Area Forecast Discussion
Issued by NWS Blacksburg, VA

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330 FXUS61 KRNK 211829 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 229 PM EDT Sun May 21 2017 .SYNOPSIS...
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Expect a cool east wind today with a front located to our south and west. Warm moist southerly flow will overrun the front resulting in widespread cloud cover in addition to areas of showers and the potential for thunderstorms.
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&& .NEAR TERM /THROUGH TONIGHT/... As of 500 AM EDT Sunday... Flash Flood Watch is in effect for the southwest part of the CWA (Grayson VA into NC High Country). Surface backdoor front has pushed through the forecast area and is now hung up over the NC mountains and into far Southwest VA. This boundary will be the focus for persistent lift and generation of rain with embedded heavier showers and Thunderstorms. HiRes models indicate anywhere from 2-3 inches of rain for the NC mountains, today and tonight inclusive. Across the remainder of the CWA, cool stable easterly winds will persist today significantly dampening the diurnal rise of temperature. Abundant cloud cover in addition to occasional rain and or drizzle can be expected. Models suggest lowest threat for rain will be this morning with a gradual increasing threat later in the day and for tonight as the surface boundary lifts back north as a warm front. Rainfall amounts of a half inch to an inch on the average are being forecast, with the exception of the southwestern part of the CWA (Grayson Highlands to Boone NC) where amounts are expected to average between 2 to 3 inches. && .SHORT TERM /MONDAY THROUGH WEDNESDAY/... As of 500 AM EDT Sunday... Mid-tropospheric pattern across much of the CONUS east of the Rockies will be dominated by an anomalous upper-level low centered over the upper Midwest/northern Great Lakes region. The upper low digs southward into the northern/central Plains by midweek. Shortwave troughs/vort maxima embedded in the circulation will rotate around it into the southern Plains, picking up moisture from the Gulf and ejecting into the Carolinas and southern Virginia area. Thus, sensible weather conditions through midweek will be heavily influenced by the position of the surface cold front/remnant baroclinic zone. Overall it is an unsettled period. 00z global guidance and 21z SREF mean MSLP fields are hazy in details as far as how far south the frontal zone makes it. Those details are pretty critical to pin down, as aforementioned vort energy interacts with the remnant baroclinic zone and may induce at least one low pressure area riding along it. This is reflected in the GFS and ECMWF, but with disagreement on timing. They do point to, in a broader sense, at some point on Tuesday as a potential period when one such low may lead to period of wetter conditions, particularly for the southern third of counties. Something to keep in mind as well is the potential for Gulf Coast convection to shut off deeper northward return of moisture, which may significantly reduce projected rainfall north of the southern Blue Ridge Tuesday. Should see a reduction in PoPs from west to east through the day on Monday, though still a limited risk for thunderstorms into the afternoon ahead of the front. As the front clears, an extension of a ridge may build into the northwestern tier of counties and allow for greater radiational cooling Monday evening. May see potential radiation fog as well in that general area, conditional on winds becoming light and variable. Highs Monday into the mid-70s east of the Blue Ridge and into the Piedmont to upper 60s/near 70 west of the Blue Ridge into southeast WV. Tuesday appears to be the wettest day in the period with a weak wave low expected to progress across the Carolinas. PoPs are highest (lower-end Likely range) particularly for counties along and south of Rt 460. In addition, potential for at least breezy northeasterlies as the wave passes by, along with limited prospect for thunder. Better thunder chances mainly southeast of our forecast area. Lower chance-ish PoPs for most of the central, northern and western tier of the forecast area. Expect cloudy and wet conditions with highs probably only making it to the mid 60s to low 70s. As the wave passes Tuesday evening, I would think a break in showers would be likely. However kept PoPs in the Chance range due to timing uncertainties associated with the next wave. Lows Tuesday night mainly in the 50s, to near 60 closest to the NC Piedmont. Generally showed an increase in PoPs/shower chances on Wednesday into the afternoon, as the upper low swings a more potent shortwave trough into the lower Mississippi Valley. As mentioned, it`s possible showers may begin as early as during the overnight hours Tuesday. If so, then PoPs probably would need to be raised for the Wednesday morning period. Forcing is quite strong with the trough, and could envision potential for stronger storms if greater instability can be realized than presently reflected in model guidance. Highs only in the upper 60s to mid 70s. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... As of 500 AM EDT Sunday... The upper trof pushes through the Appalachians and central mid Atlantic region by the end of the week. This will swing another cold front through the region and bring more thunderstorms to the forecast for midweek. As the closed low moves over with a cold pool aloft to steepen lapse rates, there looks to be a good chance for thunderstorms with some graupel/small hail on Thursday, before the upper low slowly departs and we get into some upper ridging and improving conditions heading into the weekend. Temperatures will be running slightly below normal through the end of the week. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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As of 120 PM EDT Sunday... High confidence of MVFR or lower ceilings east of KTNB to KBLF today and tonight. Medium confidence of locations that will have LIFR ceilings Stalled front over the southern Appalachians will result in abundant cloud cover into Monday. A compact short wave along the North Carolina/South Carolina border will track east this afternoon and tonight. This will enhance southeast winds and upslope along the Blue Ridge and into the eastern foothills. Medium confidence on the timing of the models which has the precipitation and best lift crossing the area from west to east from 21Z/5PM through 06Z/2AM. A surface cold front crosses through the area on Monday. Precipitation will end and ceilings will lift to VFR behind the front. Confidence on the timing of this improvement is low. Extended Aviation Discussion... Expecting several periods of precipitation with sub-VFR conditions Monday night through Friday. Any breaks between rainfall events with better flying conditions will likely be less than 12 hours.
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&& .HYDROLOGY... As of 630 AM EDT Sunday... Excessive rain threat exists from Grayson County VA into the mountains of North Carolina today and tonight. Warm moist southerly flow aloft will interact with a stalled surface front resulting in areas of rain and potential for thunderstorms. HiRes models indicate anywhere from 2-3 inches of rain for the NC mountains, today and tonight inclusive. If amounts exceed 2 inches in an hour, or 3 inches in 3 hours, flash flooding is possible. Small creeks and streams will respond first, followed by sudden rises along the larger rivers. The Weather Prediction Center has placed much of our western CWA within a Slight Risk of rainfall exceeding flash flood guidance. 1 HR FFG for our western CWA ranges from 1.5 to 2.0 inches in an hour or 2.5 inches in 3 hours. There is at least 50 percent confidence these values will be exceeded for the mountains of North Carolina where a Flash Flood Watch has been posted. ATTM forecast guidance does not have any of our main stem rivers going into flood. However, there is about a 30 to 70 percent chance for the Dan River to reach action stage with this rain event which is about 3/4 bank full... the result of about 1.25 to 1.50 basin average rainfall. Some of the rivers in NW NC, such as the headwaters of the Yadkin or the Watauga River, may experience rises closer to bank full pending the current QPF forecast of 2.0 inches basin average rainfall. Headwater guidance (amount of rain it would take to cause flooding) for these rivers is 2.5 inches in 6 hours, 3.0 inches in 12 hours, or 3.8 inches in 24 hours. && .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Flash Flood Watch until 2 AM EDT Monday for VAZ015. NC...Flash Flood Watch until 2 AM EDT Monday for NCZ001-002-018-019. WV...None.
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&& $$ SYNOPSIS...PM NEAR TERM...PM SHORT TERM...AL LONG TERM...MBS AVIATION...AMS/PM HYDROLOGY...PM

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