Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 210124 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 924 PM EDT Fri Apr 20 2018 .SYNOPSIS... High pressure will be over the area through the weekend. Low pressure will approach from the southwest Monday and slowly track northeast to the Virginia coast by Wednesday. This will bring widespread rain and below normal temperatures to the Mid Atlantic region the first half of next week. && .NEAR TERM /THROUGH SATURDAY/... As of 930 PM EDT Friday... High pressure will continue to build overhead keeping skies clear and winds light. The 00Z RNK sounding illustrates the magnitude of dry air in place with a PW value of 0.13 inches, which is the lowest PW value for 00z/April 21 for RNK (records back to 1995). While these very favorable radiational cooling conditions will likely lead to low temperatures falling below guidance values, the very dry airmass will limit the potential for widespread frost. Still, will continue Freeze Warning across the southern Shenandoah Valley where temps will drop below freezing for several hours early Saturday morning. As for the mountains, temperatures will again dip into the 20s,continuing to post- pone the growing season and delay green- up. Forecast lows tonight are a few degrees above record values for the 21st of April. After a cold start, Saturday will be another dry, fair weather sort of day with seasonably cool temperatures. The April sun angle should offset the cooler thicknesses in spite of temperatures being 5 degrees below normal. The airmass is very dry so looking at afternoon humidity minimums of 20 to 30 percent. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... As of 330 PM EDT Friday... Surface high pressure will shift east of the region Saturday night into Sunday, allowing our low level wind flow to transition from calm/light and variable to southeast at 5 to 10 mph. We will also see an increase in cloud cover thanks to increasing dew point and a weakening cap aloft allowing for better chances of cumulus development. Also, higher in the atmosphere, we will start to experience cloud cover associated with a southern stream area of low pressure advect into the region from the south. Low temperatures Saturday night will range from a mix of mid 30s to around 40 across the mountains and lower to mid 40s across the Piedmont. On Sunday, high temperatures will range from around 60 to the mid 60s across the mountains with mid 60s to upper 60s across the Piedmont. Sunday night through Monday night our weather pattern has a decent amount of uncertainty to it. While the deterministic solutions bring the Lower Mississippi Valley upper low eastward through the Deep South, there are notable differences in the timing, track, and the extent to which its associated rainfall spreads into our region. The NAM is the most progressive with the entire region receiving a generous rainfall by Monday evening. At the same time frame, the GFS and Canadian solutions barely have precipitation within the far southwest part of the region, and the European solution has measurable rainfall bisecting the region, wet southwest, and dry northeast. Our forecast will reflect a consensus of these four, which would mirror a slightly slower version of the European solution. On aspect that does has decent agreement among the models is a strengthening, and persistent, southeast flow across the entire region. This has the potential of having more precipitation generated along the crest of the Blue Ridge thanks to a strong upslope component on the east side of the region. Also, the flow tends to result in gusty to very gusty winds across the far southwest portion of the area where downsloping will be taking place. The area between Bluefield to Richlands to near Marion will be an area to watch for potential wind headlines Monday night. Temperatures during the Sunday night to Monday night time period will be a little tricky given the questions about the influence of rainfall. For now, with an anticipation of some degree of increasing dew points, will likewise trend low temperatures milder each night as compared to Saturday night. However, for the high on Monday, will trend slightly cooler as compared to Saturday given the potential for thicker clouds and the precipitation, despite likely warm air advection into the area. Based upon the above timing discussion, the best chance for moderate to heavy rainfall will come Monday afternoon and overnight. At this point our forecast will reflect its highest amounts, around two inches along the crest of the Blue Ridge, from near Fancy Gap, VA, southwest into the Northern Mountains of North Carolina. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 300 PM EDT Friday... For Tuesday, the question of the timing and track of the upper low continues. However, with the NAM not a player in the solutions, the resultant consensus between the GFS, Canadian and European solutions are more in line. Our rain will continue over the area on Tuesday with Wednesday looking like the point where we get on the backside of the system with the precipitation transitioning to steep lapse rate showers as the center of the low passes overhead, or close to the region. On the heels of the Monday/Tuesday system will be a fast moving northern stream system. Again guidance varies with this system as well. Solutions differ as to whether or not this system is a closed low, the GFS, an open low, Canadian, or relatively weak trough, European. Either solutions brings its associated upper system across our region Wednesday night through Thursday night, thus keeping showers to some degree in the forecast. Details to the extent can be worked on closer to the time of passage next week. Our progressive pattern doesn`t end on Thursday. Yet a third system is progged to cross the region within the northern stream jet. This one currently is expected to be more potent than the Wednesday/Thursday system. Temperatures during this portion of the forecast will again be challenging, but there is expected to be general trend toward milder conditions comparing Tuesday through Thursday, with slightly cooler conditions on Friday as compared to Thursday. All days are expected to average a little below normal for this time of year. && .AVIATION /01Z SATURDAY THROUGH WEDNESDAY/... As of 728 PM EDT Friday... High pressure will control our weather through Sunday. Great Flying weather expected with high confidence for VFR conditions and light winds. Extended Aviation Discussion... Clouds will begin to creep back into the area late Sunday in advance of an approaching low in the Lower Mississippi River Valley. However, cloud bases should remain VFR until deeper moisture arrives Monday. SCT MVFR showers are possible in southern Virginia and northern North Carolina Sunday night with rain becoming more widespread across the Mid Atlantic region on Monday. MVFR/IFR ceilings and visibilities are expected Monday night and Tuesday due to widespread rain and an upslope very moist southeast wind. SCT MVFR showers will continue into Wednesday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Freeze Warning from 2 AM to 9 AM EDT Saturday for VAZ023-024. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...PH/PM SHORT TERM...DS LONG TERM...DS AVIATION...KK/PM is the U.S. government's official web portal to all federal, state and local government web resources and services.