the south of a
strong...150 kt upper jet stretched across the northern Great
Lakes and SERN Canada, will help to maintain fair weather with
moderating temps and a gusty westerly wind today.

Variable amounts of cirrus clouds will drift southeast across
the region today and remain the dominant (and perhaps the lone)
cloud type until at least late this afternoon.

Temps early today will be much milder than normal by as much as
15-20 deg F across the central and NW mtns, thanks to a 12-17
kt SW wind across northern and western parts of the CWA...with
gusts to around 20 kts. A few locations across the Lower- Mid
Susq Valley could see temps hover near to a few deg F above
normal (in the low to mid 20s) as a result of mainly clear
skies and a decoupled boundary layer with nearly calm air.

Elsewhere, min temps early today will range from the mid 20s
across much of the Middle and Lower Susq Valley, to the low-mid
30s across the NW half of the CWA.

After a mostly Sunny morning in most places, the operational
NAM, GFS and RAP show a rather shallow layer of stratocu clouds
advecting ESE and into the western zones during the late
morning/early afternoon hours, and later in the afternoon or
evening across the Susq Valley. The latest National Blend of
Models and ensemble MOS pops continue to show a small chance of
some light rain...mixed with wet snow flakes...spreading into
the NW mtns and Laurel Highlands around sunset this evening.
Wet bulb profile below zero by a few deg F through all layers of
the Atmosphere favor spotty --SN and/or --PL for the first few
to several hours of the precip.

Highs today will range from near 40F north to the lower 50s
across the south. However, a brisk westerly wind (averaging in
the mid to upper teens (KTS) with gusts as high as 30 KTS during
the late morning and afternoon Western high terrain and 20-25
kts elsewhere) will make it feel notably cooler than the actual
air temp - or in the upper 20s across the higher terrain of the
north and west, and upper 30s in the SE.

&&

.SHORT TERM /8 PM THIS EVENING THROUGH 6 PM SUNDAY/...
Moisture takes the circuitous route around high pressure over
the Carolinas tonight with low clouds gradually thickening -
enough to support some areas of light precipitation as the night
wears on.

Any precip that falls prior to 09Z Sunday will be almost
exclusively across the NW mtns and Laurel Highlands and in the
form of light rain, sleet or even some spotty light freezing
rain or freezing drizzle as Wet Bulb temps will be generally
near to slightly below zero at all levels except for perhaps a
shallow layer between 900-925 mb. Wet bulbs warm to a few or
several deg C above zero by 12Z Sunday across the Laurel
Highlands and by 17Z over the NW mtns of PA, to change any
spotty light precip to rain showers.

Sunday looks to be primarily a cloudy day with some areas of
drizzle or brief showers. Temps may be sluggish to warm out of
the 30s across the north, while only reaching the low to mid 40s
in the Southern valleys. Winds will be generally light and from
the south to southwest.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Another January thaw will bring a period of relatively mild
weather to Central PA into early next week. Temperatures are not
expected to be as warm as the previous thaw late last week, but
departures should reach +10 to 15 degrees above average for mid
to late January. The progressive pattern should keep the cool
down through the middle part of next week rather brief before
temperatures moderate/trend warmer into next weekend.

A surge of warm advection ahead of a strengthening Mississippi
Valley/Great Lakes cyclone will impinge on a retreating thermal
gradient between I-80 and I-90. Forecast soundings indicate
moisture will be rather shallow initially with areas of light
rain confined mainly to the western and northern parts of the
area. Elsewhere expect low clouds to develop with pockets of
drizzle/mist and fog. Examination of thermal/moisture profiles
specifically sfc temps and maxTw aloft suggest some patchy
freezing drizzle is possible over parts of north-central PA Sat
night and Sun night. The other concern may be black ice as snow
melts during the day and then freezes at night.

The risk of ice jam flooding will continue over the weekend and
into early next week. See the hydro section for more details.

The most likely period for widespread rain is Monday night into
early Tuesday along/ahead of south-southwesterly LLJ axis and
cold/occluded front. A secondary low may form in the lee of the
Appalachians and support a period of enhanced rainfall over the
eastern 1/2 of PA Tuesday morning. At this time, the total
rainfall forecast for early next week does not appear to be as
heavy as Jan. 11-12.

Winds look rather blustery from later Tuesday into Wednesday on
the backside of the low pressure system and may approach
advisory criteria. Snow showers are likely downwind of Lake Erie
into Wed night. Dry wx with moderating temperatures Thu-Fri.

&&

.AVIATION /16Z SATURDAY THROUGH WEDNESDAY/...
Widespread VFR will continue today as high pressure maintains
control of the local weather pattern.

A MVFR stratocu/stratus deck should advect East into the
Western zones tonight. This deck will become IFR at times,
especially at BFD and JST. Some DZ may fall from this cloud
deck tonight over favorable upslope areas of the Alleghenies,
as the clouds will be 1000 feet or so thicker there than in
downstream locations. Strong winds aloft will slowly weaken as
the gradient shifts to the northeast, however LLWS will continue
today since the mixed layer will be rather shallow today (2000
to 2500ft).

Outlook...

Sun PM...SCT SHRA - mainly in the north. Otherwise MVFR S and
IFR N.

Mon...Reduced flight categories with rain showers associated
with an approaching cold front - mainly N. Improvement expected
from S-N thru the day.

Tue...VFR w/SCT MVFR in SHRA.

Tues PM-Wed...IFR SHSN NW. MVFR/VFR SE.

&&

.HYDROLOGY...
Main concern is watching water levels rise on Susq downstream on
Harrisburg. Marietta gage /MRTP1/ continues its slow rise as ice
builds up downstream. Gage above action stage and there is some
minor flood concerns in Wrightsville ongoing. The Flood Watch
for this area has been extended through the weekend, set to
expire Monday afternoon. With milder temperatures there is
potential for ice movement, as well as slow ice
deterioration/thinning with time. All areas near rivers and
streams that have significant ice buildup should monitor water
levels closely for the next several days.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Monday afternoon for PAZ065-066.

&&

$$
SYNOPSIS...Lambert
NEAR TERM...Dangelo/Lambert
SHORT TERM...Lambert
LONG TERM...Lambert/Steinbugl
AVIATION...Lambert/Ceru
HYDROLOGY...



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