Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 181547 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1050 AM EST Mon Dec 18 2017 .SYNOPSIS... Weak high pressure will extend across central NC today, then move offshore on Tuesday. A low pressure system will bring unsettled weather to central NC by Wednesday. && .NEAR TERM /THROUGH TONIGHT/... As of 1050 AM Monday... Primary forecast changes include increasing sunshine through this afternoon across all but the far NW, and nudging temps slightly upward across the south (lower cloud cover) and downward in the NW (greater coverage of clouds/fog this morning to retard heating). Latest surface analysis shows a weak surface pressure discontinuity and weak winds across central NC, and these are partly responsible for the stubbornness of the fog/stratus in the NW CWA, as convective processes are necessary to dislodge this stable air today due to the poor horizontal dispersion. Just a few high clouds and patchy cu (mainly south) are expected today with dry air through the column, stable mid levels, and a downslope westerly 850-925 mb flow. Have tweaked temps just a degree or two in the NW and S, and expect highs today from the upper 50s NW to mid-upper 60s south. -GIH Earlier discussion from 300 AM: Just minor adjustments made to the near term forecast, primarily to the sky cover and a slight uptick in the high temperatures today. Weak, baggy high pressure will extend across central NC from a parent high centered over the Deep South. The low level wly flow will advect a notably warmer air mass into our region. This air mass coupled with periods of partly sunny skies will boost afternoon temperatures into the low-mid 60s, at least 10 degrees warmer than Sunday`s high temperatures. The main caveat is that there is the potential for skies to remain overcast or mostly cloudy through mid day or early afternoon. If the sun fails to appear by early afternoon, high temperatures will likely be about 5 degrees cooler (mid-upper 50s nw to the lower 60s se). Tonight, the tranquil weather pattern will continue with no precipitation anticipated. A surge of warm, moist air from the south that the models have been fairly consistent with should lead to increasing cloud cover with the possibility of fog to develop, primarily south of highway 64. Min temps relatively mild for December with overnight temperatures ranging from the upper 30s to around 40 NW to the mid 40s SE. -WSS && .SHORT TERM /TUESDAY AND TUESDAY NIGHT/... As of 300 AM Monday... Tuesday, weak high pressure at the surface will weaken further while wly flow persists in the mid-upper levels. This flow will induce the development of a lee side trough in the western Piedmont. The resultant low level west-sw flow will continue to usher warm air into central NC. Potential for afternoon temperatures to reach 70 degrees across our southern tier counties with 60s common elsewhere. The day will start out rather cloudy with periods of sun probable in the afternoon. Similar to today, if the clouds persists into the afternoon, high temperatures will end up 5-6 degrees cooler than forecast. Main adjustment in the Tuesday night forecast was to delay the onset of precipitation across the western-southern Piedmont until midnight or a bit later when the moisture is deep enough and lift sufficient to generate precipitation. Otherwise expect increasing/thickening cloud cover southwest-to-northeast with mild overnight temperatures ranging from the mid 40s far north to the lower 50s south. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 220 AM Monday... Wednesday and Wednesday Night: Models are in much better agreement with the upper level wave moving into the region on Wednesday. High confidence in a wet forecast for Wednesday, with rain diminishing overnight as the deamplifying low shifts offshore. A the surface, though models are in fairly good agreement, small differences in the location/track of the surface low will have large impacts on the temperature and precipitation forecasts. The consensus is for the low to remain south of the area, with the backdoor cold front sliding south into the area during the day Wednesday. That being said, the expected rainfall and overcast skies should limit cooling initially so expect highs in the mid to upper 50s, though if the more aggressive NAM solution verifies, temps could be a degree or two lower. High pressure will ridge southward into Central NC Wednesday night, though clouds and light rain may linger into the overnight hours. Should skies clear more, lows could be a bit lower, but for now expect lows generally in the mid to upper 30s, a good 10- 15 degrees lower than Tuesday night. Thursday through Friday Night: A CAD wedge will set up on Thursday, lingering over the area, especially in the northwest, into Friday. With good CAA and clear to partly cloudy skies, temperatures Thursday will top out in the upper 40s to mid 50s (north to south), while lows will be in the mid 30s to low 40s. Friday could be a bit warmer, especially in the southeast, but will depend on if/when the wedge erodes ahead of the approaching frontal system. With a grain of salt, expect highs in the mid to upper 50s and lows in the mid to upper 40s Friday and Friday night. With the southwesterly flow aloft overrunning the northeasterly cold air advecting into the region at the surface, expect increasing cloudiness and chances for some light stratiform rainfall Friday and Friday night. Otherwise, this part of the forecast will be relatively dry. Saturday and Sunday: The strong low pressure system that develops off the Rockies in the Midwest on Friday will strengthen as it progresses eastward Friday night into Saturday. Central NC will be under the influence of the upper level ridge, with the increasing southwesterly flow advecting significant moisture into the Appalachians. The EC and GFS differ on the evolution of the front and pre-frontal convection during this part of the period, thus confidence is relatively low. The GFS is more progressive, moving the front through the area on Saturday with the frontal zone stalling over the area, resulting in an extended period of rainfall for Central NC. However, the EC hangs the front up along the mountains on Saturday and Sunday, with best chances for rain along the VA/NC border. As a result of these significant differences, specifics of the forecast for this time period are very much up in the air. && .AVIATION /12Z MONDAY THROUGH FRIDAY/... As of 650 AM Monday... Pockets of MVFR ceilings are confined mainly across the Sandhills and southern coastal plain. These areas of low clouds should disperse shortly after sunrise. Pockets of mostly MVFR fog across the Piedmont and northern coastal plain will lift and dissipate by 14Z. Otherwise, VFR parameters anticipated through 06Z Tuesday. After 06z Tuesday, areas of low clouds and fog is expected to develop across central NC, primarily south of highway 64. The IFR/LIFR conditions appear highest in vicinity of KFAY, but may also plague KRWI and KRDU. VFR parameters should return areawide by 16Z Tuesday. A more persistent round of sub VFR ceilings along with areas of rain will occur late Tuesday night through Wednesday night as a low pressure system affects the region. Currently, VFR parameters are expected to return for Thu and Fri. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...Hartfield/WSS SHORT TERM...WSS LONG TERM...KC AVIATION...WSS

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