Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230645 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 145 AM EST Tue Jan 23 2018 .SYNOPSIS... A lead cold front and markedly drier air will cross the Carolinas and Virginia early today, followed by a secondary cold front, with noticeably cooler conditions, this evening. Pacific high pressure will then build across the region through mid to late week. && .NEAR TERM /THROUGH TONIGHT/... As of 941 PM Monday... A couple of minor changes to the forecast this evening. The most notable is the addition of a marginal risk for severe weather out of the Storm Prediction Center over the Piedmont of central NC as a cold front approaches from the west . The main threat with this will be damaging winds as even without thunderstorms, showers could produce winds in excess of 40 mph. An isolated tornado can also not be ruled out with any isolated thunderstorms that do pop up as predicted 0-1 km helicity values climb to upwards of 400 m2/s2. The real limiting factor with this system will be the lack of instability which could keep convection limited to showers. A potential boost during the early morning hours will come from an increase in dewpoints as values in the upper 50s to lower 60s currently over upstate South Carolina and NE Georgia will advect northeastward into the forecast area between 9-12z. Despite these changes, the upshot of this event should still be widespread rain showers with breezy conditions moving through the area with a few isolated thunderstorms possible, of which it is not out of the question to see a report or two of damaging winds. Non-convective winds will also be a concern overnight. During the evening and early overnight hours, low-level wind shear will be possible as 925 mb winds of up to 50 kts will be possible above surface winds that only 5-10 kts. While these 50 kt winds aloft were fairly prevalent earlier in the models, shorter term high resolution runs are have backed off to 35-40 kts. The low-level wind shear threat will subside once winds are mixed down to the surface at which time 15-20 kts sustained winds with gusts of 25-30 kts will prevail. QPF with this event will be limited with about a quarter of an inch expected in the west with diminishing amounts further to the east. Timing will see the heaviest rain start to move into the west after 6z and into the Triangle around 9Z and 12z for the I-95 corridor. Rain will exit those same areas around 16,18,and 20z respectively. Low temperatures in the low to mid 50s are expected although current observations suggest these values may be slightly low. && .SHORT TERM /TUESDAY/... As of 325 PM Monday... Band of scattered-broken showers will be ongoing at daybreak across the heart of central NC, and is expected to move east and exit our coastal plain counties by 15-16Z. Strong kinematics and the potential for a narrow band of sfc based instability will enhance a small threat for a strong or locally damaging wind gusts with a heavier shower or isolated t-storm through 15Z. This threat appears highest along and east of I-95. Otherwise a sfc cold front will cross central NC Tuesday morning, with rapid clearing expected in its wake as the dry slot associated with a deep, mature cyclone over the Great Lakes streaks into the Carolinas. The mixing that will ensue after the clearing will translate the strong winds a thousand feet or so above the surface downward, contributing to sfc wind gusts 30-40 mph. This period of strong gusty winds expected to last an hour or two after the initial clearing, with gusts 25-30 mph probable into the afternoon. The increasingly sunny skies, breezy sw winds, and early morning temperatures in the mid 50s to around 60 support temperatures warming into the 60s across the Piedmont, and around 70 in the coastal plain and Sandhills. This seems plausible as the 850mb trough does not cross our region until mid-late afternoon, delaying cold air advection until late in the day or after sunset. && .LONG TERM /WEDNESDAY THROUGH TUESDAY/...
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As of 145 AM Tuesday... Wednesday through Saturday: Expect dry weather through Saturday. An upper level shortwave and a reinforcing dry cold frontal passage is expected Wednesday night/Thursday. Surface high pressure is expected to build into the area in the wake of the secondary cold front and remain over the area into Saturday. Meanwhile, the upper level ridge will move eastward over the Carolinas through Friday night. Highs will be generally in the upper 40s north to mid 50s south through Wednesday and Thursday, with moderation beginning Friday. Lows in the upper 20s to low 30s Wednesday and Thursday nights, increasing Friday night. Saturday Night through Tuesday: Another upper level trough and surface cold front will develop over the Midwest and progress eastward toward the Mid-Atlantic over the weekend. As southwesterly flow increases, so will the warm, moist advection into the region. As a result, cloud cover and chances for precipitation will increase late Saturday into Sunday as the cold front approaches. Some differences in the evolution of the upper level trough and the timing and duration of convection as it moves into/through Central NC have developed between the medium-range models. The ECMWF is more amplified, slower, and wetter than the GFS. As a result of the latest forecast cycle model differences, confidence in the timing of the front and convection associated with it is low.
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&& .AVIATION /06Z TUESDAY THROUGH SATURDAY/... As of 130 AM Tuesday... Generally IFR-MVFR ceilings, and MVFR visibility restrictions in showers and isolated thunder, will become VFR with the passage of a lead cold front from west to east between 12-14Z at wrn sites and 15- 17Z at ern ones. A strong sswly low level jet, on the order of 40-50 kts as low as 1500-2000 ft, with progressively stronger winds above that level, will continue to migrate newd across cntl NC this morning. The presence of this feature will support the continued newd development of the aforementioned ceilings and convection, and the maintenance of low level wind shear conditions this morning. While some of this stronger flow and momentum aloft will mix to the surface and manifest as a breezy sly surface wind, the magnitude of the flow aloft suggests llws nonetheless remains warranted until the aforementioned cold front crosses the region later this morning. Behind that front, abrupt clearing will occur, with a veering of surface winds to swly and similarly abrupt increase into the 15-25 kt range (sustained), with gusts to around 35 kts for several hours immediately following the frontal passage. These winds will then lessen and continue to veer to wly or nwly later this afternoon and evening. Outlook: MVFR ceilings possible Sat, with showers Sat night, ahead of, and in association with, a weekend frontal system. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...Ellis SHORT TERM...WSS LONG TERM...KC AVIATION...MWS

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