169 FXUS62 KILM 231144 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 744 AM EDT Fri Apr 23 2021 .SYNOPSIS... Canadian high pressure will bring unseasonably cool and dry weather through today. A storm system will move across from the Gulf Coast states Saturday bringing moderate rain to the region and possibly severe weather. High pressure builds in for early next week across the region. && .NEAR TERM /THROUGH SATURDAY/... Inland temperatures have fallen into the 30s in some areas early this morning, likely allowing frost to form in spots. The actual distribution of the coldest temperatures is more driven by soil type than by latitude or distance to the ocean, and some frost is probably occurring as far south as Georgetown and Williamsburg counties. High pressure situated across eastern North and South Carolina should move out to sea today. Light northerly winds this morning will veer southerly this afternoon as cirrus clouds arrive from the west. These clouds are associated with the subtropical jet and are being pushed our way by an upper trough moving east across AZ/NM. It appears clouds may actually diminish this afternoon before increasing again tonight. Highs today are expected to reach the lower 70s across the Pee Dee region up toward Lumberton, but only the mid 60s along the coast where veering onshore winds will bring this morning's chilly airmass back onshore. Radiational cooling will be good enough this evening for temperatures to quickly fall toward 50 inland, but temps could actually begin to increase late as clouds thicken. Advection of Gulf moisture will increase substantially Saturday morning as low pressure across the Deep South begins to organize a warm front across Georgia. Showers could begin during the morning hours, perhaps joined by thunderstorms during the afternoon as unstable air from Florida and south Georgia is lifted over the surface front. Models have painted differing solutions for days about how far north the front ultimately moves Saturday as convection will tend to slow the boundary's northward progress. At this time, the most likely solution is that the front stalls near or south of Georgetown with convection remaining mainly elevated across our forecast area. Forecast helicity is rather extreme, but surface- based CAPE would be needed for a significant severe weather risk to develop. Forecast PoPs are 70-100 percent, but QFP amounts are significantly lower in the Cape Fear area than they are along the South Carolina coast. Lastly, the 00z GFS and an earlier run of the HRRR are explicitly forecasting the development of a gravity wave Saturday afternoon across coastal South Carolina. Gravity waves are mesoscale boundary layer pressure perturbations produced by convection occurring with a surface stable layer present. The forecast synoptic pattern certainly fits the conceptual pattern. Should a gravity wave actually develop, surface wind gusts of 30 to 50 mph could occur along the back edge of thunderstorm clusters. I'll increase forecast wind gusts across coastal SC a little between 3-6 pm pending later model runs that could localize the potential convection better. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... Saturday evening could be rough and tumble as a strong cold front moves across from west to east with the area firmly in the warm sector of said storm system. Some high resolution guidance admittedly at the outer time domains are showing a strong convective line remaining intact as it moves offshore seemingly ignoring any marine layer affects. Some guidance like the GFS for instance however pushes and or keeps the stronger activity to the south. Suffice to say likely to categorical pops remain in the forecast. SPC is maintaining and or expanding the slight risk for the day two outlook. Sunday and Monday will be pleasant and dry with highs in the 70s and lows in the 50s. && .LONG TERM /TUESDAY THROUGH THURSDAY/... The main components of the extended forecast remain in place with a sharp ridge building over the area for most of next week. On either side deep troughing develops. The systems are slow to move east and the latest ECMWF has slowed a bit from previous runs. Overall dry conditions and slightly above normal temperatures will be the rule with rain chances increasing very late in the period. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... VFR conditions will continue over the next 24 hours. Areas of cirrus clouds advecting overhead this morning will be joined by scattered mid level clouds by late afternoon/early evening, but widespread ceilings not expected. Surface winds will remain light, except increasing to south 10 knots along the coast with this afternoon's seabreeze. Winds calm by sunset. Clouds increase late, ahead of a robust weather system. Extended Outlook...MVFR ceilings and visibilities are possible Saturday and Saturday night as low pressure spreads clouds and showers and scattered thunderstorms in from the southwest. VFR otherwise. && .MARINE... Through Saturday...High pressure redeveloped across the Carolinas overnight which has lead to winds veering northerly again this morning. Light north to northeast winds should turn southerly this afternoon as a moderate seabreeze develops with nearshore winds increasing to 10-12 knots. The high will move away from the area tonight as low pressure begins to take shape across the southern Mississippi Valley. A warm front of sorts will develop across the Georgia and South Carolina waters Saturday and lift north ahead of the approaching low. It's unsure how far north the front will ultimately get as a large area of rain and embedded thunderstorms streaking eastward along it should slow the front's northward progress, potentially stalling the boundary somewhere on the South Carolina coast between Georgetown and Charleston. Increasing southeast winds on Saturday could become locally strong and gusty in the vicinity of any thunderstorms, especially during the afternoon. Winds and seas will likely necessitate the issuance of a Small Craft Advisory for Saturday later today. Saturday Night through Tuesday Night...Expect a brief period of small craft winds and seas for overnight Saturday into the day Sunday depending on the eventual timing and strength of the system. A westerly flow will develop later Sunday shifting to northeast by Monday. Beyond this it seems a return synoptic flow is in place modulated somewhat by the sea breeze. Significant seas, once again beyond Saturday and early Sunday drop down to 1-3 feet. && .FIRE WEATHER... Dewpoints in the 30s across the Carolinas today should again allow relative humidity to fall as low as 25 percent. Wind speeds should remain below 10 mph except along the coast where today's seabreeze should develop. Fortunately these breezier regions should also have the benefit of picking up higher humidity from the ocean. Conditions should remain below Red Flag criteria today. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...Frost Advisory until 9 AM EDT this morning for SCZ017-023-024. NC...Frost Advisory until 9 AM EDT this morning for NCZ087-096-105- 106. MARINE...None. && $$ SYNOPSIS...ILM NEAR TERM...TRA SHORT TERM...SHK LONG TERM...SHK AVIATION...IGB MARINE...TRA/SHK FIRE WEATHER...TRA