textproduct: Raleigh

KEY MESSAGES

As of 400 PM Monday...

1) A stalled frontal boundary will produce showers and thunderstorms over central North Carolina today. Locally heavy rainfall and flash flooding will be possible.

2) Showers and storms with locally damaging winds and heavy rainfall are again possible on Tuesday, but this threat should be mostly confined to our north.

3) Unsettled weather will continue on Wednesday and possibly into late week as a cold front approaches and then slowly crosses NC.

DISCUSSION

As of 400 PM Monday...

KEY MESSAGE 1... A stalled frontal boundary will produce showers and thunderstorms over central North Carolina today. Locally heavy rainfall and flash flooding will be possible.

A Slight Risk (Level 2 of 5) for both severe thunderstorms and excessive rainfall/flash flooding is in effect across central NC today. An upper-level shortwave trough and associated MCVs will pivot across the region through tonight. At the surface, lingering frontal boundary just south of the NC/VA border will interact with good diurnal heating and the humid airmass with PWATs of ~2.0 inches will continue to trigger multi-cell cluster storms across the region this afternoon and evening. The primary hazards will be damaging winds associated with downburst and localized flash flooding from slow-moving or training storms. Areas across the Piedmont and northern Coastal Plain will have the best chance for stronger slow moving storms through at least midnight tonight. After the storms dissipate and the front lifts to the north into VA, expect low stratus clouds to move across the area before clearing skies early Tuesday morning.

KEY MESSAGE 2... Showers and storms with locally damaging winds and heavy rainfall are again possible on Tuesday, but this threat should be mostly confined to our north.

Broad troughing over the Eastern US will deepen on Tuesday due to a shortwave moving across southern Ontario and Quebec. Weak mid-level height falls will result over the Mid-Atlantic, which may reach as far south as northern NC during the day. Meanwhile a perturbation within the trough and associated weak surface low/trough will result in an area of enhanced showers and storms over the OH Valley on Tuesday morning that progresses east into the Mid-Atlantic through the afternoon and evening.

The above features will combine with a moist and moderately unstable air mass (PW values 100-125% of normal and MLCAPE values of 1500- 2500 J/kg) to result in a chance for isolated to scattered showers and storms over central NC on Tuesday afternoon and evening. However, precipitation chances will mostly be confined to our northern counties as guidance depicts a wedge of relatively drier air (PW values of 1.5-2") moving across our south as Tropical Depression Two slowly drifts west near the northern Gulf Coast. Mid- level flow will increase with the deepening trough, and 30-40 kts of bulk shear is possible across our far north. Thus the northern Piedmont and northern Coastal Plain are where SPC has a slight (level 2 of 5) risk for severe weather and WPC has a marginal to slight (levels 1-2 of 4) risk for excessive rainfall. However, capped POPs at chance given the best forcing will be across VA and coverage in the CAMS does not look widespread. Similar to recent days, the main severe threat will be damaging straight-line winds.

Additional isolated scattered convection will be possible on Tuesday night as a prefrontal surface trough sets up ahead of a strong cold front over the OH Valley. Once again this chance is mostly across our north, and the severe threat will be less with the loss of daytime heating.

KEY MESSAGE 3... Unsettled weather will continue on Wednesday and possibly into late week as a cold front approaches and then slowly crosses NC.

As mid/upper troughing with embedded shortwave impulses remains in place across the Eastern US on Wednesday, a strong cold front will approach and then slowly begin to cross central NC from the NW on Wednesday evening/night. Weak to moderate instability is again expected to develop, though it may be somewhat mitigated by morning cloud cover. Still, moisture along and ahead of the front looks plentiful with PW values of 2-2.5 inches. So the prefrontal trough may provide a focus for afternoon convection, followed by a line of showers and storms associated with the cold front moving through from NW to SE on Wednesday evening/night. POPs are categorical across the entire region, and WPC has all of central NC in a slight (level 2 of 4) risk for excessive rainfall. QPF is in the 1-2 inch range on average, and locally higher amounts will be possible. Deep- layer shear of 30-40 kts could result in some storms becoming strong to severe, and our entire region is also in a slight (level 2 of 5) risk for severe weather. Damaging winds will again be the main threat.

Uncertainty increases for late week as models differ on whether the cold front is able to push south enough to clear all of central NC or whether it will get hung up over our region. Behind the front, much drier air will bring clearing and much lower precipitation chances. Overall it looks likely for showers and storms to continue in our SE on Thursday, but the NW may be largely dry. By Friday and Saturday, the GFS sags the front to our south and keeps all of central NC mainly dry, while the ECMWF and CMC are slower with the front and keep us wetter. Given how difficult it can be for fronts to pass through this time of year, likely POPs in the SE still seem warranted, but this could change as we get closer. There is better agreement in drier conditions by Sunday and Monday as the mid/upper troughing finally pushes to our east and the flow aloft turns northwesterly.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

As of 830 PM Monday...

Another busy afternoon and early evening with scattered to numerous showers and storms across central NC. While the convective coverage and intensity has trended downward ever so slightly over the past hour or two, a small batch of showers and storms will move across the Triad through 02Z with another larger batch of showers and thunderstorms moving east across the eastern Piedmont and Triangle area should sustain itself as it moves into the Coastal Plain through around 04Z.

Convective coverage should wane toward and after 04Z with limited chances of showers and storms for much of the overnight. However, another round of late night stratus is expected with MVFR to IFR restrictions with CIGS around 1kft between 06 and 12Z, especially in the northern and eastern locations. There is a risk of marginal low- level wind across the area late tonight as a low-level jet strengthens across the area. This may result in LLWS around 1500- 2000ft with 2535 kts between 06Z and 12Z, especially across the Triad and northern locations.

Aviation conditions will improve during the morning with south/southwesterly winds at 5-10 kt increasing to 10kts by midday with gusts to around 20kts during the afternoon. LIFR/IFR stratus and LLWS should dissipate across all terminals by 14Z, returning central NC to VFR conditions into the early to mid afternoon. Guidance suggests a band of showers and storms across VA and WV may dive into northern NC after 20Z affecting the Triad. Other isolated showers and storms may develop across the Sandhills during the mid to late afternoon hours. In general convective coverage through the afternoon on Tuesday should be less than previous days although storm chances will actually increase during the evening and Tuesday night.

Outlook: Afternoon showers and storms through late week will result periods of VFR and periods of afternoon and evening sub-VFR conditions. There is a risk of low-level wind shear conditions again during the early morning hours on Wednesday morning. -Blaes

RAH WATCHES/WARNINGS/ADVISORIES

None.