Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 210918 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 518 AM EDT Wed Mar 21 2018 .SYNOPSIS... Cold high pressure under gusty NW winds today and again Thursday will keep temps 10 to as much as 15 degrees below normal. If winds subside enough during the next 3 nights and early mornings, frost and or freeze issues will increase dramatically across the area. High pressure will maintain cool daytime weather this Saturday but will give way to rain spreading in from the west Saturday night and Sunday. && .NEAR TERM /THROUGH TONIGHT/... As of 300 AM Wednesday...After a busy night of severe weather including multiple reports of up to 2" diameter hail near Myrtle Beach, all is quiet at this hour offering a welcome reprieve from the activity of earlier. The reason for the decrease in convection, which has now eroded to nothing locally, is the advection of drier and cooler air both at the surface and aloft, which has significantly changed the atmospheric profile early this morning. Whereas RAP soundings earlier showed significant residual elevated instability within an EML, lapse rates have stabilized as the entire column "shifts" colder, with the drier air aloft also squelching any convection. The dry air is mostly aloft, as a frontal inversion is trapping moisture in the form of the widespread stratus currently trapped across the CWA. As low pressure strengthens offshore along the stalled front, surface flow will back to the W/NW later this morning with dry advection thereafter causing ceilings to lift and break through the aftn. This will have little impact on temperatures however, and a January-like day is forecast as 850mb temps below 0C cause highs to climb only to around 50 far NW counties, to the mid 50s along the South Santee river. Additionally, another spoke of vorticity energy will rotate overhead late this morning into the aftn as the cold core/upper low swings into the tidewater of Virginia. Across SC, this will have little impact other than some cold air cu development. However, forecast soundings and high res guidance suggest a chance for showers across SE NC as this forcing combines with increasing column saturation up through about 700mb. Have capped POP at low-chc, with the best chance being late morning, but QPF will be very low. Despite the column being cold and 850mb temps remaining below 0C, all precip today which does occur will be of the liquid variety thanks to a dry dendritic zone and freezing levels too high for any snow in what will be light precip rates. As this final piece of energy rotates offshore this aftn, even drier air will advect into the column and many locations will see sunshine before sunset this evening. This dry advection will also be aided by westerly winds gusting above 20mph this aftn as the gradient pinches and a secondary front crosses the region. A cold night is forecast tonight, as this late March "winter" continues. Forecast mins are currently in the 35-38 degree range, which could be cold enough for some frost. However, forecast profiles and MOS guidance keep winds elevated enough overnight that frost should not be of concern. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As of 400 AM Wednesday...If you think this period is going to be boring with no appreciable weather and only few clouds, think again. Yes, skies will mainly be clear thruout this period with occasional thin cirrus. Progged atmospheric soundings across the local area indicate no moisture thru the atm column with PWs less than one quarter of an inch at times. Thus, no rainfall this period. Downslope flow during this period will scour out any moisture that does make it across the Appalachians. Thus, the mostly clear fcst. Here`s where the fcst gets difficult. Farmers, local growers, Ma and Pa`s place and any other low temperature sensitive plants, crops, shrubs and trees will become exposed to temperatures in the low to mid 30s Friday and again Saturday mornings respectively. A combination of the NAM and European MOS Guidance was used here due to their slightly lower min temp fcst with the GFS MOS. At this point, the sfc pg remains semi-tightened Fri morning and further relaxed Sat morning when compared to Fri morning. Thus, winds are forecast to stay semi-active Fri morning, NW around 5 mph, and semi-active Sat morning, NW less than 5 mph. These speeds will be enough to prevent any dew and frost development especially Fri Morning. The sfc dewpoint depression Friday morning will be the hiest of the 2 mornings in question. And if winds either potentially subside-some and/or decouple Fri morning, more-so than what guidance is saying, min temps could drop to and below 32 degrees for several hrs. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 340 PM Tuesday...this forecast period continues to show a cool down as an upper level trough will be remain along the east coast. In the large scale trough a shortwave is seen dropping southward on the west side. This will bring in a colder shot of air and a back door cold front. The 12 UTC GFS and ECMWF are not in good agreement with the GFS being much faster than the ECMWF. So uncertainty is high on timing of this front. Will favor the faster GFS and its ensemble and introduce a chance of showers late Saturday into Sunday night as the frontal boundary shifts southward. Temperatures will be below normal for the period with a frost/freeze possible Friday night. Otherwise lows in the 40s and highs in the mid to upper 50s to lower 60s. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... As of 06Z...Cold front aligned near the South Santee River is leaving NE winds in its wake at all the terminals early this morning. Within this NE flow, stratus has enveloped the local area, with cigs varying from about 600 ft to 1500 ft. This will remain the case through daybreak, but cigs are expected to slowly rise as winds back to the NW around a low pressure developing offshore. Some short duration IFR is possible, especially at the Myrtles, but MVFR should be predominant overnight. A mid-level impulse moving overhead the first half of Wednesday may create some showers at LBT/ILM, but otherwise the day will feature gradually rising cigs and increasingly gusty winds. By the aftn, winds are forecast to be westerly with gusts of 20 kts likely at all terminals. These westerly winds will help dry the column so that by the end of the period VFR will be the rule. Extended outlook...VFR. MVFR/IFR/Rain Sunday. && .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 300 AM Wednesday...Small Craft Advisory remains in effect for all waters, and will persist at least through the near term period. Gusty NE winds behind a cold front stalled south of the waters will gradually shift to the west by daybreak as low pressure develops to the east. These winds will remain 10-15 kts into the late morning before increasing as a surge of dry air associated with a secondary front crosses the waters. This will push winds back up to 20-25 kts with higher gusts, and these wind speeds are expected to persist into Thursday with slow veering back to the NW. Despite the predominantly offshore wind, wave heights will be steady in the 5-8 ft range, falling to 4-6 ft late, with a residual SW swell and NW wind wave comprising the spectrum. SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As of 400 AM Wednesday...SCA to remain in effect for the local waters thru 6 pm Thursday. Winds and respective seas will be on a slow diminishing trend this period. The sfc pressure pattern will yield a NW direction thruout this period. The sfc pg will be on a slow relaxing phase with the hier winds at the start of this period, 15 to 25 kt, slowly diminishing to 10 to occasionally 15 kt at the end of this period. Significant seas will already have a negative aspect to it for trying to build and that is the limited fetch associated with NW offshore winds. Looking at 3 to 6 ft at the start of this period, subsiding to 1 to 3 ft by the end of this period. Not much of an underlying ground swell this period just 3 to 5 second period wind waves. LONG TERM /SATURDAY THROUGH MONDAY/... As of 340 PM Tuesday...high pressure will ridge down the coast with a surge of cooler air on Sunday. The high pressure will extend down over the local waters Friday through Saturday before a cold front approaches from the north. Northwest flow of 10 to 15 knots will continue through Friday before veering to the south and west on Saturday and Sunday. Winds will increase on Sunday with a backdoor front and seas are expected to increase to 4 to 7 feet by Sunday late. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...Small Craft Advisory until 6 PM EDT Thursday for AMZ250-252- 254-256. && $$ SYNOPSIS...DRH NEAR TERM...JDW SHORT TERM...DCH LONG TERM...DRH AVIATION...JDW is the U.S. government's official web portal to all federal, state and local government web resources and services.