Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 221953 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 253 PM EST Mon Jan 22 2018 .SYNOPSIS... A cold front will approach from the west today, and cross central NC Tuesday morning. High pressure and associated seasonably cool and dry conditions will follow for mid-week. && .NEAR TERM /THROUGH THIS EVENING/... As of 1025 AM Monday... Just minor tweaks made to the near term forecast, mainly around timing of scattered showers later this evening. 12Z upper air analysis along with satellite imagery depict a deep and mature cyclone over the central Plains. This system projected to lift newd as strongest winds aloft have rounded the base of the upper level trough. While the atmosphere over central NC remains relatively dry, flow above 400mb advecting enough moisture to produce a veil of high clouds overhead. This deck of high clouds currently thick enough to inhibit sunshine across the Piedmont, though looking upstream, some thin spots noted. Thus, some potential for at least partly sunny skies this afternoon. This warmth along with a steady sly low level flow should allow temperatures to recover into the 60s this afternoon. The aforementioned sly flow will advect enough moisture to support the formation of stratocu deck late this afternoon, mainly south of highway 64. As the atmosphere continues to moisten, could see an isolated shower or two streak nwd across the region after 4 PM. Latest near term model guidance has slowed the arrival of the better lift of showers until 06Z or later. So have delayed the timing of higher pops a few hours across the western Piedmont this evening. && .SHORT TERM /TONIGHT AND TUESDAY/... As of 400 AM Monday... Tonight: A warm front and associated leading edge of surface dewpoints in the 50s will surge nwd across cntl NC tonight, with low temperatures likely to occur during the mid-evening hours, before slowly rising overnight. Despite the increasing surface theta-e, bufr forecast soundings suggest a shallow, near-surface stable layer will likely remain in place, thereby muting surface-based instability and threat of severe weather overnight. Nonetheless, HREF mean MUCAPE values are forecast to climb to around 250 J/kg between 06-12Z, in a strong WAA regime driven by a 45-60 kt sswly llj at 925-850 mb. As such, scattered to numerous (slightly elevated) showers are apt to develop and overspread cntl NC, along the axis of the llj, overnight. Tuesday: The warm front will be quickly followed by an effective (lead) cold front forecast to cross cntl NC between 12-15Z over the nw Piedmont and between 18-20Z over the Coastal Plain. The passage of this cold front --one that may be better characterized as a dry line, and accompanying broken line of convection-- will result in clearing, drying the in the low levels, and the deepening of a diurnal mixed layer that will promote the development of strong sswly winds and associated gusts that will peak in the 30s kts per bufr forecast soundings. In addition, some realization of weak surface-based instability may occur ahead of the convective line with diurnal heating, mainly east of Hwy 1, though widespread pre-frontal cloudiness will limit this heating and associated destabilization, to a several hundred J/kg or less. Nonetheless, a few strong convective wind gusts will be possible as the line moves east. Any appreciable cooling will lag this feature and not occur until a secondary front settles sewd across cntl NC after 21Z. High temperatures will consequently again climb into the 60s, to lwr 70s over the sern CWFA. Storm total precipitation amounts are expected to be around a tenth to quarter inch on average, and scattered, so a widespread soaking rain is not anticipated. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
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As of 253 PM Monday... Cold and dry air advection behind Tuesday`s front will surge east quickly overnight with low level thicknesses falling ~35 meters by 12Z Wednesday morning. Mins in the (fortunately) well mixed airmass will bottom out in the low to mid 30s, as opposed to the potential for upper 20s if we were to have better radiational conditions in combination with model consensus thicknesses ~1305 meters. The cool airmass settling into the area will linger for a couple of days, aided by modest reinforcement as a short wave skates across the Ohio Valley on Wednesday night, veering low level flow more northwesterly through Friday. Stacked ridging which originates over the northern Gulf will edge over the area, with sunny skies, rising heights and lighter flow encouraging a modest warmup on Friday. Highs Wednesday and Thursday will range mostly from 50 to 55, a couple of degrees above normal, warming to mostly mid 50s on Friday. Morning lows will be below freezing, 28 to 32 degrees, both Thursday and Friday mornings. Warm air and moisture advection will be on the increase as a return flow regime sets up Friday night. Clouds will be spreading in from the south by early Saturday, and overspreading the area during the day. Models diverge significantly as to the progression of a short wave digging south into the northern Great Lakes. The 12Z ECMWF amplifies the wave pattern south into the western Gulf, producing a deep, frontogenetic zone with a surface low developing over southern LA and lifting into central AL. This would be much slower, wetter solution than the GFS, which keeps the flow much flatter and essentially pushes a much drier, weaker frontal zone southeast through the area and offshore Sunday. Will give a nod to the ECMWF`s longer range skill and stretch fairly high PoPs from the Saturday night period through Sunday night. Hopes are raised that this might produce a widespread heavier event to alleviate moderate drought conditions across the area. Highs Saturday and Sunday will be mild...upper 50s to mid 60s, with lows in the 40s. We should be drying out Monday, with highs near or above normal (50 to 55) regardless of which model turns out to be more correct.
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&& .AVIATION /18Z MONDAY THROUGH SATURDAY/... As of 105 PM Monday... While VFR parameters will persist through 21Z across central NC, an approaching frontal system will bring an increased threat for adverse aviation conditions to central NC tonight into Tuesday. A sly flow will advect enough low level moisture to cause a deck of stratocu to develop and progress nwd late this afternoon and this evening. This deck of MVFR ceilings will likely encroach upon the KFAY vicinity by 21-22Z, and across the remainder of the TAF sites between 00Z-04Z. A low level jet will strengthen overhead this evening and into the overnight. While sfc winds at the sfc will be sly 8-12kts, winds just above the sfc (~1000-1500ft AGL) will be sly 45-50kts, creating low level wind shear conditions. The frontal system will be just west of our region prior to daybreak leading to an increase in shower activity across the western Piedmont including the Triad terminal by 06Z, and in proximity of KRDU and KFAY by 09Z. Per latest model soundings, enough instability may reside aloft to support a thunderstorm or two, primarily between 08Z-14Z. The cold front will sweep eastward across our region early Tuesday morning. Rapid clearing is anticipated, leading to VFR parameters in the Triad vicinity shortly after 12Z, and in proximity of KFAY and KRDU by 15Z, and KRWI by 16Z. Sfc winds will remain blustery ahead and behind the front with southwesterly sfc winds frequently gusting 25-35kts, with sporadic gusts 40-45kts possible. The strong wind gusts should subside after 21Z Tuesday. VFR parameters expected Tuesday night through Saturday morning as high pressure will be the main influence on our weather. Another frontal system will begin to impact the Carolinas late Saturday into Sunday, possibly leading to a period of sub-VFR parameters. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...MWS LONG TERM...mlm AVIATION...WSS

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