Area forecast discussion
National Weather Service State College PA
410 PM EDT sun may 19 2013
Synopsis...
high pressure over northern New England and the mouth of the St.
Lawrence late today will slide to the east overnight. A deep
southerly flow in the wake of this high will bring increasing warmth
and humidity to the region through the middle-week period...with well
above normal afternoon temperatures Tuesday and Wednesday. Scattered
showers and thunderstorms will occur each afternoon through Thursday
as a storm system currently over the Central Plains inches eastward.
Temperatures will cool back below normal with drier conditions for
Friday and Saturday.
&&
Near term /until 6 am Monday morning/...
a moist southeasterly flow near the surface and plume of anomalously
high precipitable water air /1.5-1.75 inches/ moving into the region from the
south will continue to lock in thick cloud cover across practically
all of the County Warning Area for the rest of today.
Portions of the northwest mountains will continue to see periods of sunshine
through the remainder of this afternoon...within the very tight
gradient of 925-850 mb Theta-E...and aided by low level downsloping to
the west of The Spine of the Appalachians.
The base of the thick stratus/stratocu deck will lift very little
from their 18z levels and continue to shroud the higher ridge tops
through this evening....before gradually lowering down the slopes
similar to last night.
Weak forcing in the form of very gentle isentropic upglide from the
south to south-southwest within the i300-310k Theta channel will help to produce
a few light showers /or more likely areas of light drizzle/ that
will drift north-northeast across the forecast area this afternoon through tonight.
The clouds will keep afternoon temperatures on the cool side and well below
normals across most of the area. Just those lucky few places over
the northwest mountains could meet or exceed climatology highs. Maximum temperatures will
be mainly in the middle 60s...but should hold at or below 60f across some of
the eastern mountains
This blanket of clouds /and relatively high surface dewpoints...in middle
50s to around 60f/ will keep temperatures 10-15f above climatology normals
overnight.
Any measurable rainfall tonight will be limited to several
hundredths of an inch or less.
&&
Short term /6 am Monday morning through Monday night/...
upper ridge will be in place from the Ohio Valley to the Gulf Coast.
500 mb heights will rise slightly /by 10-20m/ during the day
Monday...as the mean /and initially southerly/ 925-850 mb flow veers
around to the west-southwest by the midday hours Monday.
Lingering...early morning low clouds and fog should be eroded rather
quickly between 14-17z Monday...with partly sunny skies in most
areas for the afternoon hours. Deep vertical mixing of the warm
/plus 14-15c/ air at 850 mb will yield above normal afternoon temperatures
in the middle 70s to around 80f.
Some of the deeper valleys east of the Laurel Highlands...where the
greatest compressional warming will occur via an orthogonal low level west-southwest
flow...could soar into the lowers 80s if ample clearing occurs and
scattered showers/thunderstorms and rain hold off until after 20z.
Probability of precipitation for scattered showers and isolated thunderstorms and rain for Monday afternoon will be
just under 30 percent across the northwest zones...and 40-45 percent in the
southeast.
Weak low pressure aloft will finally disappear Monday night as it
passes overhead. Upstream convection will be dropping across the
Great Lakes and will only prolong the cloudy/showery forecast for
late this period.
&&
Long term /Tuesday through Sunday/...
On Tuesday...we should enter the warm sector ahead of the storm
languishing over the plains/Midwest/upper lakes. The warm sector
air will be quite warm for the time of year...having spent plenty
of time cooking over the MS and Ohio valleys with good Gulf inflow.
Temperatures there right now are about 10f above averages. Therefore...the
temperatures should round out to 80f or better in many locations Tuesday.
Along with the warm temperatures will be moist air with dewpoints
in the 60s. This warm...moist air...along with a weakening ridge
will create a good situation for scattered afternoon convection.
The medium range guidance continues to advertise a rather
unsettled period for much of the upcoming week.
We are and will remain basically under a mean upper ridge...but
the problem will be a weak frontal system that will be meandering
through central PA for much of the time into late week. So while
there is support aloft for organized precipitation...the showers will be
tied to small scale features that will be hard to see more than a
day or two down the Road. The low level forcing and increasing
humidity/instability will bring several chances for mainly diurnal
showers and thunderstorms.
The forecast has a lot of redundancy in it...implying a prolonged
period of rain/storms. The reality is likely to be more like a
period of mainly cloudy skies but with sunny breaks during the
day...along with showers that will be most likely in the afternoon
and evening. A diffuse front will attempt to push northward during
the middle of the week. Where the front will be on any given day
will have much to do with mesoscale influences from any convection
that manages to form.
Despite ample cloud cover and the chances for rain...temperatures
should remain above normal until at least late week when guidance
suggests we finally see a cold front move through ushering in
much cooler air and drier conditions.
&&
Aviation /20z Sunday through Friday/...
a wavy frontal boundary just to our south will slowly return
northward across Pennsylvania as a warm front late today into Monday.
Widespread IFR south /and low-end MVFR ceilings central and north/ with
generally 7-10sm visibilities will continue through the afternoon hours.
Lowest conditions will be found across the southern third of Penn.
Ridge tops at or above approx 1900 feet mean sea level will continue to be shrouded in
the low clouds/fog through at least early this afternoon.
Areas of drizzle and isolated to scattered -shra will be moving back
toward PA from the south this afternoon as well. The highest
probability for the light quantitative precipitation forecast will be across the southern PA taf
airfields /kjst/lns/MDT/cxy.
After some minor...brief improvement to ceilings and visibilities this
afternoon...conditions quickly slide down to widespread IFR
beginning later this evening and continuing until 13-15z Monday.
A rather unsettled and occasionally wet pattern is expected for
Monday afternoon through Thursday...with daily chance/S for convective
precipitation focused during the afternoon/early evening /and associated
restrictions/.
Outlook...
Mon-Thu...areas of early morning IFR conditions in stratus and
fog...transitioning to VFR-MVFR with scattered PM thunderstorm impacts
each day.
Fri-Sat...mainly VFR. Patchy MVFR to IFR ceilings possible 09-13z both
days.
&&
Ctp watches/warnings/advisories...
none.
&&
$$
Synopsis...Lambert
near term...Lambert
short term...dangelo/Lambert
long term...la corte/ceru
aviation...Lambert
