Area Forecast Discussion
Issued by NWS Wakefield, VA

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588 FXUS61 KAKQ 271450 AFDAKQ Area Forecast Discussion National Weather Service Wakefield VA 1050 AM EDT Tue Sep 27 2016 .SYNOPSIS... A cold front tracks across the region today, stalling along the coast tonight. The front remains along the Mid-Atlantic coast through the end of the work week, resulting in unsettled weather conditions into the weekend. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1030 AM Tue...Updated fcst through rest of today to fine tune pops/weather/wind grids. Best precip coverage depicted well in morning grids with regional radar showing light to ocnl moderate rain showers across nwrn third of the FA. Latest high res model suite (NAM12/HRRR/RAP) indicate area of enhanced 925-500mb fgen/omega developing and spreading southeastward across the central FA in a SW- NE oriented fashion roughly from KEMV to KMFV by this afternoon. Have fine tuned area of categorical pops to this region, while maintaining slght chc thunder as weak instability will develop despite the widespread stratus. Lower pops will flank the NW and far SE zones as precip will be much more widely sct in nature. Max temps look on par with latest low level thickness fcst 1370-1390 meters. Previous Discussion...Latest GOES water vapor imagery centers an anomalous upper-level low over the upper Great Lakes region. A plume of high water vapor also visible spreading into the Mid- Atlantic region ahead of a cold front located over the central Appalachians. Lead shortwave moving across the region has produced a line of showers along the front. The showers and spreading into the northwest Piedmont. Another area of showers has pushed into central Virginia to the Virginia Eastern Shore. Best chances for precip this morning will remain north of a line from Farmville to Richmond and Snow Hill. Have retained categorical POP`s for that region. The cold front reaches central Virginia by mid/late morning, and then the coast by early evening. The front then stalls along the coast in deep layer southwesterly flow. Meanwhile, the deep upper- level low drops southward over the Great Lakes. Best chances for showers will remain along and ahead of the front thanks to added height falls and increasing winds aloft. Theta-e advection and temperatures warming into the upper 70`s to around 80 will result in marginal instability across the southeast this afternoon. Effective shear is also marginal, around 20-25 knots. However, given the dynamics and moisture, expect scattered to numerous showers and thunderstorms to develop ahead of the front as it reaches the southeast this afternoon. Hi-res guidance backs this scenario up well. Convective elements will help produce locally heavy rainfall. Previous forecasts were concerned about a connection to deeper moisture located over the Gulf stream, but that appears unlikely at this time. Rainfall amounts will generally be around one half inch, with locally higher amounts across the southeast. Have opted to hold back on flood products, even with the heavy rainfall of last week. Trended highs under guidance due to precip and cloud cover, ranging from the low 70`s to upper 70`s. The upper-level low centers over the Ohio Valley tonight as the frontal boundary remains stalled along the Mid-Atlantic coast. Shortwave energy pushes offshore as low pressure lifts offshore of the Mid-Atlantic coast. Expect a generally downward trend in precip tonight, but based on the air mass in place and the dynamics provided by the deep upper low, will keep chance POP`s through the overnight period. GFS/NAM isentropic surfaces also indicate low level lift behind the front. Lows range from the upper 50`s northwest to mid 60`s southeast.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY/... Anomalous/deep upper-level low settles over the Ohio Valley Wednesday as today`s front aligns along the southeast coast. A brief lull in precipitation is expected Wednesday morning as the associated surface low lifts offshore. However, precipitation returns in earnest Wednesday afternoon as diffluent flow upstream of the upper low results in favorable upper vertical motion along an area of deep moisture convergence over the Piedmont/central Virginia. This represents a westward shift in the highest POP`s compared to the previous forecast. Thermal profiles and cross sections indicate the upper levels will begin to dry, with precipitable waters dropping to around 1-1.25 inches. However, the low levels remain quite moist with 1000-500mb relative humidity values over 75%. Deep lift indicated in the models will be enough to overrule limited moisture, resulting in widespread showers Wednesday afternoon through Wednesday night. Showers may be heavy at times. Went below guidance on temps Wednesday due to a cloudy sky and precip, with highs forecast around 70 northwest to around 80 southeast. Few thunderstorms also possible Wednesday afternoon due to elevated instability (dry air aloft). Lows Wednesday night in the low to mid 60`s. The upper low drops southward over the southern Appalachians Thursday as a prevalent dry slot works into the forecast area. Showers will lift northward through the day, with the best chances confined north of Interstate 64 late Thursday. However, based on the amount of forcing and lingering moisture, will keep chance POPs across the remaining forecast area for light showers. Highs Thursday range from the low to mid 70`s inland to upper 70`s southeast. May begin to see the sun late Thursday afternoon across the south as the sky begins to clear. Per the latest GEFS standardized anomalies, best moisture transport expected to remain north of the region through the short term. However, even with limited moisture over the region, upper level dynamics still expected to result in produce a widespread 1-2 inches inland. Less across the southeast. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... Medium range period continues to feature the presence of a deep anomalous upper level low centered over the TN/OH Valley Thu night/Fri, gradually lifting N and slowly weakening/filling across the Great Lakes by late Saturday-Sunday. At the surface, high pressure centered over eastern Canada will also weaken/slide E through the period as a surface low locates close to the upper low, with a secondary area of low pressure over the Carolinas/VA lifting N of the local area by Fri night/Sat. Fairly good agreement exists among the operational GFS/GEFS mean and the ECMWF with this overall scenario. Thus, precipitation chances will initially begin high (especially across the N) Thu night where 50-70% chances have been added to the forecast. Fri will see diminishing POPS (with southern 1/2 of the CWA only having about a 20% POP with at least partly sunny skies developing). Will maintain 30-50% chances N closer to the retreating frontal boundary. Will continue with a mainly dry forecast from fri night through the weekend (except for a 20-30% POP across the NE). Temperatures will be close to seasonal averages with highs generally in the 70s and lows in the 50s W to the 60s along the coast. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/... Cold front approaching from the northwest will spread showers and lower ceilings into the region this morning. MVFR ceilings expected to spread across the Piedmont into central Virginia by 12-15Z, remaining over the region through late Tuesday morning. VFR conditions prevail KORF and KECG. MVFR (possibly IFR) visibilities expected to light to moderate showers. Surface winds generally out of the south below 10 knots. Showers will reach southeast Virginia/northeast North Carolina by 12-15Z as the front pushes across the region. Front expected to stall along the coast late today, with showers lingering near the coast. Thunderstorms possible along the coast. Surface winds at or below 10 knots today. Unsettled weather conditions expected through the end of the work week as waves of low pressure lift along the cold front. Showers are possible each day. && .MARINE... Have dropped SCA headlines across the remaining Bay zones, though a few gusts to 20 KT will persist through daybreak (and an MWS may be issued if showers to the west push over the Bay and enhance the winds briefly). Otherwise, The approaching cold front drops into the area and stalls near SE VA tonight and Wed. Waves of low pressure forming along the front Wed night into Fri should gradually lift the boundary back to the N Thu-Fri (though confidence on exact timing is uncertain). Winds will become mainly ENE across northern areas and S/SE over southern areas Wed-Fri. SCA conditions are most likely over northern coastal waters Thu- Fri as the gradient tightens between the High to the N/NE and the front lifting back to the N. Still a fair amount of uncertainty and if the front is slower to lift to the N, SCA conditions could occur over most of the marine zones. Overall winds will avg 10-15 KT with mainly 1-2 ft waves over the Bay and 3-4 ft seas over coastal waters through this period, with seas to 5-7 ft possible N of Parramore Island. && .AKQ WATCHES/WARNINGS/ADVISORIES... MD...None. NC...None. VA...None. MARINE...None. && $$ SYNOPSIS...SAM NEAR TERM...TL/SAM SHORT TERM...SAM LONG TERM...LKB AVIATION...TL/SAM MARINE...LKB is the U.S. government's official web portal to all federal, state and local government web resources and services.