textproduct: Pittsburgh

SYNOPSIS

A very active pattern over the next few days, with multiple rounds of showers and storms. There will be flooding and severe storm concerns through the weekend.

NEAR TERM /THROUGH TODAY/

KEY MESSAGES:

- Another round of thunderstorms, possibly severe, overnight. - A Flood Watch is in effect for portions of eastern Ohio. - A second round of showers and storms returns late in the day. -------------------------------------------------------------------

Biggest forecast headache overnight is timing of the line of storms and how their structure will change as they move eastward. A strong shortwave trough will move eastward overnight. The wave will continue to support a line of showers and storms that will cross most of the area by shortly after dawn. Latest hires model data is weakening the activity as it moves eastward. This is due to a lack of instability and also, there remains ample warm air aloft which could help to limit the vertical extent of the convection. The area currently with the highest risk for severe storms would be over Ohio. SPC has put all of Ohio counties, the Northern Panhandle of West Virginia and Mercer county PA in a severe thunderstorm watch until 7am.

A shortwave trough will quickly move to the east this morning, allowing for a break in the widespread showers and storms. The trough will push a surface boundary southward through the region but it will eventually stall over the area as minor upper level height rises occur the afternoon. This stalled front will be the focus point for more activity late in the day and into the evening hours, as another shortwave moves northeastward across the region in the southwest flow aloft. A second large moisture plume will accompany the wave.

SHORT TERM /TONIGHT THROUGH SATURDAY/

KEY MESSAGES:

- Showers and thunderstorms continue through the rest of the work week as a front remains stalled over the area. - A Flood Watch remains in effect for portions of eastern Ohio. -------------------------------------------------------------------

The front is progged to drift south of the area tonight. However widespread showers and storms should continue overnight as a shortwave continues to pull a plume of deep moisture northward. Some of the model data is showing a weak surface reflection developing underneath the wave which will move across the area late tonight and Friday morning. Eventually, the moisture plume will exit the area on Friday and there should be a decrease in shower coverage from north to south as Friday's activity will become more focused near the stalled surface front.

Yet another surface low is then expected to track NE out of the Mississippi Valley region Friday night, pushing the front back north this time as a warm front. Showers and thunderstorms once again increase in coverage as the front and surface low move across the Upper Ohio Valley region Friday night into early Saturday. However, if the front moves far enough north, we may see a break in the activity late Friday night and Saturday morning as the main moisture plume will be forced northward as well.

Amounts through the day on Saturday look to be highest from Pittsburgh and north with forcing along the warm front and enhanced convergence on the nose of an impinging 850 mb jet. We'll briefly reside in the warm sector before the cold front comes through in the latter half of the day bringing yet another enhancement in rain coverage along it. The severe weather potential should be limited due to weak instability, however with repeated rounds of heavy rain and a saturated ground, flooding potential will increase, especially with rises on area rivers.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

KEY MESSAGES:

- Showers/thunderstorms with potential for heavy rainfall continue this weekend. - A cold front moves through Sunday finally bringing an end to the heavy rainfall threat. - Colder next week with lighter rain/snow chances each day. -------------------------------------------------------------------

Showers and thunderstorms continue Saturday night as the surface low tracks through the area, yet is still unable to effectively move the stalled frontal boundary. Ensembles suggest another heavier period of rain on the nose of another low level jet Saturday night into Sunday morning falling on a then very saturated ground. However, ensemble clusters all agree on a deep upper trough swinging across the eastern CONUS on Sunday into Monday which will finally shunt the surface ridge that had been firmly planted over the southeast CONUS coast and allow a cold front to advance through the area and off to the east bringing an end to the heavy rain and storm potential. There's some discrepancy in the timing of the upper pattern and how quickly the trough axis arrives with the cluster dominated by the GEPS suggesting a slower cessation of the rain in the Monday morning hours.

The upper trough is then expected to persist across the NE CONUS early next week, resulting in temperatures dipping to below normal values and daily rain/snow chances, albeit not nearly as intense as what we're experiencing this week.

AVIATION /05Z THURSDAY THROUGH MONDAY/

Residual showers and isolated thunderstorms ahead of an 850mb warm front are expected to decay under the influence of broader subsidence 2-3 hours after TAF issuance. VFR with 4-8kft ceilings are expected through 06z within the warm sector of an incoming low pressure system; development of a strong ~50kt llvl jet is likely to remain above the inversion (and llws height threshold) but enough sub-inversion mixing may foster SSE gusts between 20-30kts.

A well-developed QLCS complex of showers/thunderstorms will weaken as the enter the region after 06z, but uncertainty is fairly high on initial intensity as it reaches ZZV and its ability to maintain through western PA. TAFs noted that a likely brief wind shift out of the west will occur with the line before southerly synoptic winds prevail; thunderstorm intensity will dictate if higher speeds can occur (more likely at ZZV and trending down eastward).

Stagnation of the cold front driving the convective system is likely somewhere near/south of KPIT, which may allow for a brief period of MVFR cigs as moisture converges on the stationary boundary. Brief dry weather and mixing will improve cigs 12z-18z before additional shortwave movement aloft and any trailing sfc boundary offers lower probability showers. Timing of these showers and a more potent upper shortwave Thursday night creates greater variability in precipitation timing and restriction potential.

Outlook... The Upper Ohio River Valley will remain positioned between a Central Plains trough and Atlantic ridge that will foster multiple rounds of moderate to locally heavy rain through Monday morning. Timing and shape in each wave remains variable, which may alter the axis of highest precipitation rates and areas of greatest restrictions. Aviation customers should anticipate limited windows of VFR and dry conditions through the period as the pattern favors MVFR/IFR restrictions, heavier rain showers and occasional thunderstorms.

A deepening upper trough may overtake the region early next week, shifting towards a colder pattern featuring periodic rain/snow chances and continued ceiling restrictions.

HYDROLOGY

The potential for heavy rainfall will exist beginning tonight and continuing into Sunday. At this time, areas across Ohio have the potential to see the heaviest rainfall, where a Flood Watch has been issued. Current NBM values indicate a 60 percent chance or greater of at least 2 inches of rain over a 72 hr period in this region. Lesser probabilities exist further east, though a trend up has been noted in the latest ensemble guidance and will need to be monitored for any potential expansion of the flood watch. The exact location of the heaviest rain will depend on the position of a quasi-stationary surface front. Precipitable water is expected to remain elevated through Sunday, with some periodic jet enhanced ascent. Even after the heavy rain potential ends, river levels will likely remain elevated for a few days. Current ensemble river forecast projections also focus the most significant river rises across Ohio.

Will monitor further trends for any future adjustments to the watch.

PBZ WATCHES/WARNINGS/ADVISORIES

PA...None. OH...Flood Watch through Sunday morning for OHZ039-040-048-049- 057>059-068-069. WV...None.