Scientific Forecaster Discussion

000 fxus62 krah 220256 afdrah Area forecast discussion National Weather Service Raleigh NC 955 PM EST Thu Feb 21 2019 Synopsis... a cold front will cross central NC tonight then stall to our south on Friday. Cold high pressure will build into our region from the north. A series of surface waves along the front will result in periods of rain and drizzle while the cold air supplied by the high will result in high temperatures well below normal for late February. && Near term /through tonight/... as of 955 PM Thursday... Only minor forecast adjustments, mainly to slow pop timing a bit. The somewhat complex frontal configuration over the region includes a strengthening backdoor cold frontal zone to our N, which will drop into and through NC over the next 12 hrs. Areas of rain will continue to spread into central NC from the west and fill in overnight as moist upglide, focused on the 295k surface, slowly deepens and strengthens with backing and acceleration of the 850 mb flow over NC. The rain should be greatest in coverage and intensity over the north and west portions of the cwa, where additional forcing for ascent from passage of weak mid level perturbations on the poleward side of the ridge east of the Bahamas along with a gradual uptick in upper divergence will boost overall lift. Precipitable water will also remain 200- 300% of normal. Will trend pops up to likely/categorical northwest with good chance east overnight. Fog remains a forecast concern tonight, particularly in areas that have been slow to cloud up this evening. A few of the "usual suspects" locations are already at a half mile, but given the increase in clouds to limit the radiational cooling, confidence is not high that we'll see widespread dense fog, and in general, high-res model guidance suggests that visibilities will bottom out in the 1-2 mile range in most spots. Will go with patchy fog areawide and monitor overnight. Expect lows through dawn to range from mid 40s north to low 50s S, with The Drop in temps continuing into Fri. -Gih Earlier discussion from 325 pm: partial clearing has occurred over portions of the Piedmont this afternoon. The peeks of sun has aided to warm temperatures into the 50s. While a low overcast has persisted across the southern counties, the lack of rainfall has permitted temperatures to warm to around 60-lower 60s. This partial clearing trend will continue through early this evening. Where skies remain partly cloudy, areas of fog will develop. The fog potential currently appears highest in vicinity of the I-85/40 corridor and along Highway 64. Uncertain whether the fog will be dense enough to warrant an advisory though. A sfc cold front, currently poised just to our NW, will drift southward through central NC tonight. Chilly high pressure will build into our region overnight. Meanwhile the flow a few thousand feet above the surface will strengthen in response to a S/W lifting newd into the Tennessee Valley from the lower MS valley. This sly flow will pull warm moist air over the cool stable sfc air, leading to the renewed development of low clouds and areas of rain. This rain threat will increase after midnight across the southern counties, and areawide prior to daybreak. It will remain relatively mild this evening as most locations will see temperatures remaining in the 50s. Temperatures will eventually cool from the north, with early morning lows projected to be in the low-mid 40s north to the upper 40s/near 50 across the south. && Short term /Friday and Friday night/... as of 325 PM Thursday... The near surface air mass will continue to cool Friday as high pressure builds into the region from the north. Meanwhile sly flow above the surface will continue to advect moisture over the cool stable dome. This will tiled areas of rain/drizzle along with falling temperatures. Most locations will see their calendar high reached between midnight and 8 am as temperatures will fall into and through the 40s during the morning, then hold steady in the low-mid 40s through the afternoon. Most persistent rainfall anticipated across the northern counties. Currently expect rainfall to total between a half to one inch. This should not lead to any flash flooding but should result in appreciable rises on area creeks/streams/rivers as much of the region has received between one and two inches of rain during the past week. The wet and cold conditions will linger into Friday night as a series of mid/upper level perturbations continue to lift northeastward from the lower MS valley into the Tennessee Valley, maintaining a moist sly flow aloft. This set-up will prolong the areas of rain/drizzle into Friday night. Overnight temperatures generally in the upper 30s to around 40. && Long term /Saturday through Thursday/... as of 245 PM Thursday... Saturday: the sub-tropical ridge aloft will extend northward into the Carolinas through Saturday while the cad wedge will remain in place as the surface. Southerly flow off the Gulf will advect moisture into the deep south, while westerly flow will carry that moisture eastward into the Carolinas. As a result, light shower activity with cloudy skies with highs in the mid 40s north to mid 50s south and lows in the mid 30s to low 40s are expected. Saturday night through sunday: the next frontal system to impact central NC will develop Friday night/Saturday as the upper level trough swings through the Desert Southwest/southern rockies and into the Midwest. The surface low will come off The Rockies during that time, deepening as it accelerates northeastward toward the Great Lakes Saturday into Sunday. As the low tracks well to the north the front will slow as it impinges on the Appalachians Saturday night. The medium-range models suggest The Wedge will be strong enough to hold until the fropa early Sunday, though it could break just prior to the arrival of the front. The warm front will likely make some northward progress, resulting in a large temperature gradient across central NC ahead of the front. Expect the front to progress all the way through the area by Sunday aft/eve. Best chances for convection will be along and ahead of the front late Saturday night through early/mid Sunday afternoon. Somewhat low confidence in the high temperature forecast as it will depend on whether/how long The Wedge holds and the timing of the front. For now generally expect highs in the mid 60s to low 70s Sunday. Lows behind the front in the mid 30s to around 40 degrees Sunday night. Monday through wednesday: surface high pressure will build into the region behind the front, with dry, seasonable conditions expected through Tuesday. The upper level trough amplifies Tuesday night into Wednesday as the surface high pushes offshore. The resultant southerly return flow may result in increasing chances for rain, primarily across the south, on Wednesday. Confidence is very low at this time. && Aviation /00z Friday through Tuesday/... as of 630 PM Thursday... 24 hour taf period: conditions have broken out to vrf across the northwest Piedmont with some lower MVFR and IFR ceilings remaining mainly in the southwestern portions of the forecast area. While conditions are steadily improving, it may not be for long as areas of fog are possible after dark, particularly in areas that have cleared this afternoon and then another round of stratus is expected with MVFR conditions lowering potentially to IFR/LIFR levels as we go into Friday and Friday afternoon. This is due to another system moving into the area that is forecasted to bring more rain to the area. Not expecting much in the way of low level wind shear with this event. Winds will be light and variable overnight and then turning to northeasterly on Friday. Some gusting of the winds is possible further south and east, up to 20 kts. Long term: continued unsettled weather will bring bouts of sub-VFR conditions to the area into Sunday before a cold front moves through returning conditions to VFR for the beginning of next week. && Rah watches/warnings/advisories... none. && $$

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