Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 201049 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 648 AM EDT Tue Mar 20 2018 .SYNOPSIS... An area of low pressure will track NE along a quasi-stationary front this morning. This front will slowly push south as a cold front later this afternoon. A strong upper level system will track east across the area tonight and into the day on Wednesday. Drier high pressure will briefly return over the area on Thursday. && .NEAR TERM /TODAY/... As of 415 AM Tuesday... Compact upper low over the Ohio Valley this morning will quickly shear apart as it lifts NE up the spine of central Appalachians. On the heels of this lead wave, the consolidation of a series of trailing short-wave troughs over the Tn Valley/southern Appalachians this evening will lead to the development of a closed mid/upper level low that will cross the southern Mid-Atlantic states tonight through midday Wednesday. An area of low pressure will track NE along a quasi-stationary front this morning. This front will slowly push south as a cold front later this afternoon with another wave of low pressure spinning up across upstate SC as the front sags south of the area. This low will quickly transfer it`s energy to the old coastal low off the northern NC Coast tonight and will deepen as it lifts NE up the DELMARVA coast on Wednesday. Remnants of overnight convection/storms will likely be exiting the coastal plain counties shortly after daybreak. Subsidence in the wake of the lead wave lifting off to the north and prior to the arrival of the trailing upper wave by the late afternoon/evening, will result in considerable lull in rain chances across central NC through mid afternoon. If we see any re-development during that time frame, it will mostly likely be across the far SE zones, invof of the quasi-stationary frontal boundary that will begin to sag slowly as a cold front by late morning and into the afternoon, where models indicate 500 to 1000 J/Kg MLCAPE developing along the warm sector. The approach of the trailing shortwave trough into the southern Appalachians during the late afternoon/early evening, between 21 to 00z, will result in increasing rain chances, especially across the western Piedmont. With the front bisecting the area to start the day, followed by a slow southward push, there will be a sharp northwest-southeast temperature gradient across the area, with temperatures north of the front falling into the 40s and 50s during the afternoon, as a cool wedge of high pressure builds in from the north. Highs ranging from lower 50s north to lower to mid 70s south. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY/...
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As of 415 AM Tuesday... Precip chances will quickly ramp back up between 00 to 06z, with radar expected to look very convective in nature, courtesy of strong synoptic scale forcing/lift as the the amplifying upper trough assumes a negative tilt as it lifts across the area. Precip type is expected to fall as all rain during through this time, with a prominent warm layer aloft atop above freezing BL temperatures noted at all fcst soundings. Then between 06 to 15z, frontogenetically driven band of light precip will pivot SW to NE across the area, with precip expected to change-over to a rain-snow mix across the northern Piedmont/Coastal Plain counties, with the potential to fall briefly as all snow in small pockets where precip intensity is heavier. Bulk of precip looks to lift north of the area shortly after 15z Wednesday with DCVA resulting in intermittent light rain/showers thereafter as temperatures warm into the 40s. Highs in the lower 40s NE to upper 40s south. This system is very different than the system we saw one week ago, in which extremely high precip rates resulted in strong/deep column cooling via melting to support appreciable accumulations across the area. Instead, we are dealing with rather weak lift, especially where it intersects the dendritic growth zone. This lack of cross- hair signature, which has been proven to be a strong discriminator for reaching warning/advisory snow criteria, is absent. Without the higher rates to effectively cool the column and give us the classic deep isothermal layer, there is a very small window where we would see all snow, with more in the way of rain-snow mix in the weaker forced band as it pivots east across the northern Piedmont. Additionally, any snow that falls after 12z/daybreak Wednesday, will have a difficult time accumulating due to the time of day(tempered insolation above the cloud/precip layer and boundary layer temps above freezing. So at this time, this looks to be very marginal/nuisance event with minor snowfall accumulation of a half inch or less across the northern Piedmont/along and north of Highway 85. Am not planning on issuing any advisories at this time, allowing the oncoming day shift to evaluate the 12z suite of NWP guidance.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... As of 300 AM Tuesday... Wed night: The upper low will shift off the Delmarva and NJ coast, but a last shot of energy diving into the departing trough combined with lingering moisture at 1-3 km AGL (evident in 700 mb progs) may bring a final sprinkle or flurry across the N and NE Wed evening, but with waning lift, expect little to no additional significant precip after nightfall. Skies will trend to fair overnight, responding to rebounding heights, with just high cloudiness late as sheared trailing vorticity streaks SE over the Carolinas. Lows in the low-mid 30s. Thu through Sat: Expect mostly sunny/mostly clear skies, along with much below normal temps as a seasonably chilly high builds in from the NW/N. We may see a few shallow showers Fri with moistening in the upper reaches of a deep mixed layer as a weak mid level wave dives SE over the Southeast states. And passage of a warm frontal zone aloft SW to NE into the region will bring a few more clouds and perhaps another shower risk on Sat, mainly over the SW CWA. Highs in the 50s to around 60. Lows in the 30s. Sun/Mon: Model differences grow. The ECMWF amplifies ridging over Eastern NOAM between Rockies troughing and a deep low over the NW Atlantic, while the GFS (and to a lesser degree the Canadian) drop energy down the back side of the NW Atlantic low, causing notable height falls down the Northeast and Mid Atlantic and yielding a much more west-positioned low. This latter solution generates wintry precip over NC, mainly N/E sections, while the former (ECMWF) has all liquid precip chances mainly over the W/S. With confidence low, will lean toward climo, with only slightly below normal temps and a near-climo chance for rain. -GIH && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... As of 155 AM Tuesday... Aviation conditions will deteriorate overnight as an area of rain and a few embedded storms crosses central NC. Cigs are already MVFR at INT/GSO/RDU, and will become MVFR at RWI/FAY in the next couple of hours as the rain and isolated storms overspreads the area. All locations will trend to mainly IFR during the predawn hours, after 08z. Behind this precipitation, a return to MVFR is expected at INT/GSO/RDU by 14z, but these locations should stay MVFR through Tue within a residual cool stable pool. At RWI/FAY, however, a trend to VFR is expected by 16z. Much of central NC should be mostly dry from 14z until late today, around 22z. After this time, rain chances will start to rise once again from SW to NE, and cigs at all sites should trend back down to MVFR or IFR. Winds will be mostly under 10 kts, light/variable or from the SE or S this morning before veering to mostly N then NE during the day. Looking beyond 06z Wed, IFR conditions are likely (mainly cigs) the rest of tonight through daybreak Wed, with widespread rain. Rain will taper down to patchy light rain (or a brief rain/snow mix at INT/GSO/RDU/RWI) Wed morning before ending gradually W to E during the afternoon with a trend to MVFR or VFR. Behind this storm system, VFR conditions are expected late Wed night through Fri, although a few showers are possible Fri and again Sat as a warm front approaches the area from the SW. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL NEAR TERM...CBL SHORT TERM...CBL LONG TERM...Hartfield AVIATION...Hartfield is the U.S. government's official web portal to all federal, state and local government web resources and services.