Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 180030 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 830 PM EDT Sat Jun 17 2017 .SYNOPSIS... High pressure offshore will maintain a warm and humid airmass across the region, continuing our chances for showers and thunderstorms through the latter portion of the weekend. A weak cold front will approach the area from the northwest by Monday with a better chance for strong thunderstorms and heavy rainfall. Behind the front, a bit drier weather is expected through the middle of next week, along with a warming trend to above normal temperatures. && .NEAR TERM /THROUGH SUNDAY/...
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As of 800 PM EDT Saturday... Afternoon/Evening thundershowers are in a waning phase...HiRes models indicating activity fading by midnight. That said, will still have to keep our eye on outflow boundary from thunderstorm cluster working its way north from central NC. Evening sounding from GSO still shows SBCAPE in excess of 2000 j/kg and even though we are loosing the daytime heating, all that is needed is a lifting mechanism to sustain new thunderstorm activity within this moisture laden airmass...and the outflow boundary may do just that. For areas which received rain today, anticipate patchy fog for the overnight. Any fog and low clouds will burn off Sunday morning. Some upper short wave energy will then rotate through the upper low spinning over the Great Lakes/southern Canada and help push a cold front slowly in our direction from the northwest. This may present enough added shear to give a boost to storms developing along and west of the Blue Ridge as we get into Sunday afternoon. Temperatures will be on the warm side of normal to close out the weekend.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... As of 245 PM EDT Saturday... Low level southwest flow will be on the increase Sunday night in advance of an approaching cold front. With this flow will be increasing moisture, increasing lift, and as such, increasing chances of precipitation. The bulk of this activity will be across western sections of the region. On Monday, a cold front will enter the western parts of the area during the afternoon. Look for showers and thunderstorms to be on the increase during the course of the day. The latest Day 3 Convective Outlook by the Storm Prediction Center places the area under a slight risk of severe weather, with damaging thunderstorm gusts the primary concern, with large hail a secondary concern. Monday evening, the front will progress out of the area toward the southeast. Abundant showers and storms will be coincident to the front, along with the severe potential mentioned above. Please reference the HYDROLOGY section of this discussion for additional details concerning this event. By midnight, the bulk of the activity is expected to have exited the region with some lingering showers across the southeast parts of the area through roughly daybreak Tuesday. On Tuesday, drier high pressure will start to nose its way into western sections of the area. However, the proximity of the cold front will be such that some isolated showers and thunderstorms will be possible across the far south central and southeastern sections of the region. Temperatures during this portion of the forecast will average a few degrees above normal. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As of 245 PM EDT Saturday... Guidance varies slightly between the models, however, Tuesday night, the cold front will be exiting the region to the east. The NAM is the slowest in clearing the precipitation out of the far southeastern sections of the area, perhaps as late as six or eight in the morning Wednesday, with other solutions more progressive, with precipitation clearing about three or four hours sooner. In the wake of the cold front, the upper level pattern across the region remains in a broad trough with the pattern on Wednesday with the parent low near, then east of James Bay. There may be a few instability showers across the area on Wednesday in association with the passage of the trough axis. Also, the cold front itself is not expected to make huge headway southeast of the area on Wednesday. Its proximity, and daytime heating, may allow for refiring of showers and storms near the far southeast portion of the region. Thursday into Friday, anticipate the synoptic flow to transition to one of zonal flow with increasing spacing between the geopotential heights. This will result in a weak westerly steering flow at 500mb and higher. Closer to the surface, guidance varies on what if any low pressure system that forms in the central or western Gulf of Mexico during the early part of next week. Guidance also varies on when and what direction this system, or its remnants, progress onshore and progress northeast along the aforementioned frontal boundary, just south of our region. Depending on how the scenario pans out, moisture from this system may skirt the far southeastern sections of the area Thursday, Friday, or not at all. For now, until there is more clarity on this potential scenario, precipitation chances will be limited Thursday and Friday to typical isolated to scattered diurnal activity. Confidence is higher for precipitation to impact a greater swath of the region on Saturday with approach of a cold front. Some guidance suggest the front will reach western sections of the area by the afternoon, other hold off a little longer until the evening. Temperatures during this portion of the forecast will be around normal for Tuesday and Wednesday, but warm to a few degrees above normal for Thursday through Saturday. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/...
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As of 800 PM EDT Saturday... Warm moist airmass over the Appalachians and central mid Atlantic region will maintain threat for scattered showers and storms. Patchy fog and low level stratus also likely for the late night and early morning. In general conditions will be VFR except for ocnl lower restrictions due to the scattered deep convection. Late night fog and stratus may also result in temporary IFR for the early morning hours. Extended Aviation Discussion... Very moist and unstable conditions combine with an approaching trof to keep convective activity, with a strong diurnal trend, in the forecast through Sunday. There will also be a trend fog fog/stratus development during the predawn hours. A better chance for more widespread showers and storms including sub-VFR looks to arrive on Monday with a surface cold front perhaps crossing the region. This will allow for extended periods of sub- VFR ceilings and visibilities. In addition, radiational late night/early morning fog will be on the increase. Appears will see drier conditions Tuesday/Wednesday but not completely free from a chance for showers.
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&& .HYDROLOGY... As of 245 PM EDT Saturday... Over the past few days, today, and then again forecast for Sunday, multiple rounds of precipitation occurred, are occurring, and will occur. For the most part, this precipitation has been disjunct, with different areas on different days having their particular bout of localized heavy rain. The same is expected the remainder of today, and then again on Sunday. Monday is expected to be different. A generous swath of moderate to heavy rain producing showers and storms is progged to accompany the cold frontal passage. The latest guidance from the 00Z/8PM NAEFS Standardized Anomaly places precipitable water values of 1.50 to 1.75 inches coincident to this front, or in the neighborhood of +1 sigma in comparison to normal. While not a huge positive deviation from the normal, it may lean on the side of significant given the rounds of locally heavy rain we have been experiencing, and will continue to experience into Sunday. This scenario offers an increased potential for flooding on a broader scale than in recent days, and starts the gears churning for a potential flood or flash flood watch for portions, or all, of the region. The latest Day 3 Excessive Rainfall Outlook from the Weather Prediction Center places our in entire area in a Marginal Risk of flash flooding during this time period. Our latest forecast will make use of QPF numbers from WPC for the Monday through Monday night time frame. These values equate to an average 18 hour rainfall Monday through Monday evening of 1.00 to 1.50 inches of rain across the mountains with localized 1.75 inch average values. Across the Piedmont, amounts are less with 0.50 to 0.75 inch as an average during the same 18 hour period with localized 1.00 inch average amounts near the James River basin. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/MBS NEAR TERM...MBS/PM SHORT TERM...DS LONG TERM...DS AVIATION...PM HYDROLOGY...DS

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