Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 210254 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 954 PM EST Tue Feb 20 2018 .SYNOPSIS... A strong ridge of high pressure will remain off the southeast coast providing much above normal temperatures through the remainder of the week. Chances for rain increase late Wednesday into Thursday with a cold front, followed by a better chance of showers with another front by later in the weekend. && .NEAR TERM /THROUGH WEDNESDAY/...
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As of 940 PM EST Tuesday... Temperatures and dew points are reaching their cross-over point with fog developing within the next couple of hours, especially counties along and south of the VA/NC border. Mixing increases towards sunrise will help remove fog across the mountain valleys and foothills. The piedmont may carry fog into mid morning. Not seeing many returns upstream this evening, but models continue to carry precipitation along the southern Blue Ridge through the early morning hours. I have lowered PoPs through midnight, then raised them to 30 percent in the morning. P-type will be rain or drizzle with very little to measure. As of 620 PM EST Tuesday... Updated grids with latest observations. Minor adjustments made for breezy conditions remaining into this evening. Also made changes to dew points through the night, especially across Southside where 60F are being reported (600 PM). As of 245 PM EST Tuesday... Much of the area has finally mixed out this afternoon with residual low clouds stuck across southern sections where a weak bubble of high pressure persists. Expect these clouds to linger before again expanding back north and east after sunset when will also lose most surface mixing. However moisture will remain quite shallow into the evening so removed any pops early on to start. Moisture may deepen over far southwest sections later tonight resulting in some spotty light rain/showers/drizzle espcly NC mountains per latest HRRR but iffy given little upslope at this point. Otherwise expanse of low clouds and fog again the main issues as models show plenty of clouds east of the Blue Ridge and dense fog where skies stay clear, excluding the far west where mixing should linger. Does appear that a bit stronger southwest flow above the surface along with more clouds early may preclude widespread dense fog, but something to watch later on. Should be quite a mild overnight given rising dewpoints and warm temperatures to start most sections. Thus kept most in the 50s for lows with a few valley 40s. Weak cold front to the west will be approaching on Wednesday as the upper ridge attempts to hold tight offshore. Deepening southwest flow should enhance moisture transport into the area with forecast Pwats of better than an inch advecting in by afternoon. Forcing along the front looks quite weak given better upper support farther west closer to the actual baroclinic zone in the Midwest. Guidance does show some weak instability developing as higher dewpoints tend to coincide with heating and perhaps arrival of a weak pre-frontal type trough/outflow across the west. Appears enough ridging to hold out east to keep eastern sections dry so keeping chance pops mainly west for scattered showers with isolated coverage Blue Ridge. Cant totally rule out thunder far west but given mostly elevated instability off soundings will leave out mention for now. Could be another day of record highs with potential given a warmer start and similar warm thickness of today to see some upper 70s if more sunshine is realized out east. Otherwise bumped up highs mostly into the lower/mid 70s, while leaving areas along the southern Blue Ridge/foothills cooler per clouds and a late start due to fog. See the climate section below for details on records.
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&& .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/... As of 300 PM EST Tuesday... A frontal boundary will be sandwiched between high pressure over the Great Lakes and another high just south of Bermuda. With this boundary draped over central West Virginia and northern Virginia by Wednesday night, clouds and showers will be more likely north of Route 460. Temperatures should remain considerably mild overnight with 50s and 60s expected for lows. The upper level ridge responsible for the highly anomalous warmth should begin to sag southward toward Florida on Thursday. This change should allow the aforementioned high pressure area over the Great Lakes to head eastward toward the New England States and start wedging southward along the eastern slopes of the Appalachian Mountains. As a result from the influence of northeasterly flow, the frontal boundary should begin to push southward as a backdoor cold front east of the Blue Ridge. Models still disagree on the southward progression of this cold front during Thursday afternoon through Thursday night, which adds to the uncertainty for this part of the forecast. In addition, Friday becomes more questionable due to differences in how quickly the northeasterly flow retreats once high pressure heads offshore and the frontal boundary lifts back northward as a warm front. The latest GFS and NAM solutions continue to show the backdoor cold front reaching the Virginia and North Carolina line by Thursday night and not allowing it to leave until Friday afternoon. The ECMWF, however, sends the cold front just toward Route 460, which leads to a faster erosion of the wedge on Friday and significantly warmer and drier conditions. Confidence is higher that the more significant wedge event will take place. Thus, a non-diurnal temperature trend with more clouds and showers affecting most of the Piedmont will be advertised in this update for Thursday and Friday. Friday night should stay dry as the aforementioned high pressure area south of Bermuda nudges westward toward the Mid Atlantic. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 300 PM EST Tuesday... A low pressure system will organize over the lower Mississippi River Valley on Saturday and track eastward toward the Mid Atlantic by Sunday. Model solutions continue to slow the timing of this frontal passage for our area, which appears to be during Sunday afternoon based on the GFS. Chances of rain were lowered a little on Saturday but increased a fair amount for Sunday. It is worth noting that the ECMWF solution is a little slower with the cold front than the GFS as it displays a Sunday evening frontal passage. Therefore, some questions remain on how much the moisture could linger into Sunday night and Monday morning, especially in northwest North Carolina. Eventually, high pressure should build overhead toward Monday afternoon. The upper level pattern turns more zonal by Tuesday, which looks dry with plentiful sunshine at this point. Temperatures still will reside well above normal throughout this period although it should turn a little cooler by early next week. && .AVIATION /03Z WEDNESDAY THROUGH SUNDAY/... As of 625 PM EST Tuesday... Models again show low clouds filling back in beneath the inversion overnight with low-end MVFR to LIFR across most of the region by midnight. Fog also likely again late tonight but widespread IFR vsbys iffy with a bit more pronounced southwest trajectory just above the surface layer overnight. Latest HREF has lower vsbys initially this evening before mixing out to mainly just along the southern Blue Ridge late. Therefore opted to go with more of a MVFR range in vsbys for now. The only place that should remain VFR throughout the period is KBLF although some VFR cigs likely to affect that area late tonight into Wednesday ahead of the next cold front. Low clouds will again be slow to exit eastern sections on Wednesday given only light southwest flow and deeper moisture. Also a band of pre-frontal moisture likely to affect the far western sections by midday although appears any showers should hold off until after the valid TAF period so not including mention. Otherwise plan to keep most sites under MVFR/IFR cigs Wednesday morning with most improvement in cigs near the Blue Ridge where in between trapped moisture to the east and the frontal moisture to the west. A stronger cold front looks to arrive on Sunday from the west with a better coverage of showers including continued periods of sub-VFR. Extended Aviation Discussion... May again see widespread sub-VFR develop again Wednesday night as the front approaches but stalls allowing better pooling of moisture under weak south/southeast flow. Lingering MVFR or worse conditions in showers likely Thursday with the cold front spilling south into the area. This front lifts back north Friday with continued threat of showers and potential low clouds. This front will linger nearby into Saturday with a continued threat of showers, but cigs/vsbys should a little better south of the front. && .CLIMATE... As of 330 AM EST Monday... This afternoon and Wednesday will feature well-above normal temperatures that may approach or eclipse daily record highs and daily record high minimum temperatures. It`s also possible that all- time warmest February minimum temperatures could be approached or broken in this period. Following are daily records, as well as the all-time warmest minimum temperatures recorded in the month of February, for our five climate sites with long-established periods of record: Tuesday 2/20/2018 Site MaxT Year HiMin Year BLF 67 2017 48 1994 DAN 74 1971 51 1949 LYH 76 1930 59 1939 ROA 75 1939 54 1971 RNK 69 2017 47 1991 Wednesday 2/21/2018 Site MaxT Year HiMin Year BLF 65 1996 54 1997 DAN 75 2011 55 1953 LYH 75 1930 50 1981 ROA 73 1930 48 1997 RNK 71 1986 49 1953 Warmest HiMin - February: Site HiMin Date BLF 57 Feb 5 2008 DAN 60 Feb 18 2008 LYH 59 Feb 20 1939 ROA 57 Feb 12 1932 RNK 52 Feb 16 1990 && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH/RCS SHORT TERM...PW LONG TERM...PW AVIATION...JH/RCS CLIMATE...AL

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