Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 271904 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 304 PM EDT Sun May 27 2018 .SYNOPSIS... The combined flow between a ridge over the western Atlantic and sub- tropical storm Alberto will drive an extremely moist air mass across the Carolinas through mid week. The majority of the remnants of Alberto are largely expected to remain west of the Appalachians through the end of the work week. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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As of 300 PM AM Sunday... The latest surface analysis shows a Bermuda high pressure system extending west toward the Southeast coast this afternoo. A stationary front was noted across the mid-Atlantic while subtropical storm Alberto was moving north across the eastern Gulf of Mexico. The air mass across central NC remains quite moist although precipitable water values currently ranging in the 1.5 to 1.8 inch range are lower than previous days. Surface dew points range in the upper 60s to lower 70s across much of central NC. The 18Z SPC mesoanalysis shows a weak to moderately unstable air mass with MLCAPE values near 1500 J/Kg near the VA border decreasing to near 500 J/Kg near the SC border. Mid level lapse rates range near 5.5 deg C/Km but DCAPE values exceed 800 J/Kg across the Coastal Plain and Sandhills. With limited forcing for ascent and warm mid-levels, convection has been limited this afternoon and convection allowing models have handled this well. The greatest chances of a shower or storm will be near and especially north of interstate 85 in a region of weak convergence in the 850- 700 hPa layer and greatest instability. The southerly flow will lift the scattered showers slowly northward with this convection abating fairly quickly this evening with the loss of heating. Tropical moisture will surge back northward into central NC late tonight as the deep layer southerly flow increases. Precipitable water values will increase late tonight, exceeding 2 inches across the south by daybreak. An area of showers and thunderstorms in a weakly unstable air mass, driven by a trough at 925 and 850 hPa will lift north from GA and FL this evening into SC and eventually southern NC after 06Z. Widespread showers with heavy rain should reach the Sandhills, Southern Piedmont and southern Coastal Plain around daybreak, setting the stage for the potential of some localized flash flooding on Monday. With clouds and a light southerly flow, low temperatures should range around 70. -Blaes
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... As of 325 AM Sunday... The plume of deep tropical moisture quickly advects into southern portions of central North Carolina just after midnight on Monday morning, overspreading tropical downpours and a few embedded low topped thunderstorms from south to north. Guidance is in fairly good agreement on the timing of this band of heavy precipitation, with forecast rainfall estimates across the area between 0.75 to 1.5 inches with locally higher amounts possible before midnight on Tuesday. With already moist antecedent conditions leading to low flash flood guidance, PWAT values above 2 inches, and a large area of 1+ inches of expected rainfall totals...the potential for flash flooding cannot be ignored. This is especially true in areas that see several rounds of heavy downpours, or stronger storms that produce the highest rainfall rates. The Weather Prediction Center currently highlights a large portion of our area with a "Slight" (10 - 20%) chance of seeing excessive rainfall during this period of the forecast. With the timing of this band being so early in the day, displaced quite a bit from peak heating, the severe threat doesn`t look all that impressive. Even some of the more bullish models struggle to bring SFC CAPE Values above 1000 J/KG Monday evening. Typically, a much larger concern in a setup like this is for High Shear, Low CAPE type spinup tornadoes with any embedded thunderstorms that are able to tap into the 0 to 1km Bulk Shear. While the threat for this can`t be eliminated totally, our area will remain displaced from the better sheared environment around Alberto, keeping this threat low. && .LONG TERM /TUESDAY THROUGH SUNDAY/... As of 320 AM Sunday... Our rainy period will persist into the late week as a tropical flavored airmass lingers over the area. Periods of potentially heavy showers are expected, which will maintain the flood threat at least through Wednesday, and possibly Thursday as well. Alberto will be inland in the vicinity of central AL at the beginning of the extended period. The associated moisture plume off the Gulf will be focused into the central/southern NC mountains and Tennessee Valley. Meanwhile, a second, separate moisture plume is expected along the southern Atlantic coast due to southerly flow on the western periphery of a strong mid level ridge. The ridge will block Alberto from making much eastward progress, and will force the system slowly almost due north, to reach the Ohio Valley on Wednesday. While central NC will initially be in the `relative` dry slot between the systems, showers will be likely along with scattered, mainly diurnal storms which will produce heavy showers due to deep layer moisture (PWAT`s in this `dry slot` will still range between 1.8 & 2.1 inches) and strongly diffluent flow aloft. Highs will be from 80 to 85. Confidence is lower heading into the mid week, but it looks like we will see a surge of wrap-around moisture into central NC courtesy of Alberto`s remnants Tuesday night which will help maintain the anomalously moist airmass. While forcing will be weak, it won`t take much to produce and maintain numerous, potentially heavy, showers overnight. Alberto`s weakening remnants will be in the Ohio Valley Wednesday, but our subtropical ridging hangs tough along the Atlantic coast. The deep southerly flow maintains our moist airmass all the way through Thursday, with widespread (60-70% PoPs at this point) rain expected, but lower confidence as to pinpointing exactly when and where the very highest probabilities may lie. Low level thicknesses will be ticking upward in warm air advection, but tempered by heavy cloudiness/precip, so highs Wednesday and Thursday will be stall in the low and mid 80s. Modest drying (PWAT`s < 1.5 inches at last) ensues on Friday as the Atlantic ridge finally deamplifies and edges further offshore as a short wave moves southeast across the area on Friday. Scattered mainly diurnal convection is expected both Friday and Saturday as while we won`t be as wet, we will be warmer and more unstable as highs reach way up into the 80s and very low 90s. Morning low temperatures through the extended will be mostly persistence due to the unchanged airmass...within a couple degrees either side of 70. && .AVIATION /19Z SUNDAY THROUGH FRIDAY/... As of 215 PM Sunday... Through 18Z Monday: SCT-BKN cumulus in mainly VFR conditions were noted across central NC this afternoon with a few lingering patches of MVFR cigs in the Triad. Scattered showers and a few thunderstorms are expected this afternoon into the early evening, mainly near and north of interstate 85, which could affect the KINT and KGSO terminals. Given the limited coverage, will omit a mention of t- storms in the TAFs but note a tempo period with reduced vsbys and cigs in a shower. VFR conditions are expected for much of the evening and early overnight before increasing mid and especially low cloudiness moves north into central NC after midnight. MVFR and IFR conditions associated with rain showers spread from SC into southern NC a little before daybreak affecting the KFAY terminal. Restrictions continue north on Monday morning reaching the remaining terminals between 13 and 16Z. After 18Z Monday: An unsettled period with periods of adverse aviation conditions are expected associated with intervals of mainly afternoon and evening showers and thunderstorms and periods of morning stratus. && .EQUIPMENT... KGSO Greensboro upper-air observations will be unavailable through at least the evening release on Tuesday, May 29th because of faulty hardware. && .RAH WATCHES/WARNINGS/ADVISORIES...
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Flash Flood Watch from 1 AM EDT Monday through Tuesday morning for NCZ026>028-038>043-073>078-083>086-088-089.
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&& $$ SYNOPSIS...BLAES NEAR TERM...BLAES SHORT TERM...JM LONG TERM...MLM AVIATION...BLAES EQUIPMENT...BLAES

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