textproduct: Raleigh

WHAT HAS CHANGED

* No major changes with the system through Thursday, although forecast highs for Thursday continue to trend down.

* Increased chances for showers and storms Sunday afternoon through Monday night.

KEY MESSAGES

As of 300 AM Wednesday...

1) Periods of widespread showers and scattered rumbles of thunder are expected to shift across the Carolinas this evening and continue through Thursday.

2) Scattered showers midday Saturday will give way to a brief dry spell before a more unsettled pattern brings increased rain chances Sunday afternoon through Monday night.

DISCUSSION

As of 300 AM Wednesday...

KEY MESSAGE 1... Periods of widespread showers and scattered rumbles of thunder are expected to shift across the Carolinas this evening and continue through Thursday.

A positively-tilted mid/upper level trough axis extending from a large gyre over the Hudson Bay to the Four Corners Region this morning will slowly sag southeast and direct a deep fetch of unseasonable deep-layer moisture and strong moisture transport from the eastern Pacific, through the Carolinas, and into the Northeast today into Thurs. Waves of low pressure, and convective perturbations within the southwesterly flow, are expected to ripple along and ahead of a slow-moving cold front to bring periods of increased coverage of showers/storms to the area.

The first wave will come from upstream convection currently over southwest TN which will outpace the northeast advection of richer low-lvl theta-e and unstable air mass as it moves into the Carolinas later this morning. Central NC will remain highly stable during this time and will likely result in an area of decaying stratiform rain with limited chance for measurable amounts. The remainder of Wed will likely remain dry given a diurnal minimum in upstream convection until later this evening as convection once again develops/deepens over the southern TN Valley and advects east-northeast into the Carolinas.

Waves of moderate showers and storms will continue to be directed across the Carolina Wed evening into Thurs morning ahead of a back-door cold frontal passage slowly sagging south across the southern Mid-Atlantic. Precip chances may be negatively impacted by any gravity wave development during this time, which can rapidly erode precip generation in this type of pattern. By Thurs morning, the combination of frontogentical forcing and an area of low pressure rippling along the front will likely bring another round of widespread showers and isolated storms before shifting east of the area by late Thurs evening. Uncertainty in timing of the fropa is resulting in lower confidence in the development of an unstable air mass supportive of surface-based convection Thurs afternoon, which will be needed to realize any severe potential. Strong and increasing shear with height will be highly favorable for storm organization, but conditional on development of deep convection first.

Storm total rainfall through Thurs evening looks to bring much needed rain to the area with the reasonable low-end from the HREF still producing a swath of 0.75 to 1" somewhere over the Carolinas. However, the experimental REFS paints a troubling alternate scenario with reasonable low-end amounts closer 0.25 to 0.5". With the potential for gravity wave development and more quickly eroding shields of precipitation, this scenario can't be ruled out.

KEY MESSAGE 2... Scattered showers midday Saturday will give way to a brief dry spell before a more unsettled pattern brings increased rain chances Sunday afternoon through Monday night.

Surface high pressure will be transient through the first half of the weekend, sliding east as a weak shortwave ripples across the north. With increased moisture lingering across the south, scattered showers are expected to develop, resulting in a 30-40% chance of rain across central NC through midday Saturday. The best coverage will likely be concentrated across southern and eastern counties where moisture profiles are deepest. High temperatures will remain at or just below average, with afternoon highs ranging from the mid 70s to low 80s. By Saturday afternoon, the primary forcing and moisture axis will shift toward the coast, focusing all remaining precipitation along and east of the Coastal Plain and allowing for a brief period of clearing ahead of the next frontal passage.

The pattern becomes unsettled again Sunday into Monday as a frontal boundary lingering across the Southeast begins to lift northward. This feature is expected to merge with a cold front tracking east across the Ohio and Tennessee Valleys and into the Mid-Atlantic region late Sunday night. While model guidance continues to show inconsistencies regarding the exact timing of showers and storms, expect precipitation to begin as early as Sunday afternoon. As PWs surge to nearly 1.25 (well above the daily mean), this unsettled pattern will continue through at least Monday night as the cold front gradually moves over the region. If current trends hold, precipitation should exit the area late Monday night into early Tuesday morning.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

As of 645 AM Wednesday...

VFR conditions are expected to prevail through at least 00z Thurs. Mid-level clouds and light mainly stratiform rain is expected to move into the NC Piedmont between 11-15z, but restrictions are unlikely at this time. Waves and showers/storms will become increasingly likely throughout the late afternoon and evening and continue into Thurs. Sub-VFR restrictions will be primarily driven by convection through the 06z Thurs, with IFR/LIFR more probable behind a cold frontal passage Thurs morning.

Outlook: Periods of showers/storms will likely continue at times throughout cntl NC, ahead of and behind a cold front and surrounding band of IFR-MVFR ceilings that will sag swd and across the forecast area on Thu. A favorable pattern may exist for fog/low-stratus in the wake of the departing rain and clearing skies Thurs night into Fri morning. There will be another chance of showers/storms and flight restrictions with the approach and passage of another frontal system Sun-Mon.

RAH WATCHES/WARNINGS/ADVISORIES

None.