textproduct: Raleigh

WHAT HAS CHANGED

* Updated the aviation discussion to reflect the 18z TAFs.

KEY MESSAGES

As of 210 AM Saturday...

1) Scattered to locally numerous showers and storms are expected this afternoon into the early overnight hours and may bring a risk localized urban and small stream flooding as well as strong winds gusts.

2) Showers and storms capable of producing very heavy rain will shift east Monday and Tuesday, with mainly afternoon and evening showers and storms through the remainder of the week.

DISCUSSION

As of 210 AM Saturday...

KEY MESSAGE 1... Scattered to locally numerous showers and storms are expected this afternoon into the early overnight hours and may bring a risk localized urban and small stream flooding as well as strong winds gusts.

Southwesterly flow aloft, and deep moisture characterized by PW values of 2-2.5 inches and 125-150% of normal, currently located over the western/central NC Piedmont as well as near the SC/GA coast will continue be directed across the Carolinas and southern Middle Atlantic ahead of a mid and upper-level trough that will pivot into the OH and lower MS Valleys today and then weaken while drifting into the TN Valley and southern Appalachians through early Mon. Several MCVs will probably be embedded within that swly flow regime across the South Atlantic states and may locally focus ascent along their track.

At the surface, moist, sly/sely flow will continue across the Carolinas, in a moist/warm sector downstream of a ~1003 mb low that will drift across the OH Valley. A differential heating zone is likely to develop over the Piedmont later this afternoon as low overcast erodes from east to west and provide a surface forcing mechanism for locally numerous showers/storms.

Large scale and persistent forcing mechanisms, such as the elongated RRQ of the upper jet streak from the lower MS Valley into the Northeast and southern Canada, will likely remain just west of the forecast area amidst near neutral mid-lvl height tendencies. However, a deeply moist, uninhibited, and conditionally unstable profiles --weakly to moderately so-- will otherwise favor the development of scattered to locally numerous cells amid 25-30 kts of swly flow. A (Marginal) risk of water-loaded, damaging downbursts remains possible but highly conditional as the thermodynamic environment will only promote minimal diabatic enhancement to the downdrafts and associated surface wind gusts; storm organization may be required to produce severe magnitude wind gusts. Although heavy downpours will also result, relatively fast cell motions to the northeast will counter some tendency for back-building and consequently limit the overall risk of flooding. However, given the potential for a favorable alignment of the differential heating boundary, and subsequent outflow reinforcement, to the mean-wind in the 850-300mb layer, repeated cells training over the same area may be possible to produce locally greater rainfall amounts of 2-4+ inches.

KEY MESSAGE 2... Showers and storms capable of producing very heavy rain will shift east Monday and Tuesday, with mainly afternoon and evening showers and storms through the remainder of the week.

The base of the positively tilted upper trough west of the Appalachians will continue to sink south toward the southern Appalachians on Monday, maintaining deep moist southwesterly flow across the region. Precipitable water will continue to exceed 2 inches and be as high as 2.5-2.6 inches across the eastern half of the area. Despite weak lapse rates aloft, dew points in the mid-70s will still support at least some instability, with guidance suggesting 500 to perhaps 1000 J/kg MLCAPE by Monday afternoon. 500 mb flow also increases slightly, and bulk shear may be as much as 20- 25 knots. While the larger-scale forcing is not particularly strong with the slow- moving upper low, models suggest there may be an uptick in perturbations in the southwesterly flow to help kick off scattered to numerous showers and storms, especially across the east. The main threat from convection appears to be the potential training of storm clusters and heavy rainfall given the anomalously moist air mass and nearly unidirectional tropospheric flow. Hi-res guidance and probability-matched mean (PMM) precipitation fields suggest localized areas of 1-3 inches will be possible, although we have seen as much as 4-6 inches with some focused cells in the foothills today.

A similar pattern is likely to repeat across mainly eastern areas on Tuesday and to a lesser extent into Wednesday as the upper low lifts and weakens. Moist flow around the Bermuda high will continue through the end of the week, so diurnal convection will be possible each day.

AVIATION /17Z SUNDAY THROUGH FRIDAY/

As of 120 PM Sunday...

TAF period: While INT/GSO will have MVFR ceilings to start the 18Z TAF period, ceiling heights should not take too long to lift to VFR levels, and several hours of VFR conditions are expected at all terminals. After a lull in shower coverage this morning, expect scattered showers to develop at all sites, with TEMPO groups maintained from the previous forecast during the time period when thunderstorms are most likely (although thunderstorms are possible at any time during the afternoon). As the coverage of showers decreases, widespread ceilings with flight restrictions are expected to develop after dusk. The greatest uncertainty is how far east IFR conditions will extend, and have included FAY in only a TEMPO group for now while all other sites have IFR as prevailing conditions. Have not seen many gusts so far today, and have removed gusts between the lack of observations and the widespread cloud cover that is expected. While there will be some speed variation, the wind will generally be out of the south this afternoon and veer to the southwest overnight.

Outlook: Coverage of showers and thunderstorms on Monday should be similar to today, then expect greater coverage of precipitation on Tuesday and Wednesday. Shower/storm coverage should decrease again on Thursday and Friday to more summer-like scattered. Flight restrictions will be likely each night, especially at any terminal that receives moderate to heavy rainfall the previous day.

RAH WATCHES/WARNINGS/ADVISORIES

None.