Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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846 FXUS62 KRAH 180044 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 844 PM EDT Sat Mar 17 2018 .SYNOPSIS... An area of low pressure will cross central NC tonight. Cool high pressure will build into the region Sunday, then drift offshore by early Monday. A series of low pressure systems will affect our area Monday through Wednesday, causing a period of unsettled weather. && .NEAR TERM /TONIGHT/... As of 650 PM Saturday... Main adjustment to the near term forecast was to raise hourly temps few degrees to reflect observed trends, and to decrease the PoPs through early evening as better forcing for scattered convection not expected to arrive until 02Z or a little later. Scattered strong convection has developed over sections of southeast KY-sw Virginia, in vicinity of the 925mb front and immediately ahead of a 1006mb sfc low. Near term models maintain this low level boundary in vicinity of the NC/VA border, dipping into the northern coastal plain. this feature will provide an avenue for scattered storms to travel long late this evening-overnight, so expect the greatest coverage north-northeast of KRDU. As the storms encounter an air mass with slightly better moisture advection and low level instability, expect a few more storms to develop over the Piedmont as well. Bulk of the convection will move east-se and projected to exit our far east-se counties prior to 09Z. Potential still exists for an isolated severe storm or two with the primary severe weather threat being large hail up to the size of quarters. Some convective presentations suggest a wind threat developing over the far northeast Piedmont into the coastal plain overnight as the storms evolve into a bowing segment. Prior to the storms arrival, expect a mild evening with temperatures primarily in the 60s, cooling into the 50s after 03Z. Min temps by early Sunday should vary from the upper 30s to near 40 far northeast to around 50 across the south. && .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/... 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. Temperatures 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. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/...
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As of 835 PM Saturday... 24 Hour TAF Period: An area of low pressure is expected to track across central NC late this evening/early Sunday morning along a stalled surface boundary. This will result in scattered showers and storms across the area, generally in the 02-08Z time frame from north/west to south/east. Lightning, strong gusty winds, and hail will accompany the strongest convection. Behind the cluster of showers and storms and accompanying surface low, expect the surface front to shift back southward, with IFR/MVFR cigs quickly developing in its wake. Cigs will be slow to lift Sunday morning, with VFR conditions expected to return by mid to late morning at most sites, with MVFR cigs possibly lingering into the early afternoon at KFAY. Afterwards, VFR conditions are expected to prevail for the remainder of the 24 hour TAF period as surface high pressure settles into the region. Outlook: Another threat of sub-VFR conditions will arrive 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 NEAR TERM...WSS SHORT TERM...mlm LONG TERM...MWS AVIATION...BSD/mlm is the U.S. government's official web portal to all federal, state and local government web resources and services.