Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 201523

Area Forecast Discussion
National Weather Service Blacksburg VA
1023 AM EST Tue Feb 20 2018

A strong ridge will set up off the southeast coast providing
much above normal temperatures through the week. Chances for
rain increase late Wednesday into Thursday with a front.


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

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.


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.


As of 611 AM EST Tuesday...

Low clouds and fog, dense in spots will be sticking around
along/east of the Blue Ridge this morning then some minor
improvement east of Roanoke this afternoon, with better flying
conditions developing from Roanoke west as we clear out. Early
this morning, a low level wind shear threat remains across
portions of the area, but should be gone by 14-16z.

As we head toward evening/tonight will see clouds of the low end
MVFR to LIFR arrive again as far west as BCB/ROA in the 04-06z
time frame.

The only place that should remain VFR throughout the period is

Cannot rule out drizzle this morning in Danville, but vsby
impacts will be more from fog.

Confidence high on starting out sub-VFR then medium this
afternoon, in the east to high in the west on all parameters
with medium to high on overnight-Wed morning conditions.

Extended Aviation Discussion...

Scattered MVFR showers are expected Wednesday into Thursday
with the approach of a cold front. 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.


As of 330 AM EST Monday...

Today 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


VA...Dense Fog Advisory until noon EST today for VAZ022-032>034-
NC...Dense Fog Advisory until noon EST today for NCZ003>006-019-020.


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