Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 201706 AAA AFDRNK Area Forecast Discussion...UPDATED National Weather Service Blacksburg VA 1206 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 week. Chances for rain increase late Wednesday into Thursday with a front. && .NEAR TERM /THROUGH TONIGHT/... Update as of 1202 PM EST Tuesday... With fog having all but dissipated, updated to let the going dense fog advisory expire. However still expecting clouds and patchy fog to linger over southern sections with some of this cloudiness likely spilling back north as southwest flow increases. Also bumping up highs again given a quick spike in readings once the fog/clouds fade with thickness again supportive of 70s where there is more insolation this afternoon. Otherwise few other changes for now. Previous discussion as of 1020 AM EST Tuesday... Fog remains the main issue this morning with an axis of dense fog lingering from ROA east to around LYH as well as spots near the NC/VA border beneath a canopy of low clouds. Since visibilities have been slow to improve went ahead and extended the dense fog advisory a couple more hours south/east of ROA while cancelling to the west where skies have cleared. Also morning soundings show lack of deep moisture so have removed pops through midday and have doubts to much more than isolated coverage mainly southern Blue Ridge later on. Otherwise running with more sunshine west and more clouds south where guidance suggests that some spots may not clear if at all until late this afternoon. This makes the temp forecast a bit tricky as thickness warm enough for 70 or warmer across much of the area, while if clouds linger then looking at mostly upper 50s to mid 60s in those locations. For now bumped up across the west where 70s possible while tweaking down southern tier and playing for more late days highs. Expect in spots where the fog lingers and then clears will see readings really jump with light southerly mixing. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... As of 346 AM EST Tuesday... Anomalous 592+ dm 500 mb ridge off the coastal Carolinas and nearly full latitude mean 500 mb troughing in the intermountain West/Desert Southwest will govern mid-level circulation pattern through the rest of the workweek. At the surface, a cold front will be slow to progress eastward into the western Appalachians, stuck in the SW-NE oriented mid-level height gradient between the two mid-level features. Toward Thursday and Friday, a backdoor cold front and reinforcing northeast surface winds begins to build south/southwestward from the northern Mid-Atlantic. Though 00z guidance is in general agreement on the broad details, there is a larger than desirable amount of disagreement and little model consensus noted on specifics. This is more the case for the backdoor frontal evolution Thursday-Friday in terms of how far south this feature builds, which may potentially return northward as a warm front late in the day Friday. Though the best chance for rain is on Thursday, stated uncertainty leads to a lot of Chance-level PoPs, a lot more cloud cover, and highs on the colder side and lows on the milder side of guidance. Still some indication on Wednesday of potential convective showers/possible rumbles of thunder across the western third of the CWA, with forecast guidance QPF being the greatest in areal coverage in the global models. That larger coverage could be from the coarser resolution in each though, as the mesoscale NAM keeps much of the region largely dry as the frontal boundary remains to our west. Continued to carry lower-end Chance PoPs for showers/possible thunder mainly west of the New River, with slight/low Chance from I- 81 east. Wednesday`s the warmest day in the period, with highs reaching the lower/middle 70s with 850 mb temps between 12 to +14C. A very mild evening in store for Wednesday night with a good deal of cloud cover and a relative min in rain chances. Showed lows in the mid 50s to around 60, warmest in southeastern sections. Thursday appears to be the wettest day in the period, due to the sagging front nearing our southeast WV counties, all the while the backdoor front will effectively link up with it from the north/northeast. GFS and NAM are the furthest south with the backdoor boundary and NE`ly wind shift, while the ECMWF shows limited southward intrusion to maybe as far as Route 460. Kept Likely PoPs north/along Rt 460, with Chance across the southern half. Used non-diurnal temperatures for Thursday/Thursday night, with early highs in the lower 60s northern sections before falling in the day with cooler northeast flow. May be enough of a diurnal trend south of a Roanoke/Blacksburg to Brookneal line to keep highs in the mid/upper 60s. Best chance of rain is Thursday night across NC into Southside as boundary slips south toward the VA/NC border, as shown by the GFS/NAM, but given uncertainty between NAM/GFS and ECMWF kept PoPs no higher than about 30%. Lows mid 40s to the low/mid 50s, mildest further south. Question on the location of the backdoor front will also affect the forecast for Friday. This feature should return northward as a warm front ahead of organizing low in the Ohio Valley. How far north it has to travel will affect temps, cloud cover and rain chances Friday. Sided toward the NAM/GFS idea for initial frontal position which keeps temperatures on the cooler side east of the Blue Ridge and into the foothills, where values are in the 50s. In the western mountains, temperatures should top out in the 60s, especially south of the New River. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... As of 200 PM EST Monday... The cool wedge should weaken as surface high pressure heads offshore on Friday. However, the latest GFS shows a slower weakening trend compared to the ECMWF model. Another area of low pressure will try to develop over the southern Gulf Coast as an upper level trough progresses eastward from the Rocky Mountains. More clouds and chances of rain seem likely for the upcoming weekend as this area of low pressure pushes eastward. Meanwhile, the upper level ridge over the Southeast should finally give way and heads toward Florida. That pattern change should free up the atmospheric flow to allow a cold front passage over the Mid Atlantic toward Sunday night. High pressure should allow drier conditions by Monday. Despite the overall trend of more clouds and rain, temperatures still should remain considerably above normal with no anticipated threat of freezing temperatures. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... As of 1201 PM EST Tuesday... Low clouds and fog have scoured out across all except the far southern sections including KDAN early this afternoon. This should make for at least a period of VFR around KROA/KLYH this afternoon while KDAN likely stays sub-VFR with IFR cigs and periods of MVFR vsbys in lingering fog. Clouds may return back north into KBCB/KROA by mid to late afternoon but thinking still VFR for now. 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...PC/WP NEAR TERM...JH/WP SHORT TERM...AL LONG TERM...PW AVIATION...AL/DS/JH/PC/WP CLIMATE...AL is the U.S. government's official web portal to all federal, state and local government web resources and services.