textproduct: Raleigh

WHAT HAS CHANGED

* Heat Advisory has been expanded to cover much of the eastern Piedmont, Sandhills, and Coastal Plain including the Triangle Area, Rocky Mount, Goldsboro, Fayetteville, and Southern Pines.

* Marginal risk of severe thunderstorms, mainly late this afternoon into the evening.

* Flash Flood potential continues to increase for portions of the eastern Piedmont through the Coastal Plain Friday and Friday night. Severe storms also possible Friday.

KEY MESSAGES

As of 255 AM Thursday...

1) Today will be the last day in this series of high heat and humidity. Heat indices of 105 to 109 in the Heat Advisory area.

2) There is a marginal risk of severe thunderstorms late this afternoon into the evening. The highest probabilities of damaging wind gusts are from in the north-central and NE part of the region, from the Triangle to Roanoke Rapids. There is also a threat of a few severe storms within the Piedmont trough this afternoon.

Another threat of wet microbursts Friday afternoon and evening.

3) A wet period is expected for most areas of central NC Friday into Friday night. The highest probabilities of heavy rainfall are in much of the area Friday and Friday evening (east of the Triad region), shifting to the east of I-95 Saturday. Locally very heavy rainfall (2-4+ inches) and a few instances of flash flooding will be possible, especially from the Triangle Area south and east through the I-95 corridor Friday into Friday evening. The Triad should still get 1+ inch of rain for the event.

DISCUSSION

As of 243 AM Thursday...

KEY MESSAGE 1... A Heat Advisory is in effect for a good portion of central and eastern North Carolina through 800 PM this evening. Today will be the last day in this series of high heat and humidity. Heat indices of 105 to 109 in the Heat Advisory area.

There is a pattern change coming, but we will have another day with high heat and humidity ahead of the approaching trough/cold front that will arrive later tonight into Friday. Expect mostly sunny skies for much of the day as a warm front will surge well to our north this morning. It will be very late today before the pre- frontal trough approaches the Triad and northern Piedmont from the north with the threat of a few thunderstorms. Since most of the day will be mostly sunny highs will top out well into the mid 90s outside of the far NW Piedmont where 90 will be likely in the Triad. Low level moisture will be on the increase dew points in the lower to mid 70s, mixing out into the upper 60s NW. Therefore, the heat indices will likely reach advisory criteria of 105 to 109 this afternoon. In addition, the experimental HeatRisk product has expanded the high and extreme risk deeper into the heart of the state. The Heat Advisory will run late morning through 800 PM.

The chance of some cooling thunderstorms will increase from the north late this afternoon and evening as outflow boundaries approach the Triad and Triangle Areas.

Key message 2... There is a marginal risk of severe thunderstorms late this afternoon into the evening. The highest probabilities of damaging wind gusts are from in the north-central and NE part of the region, from the Triangle to Roanoke Rapids. In addition, a lesser threat of a few severe wind gusts will be possible within the Piedmont trough this afternoon.

The Piedmont trough is expected to become enhanced during the afternoon and there should be scattered thunderstorm development within the trough this afternoon with strong heating and increasing instability. A few of these storms may produce damaging wind gusts with peak heating mid to late afternoon.

A more formidable chance of thunderstorms and severe threat will likely exist later today into the evening over north-central and eastern NC.

CAMS continue to indicate late today into the evening as the most favorable timing for scattered thunderstorm development. As a mid-level short wave trough tracks east from the Ohio Valley to the Mid-Atlantic coast through this evening, a 25-35kt SW H85 flow will develop a top the increasingly unstable boundary layer, strongest over the NE part of NC later today.

Thunderstorms that track across MD/VA this afternoon should send outflow boundaries southward into northern and eastern NC this evening. Thunderstorms may either track ESE or develop on the boundaries as they move SE into our region after 500 PM. Some storms may merge into clusters as they interact with the Piedmont trough.

The main threat will be a few damaging wind gusts between 500 PM and 1000 PM.

KEY MESSAGE 3... A wet period is expected for most areas of central NC Friday into Friday night. The highest probabilities of heavy rainfall are in much of the area Friday and Friday evening (east of the Triad region), shifting to the east of I-95 Saturday.

Locally very heavy rainfall (2-4+ inches) and a few instances of flash flooding will be possible, especially from the Triangle Area south and east through the I-95 corridor Friday into Friday evening. The Triad should still get 1+ inch of rain for the event.

Models continue to hone in on east-central NC for a heavy rainfall event Friday into Friday night. This seems to be a sweet spot for 2-4+ inches of rainfall given the cold front settling south into the region, strong convergence, strong and deep instability, and very high PW's of 2.25+ inches. The shear will become weak supporting very slow storm movement and merging cells.

Had there not been a relatively dry several weeks over most of the region, we would have a Flash Flood Watch out for Friday and Friday night for the NE Piedmont into the Coastal Plain. However, antecedent conditions are relatively dry, with FFG values running higher than normal. However, if the heaviest rain falls in urban areas such as Fayetteville or Raleigh, flash flooding appears likely. The models suggest possibly the Triangle or just to the south and east may have the highest risk, yet a swing of 50 miles or so in upcoming forecasts will have significant ramifications to the forecast. Expect a portion of the region to have a Flash Flood Watch issued later today or tonight if models continue to focus on the same east-central portion of NC into SC.

In addition, the strong instability south of the front Friday with MLCapes of 2500+ J/kg and weak shear suggest wet microbursts will be possible, thus a marginal risk of severe storms will exist along and ahead of the front by early afternoon.

AVIATION /06Z THURSDAY THROUGH MONDAY/

As of 220 AM Thursday...

24-hour TAF period: VFR conditions will prevail across most of central NC through Thursday morning, but there is a chance of patchy fog and low stratus in mainly the eastern Sandhills and Coastal Plain (including FAY and RWI) from about 09z-14z as low-level moisture return increases with the flow turning southerly. Any fog or low stratus will lift/scatter out by mid-morning.

Scattered cumulus and mid/high clouds are expected through the day. Isolated to scattered showers and storms may develop in the south and east (including RDU/RWI/FAY) in the afternoon, with the sea breeze potentially acting as a focus, though high-res guidance has a large spread in how much coverage there will be, decreasing confidence. A better chance of convection comes across the northern TAF sites (INT/GSO/RDU/RWI) in the evening hours from about 23z to 06z. So added PROB30 groups for this potential at all TAF sites, with their timing based on where convection appears most likely, starting earliest in the SE and latest in the NW. Strong gusty winds and brief sub-VFR restrictions will be possible with any storms. Otherwise, winds are expected to be generally from the SW around 5- 10 kts through the day.

Outlook: Expect widespread showers/storms Friday into Saturday, with coverage decreasing from west to east Saturday night into Sunday. Flight restrictions are expected Thursday night through Saturday and possibly into Sunday morning, with mostly VFR and dry weather for the rest of Sunday into Monday.

CLIMATE

Record High Minimum Temperatures:

August 20: KRDU: 75/2007 KFAY: 78/2005 August 21: KRDU: 76/2009 KFAY: 77/2005 August 22: KRDU: 74/2017 KFAY: 75/2018

RAH WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 11 AM this morning to 8 PM EDT this evening for NCZ024>028-040>043-075>078-084>086-088-089.