textproduct: Raleigh

WHAT HAS CHANGED

* There is a Marginal/Slight Risk of severe storms Today and Sunday.

* A heat wave continues to be expected Wednesday through the July 4th weekend.

KEY MESSAGES

As of 305 AM Saturday...

1) Severe thunderstorm risk both today and Sunday.

2) A dangerous heat wave is expected Wednesday through the July 4th weekend.

DISCUSSION

As of 305 AM Saturday...

KEY MESSAGE 1: Severe thunderstorm risk both today and Sunday.

The primary weather concern this weekend is the risk of afternoon and evening severe thunderstorm activity. The setup for this risk today includes afternoon temps rising into the lower 90s, dwpts in the lower to mid 70s, MLCAPE generally 2000+ J/kg, weak but adequate 20-30 kt deep-layer shear, and forecast DCAPE AOA 1000 J/kg. The timing of today's storms depends largely on ongoing upstream storm activity across the Ohio Valley. As those storms move east, look for new convection to develop eastward along outflow boundaries. Storms will arrive or develop by mid-afternoon over the western Piedmont before expanding eastward and moving through the Triangle and Coastal Plain regions during the late afternoon and evening. Given the aforementioned setup, the primary severe threat remains damaging straight-line winds (50-70 mph), localized wet microbursts, and marginally severe hail with the strongest cores. Storms will move east of central NC during the later evening hours and should be followed by a mainly dry overnight period given a post-storm stable low level airmass that will be place. All of central NC is in a slight (level 2 of 5) risk for severe storms today per the latest SWODY1.

Sunday appears to present a somewhat different convective evolution, yet still having a possible severe risk. While mid-level heights begin rising as the lower Miss ridge expands, both the GFS and ECMWF continue to indicate that a weak shortwave riding around the northern edge of the ridge will accompany a decaying MCS moving southeast from the Ohio Valley into Virginia during the morning. The main question is to what extent this convection survives into our area. If morning convection weakens substantially, cloud cover should erode sufficiently to allow another strongly unstable afternoon with MLCAPE exceeding 2500 J/kg. If sufficient recovery occurs, organized multicells could develop during the afternoon, particularly across northern and eastern portions of central North Carolina. Damaging winds again appear to be the main concern due to large sub-cloud lapse rates and high DCAPE. Heavy rainfall is also possible given precipitable water values approaching 2 inches. Nearly all of central NC (with the exception of our far western zones) is in a Marginal (level 1 of 5) risk for severe storms on Sunday per latest SWODY2.

KEY MESSAGE 2... A dangerous heat wave is expected Wednesday through the July 4th weekend.

Models continue to forecast an anomalous mid/upper ridge over the Ohio/Tennessee Valley/central Appalachians beginning mid next weak, then peaking on possibly the July 4th weekend. This is historically a favored location of the mid/upper ridge position that have brought past heat waves to our region.

The combination of strong subsidence, lack of clouds, dry ground, and a downslope low and mid level flow - could potentially yield a near historical heat event bringing very dangerous conditions. Some guidance is now forecasting a few disturbances in the NW-N flow aloft riding into the region by Friday and Saturday the 4th. This may be enough to trigger either widely scattered or scattered (20-30 percent chance) of afternoon or early nighttime thunderstorms perhaps bringing some temporary cooling. Models are also hinting at the ridge breaking down and shifting west beginning the 4th or 5th. We typically end heat waves with some strong thunderstorms before the cooling.

Bottom line... Begin preparations now for the potential for highs possibly 100+ for 3-4 consecutive days, daytime apparent temperatures possibly peaking at 105+ in the Triad, and 105-112 elsewhere Wed-Sat. To make matters worse, overnight lows may have a hard time dropping below 80, especially in urban areas.

Outlook

Generally VFR conditions are expected into Monday and Tuesday.

CLIMATE

Record High Temperatures:

July 1: KGSO: 99/2012 KRDU: 103/2012 KFAY: 104/1959

RAH WATCHES/WARNINGS/ADVISORIES

None.