Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230631 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 131 AM EST Tue Jan 23 2018 .SYNOPSIS...
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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.
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&& .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 /TUESDAY NIGHT THROUGH MONDAY/... As of 253 PM Monday... Cold and dry air advection behind Tuesday`s front will surge east quickly overnight with low level thicknesses falling ~35 meters by 12Z Wednesday morning. Mins in the (fortunately) well mixed airmass will bottom out in the low to mid 30s, as opposed to the potential for upper 20s if we were to have better radiational conditions in combination with model consensus thicknesses ~1305 meters. The cool airmass settling into the area will linger for a couple of days, aided by modest reinforcement as a short wave skates across the Ohio Valley on Wednesday night, veering low level flow more northwesterly through Friday. Stacked ridging which originates over the northern Gulf will edge over the area, with sunny skies, rising heights and lighter flow encouraging a modest warmup on Friday. Highs Wednesday and Thursday will range mostly from 50 to 55, a couple of degrees above normal, warming to mostly mid 50s on Friday. Morning lows will be below freezing, 28 to 32 degrees, both Thursday and Friday mornings. Warm air and moisture advection will be on the increase as a return flow regime sets up Friday night. Clouds will be spreading in from the south by early Saturday, and overspreading the area during the day. Models diverge significantly as to the progression of a short wave digging south into the northern Great Lakes. The 12Z ECMWF amplifies the wave pattern south into the western Gulf, producing a deep, frontogenetic zone with a surface low developing over southern LA and lifting into central AL. This would be much slower, wetter solution than the GFS, which keeps the flow much flatter and essentially pushes a much drier, weaker frontal zone southeast through the area and offshore Sunday. Will give a nod to the ECMWF`s longer range skill and stretch fairly high PoPs from the Saturday night period through Sunday night. Hopes are raised that this might produce a widespread heavier event to alleviate moderate drought conditions across the area. Highs Saturday and Sunday will be mild...upper 50s to mid 60s, with lows in the 40s. We should be drying out Monday, with highs near or above normal (50 to 55) regardless of which model turns out to be more correct. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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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.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...Ellis SHORT TERM...WSS LONG TERM...mlm AVIATION...MWS

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