Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS61 KRNK 241001
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
501 AM EST Sat Feb 24 2018

.SYNOPSIS...
A strong cold front will slowly approach the Appalachians and 
central mid Atlantic region from the west resulting in an 
increased threat for showers this weekend. Until the front
crosses the area Sunday, temperatures will remain well above 
normal.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 500 AM EST Saturday...

Areas of fog mainly east of the Blue Ridge this morning, with
lowest visibilities east of Highway 29 with many observing
stations AOB 1/2sm. This fog is part of the remnant cool wedge
from yesterday, all of which will erode later this morning as
the sun comes up and mixing begins.

A cold front over the Ohio and Tennessee Valleys stretches from
NE-SW along a PIT-LEX-MRM-LIT line. This front will begin to
move east later today as an upper level trough over the Rockies
begins to move east. Meanwhile, the subtropical ridge of high 
pressure near the southeast Atlantic Coast will begin to 
weaken, but not before we go through another 24 to 36 hours of
above seasonal warmth.

The entire CWA will be under the influence of a warm moist
southwesterly winds today with temperatures returning to the 
70s areawide, and near record warmth. Shower threat will be the
greatest across far western VA and into WV, closer to the 
front. The HRRR model seems to capture the moisture over the TN 
valley the best, so went with a high likelihood threat for 
showers later this morning as this area of rain moves NE toward 
Bluefield and Lewisburg. Not sure how much of this activity will
survive the trip over the mountains, but think areas near I-64 
will have the better chance of showers crossing the Highlands 
into the Shenandoah Valley and areas north of Roanoke-Lynchburg.
Farther south, do not think much wetting will occur today. If 
anything, fewer clouds and greater potential for afternoon sun 
and deeper mixing will help elevate the temperatures. Can't 
totally rule out a rumble of thunder for areas west of I-81 this
afternoon per CAPE values increasing into the 500-1000 j/kg 
range in the upper TN valley ahead of the front.

For tonight, forecast area remains on the warm side of the
advancing front, temperatures remaining above 50. Shower threat
will again be mainly confined to the western and northern CWA 
closer to the approaching frontal boundary. 

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
As of 400 AM EST Saturday...

For a while now, our forecast has been reflecting the passage of a 
cold front during the course of the day Sunday. This is still the 
case. Guidance across the board is demonstrating good consensus of 
the front arriving across the mountains during the morning to mid-
day, and then crossing the Piedmont during the afternoon. What has 
worked its way into the guidance solutions is the potential for a 
bit of breaking up of the activity once it is east of the mountains. 
This is not an uncommon event with cold frontal passages as the 
associated precipitation begins to experience the affects of 
northwest flow subsidence off the crest of the Blue Ridge. 
Reformation of a more substantial line of convection is possible 
just as the activity has exited, or is exiting the forecast area to 
the east out of the direct influence of this downslope component. 
Our forecast will reflect categorical coverage of showers across the 
mountains, but with only likely coverage in the east. Timing of the 
front in the east will coincide with peak heating of the day. Weak 
surface based instability is forecast, and SPC Day 2 Convective 
Outlook has general forecast for the area. These two elements will 
continue to allow for the mention of a slight chance of storms east 
of the mountains during the afternoon and early evening. 
Temperatures will continue to be on the very mild side for this time 
of year. Anticipate highs of the mid to upper 60s across the 
mountains with readings around 70 to the lower 70s across the 
Piedmont. Breezy conditions are expected on Sunday with a 35kt low 
level jet ahead of the front with some gusts at the higher ridge 
tops in the 30 to 40 mph range. Gusts of 15 to 25 mph will be more 
common with the mountain valleys and across the Piedmont.

Sunday night into Monday, guidance has trended to be in better 
agreement as compared to 24 hours ago. Previously, the GFS had been 
on its own with a definite exiting of the cold front, with no impact 
across the region in assocation with a Gulf Coast wave that was 
progged to ride northeast along the exiting front. Its foreign 
counterparts to our north and across the Pond along with its NAM 
cousin were leaning more on this Gulf wave stalling the front, and 
allowing a return of precipitation across at least the southern half 
of the forecast area. Granted, each of these other models differed 
on timing and qpf, but nonetheless, did not have a dry forecast 
rapidly entering the forecast picture. Fast forward 24 hours and the 
GFS is now onboard with a similar scenario.  Our forecast had been 
reflecting the wetter solutions of the consensus of guidance.  That 
strategy is looking more promising. Look for showers across mainly 
areas south of Route 460 Sunday night and Monday, with the bulk of 
the activity across portions of Southside Virginia and neighboring 
sections of north central North Carolina. While temperatures will 
trend cooler, they will still average 10 to 15 degrees above normal, 
rather than the 20 to 25 degrees above normal prior to the cold 
frontal passage.

Monday night into Tuesday, there is good model consensus for a 
surface high around 1030 mb to build overhead. This will provide for 
a push for drier and cooler air to work its way into the area. Still 
readings will be slightly on the mild side for this time of year 
with values 5 to 10 degrees above normal.

Tuesday night will be characterized by increasing cloud cover from 
southwest to northeast thanks to an approaching area of low pressure 
through the Lower Mississippi Valley. Temperatures as a result will 
be slightly milder than those realized Monday night. Precipitation 
is expected to work its way northeast as close as the Smokies, but 
not reach the southwest sections of the area until Wednesday morning.

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
As of 100 PM EST Friday...

The run of quiet weather looks to be brief as the flow again 
backs/amplifies by midweek ahead of a strong upper low that will
eject out of the southwest states into the Great Lakes and Mid-
Atlantic region by Friday. Pieces of mid level energy ahead of 
the main upper system will give rise to overrunning across a 
warm front to the southwest by later Wednesday with this 
boundary crossing the region Wednesday night/Thursday as the 
main surface low tracks into the upper Midwest. Quick return of 
deeper moisture per model Pwats suggests an increasing threat 
for showers/rain from southwest to northeast later Wednesday and
across the region Wednesday night into Thursday ahead of the 
trailing cold front. Upper trough looks to deepen across the 
region with a switch to colder northwest flow by Friday in the 
wake of the cold front. This likely to produce some upslope 
rain/snow showers with breezy/windy conditions elsewhere with 
more sunshine returning out east.

Temperatures mainly 50s/60s for highs early on with warmest Thursday 
under strong southwest flow ahead of the front and then much cooler 
Friday under cold advection resulting in highs 40s mountains to 
low/mid 50s east.

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
As of 1145 PM EST Friday...

Dense fog has set in at LYH/DAN and other airports in the
piedmont. At the moment think these two terminals will be the
only ones affected in the tafs thru 12z-14z. Aside from this
will be seeing VFR ceilings further west, with potential for fog
in the high end IFR range at ROA.

Models continue to slow down rain threat until later this
morning and mainly affecting airports west of the Blue Ridge and
north of a BLF-ROA line.

Will see dense fog wane at LYH/DAN by 13-14z, then improve to
VFR by 15-16z.

Ceiling and vsbys should stay at or above 3kft for most of the 
afternoon outside of showers. Will have to see how much fog and 
low clouds form Sat evening as winds subside, but appears where 
it rains Saturday will see some fog into Sat night. 

Confidence high on cigs/vsbys in the east through 12z and less
beyond for all sites, especially after 00z Sunday.

Extended: 

During the day Sunday, showers become more widespread as a cold
front crosses the region from the west. Sub-VFR conditions may 
well extend through Monday as the front stalls out to the south 
allowing for showers to linger in the region throughout the day.
High pressure builds over the region Monday night into Tuesday 
which should bring drier conditions, at least temporarily.

&&

.CLIMATE...

Records For...

Saturday 2/24/2018 
Site MaxT/Year HiMin/Year
BLF    71/2017    53/2017 
DAN    79/2012    51/1996
LYH    79/2012    53/1985
ROA    80/1985    52/1943
RNK    73/1975    49/1975

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$
SYNOPSIS...PM
NEAR TERM...PM

National Weather Service Glossary of Abbreviations