textproduct: Raleigh

WHAT HAS CHANGED

* Waves of showers and thunderstorms expected; however, models including the HRRR, NAMNEST, and others have done an extremely bad job with this pattern in convective initiation and general location, timing, and amounts. Thus, we will not have high confidence in this forecast when it comes to specifics. Yet, it is a pattern that supports flash flooding potential.

* A Flood Watch will be issued for portions of the southern and eastern Piedmont, Sandhills, and Coastal Plain given the potential for 3-6 hour very heavy rainfall rates late this morning through the evening.

* The threat of severe thunderstorms along and south of the boundary has increased, especially SW of RDU this afternoon.

KEY MESSAGES

As of 155 AM Friday...

1) A Flood Watch will be issued for a good portion of Central NC, except for the Northwest Piedmont (Triad Area) east along the NC/VA border.

2) There is a Slight Risk of severe thunderstorms today into this evening, mainly over southern portions of central NC.

DISCUSSION

As of 155 AM Friday...

KEY MESSAGE 1... A Flood Watch will be issued for a good portion of Central NC, except for the Northwest Piedmont (Triad Area) east along the NC/VA border. Timing most likely will be 16z today through 12z/Saturday.

The main synoptic boundary was located over our region early today, analyzed from near the Tidewater of VA southwest through the northern Triangle Area and southwest to Charlotte. A couple of clusters of showers and thunderstorms have been developing ENE along the boundary from CLT to near RDU. GSP recently had Severe Thunderstorm Warning around Charlotte. Another, much larger complex of thunderstorms was located over NE Alabama into GA and southwestern NC, moving NE. Current PWs were running between 1.8 and 2 inches along and south of the boundary, with a max of 2.2 over NE AL. This complex should continue moving NE as the mid/upper trough approaches the southern Appalachians this morning. PWs will increase to 2 to 2.5 inches along and ahead of the feature (boundary).

We will place the Flash flood watch along and south of the expected average placement of the boundary today. The latest CAMS suggest it will drift southward through 12z, possibly sagging through the Triangle. By 18z... it should be somewhere near a line extending from near Rocky Mount southwest to around our south of Raleigh and southwest to around Wadesboro. As the mid level trough approaches, the boundary may waver back north 25 to 50 miles as a surface wave approaches this afternoon. So, the Flood Watch would be favored mainly south of I-85/I-40 east to near Wilson (southward). Several waves of showers/thunderstorms are expected along and south of the boundary, with showers north. The highest QPF should be found within the Watch area, with 2-4+ inches likely - mostly in 3-6 hours. Some local 5+ inch amounts will be possible.

Most guidance (which again has been unreliable thus far) suggests convection will taper off this evening, with additional showers/storms possible again later tonight (mainly south and east portions of the current watch. We will see.

KEY MESSAGE 2... There is a Slight Risk of severe thunderstorms today into this evening, mainly over southern portions of central NC. This is surrounded by a marginal risk (near and south of the boundary).

Morning cloudiness and convection should taper off along and south of the aforementioned boundary. There should be a rapid development of new showers/storms with the approach of the mid level support late this morning or early afternoon. SPC has upgraded southern portions of our region into the slight risk category as the potential for wet microbursts has increased. Strong heating in the warm sector will lead to very unstable conditions ahead of the wave of low pressure and cold front. Storms will spread east through the warm sector, with the threat of some 60 mph winds and associated torrential rainfall this afternoon.

AVIATION /07Z FRIDAY THROUGH TUESDAY/

As of 250 AM Friday...

24 hour TAF period: A couple clusters of showers and storms are currently moving NE across central NC, with one just to the south of RDU. These should continue weakening over the next few hours. Meanwhile IFR stratus should begin developing shortly, first in the NE before spreading south and west overnight. The highest confidence in this occurring is at RWI, followed by RDU, where LIFR ceilings can't be ruled out. Guidance is still in more disagreement on whether it will get as far south and west as INT/GSO/FAY. So have TEMPO groups for IFR at those sites and prevail it at RDU and RWI. Some reduced visibilities from fog/mist are also possible, but mainly in the MVFR range.

Ceilings will lift to MVFR everywhere by late morning, but they may be slow to lift to VFR particularly in the NE. By early afternoon, numerous showers and storms will then rapidly develop along and south of a boundary stretching across the area, before moving north into the rest of the area. Gusty winds and sub-VFR restrictions will be possible with any storms. Otherwise winds will be largely from the E/NE and light (around 7 kts or less).

The most widespread precipitation will come to an end by about 04- 06z Saturday, but some lingering showers are still possible overnight. Otherwise, expect IFR stratus to quickly overspread the region during the evening and linger into the overnight hours, with LIFR ceilings and IFR visibilities a possibility as winds go very light to calm.

Outlook: Showers and storms with possible heavy rain and sub-VFR conditions will primarily shift to FAY/RWI on Saturday. Drier conditions with less flight restrictions are expected for Saturday night through Tuesday.

CLIMATE

Record High Minimum Temperatures:

August 21: KRDU: 76/2009 KFAY: 77/2005 August 22: KRDU: 74/2017 KFAY: 75/2018

RAH WATCHES/WARNINGS/ADVISORIES

Most of central NC is under a Flash Flood Watch.