Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 141457 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 957 AM EST Tue Feb 14 2017 .SYNOPSIS... Weak high pressure over the area this morning will move east to along the coast this afternoon. A cold front moving across the upper midwest will approach the region tonight. In addition, an area of low pressure over Texas will be moving east across the southern states. A period of light rain or snow is likely late tonight and early Wednesday, rain for the lower elevations, and snow for the higher elevations. Blustery conditions will return Thursday will colder temperatures. && .NEAR TERM /THROUGH WEDNESDAY/...
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As of 950 AM EST Tuesday... Limited adjustments were made to the ongoing forecast. Changes include minor tweaks to the cloud cover to reflect the leading edge of the main area of clouds that is progressing southeast through the area. Likewise, the breaks in the overcast behind th leading edge are now also better represented in the sky cover grids. The result though for the day will remain the same. Still anticipate a trend towards more cloud cover, especially this afternoon, and in the west. Also, temperatures, dew points and winds were refreshed to reflect the latest observations and expected trends as we progress into the early afternoon. As of 500 AM EST Tuesday... Split flow in the upper levels of the atmosphere will make for a little bit of a weather challenge over the next 24 hours. A short wave trough will dig southeast through the Great Lakes Region today, its associated cold front moving southeast across the upper midwest and into the Ohio Valley. Meanwhile, a short wave trough over Texas will move east, spreading clouds and moisture northeast across the Mid-MS valley and toward our region. Model consensus is for increasing cloudiness today with potential for some light rain late in the day across our far western CWA. Radar may indicate potential for precip to arrive earlier, but antecedent airmass is dry with dewpoints in the teens and 20s, and radar returns will likely be from virga, or precip which evaporates before reaching the ground. Temperatures today will be relatively cool, the increasing cloud cover preventing much in the way of a warm up. Undercut MOS temps by 4 to 8 degrees to account for the increasing cloud cover, with most areas climbing into the 40s for highs, maybe 50 across southside VA and the Piedmont. By tonight, the column is forecast to eventually saturate with likely to categorical pops for most of the CWA after midnight and into early Wednesday. Greatest QPF is expected across the southern half of the CWA (south of highway 460), with a quarter to a half inch liquid equivalent. The northern CWA is expected to receive a tenth of an inch or less. The key word here is liquid equivalent because some areas of the CWA will receive snow as opposed to rain. Temperatures early tonight will initially will be in the 40s, but as evaporational cooling of the column takes place it will gradually bring temperatures down with the Freezing Level dropping to within about 4kft above MSL. This lowering of the freezing level will result in a p-type change from rain to snow across the higher elevations, with some minor to modest accumulations anticipated for elevations above 3500 feet. Places like Mount Rogers (elv. 5700 ft) will have the best chance for the precip to be all snow, so a quarter to half inch QPF would translate into several inches of snow for these higher ridge tops. Model snow ratios are suggesting something close to 7:1 so 1 to 4 inches of snow would be possible for elevations above 3500 feet late tonight and early Wednesday with focus on the far western CWA from Bluefield south, including the Mountain Empire, Grayson Highlands and the higher terrain surrounding Mount Rogers, and then into the high country of NC for the higher elevations just west of West Jefferson and into western Watauga including Beech Mtn. At some point we may need to consider an elevation defendant winter weather advisory, but want to take a look at the next model run or two to assess amount of cold air available and whether the cold front approaching from the northwest will aid or inhibit precip production and/or lowering of the freezing level below what is already forecast. Confidence attm is for at least an inch or two of snow for the highest elevations and just plain ol rain below 2000 feet MSL. It`s the layer between 2000-4000 feet MSL that could have more or less of a mix of snow pending available cold air, and a lot will depend on how warm the boundary layer gets today and whether this warmth can be maintained during the balance of precipitation event. Based on what I am seeing in the current set of models (00Z run), the NAM is the coldest with temperatures getting into the 28-33 degree range for elevations above 3500 feet. From 3500-2000 MSL temperatures cool into the mid-upper 30s. And below 2000 feet MSL in elevation readings will be closer to 40 deg F. The GFS is a little warmer, by a degree or two F, and a little wetter, a tenth of an inch more QPF compared to the NAM. This would have little impact on the lower elevations, yeilding a little more rain, but would mean elevations above 4000 feet, such as Mount Rogers could have closer to 4-5 inches of snow. The most likely scenario is something in between, so 1-4 should cover it.
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&& .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/... As of 415 AM EST Tuesday... Classic split flow pattern will remain in place into Wednesday, then evolve into a dominant northern stream flow across the Mid-Atlantic region. Southern stream closed low coming out of Texas will begin to fill and lift northeast as a deepening northern stream upper low across the Great Lakes digs toward the Mid-Atlantic. The RNK CWA will find itself on the northern extent of the baroclinic zone associated with the merge of these two systems, which will take place overnight Tuesday into Wednesday. The best lift will reside near the I-40 corridor, just to the south of the CWA. However, substantial lift will exist to the north with the strengthening northern stream upper trough such that an additional 1/4 inch of QPF can be expected along and east of the Blue Ridge Wednesday morning. In the 15Z- 18Z time frame, models show a sharp northwest shift to the low- level flow, which will result in a rapid recession of the precipitation to the southeast, generally exiting the Piedmont region around or shortly after 18Z. Pops at 12Z are generally categorical along and east of the Blue Ridge to just slight chance by 18Z southeastern Piedmont. By late morning/early afternoon, any remaining precipitation will transition to upslope across the usual western upslope areas. As has been noted the past two days, there will be a race between the advection of cooler air/evaporational cooling from precipitation western mountains and the exit of the precipitation to the east. Much of the time, temperatures will be above freezing while precipitation is occurring. However, toward daybreak and into the first half of this period, through roughly 15Z Thursday, precipitation and temperatures at the higher elevations of the western mountains should combine such that minor accumulations of snow can be expected. This would be mainly for elevations above 3500 ft. Looking at BUFKIT model soundings, it is a fairly classic case where the sounding cools from top down, leaving the surface layers, even at locations such as Boone (TNB) just above freezing until right at daybreak. Using a general snow/liquid ratio of 7-8:1 gives an additional one inch or less of additional snowfall at these higher elevations in the 12Z-18Z time frame Wednesday morning. Other p-types do not appear to be an issue with precipitation transitioning simply from snow to rain, or must remaining all rain throughout the event at lower elevations and across the Piedmont. Through Wednesday afternoon and into Thursday morning, precipitation will end east of the Alleghany front as activity transitions entirely to upslope snow showers on the heels of another significant surge of Arctic air. 850mb temps will plunge toward -12C by 12Z Thursday. Gusty northwest winds will become a concern once again and appear to approach Wind Advisory criteria in some areas along and west of the Blue Ridge once again. However, the 850mb jet is generally in the 30-40kt range as opposed to the 50-60kt range with the most recent event. Some MOS guidance suggests, however, that some locations as noted will be near Wind Advisory criteria. For now, just plan to highlight this in the HWO and evaluate the need for an advisory with later shifts. Temperatures Wednesday with cloud cover and modest cold advection will be near seasonal values for lows, while below for highs. Thursday will be a cold day and generally about 10-15 degrees below normal, especially maximum temperatures. Friday, the upper trough shifts east of the area and a rapid warmup ensues once again. Several models depict an area of warm advection clouds and perhaps a few sprinkles traversing the area in the departing northwest flow, but this is likely overdone. However, an area of dense cirrus could develop in such a pattern and retard the otherwise expected warmup. && .LONG TERM /SATURDAY THROUGH MONDAY/... As of 445 AM EST Tuesday... This period will be characterized by an extended time frame of above normal temperatures as the flow aloft amplifies across the U.S. in response to an eastern Pacific trough/energy shifting into the western and eventually central U.S. early next week. Long wave ridging sets up just to the west of our region over the weekend and gradually translates eastward into early next week. Expect a mostly precipitation free period with well above normal temperatures. A weak southern stream weather system may mar an otherwise precipitation free weekend as it lifts northeast from the southeast states early Sunday. Given that this system is moving into an area of large scale ridging, only minor amounts of precipitation are expected and it is expected to lift out of the area by Sunday afternoon. 850mb temperatures will approach +12C by late weekend into early next week, supporting maximum temperatures in the 60s west to lower 70s Piedmont by Sun-Mon. We will have to wait until the mid part of next week for the aforementioned weather system to reach our region and bring the next significant chance of precipitation. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/... As of 800 AM EST Tuesday... VFR conditions will prevail today...or the first half of the TAF cycle. Mid/High level cloudiness will be increasing in advance of a storm system over Texas, but the low levels of the atmosphere is dry, thus little or no low cloud element attm. By tonight, moisture will increase enough to begin saturating the lower levels of the atmosphere. MVFR cigs are expected to develop from west to east across the region along with the development of light rain. Freezing levels will lower to near 4kft tonight, yielding some snow accumulation in the mountains of NC and far western VA. Extended Aviation Discussion... Sub-VFR conditions likely Wednesday. Passage of a cold front and development of a coastal low will bring another round of stronger winds and colder temperatures for Wednesday night and Thursday. The northwest flow will continue to promote some MVFR cigs in the mountains through Thursday. Friday and Saturday will be milder and dry with VFR conditions. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...DS/PM SHORT TERM...RAB LONG TERM...JH/RAB AVIATION...PM is the U.S. government's official web portal to all federal, state and local government web resources and services.