Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
854 am EDT Tuesday may 26 2015
high pressure at the surface and aloft will remain just off the East
Coast through the end of the week. A cold front will likely push
across the region next weekend...then stall just south of the
state early next week.
Near term /until 6 PM this evening/...
deep south-southwesterly flow on western periphery of the
dominant coast ridge will transport Gulf of mex moisture
northward into central PA early this morning. Sat-derived precipitable waters near
1.7 inches across northern/western PA at 09z. Regional radar at
1230z showing isolated-scattered rain showers developing in this moist
air mass in advance of a middle level vorticity maximum lifting NE out of
southern Ohio. Based on radar trends and latest hrrr output...will
carry chance of rain showers across the northwest mountains this morning. For areas
south and east of kjst/kunv/kipt...the rest of the late morning
hours should remain dry.
After the passage of the shortwave later this morning...lg scale
forcing becomes quite very weak. However...heating of moist air
mass with precipitable waters 2-3sd above climatology should yield scattered diurnally-driven
rain showers/thunderstorms and rain this afternoon with latest NAM and sref mean surface-convective available potential energy maxing
out around 1000 j/kg. The best chance for convection will likely be
across the central mountains and middle susq valley...where 50 percent probability of precipitation
painted. Early clouds/showers across the northwest should result in less
instability and a diminished chance of convection up there. Across
southeast PA...warmer temperatures aloft should help suppress convection
over the lower susq valley.
Mean sref/gefs convective available potential energy top out near or slightly less than 1000 j/kg
this afternoon and shear not particularly strong. Thus...Storm Prediction Center outlook
keeps the marginal risk west of our County Warning Area.
Early am infrared loop suggests areas south of I-80 will dawn mostly sunny
today. However...a good deal of cumulus should develop by midday based
on model soundings. Temperatures are likely to be a bit cooler than Monday
due to more clouds...with maxes from the u70s over the Allegheny
plateau...to the m80s across the lower susq valley.
Short term /6 PM this evening through 6 PM Wednesday/...
diurnally-driven convection should die off after sunset this
evening. The rest of the night will likely feature warm and
tranquil conds. High precipitable water air mass will result in middle Summer min
temperatures...with lows in the M/u60s most places.
A potent shortwave lifting through the Great Lakes will introduce a
better chance of rain showers/thunderstorms and rain on Wednesday. Model timing of this feature
suggests showers could arrive early in the day across the west
counties...with a greater risk of PM thunderstorms and rain across eastern PA. Gefs
mean convective available potential energy peak around 1200 j/kg across the southeast counties Wednesday afternoon.
However...middle level winds and associated deep layer shear not particularly
strong in model data...so severe weather threat just marginal. Mcldy skies will
hold temperatures down a bit...but consall still suggests high temperatures between
the m70s-l80s...several degree above normal for late may.
Long term /Wednesday night through Monday/...
high pressure at surface and aloft just off the East Coast should keep
warm/humid air over central PA through the end of the week. Passage
of middle level shortwave on Thursday should support additional scattered
rain showers/thunderstorms and rain Thursday. By Friday...bulk of medium range guidance implies
drier weather /esp northern PA/...as lower precipitable water air mass works into
central PA in associated with high pressure working across S Ontario into north
A cold front will likely bring an end to the summerlike
warmth/humidity by next weekend. A fair amount of agreement among
medium range guidance that a period of rainy weather could follow late
Sunday/Monday as wave of low pressure rides eastward along stalled
frontal boundary south of PA.
Aviation /13z Tuesday through Saturday/...
isolated shower activity has developed along a SW to NE line from
near kjst to north of kipt. Activity is light and should not
cause a direct impact. However...moisture from the showers will
lead to lowered ceilings over the next couple of hours across that
VFR should continue at most sites over the next 24 hours. Highest
confidence in brief 1-3hr period of 3-6sm visibility restrictions
in br/haze is at MDT/lns around daybreak Tuesday. Given relatively
low probability/expected coverage...decided not to explicitly
mention thunderstorms in terminals at this time. Later issuances should
introduce a thunderstorms in the vicinity from group with either the 06 or 12z issuance.
A Summer-time pattern will persist through the week with replay of
patchy morning fog giving way to VFR and afternoon isolated-scattered thunderstorms. A
more defined thunderstorm risk should arrive by next weekend as a cold
front presses southeastward from the Great Lakes.
Tue-Fri...patchy a.M. MVFR fog/br. Isolated-scattered p.M. Thunderstorms.
Sat-sun...patchy a.M. MVFR fog/br. Scattered-numerous p.M thunderstorms west/cold