Area Forecast Discussion
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FXUS61 KRNK 211015
Area Forecast Discussion
National Weather Service Blacksburg VA
515 AM EST Sat Jan 21 2017
A warm frontal boundary situated across the Gulf Coast states
will move north today, moisture increasing across the southern
Appalachians with the potential for rain as far north as
southern West Virginia and Virgina by this evening. There will
be several surges of moisture that will flow northward from the
Gulf Coast this weekend, with the potential for the rain to be
moderate to heavy at times. The heaviest of the rain will occur
Sunday night into Monday as a bonafide area of low pressure
develops over the Mid-Mississippi Valley and moves northeast
across the area.
.NEAR TERM /THROUGH TONIGHT/...
As of 445 AM EST Saturday...
Areas of dense fog were common from the New River and Greenbrier
Valleys, east across the Blue Ridge and into the Piedmont of
Virginia and North Carolina. The airmass over the eastern 2/3rds
of the forecast area is stable with shallow wedge of cool air,
clear skies, and moist ground helping to promote the fog. The
HRRR model suggests the fog will be stubborn to dissipate,
lingering as long as 10AM.
Today`s challenge will be high temperatures. Fog this morning
and increasing clouds from late morning into the afternoon may
keep temperatures from reaching their full potential in spite of
warmer air aloft. As such, went 2 to 5 degrees blo the MOS
forecast highs. Never the less, readings will still be 8 to 15
degrees above normal for Mid January. There are a couple of
record highs that may be tested today, Bluefield 60 and
Blacksburg 59. However, much will depend on how fast the fog
dissipates and the clouds increase this afternoon as to whether
these numbers can be tied or exceeded.
Today`s rain threat will come from the Gulf Coast Region. A
strong upper level disturbance, short wave trough, was ejecting
northeast from the Lower Mississippi Valley. Widespread showers
and Thunderstorms will be the result across much of the Deep
South, but some of this activity is forecast to surge as far
north as southern West Virginia and Virginia this evening,
although not expecting any thunderstorms this far north. Per
the west-east orientation of the warm front along the Gulf
Coast, think the bulk of the heavy rainfall and thunderstorms
will remain well south of our region. QFP amounts across our
forecast area will be significantly less with models suggesting
a quarter inch or less from this particular surge of moisture.
Onset looks to be mid to late afternoon today with the rain
lasting for about 4 to 6 hours before ending. Greatest
potential for rain will be east of the Interstate 81 corridor
with probabilities increasing substantially from eastern
Tennessee and North Carolina southward.
Once the rain threat abates during the late evening we will once
again be left with a moist ground and the potential for at least
patchy fog. Do not think we see any widespread clearing so
the probability for dense radiation cooling induced fog appears
to be less.
Another surge of moisture will arrive late tonight into Sunday.
The upper trough that is over the western United States is
littered with short wave energy from storm systems which
continue to pound the west coast. Yet another piece of energy
will eject northeast late tonight with rain overspreading the
area by daybreak Sunday, a precursor for more to come later
Sunday into Monday, each wave having the potential to produce
higher amounts of rain due to the increasing available moisture
flowing north from the Gulf Coast Region. By Sunday, a bonafide
area of low pressure will develop over the Mid-Mississippi
Valley, a feature that will have to be closely watched as it may
focus a much larger fetch of rain into our region leading to
the potential for excessive rainfall.
.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
As of 345 AM EST Saturday...
During this portion of the forecast, the feature we will be watching
is a closed upper low that will track from the mid-Mississippi
Valley eastward, and then northeastward across our region. In
advance of this system, low level flow with Gulf of Mexico origins
will advance across our area. Contained within this southwest flow
will be individual shortwave troughs ahead of the main upper low.
A generous rainfall is expected during this portion of the forecast.
Anticipate distinct rounds of precipitation. The first will be during
the day Sunday, primarily the morning hours. After perhaps a six hour
decrease in coverage Sunday afternoon into early Sunday evening,
precipitation on the northeast flank of the parent upper low is
expected to arrive across the area. As Sunday night progresses, the
center of the upper low is progged to move northeast over the
forecast area. Guidance offers solutions that place first the
northern and the western flank deformation zones over the region,
with western sections of the area having the best chance for
continuous moderate rain heading into Monday morning.
By Monday afternoon and evening, the center of the low is expected
to heading northeast of the region with the flow over our region
becoming northwest. Latest guidance continues to ramp up the 850mb
winds across the region, maintaing upslope precipitation in the west
with wind speeds and gusts potentially reaching at least wind
advisory levels at the highest elevations across at least the
Northern Mountains of North Carolina. Another concern with the
stronger winds will be a greater chance of downed trees without
typical wind thresholds for doing so thanks to what will become very
saturated soils across the region.
Rainfall amounts within the Sunday through Monday night time frame
are expected to average between 1.50 and 2.00 inches, with locally
higher amounts near 2.50 inches along the crest of the Blue Ridge
and other higher peaks where a southeast upslope flow amplifies
precipitation production. With this amount of precipitation expected
over a multi-day period, localized flooding, as well as, rises or
rivers and streams towards the end of the period, or slightly after
the end of the period cannot be ruled out. This concern will
continue to be mentioned in the Hazardous Weather Outlook.
Cold air advection will also establish itself across the area, with
some of the precipitation in the west transitioning to light snow
across the higher elevations of southeast West Virginia, south into
the Northern Mountains of North Carolina late Monday night into
Upslope rain/snow showers will continue into Tuesday night but with
decreasing coverage through the night. Winds will decrease as the
low level flow veers southwest as the axis of an upper ridge shifts
east of the region.
Isolated thunderstorms are possible at times during this portion of
the forecast. Currently we are watching Sunday and Sunday evening as
potential time frames across primarily the southern third of the
region based upon model forecast Lifted Indices slightly below zero,
and surface based CAPE of a few hundred J/KG.
Temperatures during this portion of the forecast will average
fifteen to twenty degrees above normal through Tuesday. Cooler, but
still above normal, temperatures are expected Tuesday night.
.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
As of 415 PM EST Friday...
Shortwave ridging continues Wednesday into Wednesday night with weak
surface high pressure extending north into the region. This should
result in a period of dry/mild weather for midweek with highs well
above normal and likely 60s east of the mountains.
This will begin to mark the end of the period of above normal temps
and bring the reality of winter back as a longwave trough pushes
southeast and across the region by the end of the week. Cold front
preceding this upper trough will cross the region on Thursday which
will be the transition day to colder air that arrives on Friday.
Weak impulse sliding up ahead/along the boundary could produce a
band of rain/showers by Thursday morning before things switch to a
mainly mountain upslope snow shower regime under cold northwest flow
within the cyclonic trajectory by Friday. Expect highs by then to
only range from 30s west to low/mid 40s east under blustery
.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
As of 1150 PM EST Friday...
Poor aviation conditions expected through the TAF valid period.
Dense fog has settled in across much of the CWA at this hour as
a result of earlier rainfall, late day clearing leaving wet
ground, along with calm winds. All airports expect KBLF are
being affected at this hour. KBLF has a slight southwest wind
that has prevented them from being affected so far, but
indications are that visibility/ceiling there will also go down
later tonight as the winds come around to the southeast. KLWB is
likely a local situation at the moment. Most TAF sites are IFR-
LIFR and will likely remain so until 14Z-15Z as there is little
synoptic situation that will bring about improvement.
Whether or not clouds diminish/dissipate after 15Z Saturday is
questionable. Model cross sections show near saturation
continuing in the lower layers and with a light southeast wind
most sites, clouds may hold in. Would expect ceilings to
improve to MVFR at best, but IFR may hold some areas, especially
east of the Blue Ridge.
Around 21Z or so, the next short wave from the deepening upper
low across the Midsouth will move toward the area with the next
wave of rain. Expect mainly MVFR cigs/vsbys as this rainfall
moves through during the late afternoon/evening. Thereafter,
drizzle and fog will likely settle in once again with IFR-LIFR
cigs/vsbys once again for the remainder of the TAF valid period.
Winds should be mostly calm overnight, then southeast 4-7kts
most sites after 14Z Saturday.
Medium confidence in ceilings and visibilities through the TAF
Medium to high confidence in wind speed/direction through the
TAF valid period.
Extended Aviation Discussion...
A slow-evolving but potent frontal system affecting a good part
of the southeast CONUS will lead to a potentially long period
of sub-VFR conditions this weekend due to low clouds/stratus
and rain. Rain may at times be steady to heavy and may include a
rumble or two of thunder, particularly Saturday night into
Sunday. Precipitation begins to trend more showery/intermittent
on Monday before trending VFR/MVFR for Tuesday.
As of 305 PM EST Thursday...
Record warm Mins for January 21
Blacksburg....42 in 1954
Bluefield.....47 in 1999
Danville......56 in 1954
Lynchburg.....51 in 1927
Roanoke.......51 in 1959
Record highs for January 21
Blacksburg....59 in 1954
Bluefield.....60 in 1999
Danville......68 in 1959
Lynchburg.....72 in 1932
Roanoke.......74 in 1932
VA...Dense Fog Advisory until 10 AM EST this morning for VAZ010>014-
NC...Dense Fog Advisory until 10 AM EST this morning for NCZ002>006-
WV...Dense Fog Advisory until 10 AM EST this morning for WVZ044-507.