Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 220158 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 958 PM EDT Sun May 21 2017 .SYNOPSIS... A weak front stalled near the coast will return northward late tonight ahead of a stronger frontal system approaching from the west. A series of storm systems moving across the Southeastern United States will bring an increasing potential for showers and thunderstorms this week. The best chances of rain should occur from late Monday through Wednesday. Conditions may dry out somewhat toward the end of the week. && .NEAR TERM /THROUGH MONDAY/... As of 1000 PM Sunday...Our swath of bone-dry mid-level air is obviously in place given what happened to the band of rain that completely died upon reaching the Florence vicinity a couple hours ago. Latest RAP forecast soundings show this dry air persisting along the coast overnight, so I have removed PoPs completely for coastal South Carolina and left only a narrow zone of 20 PoPs for coastal North Carolina where a few very shallow convective showers and sprinkles have managed to develop beneath the warm/dry subsidence inversion aloft. In fact the only "good" chance of rain overnight may develop around 1-2 AM back in Darlington and Marlboro counties as the next band of rainfall approaches from the west, although its eastward progress will be again limited by the dry mid-level air currently feeding onshore on a south-southeasterly 850 mb flow. Discussion from 630 PM follows... A close look at visible satellite imagery this evening shows the old stalled front out over the ocean south of Cape Fear. The boundary becomes harder to locate as it curves northwestward and inland toward Whiteville and Elizabethtown. This boundary should move little through the evening hours, but is expected to begin moving northward later tonight as the overall environmental flow veers more southerly in advance of a more significant synoptic front advancing eastward through the southern Appalachians. Showers and thunderstorms in a north-south oriented band across central North and South Carolina are collocated with a band of good mid and upper level moisture and a weakening upper level vorticity maximum. Linear motion extrapolation insists these showers would race to the South Carolina coast between 9-10 PM, however this appears unlikely due to a layer of intensely dry air located above a subsidence inversion located between 3500-5000 feet AGL. Both the GFS and NAM show dewpoints as low as 0-10 degrees F through a deep layer between 4000-10000 feet AGL. Regardless of what`s going on to our west, this kind of dry air is going to eat up any precip attempting to fall through it and is the reason I have dramatically trimmed back PoPs through the evening hours. I didn`t entirely remove PoPs however, as below the inversion layer lapse rates are steep and with small temp- dewpoint depressions at the surface it`s not impossible some small towering cumulus clouds could push onshore over the next 5 or 6 hours dropping a few hundredths of an inch of rain in spots. Models show the dry air will eventually get nudged closer to the coast, and offshore by sunrise Monday morning as the mean layer flow veering more southerly. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 300 PM Sunday...Broad 5h troughing over much of the CONUS during the period will maintain unsettled weather through Tue night. Weak cold front stalls in the region Mon evening, ending up parallel to the steering flow. Weak low developing along the front over AL will track northeast along the front Mon night into Tue. Deep southwest flow, directing the low northeast, will help spread deep Gulf moisture over the region. Precipitable water values are forecast to approach 2 inches Tue and the weak, but developing low will help provide upward motion. Shortwaves within the large scale 5h trough will further enhance upward motion, timing dependent. Worth noting that the bulk of the guidance is about 12 hours slower with the development of this scenario compared to Sat, mainly due to the 5h trough being more amplified. However, thinking has not changed much and all of the guidance still has the low developing and tracking northeast, across the western Carolinas. Best rainfall chances and highest rainfall totals are expected across inland areas, closer to the surface low, but just about the entire region should receive rainfall from this event. Surface low moves off the coast near the NC/VA border Tue evening, but with the 5h trough axis remaining west of area the front will continue lingering in the area. Larger differences between the guidance are evident Tue night but most agree that at least a portion of the Tue night period has a good chance of being wet. Coverage will be a little less than during the day Tue but still potential for showers and thunderstorms with both PVA ahead of any shortwaves and lingering front having an abundance of deep moisture to tap into. Also evidence of an increasing low level jet Tue night, some disagreement on exactly how strong but range of solutions is 40- 50 kt which is sufficient to enhance dynamics. Clouds and low level moisture/rain will keep highs near to slightly below climo but help keep lows above to well above climo. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 300 PM Sunday...Relatively narrow warm sector spreads into the area on Wednesday. The warm front early may bring some light rain but the approach of the cold front yield the more significant rise in POPs as well as the chance for higher rainfall amounts. Wind fields not quite as strong as previously forecast and instability should be limited by cloud cover so severe weather threat seems minimal. The main energy associated with the upper trough crosses Thursday but the deepest moisture will be well offshore by then. Even so, the strength of the main vorticity max should be able to wring out a few showers especially if breaks of sunshine provide some instability. Deep layer westerly flow will bring sunshine on Friday with much lower humidity levels that will recover slightly into Saturday. A healthy disturbance may come across in this zonal flow on Sunday bringing some minor rain chances. && .AVIATION /00Z MONDAY THROUGH FRIDAY/... As of 00Z Monday...A weak stalled front near North Myrtle Beach and Lumberton will begin to return northward late tonight. Showers across central South Carolina will weaken later this evening as they push eastward into very dry mid-level air across the eastern Carolinas. What`s left of these showers should move overhead and offshore late tonight into Monday morning. Extended Outlook...Reduced flight categories will be possible in periodic showers and thunderstorms Monday evening through Wednesday. The strongest convective activity will occur Wednesday. && .MARINE... NEAR TERM /THROUGH MONDAY/... As of 630 PM Sunday...An old stalled front located about 20 miles south of Cape Fear curves northwestward and comes onshore somewhere in the North Myrtle Beach vicinity. This front has become weaker with time and currently separates easterly winds to the north from more southeasterly winds to the south. While the front is expected to hardly budge through the evening hours, the overall environmental wind should veer more southerly late tonight as a stronger frontal system begins to move our way from the southern Appalachians. Seas currently around 3 feet (2 feet nearshore) should build by maybe an additional half foot overnight as onshore wind trajectories continue. SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 300 PM Sunday...High pressure remains well off the coast but its influence continues to weaken. Southwest flow will be maintained through the period but gradient will be pinched between the high and a series of developing surface waves that will track northeast along the front stalled just west of the waters. Southwest flow increases from 10 to 15 kt Mon night to 15 to 20 kt Tue before dropping closer to 15 kt later Tue night. Increase in winds is related to the passage of the surface waves so an difference in timing or strength of these features would have a direct impact on wind speeds. Seas around 3 ft Mon evening build to 3 to 4 ft Mon night, peaking at 3 to 5 ft Tue and Tue night. LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 300 PM Sunday...Broad swath of southwesterly flow in place on Wednesday between large Atlantic high and cold front approaching from the west. Winds may not ramp up to advisory levels but seas likely will. This boundary will turn the flow from SW to W by Thursday as well as a small decrease in overall wind speed allowing seas to abate below advisory thresholds. The circulation and gradient associated with the front will finally move east of the area by Friday as high pressure builds into Florida and the Bahamas. Locally wind will remain westerly and continue to abate. Seas will follow suit. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ SYNOPSIS...TRA NEAR TERM...TRA SHORT TERM...III LONG TERM...MBB AVIATION...TRA is the U.S. government's official web portal to all federal, state and local government web resources and services.