Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 172016 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 416 PM EDT Sat Mar 17 2018 .SYNOPSIS... An area of low pressure will move southeast along a frontal boundary which stretches across the area tonight. Cooler high pressure will build into the Carolinas behind the low on Sunday. && .NEAR TERM /TONIGHT/... As of 330 PM Saturday... Dew points have been creeping up all day, having edged into the lower 50s across the southern tier this afternoon. The surface frontal boundary stretches from the Central Coastal Plain to the far Northwest Piedmont, setting up a nice corridor along the path the upper short wave will be following tonight. Expect to see convection multiplying by late afternoon then edging southeast along the boundary and into the Northwest Piedmont shortly after sunset. Convective parameters would be alarming if the diurnal timing were more favorable, with very steep mid level lapse rates and a strong shear profile to support at least modest potential for elevated strong storms...perhaps multicellular or supercellular structure, per the day one outlook for a slight risk across the heart of the area. Mesoscale models have been reasonably consistent, timing-wise, in depicting the best convective potential between 700 AM through 300 AM, and have made some fine tuning to the likely PoPs and thunder potential along the favored frontal zone accordingly. Cannot entirely rule out a rouge storm in the southeast, where initial low level instability and updraft helicity will be more favorable. Mins will be mild, low 40s north to upper 40s south, except for the northeast, where earlier (predawn) onset of cold air advection in northerly flow behind the system will allow mins to fall into the upper 30s. && .SHORT TERM /SUNDAY AND SUNDAY NIGHT/... As of 330 PM Saturday... Sun will break out during the morning with cool and dry advection underway early Sunday in northeast flow as high pressure scoots across the Ohio Valley and pushes down the Atlantic coast. Highs will benefit from insolation, reaching the upper 50s to lower 60s. The tranquility will be short-lived, however, as the high moves offshore quickly, allowing for southwest return flow ahead of our next system, which will begin to spread increasing cloudiness across the area by late Sunday night, with a chance of light rain in the southwest Piedmont by morning. Lows will be cooler in the northeast Sunday night, mid 30s, while cloudiness and earlier onset of return flow will limit the diurnal fall to the mid 40s in the west. && .LONG TERM /MONDAY THROUGH SATURDAY/...
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As of 410 PM Saturday... Confidence is increasing regarding two episodes of precipitation across cntl NC early to mid week, including some wintry precipitation Tue night-Wed, in a somewhat similar fashion as last Mon. A progressive trough aloft now over the Intermountain West will evolve into a closed mid-upr low east of the cntl Rockies by Sun night, then get sheared/crushed as it migrates through confluent flow around the base of a negative height anomaly over sern Canada, to the middle Atlantic coast by Tue. This trough will support Miller "Type B" cyclogenesis along the srn fringe of a cold air damming high anchored by the confluence aloft, with an associated wave of low pressure likely to deepen while tracking the srn Piedmont to the srn-cntl Coastal Plain Mon night. The result will be lowering overcast on Mon, with an increasing probability of rain from sw to ne with time on Mon, and a subsequent likely to categorical probability of rain/showers Mon night, with nrn (climatologically- favored) counties likely to remain in a cooler and more stable wedge air mass, while srn ones experience milder and slightly unstable conditions, with an associated slight chance of thunder Mon night. Elevated thunder will be possible as far north as roughly the HWY 64 corridor. Thereafter, medium range guidance solutions are similar to those of 24 hours ago, which had trended toward a deeper and more srn solution regarding a trailing shortwave trough rotating through the base of the l/w ern U.S. trough - a pattern very similar to that of last Sun night-Mon. Such a scenario would favor the development of a mid level deformation precipitation band over the srn and s-cntl Appalachians and subsequent enewd migration across the srn middle Atlantic states Tue night-Wed, on the nw side of a developing surface cyclone over and offshore of SC. Given this stable signal in model guidance, the dominant large scale pattern described in recent days that will favor a srn storm track, and the (overnight Tue-Wed) timing of the deformation band, it appears increasingly probable that portions of at least the nrn Piedmont and nrn Coastal plain will experience a period of wintry precipitation. Similar to the last event, partial thicknesses/deep layer thermodynamic profiles will trend sufficiently cold for snow aloft, with the primary inhibiting factor being a slightly above freezing boundary layer. With that in mind, and also similar to the last event, melting of snow and associated cooling of the boundary layer --to freezing-- would likely occur when the heavier precipitation rates occur, and/or when insolation will be at a minimum late Tue night-early Wed. As such, a light accumulation of snow will be possible, though it is far too early to determine any specific amounts or details. A drying and clearing trend will commence as the ern U.S. trough gradually relaxes and lifts away through the end of the week. Temperatuires will consequently gradually modify, though still below average, and with freezing conditions probable Fri and Sat mornings. The next weather system will migrate east and spread a chance of rain east of the Appalachians next weekend.
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As of 130 PM Saturday... Initial surge of heavier mid cloudiness and patchy light rain has moved east of the area, with a clearing trend spreading east which will continue through around 00Z. Expect cloudiness to increase in from the northwest between 00-03Z as a low pressure area moves east along a stalled frontal zone across the area. This low will lead to a 4-6 hour window for showers (and potentially some stronger thunderstorms capable of producing some hail and damaging wind gusts), from 02Z-07Z in the Triad, and from 02-08Z at the eastern sites. Storms would be moving quickly and will not be extensive, so will not include in the TAFs at this point due to low confidence in the timing and just where they will track. MVFR/IFR ceilings will linger into early morning (12-14Z), with dry northeast flow scattering out the clouds to VFR late in the TAF period. VFR conditions are expected Sunday afternoon as high pressure settles into the area, then another threat of sub-VFR conditions arrives on Monday, with the sub-VFR threat persisting into the middle of next week as a series of disturbances passes overhead. Confidence is not high regarding timing of these disturbances, so aviation interests should check back for the latest thinking.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS/mlm NEAR TERM...mlm SHORT TERM...mlm LONG TERM...MWS AVIATION...mlm is the U.S. government's official web portal to all federal, state and local government web resources and services.