textproduct: Raleigh

WHAT HAS CHANGED

* A Heat Wave is expected Wednesday through the July 4th weekend. A potentially dangerous and extended heat wave is possible.

* There is a Marginal Risk of severe storms on Saturday for most of central NC, except north and east of Raleigh where Slight Risk is located.

KEY MESSAGES

As of 1245 AM Friday...

1) A Heat wave is expected Wednesday through the July 4th weekend. A potentially dangerous and extended heat wave is possible.

2) Thunderstorm chances return today and remain elevated through the weekend, with a severe weather threat Saturday.

DISCUSSION

As of 225 AM Friday...

KEY MESSAGE 1... A Heat wave is expected Wednesday through the July 4th weekend. A potentially dangerous and extended heat wave is possible.

There will be a lot more discussion on the heat topic in the coming days, but suffice to say the time is now that preparations are made for a potentially dangerous and extended heat wave beginning Wednesday, July 1st, then peaking the weekend of July 4th.

Models forecast an anomalous mid/upper ridge over the east-central United States beginning mid next weak, then peaking on possibly the July 4th weekend. For North Carolina, the forecast mid/upper ridge axis is expected to be over the OH valley south to the TN Valley next week. This is historically the favored location of the mid/upper ridge positions that have brought past heat waves to our region.

In addition, forecast temperatures aloft in the H85 range are currently projected to be near or exceed record high values. This combined with the 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.

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, with 90 still possible at midnight.

The specifics will be ironed out in the coming days. However, begin prepping for excessive and dangerous heat beginning Wednesday, July 1.

KEY MESSAGE 2... Thunderstorm chances return today and remain elevated through the weekend, with a severe weather threat Saturday.

After several days of predominantly dry weather, convective chances will return to central NC this afternoon and evening as weak mid- level perturbations track northeast across the Carolinas within modest west-southwesterly flow aloft. Differential heating and a developing inland-moving sea breeze should provide additional low- level convergence, supporting scattered showers and thunderstorms during the afternoon and evening hours today.

A stronger shortwave trough will traverse the Ohio Valley on Saturday while an approaching cold front and pre-frontal trough advance toward the Appalachians. Model guidance remains in good agreement in increasing instability, with afternoon MLCAPE potentially exceeding 1500 J/kg beneath steep low-level lapse rates and a deeply mixed boundary layer. At the same time, effective bulk shear increases into the 25 to 35 kt range, supporting better storm organization than today. SPC continues to maintain a Marginal to Slight Risk for severe thunderstorms across central NC on Saturday (Slight northeast of RDU with Marginal elsewhere) with the primary severe hazard will be damaging straight-line wind gusts, aided by inverted-V sounding characteristics, large sub-cloud dewpoint depressions, and efficient downward momentum transfer. Thunderstorm coverage should increase during the afternoon and continue into Saturday evening as the pre-frontal convection progresses eastward.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

As of 600 AM Friday...

Through 06Z Saturday: VFR conditions are expected to prevail through much of the period, although scattered afternoon and evening thunderstorms may bring localized reduced flight conditions to portions of central NC, particularly south and east of RDU today. The greatest chance of convection during the TAF period will be at KFAY between 21-03Z. Outside of thunderstorms, southwest winds will increase to 8 to 12 kt this afternoon with occasional gusts around 18 kt before diminishing this evening. Any storms that develop can produce localized outflow boundaries with quick wind shifts. VFR conditions are expected overnight at most terminals, although very patchy IFR fog or low stratus can't be ruled out around daybreak Saturday, especially where evening rainfall occurs.

Outlook... Daily afternoon and evening thunderstorm chances will continue through the weekend as an approaching cold front moves toward the region. The greatest aviation impacts are expected Saturday afternoon and evening, when scattered to numerous thunderstorms may produce periods of sub-VFR conditions, strong and erratic wind gusts, frequent lightning, and heavy rainfall at all central NC TAF sites. Conditions should gradually improve Sunday night into Monday as the front shifts east of the area, with predominantly VFR weather expected for much of next week aside from isolated diurnal convection.

CLIMATE

Record High Temperatures:

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

RAH WATCHES/WARNINGS/ADVISORIES

None.