Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 241909 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 309 PM EDT Wed May 24 2017 .SYNOPSIS...
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A warm front over extreme northwest North Carolina will lift north tonight, followed by a cold front moving from west to east. By Thursday morning low pressure will be over the Ohio Valley with the cold front trailing across far eastern Virginia and North Carolina. Friday will be a dry day with the low well into New England.
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&& .NEAR TERM /THROUGH THURSDAY/...
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As of 245 PM EDT Wednesday... Deep synoptic scale long wave trof was over the eastern United states with two upper low within the trof. The trof crosses the Ohio Valley on Thursday with the main trof axis moving toward the East Coast on Tuesday afternoon. Surface wedge retreats tonight as winds turn to the south then southwest. Looking a three batches of showers and thunderstorms. The first coming in at 3PM from the warm, unstable side of the wedge from the west,and the next developing in North Carolina and moving northeast through Virginia, similar to the precipitation forecast trends in the 17Z HRRR and to a lesser extent the Hi- Resnmm and HI- RESarw. The third will be along the front later tonight. This put the best timing for precipitation and storms from 21Z/5PM until midnight. With a retreating warm front and high shear the near-storm environment is favorable for rotation in any of the stronger cells. The threat for flash flooding will also continue until the cold front passes. Expecting rainfall rates typical in convective storms. Precipitable water values increase to around 1.5 inches in the piedmont of Virginia and North Carolina tonight. Rates of 1.0 to 1.5 inches per hour would exceed what is needed for flash flooding in parts of southwest Virginia and northwest North Carolina. The Flash Flood Watch will be expanded to include another tier of counties from Pulaski to Halifax. Cold pool aloft with the southern of the two upper lows passes over the Carolinas but 500MB temperatures still lower enough to produce large lapse rates over southeast West Virginia, southwest Virginia, and northwest North Carolina on Thursday. CAPES are forecast around 800 J/Kg in the piedmont Thursday afternoon. Low level moisture will erode east of the Blue Ridge Thursday but will increase with the increasing west wind on the west side of the central Appalachians. Will keep a high probability of precipitation ahead of the cold front which crosses the region tonight. As the warm front moves north temperatures will temporarily go up then they will cool off again behind the cold front. Definitely a non-diurnal trend. Expecting enough breaks in the clouds east of the Blue Ridge on Thursday for some sun and temperatures warmer than Wednesday. Blend of MET/MAV guidance looks reasonable for maximum temperatures on Thursday.
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&& .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY NIGHT/...
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As of 145 PM EDT Wednesday... Thursday night into Friday, the center of the upper low responsible for the wet weather on Thursday will translate from near the Mason- Dixon line to off the coast of Cape Cod. This transition will place the region on the backside of the departing system within a tightening pressure gradient. Look for winds to become gusty from the northwest as 850mb flow is progged to peak around 40kts in the evening. Ridgetop gusts in the 30 to 40 mph range will be common with the highest peaks experiencing a few mph about that. Mountain valleys will experience gusts more in the 20 to 30 mph range. Gusts of any substantial nature are not expected downstream of the crest of the Blue Ridge. Precipitation will linger through the overnight hours in the form of upslope scattered showers. The best coverage will be across southeast West Virginia with decreasing potential heading eastward to roughly the Interstate-81 corridor. On Friday, we will gradually lose the upslope scattered showers, and associated cloud cover, as low level winds start to back more westerly through the day with the approach of an upper level shortwave ridge. The arrival of drier air will also yield a decreasing trend in coverage. The best coverage will still be over parts of southeast West Virginia, mainly during the morning hours. The vast majority of the forecast area will experience limited cloud cover and weak winds. Our period of drier weather will be short lived. Friday night into Saturday we will be watching a cold front approach and then enter the region. Isolated showers may on the radar screen by sunrise Saturday in the western portions of the area, but the better chances will be during the day Saturday with the arrival of the front combined with daytime heating. Guidance varies on what extent the front makes it through the region before stalling. Model are leaning towards a thunderstorm complex potentially riding along this stalled front late Saturday night and entering the area by Sunday morning. Temperatures during this portion of the forecast will start near normal, but trend to a little over five degrees above normal by Saturday.
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&& .LONG TERM /SUNDAY THROUGH TUESDAY/...
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As of 205 PM EDT Wednesday... During this part of the forecast, a broad, slow moving upper low will transition from the Great Lakes region to over southeast Canada. This will keep our region in a pattern dominated by a persistent trough of low pressure across the eastern portion of the U.S. Within this trough pattern a series of cold fronts are expected to enter, and then move through the forecast region. Guidance offers the first of these two to have a Sunday night into Monday time frame, with a second during late Tuesday night into Wednesday. Both of these time frames will be periods with a greater potential for showers and storms, with the first being more robust than the second. However, while the area is within the overall trough pattern, daytime heating will be enough to produce at least isolated showers during other parts of this time period. Temperatures during this portion of the forecast will start the period about ten degrees above normal. By Wednesday, readings within a degree or two of normal are forecast.
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&& .AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
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As of 250 PM EDT Wednesday... Wedge holding with IFR to MVFR ceilings over southeast West Virginia and southwest Virginia TAF sites. Warm front slowly lift north this evening which will brings winds around to the south then southwest and raise ceilings to VFR. Numerous thunderstorms are expected tonight with MVFR ceilings and visibilities. Some of these storm may be severe across southern Virginia. Confidence is low on the timing of when this improvement will be. High confidence that a cold front will cross the area and turn winds to the southwest to west before 12Z/8AM. Surface and low level winds from the west will result in downsloping east of the Blue Ridge to sct to broken VFR stratocumulus. Scattered showers and isolated thunderstorms are expected where MVFR conditions are possible. Ceilings will remain MVFR from KROA and KBCB along the Blue Ridge and to the west. Extended Aviation Discussion... Friday will be drier with a better chance of VFR ceilings and visibilities. Another front reaches the area for Saturday with more showers and thunderstorms including sub-VFR conditions into Monday.
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&& .HYDROLOGY...
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As of 300 PM EDT Wednesday... Flash Flood Watch expanded another tier of counties for late this afternoon into late this evening. Given wet antecedent conditions in much of the southern and southwest sections, hoisting a Flash Flood watch given potential for higher rate rainfall over wet ground into this evening. The river forecast for the Dan and Roanoke Rivers will be updated again after 8PM based on rainfall through that time and expected rainfall for overnight. Minor Flooding is expected to continue on the Dan River at South Boston and is forecast on the Dan River at Paces and on the Roanoke River at Randolph.
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&& .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Flash Flood Watch until midnight EDT tonight for VAZ007-009- 010-012-015-016-032-043. NC...Flash Flood Watch until midnight EDT tonight for NCZ001>006- 018>020. WV...None.
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&& $$ SYNOPSIS...AMS NEAR TERM...AMS SHORT TERM...DS LONG TERM...DS AVIATION...AMS/JH HYDROLOGY...AMS

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