textproduct: Raleigh

WHAT HAS CHANGED

As of 430 PM Sunday...

* The potential remains for widespread severe weather Monday, some significant (eg. EF2 or stronger tornado and 75 mph or greater gusts), highlighted by a Level 4 of 5 Moderate Risk. Residents should finalize severe weather preparedness plans and ensure means to receive severe weather alerts. More info at www.weather.gov/rah/severeprep

* Wind gusts between 30 and 40 mph will occur away from storms throughout the day and may briefly maximize between 40-50 mph immediately ahead of a cold front that will both focus some of the most intense storms and also serve as the back edge of the severe weather threat, as it progresses across the forecast area.

KEY MESSAGES

As of 430 PM Sunday...

1) Marginal risk (low probability and coverage) for shallow, rotating cells capable of an isolated spin-up/tornado or strong to locally damaging wind gust this evening through Monday morning.

2) The potential remains for widespread severe weather Monday, some significant (eg. EF2 or stronger tornado and 75 mph or greater gusts), highlighted with a Level 4 of 5 Moderate Risk.

3) Wind gusts between 30 and 40 mph will occur away from storms throughout the day Mon and may briefly maximize between 40-50 mph immediately ahead of a cold front that will both focus some of the most intense storms and also serve as the back edge of the severe weather threat, as it progresses across the forecast area.

4 Cooler than normal temperatures expected early Tuesday through Wednesday night across central NC, with highs running 1520 degrees below mid-March averages.

DISCUSSION

As of 430 PM Sunday...

KEY MESSAGE 1... Marginal risk (low probability and coverage) for shallow, rotating cells capable of an isolated spin-up/tornado or strong to locally damaging wind gust this evening through Monday morning.

Aloft, a s/w will continue lifting nwd along the NC coast through this evening. Another s/w will follow overnight tonight through Monday morning. At the surface, as of 18Z the ~1032 mb high off New England was ridging swwd across the mid-Atlantic and Carolinas, with the front draped across the Southeast US. Surface winds have become sely across central NC, with gusts of 15-25 mph. Low-level moisture is still limited, but will gradually increase through tonight as the front lifts back into the area as a warm front.

The LLJ is expected to increase tonight, from 30-40 kts this eve to 40-50 kts tonight into early Mon morn. The 6 km Bulk Shear is 30-40 kts, but MLCAPE remains limited (less than 400 J/Kg, focused across the south, with MLCIN across much of central NC). Expect instability to increase nwwd into the area as the warm front lifts into the area and low-level moisture increases. Hi-res guidance suggests continued 6km Bulk Shear of 25-35 kts through tonight, with LCLs dropping to 100-500m. The biggest question parameter-wise will be the CAPE/CIN.

Given the only modest low-level moisture recovery coincident with heating this aft, instability should remain weak and confined primarily to the srn Piedmont and Sandhills through this evening. While showers are lifting into the Sandhills and srn Coastal Plain, the deeper convection remains well off the SC coast. There is still a conditional risk of at least transient mini-supercells capable of strong to locally damaging gusts and/or an isolated spin-up/tornado. The latter risk would be more likely toward evening, as LCLs lower with nocturnal cooling, and strengthening low-level wind fields yield stronger SRH. As cells progress nwwd across the nrn Piedmont through evening, they should become increasingly elevated and/or weaken, though associated rain may briefly diabatically- strengthen/reinforce the (wedge) front as it slows over w-cntl NC/VA. As earlier discussions noted, low probability/risks of severe will exist through Mon morning from scattered cells moving onshore over sern/ern NC and into the srn-cntl Coastal Plain, as well as other scattered cells more-broadly throughout cntl NC, amid warm/moist advection, both of which would pose a risk of isolated tornadoes and strong to locally damaging gusts.

KEY MESSAGE 2... The potential remains for widespread severe weather Monday, some significant (eg. EF2 or stronger tornado and 75 mph or greater gusts), highlighted with a Level 4 of 5 Moderate Risk.

A basal shortwave trough, embedded within a highly amplified/ meridional synoptic trough, will assume negative tilt while pivoting and lifting rapidly from the lwr MS Valley to lwr Great Lakes from 12Z Mon to 12Z Tue. Preceding height falls and strengthening lwr/mid- tropospheric flow will result downstream of the trough and across cntl NC (strongest nw) through the day and early evening Mon, within the right entrance of a powerful, cyclonically-curved jet streak. In addition to related strong dynamics and kinematics with those developments, there are indications from forecast mid-level lapse rates, Hysplit back trajectories, and upstream observed soundings from the srn Rockies and srn Plains, that a residual elevated mixed layer will have been advected newd and across cntl NC by late tonight-early Mon.

At the surface, a strong polar front, extending at 12Z Mon from ern OH sswd through the srn Appalachians and FL panhandle, will sweep ewd and across the South and Middle Atlantic through 12Z Tue. A composite outflow/effective front, from an upstream QLCS at the start of the period, may slightly outpace the polar, synoptic one. As often occurs, the synoptic front may fracture, with the nrn portion slowed by the cntl Appalachians and the srn portion, unimpeded around the srn Appalachians, likely to move more quickly and across and offshore the South Atlantic states through early evening. A triple point may develop along that srn frontal segment and track across the Piedmont of the Carolinas and VA, where isallobaric forcing, convergence, and low-level shear may maximize and focus a regional maximum in severe potential.

There may be an early day, ongoing severe risk from overnight-early morning convection, amid a regime of strengthening, warm/moist air advection and transport of 60s F surface dewpoints nwd across cntl NC, and beneath the aforementioned steadily falling heights aloft. A risk of isolated tornadoes and strong to locally damaging gusts will result.

The severe risk will subsequently increase by mid to late morning, as the stronger forcing and flow aloft overspread a destabilizing warm/moist sector over cntl NC, where a combination of diurnal heating into-through the 70s F and with surface dewpoints generally in the lwr 60s F should yield 750-1500 J/kg of MLCAPE. Strong to extreme shear profiles will favor elongated, and clockwise- curved hodographs, with the latter favored and maximized where surface flow may back to ssely/sely immediately ahead of the pressure falls related to the approaching front and possible triple point. While that parameter space will be favorable for organized, probably mixed convective modes, it remains unclear which mode may be dominant. Discrete or semi-discrete modes would favor supercells with all hazards (including large hail with tornadoes, possibly strong, and damaging wind gusts) versus a QLCS with little to no hail and instead widespread strong to damaging wind gusts and swaths/corridors of significant wind and/or mesovortex tornadoes. There will probably be some combination of the two modes, with a dominance that may not become apparent until Mon morning.

KEY MESSAGE 3... Wind gusts between 30 and 40 mph will occur away from storms throughout the day and may briefly maximize between 40- 50 mph immediately ahead of a cold front that will both focus some of the most intense storms and also serve as the back edge of the severe weather threat, as it progresses across the forecast area.

Already strong and gusty, background gradient winds may be briefly enhanced immediately ahead of the pressure trough/pressure fall corridor accompanying the cold front, as it progresses across the forecast area and both focuses some of the most intense storms and also serves as the back edge of the severe weather threat.

KEY MESSAGE 4.. Cooler than normal temperatures expected early Tuesday through Wednesday night across central NC, with highs running 1520 degrees below mid-March averages.

In the wake of Mondays cold front, cooler and drier air will filter into central North Carolina late Monday night into Tuesday. A broad upper-level trough settling over the eastern US will reinforce cold air advection through the day Tuesday, keeping temperatures below mid-March climatology.

High temperatures are expected to reach the mid 40s to near 50 across the region Tuesday, which is roughly 15-20 degrees below normal for this time of year. Breezy west to northwest winds may also persist through much of the day Tuesday, with early morning wind chills in the 20s. Tuesday night lows will be in the mid to upper 20s with some cooler spots in NW the low 20s.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

As of 200 PM Sunday...

24 hour TAF period: Periods of showers and thunderstorms will be possible at all terminals this afternoon through Monday morning. While some gusts into the 20s kts will be possible with convection this evening, the potential for stronger gusts increases overnight and especially into Mon morning. Cannot rule out an isolated tornado, however the greater chances for that will be with the line of storms expected to move into and across the Triad late in the TAF period, though timing of the line remains a little uncertain. Generally expect VFR conditions this afternoon, with MVFR to IFR cigs possibly accompanying the showers/storms this eve through tonight. LLWS will be possible tonight, with 35-40 kts around 2kft, but with continued winds of 5-10 kts overnight and increasing toward daybreak Mon, have left out mention in the TAF.

Outlook: Expect the line of storms to move across central NC Monday afternoon, accompanied by strong (severe with the strongest of storms) gusty winds and the usual cig/vsby restrictions where storms impact a terminal. All manner of severe weather is possible with the storms on Monday, including tornadoes and gusts of 60-80 mph or greater. Outside of convection, winds will still be strong, with gusts of 30-40 mph. Storms should move out of the area Mon eve, with dry weather and VFR conditions expected thereafter.

RAH WATCHES/WARNINGS/ADVISORIES

None.