Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 271734 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 134 PM EDT Sat May 27 2017 .SYNOPSIS... Increasingly hot temperatures are expected over the weekend as high pressure over Florida moves slowly eastward. By Tuesday the high should be far enough away to allow a weak cold front to stall across the Carolinas. This front will linger through Friday, enhancing the potential for scattered thunderstorms. && .NEAR TERM /THROUGH TONIGHT/... As of 130 PM Saturday...No significant changes to the forecast at this time. A batch of convection will pass north of the area between 5pm and 10 pm. A bit later in the evening a few storms will approach our inland I-95 zones between 10pm and midnight but diurnal cooling will cause them to weaken on approach at far western doorstep. As of 300 AM Saturday...A hot and dry start to the Memorial Day weekend as flat elongated ridge across the Gulf Coast exerts its influence locally. However, the weather will become potentially more active late tonight as a vorticity impulse embedded in a band of increased westerlies aloft rotates just north of the area. Satellite imagery early this morning shows mostly clear sky conditions minus the odd patch of cirrus moving overhead. Broad high pressure centered across FL remains the dominant feature, while a weak baroclinic boundary oriented E-to-W lies across VA and back into the OH Vly. It is the confluence between the upper features driving this surface setup that will create the enhanced westerlies and potential convection late. Before this occurs, a thermal plume of very warm air will advect from W to E across the region causing 850mb temps to climb towards +16C, and 700mb temps up over +11C. This warming of the column combined with dry W/NW flow aloft will drive temps into the low 90s away from the beaches this aftn, while also serving as a cap to any diurnal convection. In fact, even as PWATs climb towards 1.5 inches, diurnal CU will struggle to gain much depth thanks to this dry and warm air aloft. In other words, it should be a great start weather-wise for the holiday wknd, with S/SW wind gusts of 20-25+ mph being the only non-ideal parameter this aftn. Winds will ease after dark, and the focus will shift to an area of convection likely to be ongoing to the west this evening. A potent vorticity impulse will race eastward tonight, likely shifting just north of the area before exiting offshore early Sunday morning. A very warm and increasingly unstable airmass ahead of this feature suggests that tstms will develop beneath it, and may form into an MCS as is typically seen in these "Ring of Fire" type setups. The question is how much of this will persist as we lose instability tonight, as well as how far south will the convection sink as it becomes impeded by the ridge. Most high res guidance suggests that tstms will just graze the northern fringe of the CWA after midnight, and even this will be in a weakening state. It is interesting to note that both the NSSL WRF and latest ARW suggest convection falling apart as a cold pool surges ahead of the tstm complex. Not sure how much to trust that exact scenario, but weakening of tstms seems likely as they zoom past the area despite strong ML lapse rates. SPC has placed the far NW tier of counties into a MRGL risk for tonight, and with increasing mid-level winds and steep lapse rates, if the cap is broken and convection can sustain itself tonight, a strong wind risk does exist. However, expect most of the strong storms to be north and west of the CWA, and have carried CHC POP along and north of a line from Hartsville, SC...to Maple Hill, NC, with barely any chance at all south of there. Continued warm SW winds much of the night will keep mins from falling much below 70 most locations tonight. Lower rain cooled mins are possible in a few locations if tstms develop, but this will be a very near term forecast challenge if it occurs. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 300 AM Saturday...500 mb high pressure will strengthen as it slowly moves eastward across Florida early next week. Surface high pressure typically sets up around 30 degrees north latitude this time of year, but will be located much farther south this time given the position of the upper center. Deep westerly flow is expected across the Carolinas both days bringing hot temperatures and a chance of thunderstorms. A series of 500 mb vort maxes will move across North Carolina and northernmost South Carolina Sunday afternoon through Monday night. Each model has a different timing and path for the individual disturbances, so it`s not possible to have much confidence about the timing and location of the best risk for storms. Despite steep lapse rates aloft and relatively good low-level moisture, there is a good deal of dry air noted in all models at and above the 800 mb level which should reduce the number of storms able to develop. Even if only a couple storm cells do develop, strong westerly winds throughout the depth of the troposphere should push these cells eastward at 30 mph. Given even limited storm organization, dry air aloft would favor storms propagating rapidly eastward on "running" outflow boundaries that could push the effective storm motion up over 40 mph. Forecast PoPs each 12-hour period range from 20-30 percent, generally higher north of Florence, Myrtle Beach, and Southport. With high angle sunshine and 850 mb temps of +18C to +19C, high temperatures should soar into the 90s both days away from the beaches. Monday looks like the warmest day when temps in the mid 90s could occur inland with heat indices eclipsing 100 degrees during the afternoon hours. There is only one record high that might be in jeopardy: Monday`s record in Florence is only 95 (set in 1991) versus our latest forecast of 93. Moderate to strong seabreeze circulations are expected both days with afternoon wind gusts near the beaches likely reaching 25 mph. Synoptic westerly wind should keep the seabreeze front very close to the coast with only limited penetration (~10 miles) inland expected. Very warm nighttime lows 70-75 degrees are expected both nights. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 300 PM Friday...Primary headline for the extended forecast period is `unsettled`, as a low-amplitude but broad upper trough translates slowly east across New England and the Mid-Atlantic seaboard. Near the base of the trough, phasing impulses from the southern jet stream over top the Gulf ridge will allow a series of whisking disturbances to bring TSTM chances much of the upcoming week when coupled with late May and early June heating and an active afternoon sea breeze boundary. Temperatures however will remain slightly above normal as the thermal influence is exerted more by the Gulf and SE states upper ridge. No shortage of column mositure or dry mid level caps to impede convection with PWAT values of 1.5 to 1.75 inches next week, so what convection does form will be capable of dumping good rain amounts. Storm motion generally will be from west to east, so all locations will have a good chance of rainfall and TSTMS at one point during the upcoming week. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... As of 18Z...VFR conditions will prevail as a near typical summer pattern prevails across the Carolinas this TAF cycle. West-southwest winds will become stronger this afternoon and from the from the south at the MYR and CRE airports with this afternoon`s seabreeze. A disturbance approaching from the west may help develop cumulus/altocumulus clouds with bases in the 6000-foot range tonight with isolated showers possible mainly at KLBT and KILM aft 00Z. Extended Outlook...Brief MVFR/IFR conditions are possible in scattered showers and thunderstorms Sunday Evening, and again Monday Night through Tuesday. && .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 130 PM Saturday...Elongated high pressure across the Gulf Coast will exert its influence on the local waters today. SW winds in the vicinity of this high will increase steadily this aftn, becoming 15-20 kts across all waters. A local backing and increase in speed is forecast within the near-shore sea breeze circulation as well where wind gusts of up to 25 kts are expected through this evening. Winds will remain primarily SW through tonight with only a slight decrease in speeds. However, a complex of tstms moving to the north may create some variable wind directions due to boundary interactions overnight. Seas will be 3-4 ft most of this period, with the SW wind chop of 5 sec masking the lower amplitude 8-9 sec SE swell. SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 300 AM Saturday...Surface high pressure that typically sets up around 30 degrees north latitude (North Florida) this time of year will instead be displaced a few hundred miles to the south. West-southwesterly winds are expected both Sunday and Monday along the northern periphery of the high. Given the unusual southern position of the high, a backdoor cold front may get close to our portion of the coast Sunday afternoon and evening. Model consensus drops the front no farther south than the southern Outer Banks currently. Inland temperatures should soar into the 90s both Sunday and Monday, which should generate a moderate to strong seabreeze both days. Wind gusts could reach 20-25 knots near the beaches between 3-7 PM. Also of concern, there is a chance of fast- moving thunderstorms Sunday evening and again Monday afternoon/evening. Winds aloft are such that any storms could be moving eastward at 30-35 knots, making it imperative that offshore mariners keep a close watch on weather conditions well off to the west. LONG TERM /TUESDAY THROUGH WEDNESDAY/... As of 300 PM Friday...SW winds this period 15 KT or less except higher afternoon gusts associated with an active sea breeze. Sea heights generally a 2-3 foot range this period, along with isolated to scattered inshore TSTMS mainly in the late afternoons and evening as storms push off land over the 0-20NM waters. Troughing inland may bring the highest winds of 15-20 KT sustained Wednesday. Mariners should get a radar update before heading out this period as there will be a chance of storms each day. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ SYNOPSIS...TRA/8 NEAR TERM...JDW/8 SHORT TERM...TRA LONG TERM...MJC AVIATION...TRA/8

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