textproduct: Raleigh

WHAT HAS CHANGED

* No significant changes to the heavy rainfall threat through Thursday night, but a slightly drier trend for Friday and Saturday.

KEY MESSAGES

As of 400 PM Wednesday...

1) Waves of showers and storms with a risk for excessive rainfall and flooding will focus along a slow-moving, outflow- reinforced/strengthened cold front that passes through central NC from now through Thursday night.

2) The slow moving cold front across the Carolinas will settle south and stall across the southern Carolinas for Friday and into the weekend with the area of widespread rain shifting south and the rainfall intensity decreasing on Friday and especially Saturday.

DISCUSSION

As of 400 PM Wednesday...

KEY MESSAGE 1... Waves of showers and storms with a risk for excessive rainfall and flooding will focus along a slow-moving, outflow-reinforced/strengthened cold front that passes through central NC from now through Thursday night.

Scattered thunderstorms will begin to move into the NW Piedmont over the next few hours. These cells will then develop into a broken convective line pushing eastward through the Triangle during the evening and overnight hours, maintaining the threat for isolated to scattered strong to severe storms. It is fairly cool and stable near the VA border with temperatures only in the upper-70s, so it may be difficult to get much of a damaging wind threat there. Across the NW Piedmont, on the other hand, temperatures are still in the upper-80s with SBCAPE around 1500-2000 J/kg. If storms can outrun the boundary into the warm sector, this is likely where the greatest severe threat will be. However, while 0-1 km shear is not impressive (10-15 kts), still can't rule out a quick spinup with any convection, even on the cool side of the boundary. Localized excessive rainfall and flash flooding will be the bigger threat, particularly in urban areas.

By late tonight and Thursday morning, the activity is expected to spread south and east into the Sandhills and Coastal Plain as well. While the severe threat will generally subside tonight with loss of instability, storm motion running parallel to the stalling boundary will cause repeating waves of heavy rain to train over the same areas, shifting the primary hazard to more widespread flash flooding. Thus a Flood Watch is in effect for all of central NC, beginning across the northern half this afternoon and evening, then across our southern half overnight tonight. The WPC Excessive Rainfall Outlook for today/tonight highlights a Moderate Risk (Level 3/4) clipping the northeastern Coastal Plain (near Roanoke Rapids) with a Slight Risk (Level 2/4) across roughly the northern half. Urban areas, small streams, and creeks will be particularly vulnerable to rapid runoff and localized flash flooding. Mainstem rivers will also begin to rise in response to heavy rainfall, though they are expected to remain below minor flood stage through at least Thursday morning.

The cold front will move only slightly south and east on Thursday, and with rich Gulf moisture from Tropical Storm Bertha continuing to funnel NE along the front, another day of heavy rainfall and possible flash flooding is expected, this time across all of central NC. WPC QPF on Thursday and Thursday night is 1-2 inches on average across the region, except across our far north as the front sags south of there. As always, locally higher amounts will be possible, with 12z HREF LPMM depicting potential for 2-4 inches to the south and east of Raleigh. The 12z HREF and REFS depict neighborhood probabilities of exceeding 6-hour FFG peaking across our area from 12-18z Thursday. Thus there is a moderate (level 3 of 4) risk for excessive rainfall across eastern NC that includes much of our Coastal Plain, with a slight (level 2 of 4) risk for the rest of central NC outside of the far NW. No mainstem rivers are forecast to reach minor flood stage at this time, but it can't be ruled out if excessive rain falls directly over a basin. Still, urban and small stream flash flooding will be the biggest and most widespread threat.

One silver lining is the severe threat at least looks to decrease on Thursday. This is due to the Eastern US troughing beginning to flatten, resulting in a weakening of the mid-level flow, and the best instability shifting to our southern zones where shear will be less. However, a few strong storms with water-loaded downbursts or a quick spinup still can't be totally ruled out, mainly in the south.

KEY MESSAGE 2... The slow moving cold front across the Carolinas will settle south and stall across the southern Carolinas for Friday and into the weekend with the area of widespread rain shifting south and the rainfall intensity decreasing on Friday and especially Saturday.

While there will still be a threat for heavy rainfall on Friday, it looks to be less overall compared to today and Thursday and shifts to mainly south and east of Raleigh, as the cold front shifts south and stalls from SC to the southern NC coast. WPC QPF from Friday into Saturday has decreased compared to yesterday, with just 0.25- 0.5 inches expected on average for most of the area, except 0.5-1.5 inch totals across the southern Piedmont, Sandhills, and central/southern Coastal Plain. A shortwave trough moving through the TN Valley and southern Appalachians may actually enhance the moist air mass again on Saturday as PW values increase to 2-2.5 inches, along with providing additional forcing for ascent. However, we will still be on the cool side of the boundary, which should keep rainfall rates lower than they had been.

Conditions will turn drier from Sunday through early next week as the mid/upper troughing finally pushes to our east and the flow aloft turns northwesterly. However, as temperatures heat back up, isolated diurnally-driven convection still can't be ruled out each day, especially by Tuesday and Wednesday.

AVIATION /06Z THURSDAY THROUGH MONDAY/

As of 235 AM Thursday...

An axis of convection --widespread, moderate to heavy showers and widely scattered storms-- will continue to focus along a 925-850 mb front across Piedmont sites this morning. Other, scattered convection will meanwhile continue to launch along and north of a nearly stationary outflow boundary/effective front MCZ-POB-CLT, including at RWI, as moist swly flow impinges upon and rides atop the outflow boundary. While flight restrictions will occur first and be maximized in this convection, areas of IFR-MVFR ceilings are likely to develop in the rain-cooled air north/poleward of the outflow boundary through the morning. This boundary, and both focus for convection and areas of flight restrictions, will probably not reach FAY until this afternoon. Some lifting of ceilings through MVFR range will meanwhile result at Piedmont sites, accompanied by briefly gusty nly surface winds, as a reinforcing synoptic front and following drier air now over cntl VA continue swd and into cntl NC later today. Nocturnal cooling will support a lowering of ceilings into IFR range once again tonight, accompanied by continued waves of elevated convection along and north of the aforementioned fronts that will have probably merged and stalled over the ern Carolinas.

Outlook: Cold air damming to the north and west of the fronts will favor overnight-morning IFR-MVFR ceilings and possibly some periods of light rain over cntl NC Fri-Sat. Some continued elevated convection will also continue at FAY during that time, in relatively closer proximity to the stalled front and instability axis.

CLIMATE

Record Low Maximum Temperatures:

July 23: KGSO: 73/2019 KRDU: 76/1921

July 24: KFAY: 81/1985

RAH WATCHES/WARNINGS/ADVISORIES

Flood Watch through Thursday evening for NCZ007>011-021>028- 038>041. Flood Watch from midnight EDT tonight through Friday morning for NCZ042-043-073>078-083>086-088-089.