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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

                            
000
FXUS61 KCTP 221829
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
229 PM EDT Wed May 22 2019

.SYNOPSIS...

A ridge of high pressure will drift off the Mid Atlantic Coast 
this evening. A cold front will move through the area Thursday, 
followed by breezy, cooler and dry weather for Friday.

The holiday weekend will bring warmer temperatures, higher 
humidity and a renewed chance for showers.

&&

.NEAR TERM /THROUGH TONIGHT/...

Mid and high clouds are bleeding over the top of the big 
eastern upper ridge. As the surface high moves off to our east, 
bringing the low level flow around to the SE, eventually gusting
10-15 mph.

The HRRR is consistent in bringing the chance of showers back
into my NWRN zones between about 4-6PM. Radar and observations
do show some light showers moving out of OH with rain at ERI as
of 2PM. While most of the region will remain dry, we did 
include a 20-30% POP from mid afternoon into the evening.

The prospects for the overnight a tad less certain. There is a
good chance a dying area of showers will work across, but 
looking at the various flavors of the HREF lends little 
confidence about how big an area actually sees rain by morning. 
I used blended POPs that tracked the best chance of rain from NW
PA down over central PA into my SERN zones by sunrise.

With clouds and showers, lows will only drop back into the 50s.

&&

.SHORT TERM / THURSDAY THROUGH 6 AM THURSDAY/...
After a fair evening with a generally light southerly wind, 
clouds will thicken up from the west late and showers/isolated
embedded TSRA will move into our northwestern zones after 02z 
near the nose of a +2-3 sigma westerly 850 mb wind max and much 
higher PWAT air (30-35mm or +2-3 sigma). Min temps will only
settle into the 50s to around 60F.

&&

.LONG TERM /THURSDAY THROUGH WEDNESDAY/...
The showers will spread further east across the northern half 
of PA through the mid morning hours Thursday (and end 
temporarily over the Lower Susq Valley and Western Poconos for 
about a 4-hour period during the late morning and early 
afternoon), before a rather extensive area of convection 
(highlighted by embedded supercell storms) moves east through 
the entire region near a sfc warm front/lee trough between 
17-23Z Thursday. A cold front will slide SE across the region 
late Thu afternoon and evening, accompanied by another, albeit 
more brief round of showers and TSRA. 

Multiple ingredients are present Thursday afternoon and evening
for a rather significant severe storm outbreak with numerous 
fast-moving supercells and mini bow echoes within short TSRA 
lines. SPC upgraded the previous Slight Risk for Day 2 to an 
Enhanced Risk for much of our CWA. This upgrades seems warranted
as llvl directional shear is impressive and strong leading to 
broad-loop hodographs and the base of 50kt westerly and WNW 
winds only around 5KFT agl during the mid afternoon to early 
evening hours Thursday. 

This is a classic supercell and potential large hail-producing 
environment as storm relative inflow/updraft helicity will be 
unusually high INVOF sfc warm front/lee trough across Ncent PA 
and the Susq Valley during the afternoon hours. Low LCLs under 
3000 FT AGL covering much of Central and Eastern PA and strong 
LLVL shear will lead to 0-1km EHIs of 2-4(m^2/sec^2) near the 
warm front/lee trough over the central and eastern zones between
21Z Thu and 00Z Fri. The threat of several tornado-producing 
supercells will occur across this same region from mid afternoon
to around dusk.

In addition to the anticipated hail and isolated tornado 
threat, the supercells and their strong updrafts will 
occasionally collapse into fast moving mini bow/spearhead echoes
that have the potential for producing localized, straight line 
winds in excess of 75 mph and swaths of wind damage.

Storm total QPF will likely average between 3 and 6 tenths of an
inch given the fast storm motion. Localized one inch amounts are
possible where two or more TSRA occur. 

In the wake of the Thursday's active weather, a slightly cooler
and drier day is in store for Friday. By Saturday the humidity 
will begin to return along with a chance of mainly diurnal 
showers/storms. as a weak shortwave once again races over the 
top of the eastern ridge. We look to get a break on Sunday as 
the 12Z models show a mainly dry day, but by the Monday-Tuesday 
timeframe more showers become possible as we stay in fast flow 
aloft near the top of the subtropical ridge.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
A ridge of high pressure today will slide off the Mid Atlantic 
Coast this evening.

Widespread VFR can be expected today and much of tonight, with 
scattered restrictions possible with a dying area of showers and
thunderstorms that will traverse the area during the evening and
overnight.

Winds starting off light and variable will become S-SE 5-10 kt,
eventually gusting into the teens.

Enhanced weather impacts expected on Thursday from mid- 
afternoon through the evening hours as strong to severe 
thunderstorms impact the region. 

.Outlook...

Thu...VFR into the afternoon, then strong thunderstorms 
developing mid-afternoon lasting into Thu night.

Fri...Ceiling restrictions poss NW early, then mainly VFR.

Sat...Restrictions in vicinity of showers/thunderstorms. Tstms
more numerous north, scattered south.

Sun...Restrictions possible near scattered tstms, mainly west.

&&

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

&&

$$
SYNOPSIS...La Corte
NEAR TERM...La Corte
SHORT TERM...Lambert
LONG TERM...La Corte/Lambert
AVIATION...La Corte