Area Forecast Discussion

WHAT HAS CHANGED

* Increasing confidence in severe weather this afternoon and evening * WPC has included the Laurel Highlands in a Slight Risk for excessive rainfall

KEY MESSAGES

1) Severe weather is increasingly likely this afternoon and evening.

2) Warmth surges again next week with a classic summerlike pattern expected - though uncertainty remains.

DISCUSSION

KEY MESSAGE 1: Severe weather is increasingly likely this afternoon and evening.

An upper shortwave trough diving out of the Great Lakes region will bring an increasing chance for showers/t-storms this afternoon and evening. SPC maintains a level 2/5 (SLGT) severe t-storm risk, which includes all of central PA. Of note, the wind probabilities are 30 percent south of Route 6 (higher than the 15 percent probabilities that we see with most Slight risks).

Latest hires guidance continues to show spread with respect to the number and timing of convective elements through the day Saturday. Radar trends do show morning rain showers upstream of our NW zones, but anything before 1 PM will likely be nonsevere.

The highest chance for severe weather will be in the late afternoon and evening hours, sparked by a boost of larger scale ascent linked to the thermally direct, right entrance region of a robust mid and upper level jet segment as well as positive vorticity advection aloft. SBCAPE climbs to near 1500 J/kg this afternoon and early evening with sfc-6 km shear of 30-40 kts. The primary triggering mechanism for convection initiation will likely be a prefrontal sfc trough set up just inland from Lake Erie, as well as any cold pools from preexisting convection moving in from the west. Hodographs are primarily straight (west to east) and may support splitting supercells early on, but as cold pools grow/merge we anticipate upscale growth into line segments with preferred storm motion towards the southeast.

Damaging winds and large hail will be the primary severe hazards, but a tornado can not be ruled out particularly in western zones where there is a little more turning in the low levels of the atmosphere. Large hail would also be favored more in the western zones, especially in any supercells that may develop prior to linear modes. In terms of wind, several of the HRRR runs over the past 24 hours have shown fairly large bow echos developing with wind gusts of 50-65 mph embedded within them, but there are considerable run to run differences in the track of these convective elements.

Model guidance remains generally dry on Sunday, with much of the precip ending around midnight tonight. Rainfall totals are expected to be highest in the west and southwest (up to 1 inch), with storms producing less rainfall over the southeast as the evening wears on and daytime heating is lost. This pattern does not bode well for ongoing drought conditions. There is a WPC SLGT ERO in the Laurels, but dry antecedent conditions will likely offset the flood potential.

Patchy fog will be possible late tonight into Sunday morning as clouds clear out behind the cold front and the wet ground allows for sfc RH near 100 pct in spots. Key limiting factor will be how quickly the winds diminish as skies clear out.

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KEY MESSAGE 2: Warmth surges again next week with a classic summerlike pattern expected - though uncertainty remains.

Following a modest cool down on Monday, emerging model data continues to signal above normal temperatures for the rest of next week. Even the cool down on Monday will see above normal high temperatures for most of the area. Long range models diverge on potential solutions toward the end of next week. A prevailing ridge in place over the eastern US will ensure above normal temperatures (80s and 90s), but models diverge in how they handle an upper closed low moving out of the South Central US up towards the Great Lakes. Latest NBM has 20-40 pct PoPs covering at least a portion of central PA each day Tuesday through the end of the coming week, but realistically much of that time period may end up dry if the unsettled weather with the aforementioned upper disturbance passes to our west.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Confidence has increased regarding the potential for thunderstorms across the southern terminals (KJST/KAOO/KMDT/KLNS) as an MCS tracks eastward from the Ohio Valley. MCS maintenance is supported by adequate MLCAPE (around 1000 J/kg) / line-normal bulk shear (20-30 kts). Although the extent of strong wind gusts remains uncertain, conditions are supportive of severe-level wind gusts (50+ kts) embedded within this line.

A cold front dropping across the region through this evening will bring additional showers/storms, with convection related to this front primarily handled by PROB30s for the 18Z TAF issuance due to uncertainties regarding timing/impacts. Nevertheless, gusty winds may occur with these storms as well, including locally severe-level wind gusts.

In the wake of these storms, clearing skies and lighter winds may support mist/fog development overnight, especially across areas where rain had fallen. Flight restrictions are most likely across KBFD/KJST due to the combination of upslope flow & low-level moisture. Fog/stratus will dissipate after 12-15Z Sunday as the boundary layer mixes out, followed by a SCT VFR cumulus deck + post-frontal northwesterly winds at 10-15 kts gusting to around 20-25 kts during the afternoon.

Outlook...

Sun-Tue...VFR favored outside of isolated morning fog. Restrictions possible in -SHRA/-TSRA early Sun & again late Tue.

Wed-Thu...Restrictions possible in -SHRA/-TSRA.

CTP WATCHES / WARNINGS / ADVISORIES

None.