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Area Forecast Discussion (AFD)

Issued by Blacksburg, VA (RNK)

                            
000
FXUS61 KRNK 171413
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
1013 AM EDT Wed Oct 17 2018

.SYNOPSIS...
A weak cold front will drift south of the area this morning
bringing an initial surge of drier air through midday. A 
secondary dry cold front arrives this afternoon resulting in 
another round of cooler and drier air in advance of high 
pressure that should build east into the region later today 
through Friday. Another cold front brings a return of wet 
weather Friday night into Saturday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1013 AM EDT Wednesday...Overall forecast is in pretty 
good shape to this point in the morning. Only notable change was
to increase dewpoints through noon in Southside and the 
foothills/NC Piedmont areas where dry advection on northerly 
flow has been slower to occur. As one gets on the other side of 
the Blue Ridge, the air mass is comparatively much drier with 
dewpoints in the 30s. Really two distinct air masses which can 
also be seen on observed morning RAOB soundings at RNK and GSO, 
with much drier air aloft noted on the RNK sounding. Morning 
sounding at RNK shows the mixing depth is still pretty shallow 
which has kept stronger winds at bay, at least to to this point 
in the morning. Combined effects of continued surface heating 
and cold advection aloft occuring later in the afternoon will 
help steepen low-level lapse rates. This will allow for gusts to
increase into the afternoon, which may approach Advisory-level 
gusts toward midafternoon at Roanoke and on the higher ridges. 
Though trees still are fully leafed, will hold on any wind 
headlines based on progged high inversion heights and short 
duration of peak winds. Still think gusts in the lower 
elevations may approach 35 mph at times this afternoon in the 
lower mountain valleys, with gusts out in the Piedmont in the 
25-30 mph range. All told, a pretty blustery day is still 
expected. As clouds clear out across the south, full sunshine is
expected with highs in the 60s to low 70s still on pace.

Previous discussion from 300 AM...

Cold front and its associated surface wave across the Carolinas will 
continue to sink to the southeast this morning in advance of a dry 
secondary boundary that will slide through the region this afternoon. 
This should allow any ongoing light rain to exit before daybreak with 
much drier air aloft working in this morning, and to the surface as 
mixing with the 850 mb boundary increases by afternoon. Latest guidance 
showing the jet aloft also ramping up to 35-40 kts by midday but a bit 
our of sync with the best cool advection and subsequent pressure rises 
that wont arrive until this evening. Although will be close to advisory 
winds around Roanoke given channeling across the Blue Ridge per 
westerly flow, wont hoist a headline at this point given a rather high 
inversion level and overall marginal nature at this point despite 
leaves on the trees. Thus bumping up speeds given forecast deep mixing 
with quite a breezy afternoon in store espcly along the ridges. 
Otherwise with best cooling aloft lagging a bit, bumped up highs given 
heating of drier air and downslope warming out east where some 70s 
possible. 

Upper trough passes to the north overnight with deeper cold advection 
continuing to work in ahead of high pressure building in from the west. 
However the surface high will remain far enough to the west to prevent 
the gradient from relaxing until perhaps deeper western valleys just 
before daybreak, and out east where mixing will be weaker. This would 
suggest less threat for frost outside of the deepest valleys, but 
possibly more of an advection type freeze across the higher northwest 
ridges where 850 mb temps are progged to drop to around -3 deg/C around 
dawn. Otherwise should be mainly clear outside of some high clouds but 
remain blustery during the evening before speeds slowly edge down after 
midnight. Lows mostly 30s west to around 40 east pending degree of 
decoupling with some 20s higher elevation across the northwest. This 
supports including a freeze warning for Western Greenbrier/Bath tonight 
and a bordering frost advisory for the valleys late.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 300 AM EDT Wednesday... 

High pressure centered over the Ohio Valley Thursday morning will 
build east Thursday afternoon and reach Virginia Thursday night into 
Friday.  The high center will slide east into the Atlantic ocean 
Friday into Friday night. 

High temperatures Thursday will range from the mid 40s in the 
northwest mountains to around 60 degrees in the piedmont. It is 
going to be cold Thursday night with low temperatures from the upper 
20s in the mountains to near 40 degrees along the southern Blue 
Ridge mountains. If the winds die off expect pockets of frost to 
develop in the mountains depending on high clouds. Will keep mention 
of frost in the Hazardous Weather Outlook for the sheltered valleys 
west of the Blue Ridge.  

A cold front will approach us from the west Friday and travel across 
region Friday night into Saturday. Moisture from the Gulf will get 
entrained into the frontal zone accompanying the deepening northern 
tier trough. Utilized the superblend for pops on Friday with 
generally increasing as the day progresses. High temperatures Friday 
will vary from the lower 50s in the west to the lower 60s in the 
east. With clouds and scattered showers, Temperatures Friday night 
will moderate into the lower 40s in the northwest mountains to the 
lower 50s in the Piedmont.   

Confidence is average for all elements.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 325 PM EDT Tuesday... 

500 MB trough will deepen over the eastern United States Saturday. 
Next surge of colder air reaches area on Sunday morning then 
moderates on moves northeast on Monday. Timing of cold front on 
Saturday is slightly faster, with boundary into southern Virgina in 
the morning. Overall along with the colder air, the air mass dries 
out behind the front on Saturday. Pressure rises, cold air advection 
and 40 to 50 knot low level jet will result in gusty wind Saturday 
afternoon through Saturday night. 

Trough gets re-enforced Monday and Tuesday. More cold air arrives 
for Tuesday. Moisture in the 1000-700 MB level will return to the 
western mountains, deep enough for some upslope showers on Tuesday.  

Confidence is average for all elements.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
As of 655 AM EDT Wednesday...

Improving conditions are expected through the morning as drier
air works in behind a passing weak cold front to the south. May
still see areas of sub-VFR ceilings espcly across the west with
upslope flow around KBLF, and with residual low clouds just
north of the front at KDAN before clearing takes place by mid
morning if not sooner. Otherwise looking at mostly VFR mid/high
canopy including mountain wave clouds once all residual low 
clouds scour out with a return to widespread VFR by this 
afternoon. 

While ceilings should trend VFR, the bigger aviation operational
concern will be the increase in northwesterly winds/gusts. 
Sustained winds should steadily increase to 10 to 25 kts espcly 
mountain locations through early this afternoon, with gusts up 
to 40 kts possible with the stronger gusts early/mid afternoon 
per enhanced mixing. Winds should be lighter east of the 
mountains but still some gusts to 20+ knots could occur.

Northwest winds/gusts will slowly diminish overnight into 
Thursday morning as surface high pressure builds into the 
region. May see some low clouds redevelop under upslope flow
around KBLF tonight with only high clouds elsewhere, supporting 
VFR overnight into Thursday when winds should be much lighter.

Forecast confidence is high to moderate for all TAF elements.

Extended Discussion... 

By Friday into Saturday, the center of the surface high will 
shift east of the area and a cold front will approach from the 
west. This will bring a return of rainfall to the area Friday
night into Saturday with the potential for a generous coverage 
of sub-VFR ceilings and localized sub- VFR visibilities during 
this time frame. High pressure builds in behind the front on 
Sunday with VFR returning across the region after any residual 
low clouds clear the northwest slopes.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Frost Advisory from midnight tonight to 9 AM EDT Thursday for 
     VAZ007-010-011-018-019.
     Freeze Warning from midnight tonight to 9 AM EDT Thursday for 
     VAZ020.
NC...None.
WV...Frost Advisory from midnight tonight to 9 AM EDT Thursday for 
     WVZ042>044-507.
     Freeze Warning from midnight tonight to 9 AM EDT Thursday for 
     WVZ508.

&&

$$
SYNOPSIS...JH
NEAR TERM...AL/JH
SHORT TERM...KK
LONG TERM...AMS
AVIATION...JH