Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 201823 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 230 PM EDT Tue Mar 20 2018 .SYNOPSIS... A series of low pressure areas will move eastward along the surface front through Wednesday. An upper level low will sweep through the area late Wednesday through Wednesday night, finally allowing the surface frontal system to move east and out of our region. Cool high pressure will build into the area for Thursday and Friday. && .NEAR TERM /TODAY/... As of 1010 AM Tuesday... Morning update: Some light rain continues across the northeastern quadrant of the forecast are this morning but very little QPF is expected with this through the rest of the morning. After this moves out to the east, A lull is expected in precipitation for much of the day before another round of rain moves into the Triad area late this afternoon/evening. With a wedge front hanging across the area, temperatures continue to be a difficult forecast for this afternoon. Currently temperatures in the VA border counties are in the low 40s with low to mid 60s across the southeast. This southeastern area will rise into the 70s today and some instability moving along the NC/SC border will give a chance for some thunderstorms later this afternoon/evening but this will be confined to the extreme southeastern portions of the forecast area. Previous discussion follows. -Ellis 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/... 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. && .LONG TERM /THURSDAY THROUGH TUESDAY/... As of 230 PM Tuesday... The upper trof will shift offshore, with ensuing deep northwest flow settling in through the weekend to maintain unseasonably cool temperatures. Initial surface high pressure building in on Thursday and Friday will be dry, with increased sunshine allowing mins in the morning to fall below freezing across the north to mid 30s south on both Friday and Saturday mornings. A lot of divergence in model solutions this weekend, as a short wave will be racing southeast in the mid level flow, with deep cooling of the airmass with potential precipitation breaking out on Saturday and continuing through Sunday. This wave could produce some mixed precip across the northern tier where CAD airmass will be deeper, but will let this ride for now given low confidence. The cool airmass will remain entrenched over the area on Monday, with modest warming and perhaps some sun Tuesday as upper ridging amplifies up the east coast. High temperatures all the way through the extended will be hard-pressed to climb out of the mid 50s over most of the area, with some 60s across the southern tier dependent on the extent of the CAD airmass. Low temperatures will be mostly in the mid to upper 30s, with perhaps some lower 40s in the south and east. && .AVIATION /18Z TUESDAY THROUGH SATURDAY/... As of 225 PM Tuesday... Residual cool airmass across the area will maintain predominantly IFR conditions over the TAF sites through 00Z, with LIFR returning along with widespread rain by 03Z. Visibility will remain mostly VFR as winds remain 6-10 knots over night from the NW-NE. Expecting rain to mix with, perhaps change over to snow in the Triad sites towards 09Z, concurrent with exit of upper short wave and arrival of deep cooler air. Precip will be tapering off to more showery nature by 12Z, with ceiling improving slowly by late morning, perhaps reaching MVFR by 18Z. VFR conditions are expected to return Wednesday night through Friday, with unfavorable aviation conditions possible on Saturday and into Sunday as a warm front approaches the area from the SW. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL NEAR TERM...Ellis/CBL SHORT TERM...CBL LONG TERM...MLM AVIATION...MLM is the U.S. government's official web portal to all federal, state and local government web resources and services.