Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 201132
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
632 AM EST Tue Feb 20 2018

.SYNOPSIS...
Areas of dense fog and light wind will occur across much of
Central Pennsylvania and the Susquehanna River Valley early
today. Unseasonably warm air with near record afternoon
high temperatures will push north across the region
for today and Wednesday. Temperatures 25 to 35 degrees above
average will be common this afternoon, and again on Wednesday,
making it feel more like May than February. A strong cold
front will move southeast across the commonwealth during the day
Wednesday, followed by several weak waves of low pressure
bringing bouts of rain for the second half of the week, and even
some light freezing precipitation Thursday morning.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Mild air, light wind and relatively high sfc dewpoints (ranging
from the 30s in most places, to as much as 50F across the
Laurel Highlands) will lead to low visibilities in locally
dense fog early today. The best combination of high sfc
dewpoints and light wind was occurring across the lower and
Middle Susquehanna Valley early today, which is where to expect
to see the most widespread and longest lasting dense fog this
morning.

Clouds and the fog should erode fairly quickly during the
midday hours in most places throughout Central and Western PA.
However, a shallow but strong llvl inversion anchored by light
sfc wind and an persistent, moderately strong 1000 mb
Ageostrophic wind could keep, cloudy, cool and foggy conditions
intact across the Lower and Middle Susquehanna Valley into this
afternoon.

Forecast max temps of the low 50s across the NE zones and upper
50s to upper 60s elsewhere were lowered by some 8-10f after
being weighted about 80 percent toward the latest 00Z Tue NAM
with the rest being the Nat`l Blend of Models (which typically
washes out the detail and llvl cool air damming thanks to it`s
ensemble average). Persistent and moderately strong LLVL
northeasterly Ageostrophic Flow should help to maintain a rather
strong inversion within 1KFT AGL SE of the Allegheny Plateau.

We could still easily bust on the high side by as much as 10
deg F in some locations scent PA and the Lower Susquehanna
Valley.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
*Record-challenging warmth Tuesday night through Wednesday

Increasing deep layer south to SW flow will continue to advect
unseasonably warm air into the region tonight and Wednesday.
Expect a spring-like start to the period with patchy fog tonight
into AM Wed. giving way to a mix of clouds and sun with near-
record to record high temps across much of portions of Central
and Western PA.

925 mb Temp anomalies stay in the 2-3 sigma range during this
period with persistent 14-16C air across the CWA. Temps will
only fall to the upper 40s to mid 50s tonight, then rebound into
the 60s...to mid 70s SE. The mid to late morning timing of a
strong cfropa Wednesday across the NW mtns will likely limit the
temp climb to only highs in the mid to upper 50s there, before
readings fall quickly during the afternoon and evening.

Conditions stay rainfree well ahead of the cfront for tonight.
However, rain will develop near and in the wake of the cfropa
later Wed morning (NW) and during the afternoon and evening
elsewhere across the Central Mountains, then Lower Susq Valley.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
*Periods of rain with a several hour period of icing from
 freezing rain expected late Wed night through Thur morning.
 brief interval of mixed precip possible Wed. night- AM Fri. A
 light accumulation of snow could precede the freezing rain
 across the Northern Mountains early Thursday.

*Pattern favors above average temperatures this weekend into
 next week

The cold front will become quasi stationary near the MD line
Wed. night. The wavy frontal boundary will bring periods of rain
into the weekend, with several (to as much as 8) hour period of
mixed wintry precip likely across mainly the Central and
northern mountains Wed. night and Thu morning, thanks to an
increasing and moderately strong northerly, 1000 mb
Ageostrophic flow that will serve to pump shallow/sub-freezing
air south into the region.

Confidence is moderate with respect to the exact ptype details
(and the location and amount of fzra). However, there is at
least some risk for a light snow and/or ice accumulation. We`re
in good agreement with WPC`s Winter Weather graphics ATTM, with
a coating to 2 inches of snow possible at the onset across parts
of northcentral and NW PA, before a changeover to sleet and or
Freezing Rain.

Another round or two or rain is likely over the weekend. We
will need to monitor the potential for flooding especially
across south- central PA where the ground is very saturated from
previous rain/snowmelt and streamflows are well above normal.

&&

.AVIATION /11Z TUESDAY THROUGH SATURDAY/...
Areas of dense fog will impact much of the Susquehanna River
Valley and parts of the Central Mountains through mid morning,
though it may take until late morning until visibilities improve
to VFR over the SE. However, a shallow but strong llvl inversion
could keep ceiling restrictions ongoing there into this
afternoon. The remainder of the CWA is already or will become
VFR by 14z. Though BFD could see sub-VFR conditions return
this afternoon with a warm front nearby. In addition, LLWS will
continue through first half of the day with SW flow just off
the deck around 35-45 kts.

Patchy fog / low ceilings edge back into the Susq Valley from
the east tonight into Wed morning, then give way to VFR
conditions across most of the region. Exeption will be over the
NW Mtns as a sharp cold front will spread rain and restrictions
in from the NW, with restrictions possible in the
afternoon/evening into the central mountains.

That will be followed by several weak waves of low pressure
bringing bouts of rain for the second half of the week, and even
some light freezing precipitation Thursday morning.

Outlook...

Wed...Restrictions likely in rain showers mainly NW half.

Thu...Widespread restrictions in rain south/wintry mix north.

Fri-Sat...Widespread restrictions in periods of rain.


&&

.HYDROLOGY...
Recent rainfall and snowmelt has resulted in very saturated soil
and well above normal streamflows across much of the area
especially south-central PA. Rainfall of 0.50 to 1.00 inch is
forecast between Wednesday and Thursday night, with additional
rainfall likely into/over the weekend. River flood potential
(including small streams, creeks and tributaries) is above
normal and we will continue to monitor the risk in the coming
days.

&&

.CLIMATE...
Record highs for selected sites for Tuesday and Wednesday
February 20th and 21st:

MDT: 72/1930;71/1997
IPT: 67/2016;69/1930
BFD: 58/2016;64/1997
AOO: 62/2016;71/1997

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 10 AM EST this morning for PAZ049>053-
057>059-066.

&&

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



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