textproduct: Pittsburgh

WHAT HAS CHANGED

Confidence in severe weather occurrence continues to rise for Sunday. However, some timing and placement uncertainty continues among the various model guidance. There is also some slowly increasing coalescence around the timing of our next good rain chance on Thursday.

KEY MESSAGES

1) Severe thunderstorms remain a good possibility ahead of a crossing cold front Sunday, with hail, wind, and tornado threats all in play.

2) Mostly quiet weather Monday through Wednesday, with the next shower and thunderstorm chance around Thursday.

DISCUSSION

KEY MESSAGE 1... While some exact details remain questionable, there remains a good signal for severe weather Sunday afternoon and evening. All severe threats remain in play, although large hail and damaging wind chances will dominate, with a lesser but still present tornado threat.

Surface low pressure crossing the Great Lakes provides a warm frontal passage tonight, introducing a warmer airmass to the region. This will be followed by a cold frontal passage during the late afternoon and evening hours that will serve as a focus for thunderstorms. 850mb temperatures will likely rise into the 13C to 15C range by 12Z south of the approaching cold front, which support high temperatures well into the 70s south of US-422, and even in the 80s south of the Mason-Dixon Line. Increased moisture (precipitable water of 1.0 to 1.2 inches just ahead of the front) helps to drive dewpoints into the mid to upper 50s during the afternoon, helping to push surface-based CAPE into the 750 to 1500 J/kg range. Much of this instability lies in the favorable -10C to -30C layer for hail growth, with mid-level lapse rates of 7C to 7.5C present as well. 700mb WSW wind of 50-60 knots will contribute to 0-6km shear of 45 to 55 knots ahead of the front, along with a slowly veering vertical profile. These parameters would be supportive of supercell structures.

Much of the uncertainty for this event comes from:

1) Timing, as the various CAMs still disagree on when convective initiation occurs. This timing generally ranges from about 18Z to 22Z, and this has implications for the placement of the highest threat corridor depending on frontal progression. Right now, most outcomes would overlap the 4 PM to 8 PM period, so this could currently be considered the most likely timing of any severe weather. In most cases, the areal placement of the greatest threat would lie from near Pittsburgh and south.

2) The amount of warm air/inhibition in the lower and mid levels. Some HREF members suggest a warm layer around the 800mb level that may act as a hindrance to surface-based convection initially. Also, temperature/dewpoint spreads ahead of convection may be as large as 20F. In this case, large hail would the highest threat with any initial discrete storm development given the shear and favorable CAPE profile, with potential wind and tornado threats possibly ramping up a bit later once any capping is eroded or (in the case of straight- line wind) when storms begin organizing into line segments. This is considered to be the more likely scenario at this time. A less-likely scenario involves less warm air aloft, allowing for more surface-based convection at the start. In that case, wind and tornado threats would be higher and more widespread from the beginning of the event.

In any case, the overall confidence in severe weather occurrence continues to increase, and at least one area Severe Thunderstorm and/or Tornado Watch seems likely tomorrow. We will also need to monitor the potential of training thunderstorms, as mean storm motion should be fairly parallel to the crossing cold front. Thus, isolated heavy rainfall totals (exceeding 1.5 inch) and localized water issues cannot be ruled out.

Storm intensity should begin to ramp down by the mid to late evening with diurnal stabilization, although convection is likely to continue along and ahead of the cold front until it exits the region to our south after midnight.

KEY MESSAGE 2... Largely quiet weather follows for the Monday to Wednesday period. Although lake-850mb delta T values appear marginal, the passage of a shortwave to our north may support some isolated lake-enhanced showers north of Pittsburgh on Monday. PoPs may need to be added if confidence in this increases. In the wake of the Sunday night cold front, temperatures start out below- normal on Monday, then will likely moderate to and just above climatology by Wednesday.

The next chance of showers and thunderstorms may arrive on or around Thursday, although ensembles differ on timing currently. Given a warm surge followed by another cold frontal passage during this period, this may represent our next chance for at least isolated severe thunderstorms. Extended machine-learning guidance from CSU and NCAR support this idea.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

VFR will prevail for the TAF period with only high clouds and perhaps a brief scattered VFR deck late morning and early afternoon.

There is medium confidence in a restrictions making their way to FKL/DUJ in the 12Z to 18Z window tomorrow as the next system encroaches, so opted to message in TAFs. Restriction chances seem quite confined to these ports, with chances less than 30% elsewhere (less likely to the south).

Thunderstorm chances increase area-wide in the 18Z to 00Z window as a cold front passes. Strong gusts exceeding 50kts and large hail are possible in the most intense storms. Likelihood of hail/wind increases from north to south. Brief restrictions in vis/cig are also likely in any more intense rain.

Outlook... Restrictions will likely linger into Monday morning before VFR returns Monday afternoon.

PBZ WATCHES/WARNINGS/ADVISORIES

PA...None. OH...None. WV...None.