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FXUS61 KRNK 240607

Area Forecast Discussion
National Weather Service Blacksburg VA
207 AM EDT Tue Apr 24 2018

Low pressure will slowly move from the Tennessee Valley to the
Carolinas through Tuesday. Prolonged southeast winds will keep
the area wet through Tuesday night, when the low lifts to the
Delmarva. As this system departs on Wednesday, two additional 
disturbances look to cross over or near our area Thursday into 


As of 200 AM EDT Monday...

The wind advisory was allow to expire at 2 am with winds 
decreasing this morning. Mesonet observations indicated that 
winds are remaining below wind advisory levels. An isolated 
wind gust may be possible the next couple of hours in the 
highest elevation. Made some minor adjustments in temperatures
this morning. 

As of 956 PM EDT Monday...Raised temperatures up some through 
the rest of the overnight, but this amounts to essentially 
temperatures remaining roughly steady-state. 

Mesonet observations indicate southeast wind gusts in the Wind 
Advisory area have been between 35-45 mph. 18z GFS appears to
have better handle on low-level winds per comparison with
Doppler VAD profiler data than does the bullish NAM, and there
is still a small window of opportunity for further 
strengthening to occur until midnight. Thereafter winds should
begin to ease. Current Wind Advisory through 2 AM looks valid 
at this point in time. 

As expected, rainfall rates have been pretty gradual with a
largely stratiform precipitation character. Radar derived rain
totals likely are still underestimating in the southern Blue
Ridge with rain amounts through several sources in Watauga
County on the order of 2 inches, resulting in some rises on the
New River and likely into surrounding smaller creeks/streams. No
current hydro problems noted per county officials in spite of
these rains. Amounts taper to an inch or less as you get 
further away from the Flood Watch area. Still looking at 
additional rains tomorrow but don't presently feel extension of 
the Watch is warranted attm. Finer- resolution guidance 
indicates another round of potentially steadier/heavier rains
after 06z along/east of the Ridge as convection over GA weakens
into a more stable environment and lifts north/northeast. These
ideas generally handled in the forecast so no major changes
needed to PoPs/Wx attm.

Previous discussion issued at 225 PM follows...

Southeast flow accelerating this afternoon, with gradient tightening 
between low pressure over Western TN and high pressure off the mid-
Atlantic/Northeast coast. Getting some gusts over 40 mph in the 
higher terrain of NC, with 25 to 35 mph further north. Models are 
indicating stronger part of low level wind field arrives around 00z-
04z this evening, and have added western Greenbrier to the wind 
advisory. Also strong winds should linger past midnight, so extended 
the advisory til 2am Tuesday.

The flood watch still looks good, see hydro discussion below for 

Looking at periods of rain working in from the south as plume of 
deep moisture drives northwest from the Atlantic through the 
overnight. Models in reasonable agreement in placing emphasis of 
higher rates over the southern Blue Ridge with light-moderate rain 
surround this. Latest high-res models are keeping the northeast 
piedmont dry until overnight/after midnight and leaned pops toward 
that solution with categorical over the southern CWA to chance in 
the far northeast and northwest.

Toward morning, southeast flow at 8h continues but better lift 
starts shifting to the northeast with potential rain shadow effect 
taking shape over the mountain empire of SW VA into southern WV. 
Seems enough moisture transport to keep pops greater than 50 percent 
across the west, with 100 pops in the east. As the 8h low moves over 
the Mountain Empire in the afternoon, a pocket of lower rainfall 
works from southwest VA into central WV, while upslope flow in the 
east persists but weakens somewhat in the afternoon. Still looks 
like a good period of rain along/east of the Blue Ridge Tuesday. 

Some SBCAPEs greater than 500 J/KG start to get close to our NW 
piedmont of NC in the afternoon, and includes isolated thunder 
threat here, but overall looks too cloudy and wedged in for deeper 

Kept lows tonight similar to previous forecast with mid to upper 40s 
across much of the area. Highs Tuesday will be cool again especially 
across the Blue Ridge, and Alleghanys, with upper 40s to mid 50s, 
with upper 50s to lower 60s expected west of I-77 in WV, and over 
the upper TN VAlley in SW Va, as well as southeast of a Lynchburg to 
Stuart line.


As of 200 PM EDT Monday...

The center of the slow moving upper low that would have been 
associated with our rainfall Monday and Tuesday, is expected to 
start Tuesday evening over eastern Kentucky and progress due east 
across the forecast area by early Wednesday morning. Showers will 
prevail through the night in association with the system's steeper 
cold pool lapse rates. Early in the evening, the far southeast 
portion of the area will be very close to region of weak CAPE. SREF 
numbers keeps any amounts greater than 500 J/Kg just outside the 
forecast area. At this point, am leaning toward keeping the forecast 
simply as showers.

On Wednesday, the upper low will begin a turn northeast in response 
to yet another shortwave trough heading southeast into NY/PA. The 
first upper low will lose its definition as it becomes enveloped 
within the second. Flow across our area will turn northwest on the 
back side of the system, trending the precipitation to be focused in 
two regions. The first will be an area of upslope showers across 
southeast West Virginia. The other will be showers on the western 
flank of the upper trough axis, still influenced by the steeper 
lapse rates of the associated cold pool. Close to adding thunder in 
the east on Wednesday, but the best lift looks to remain just east 
of the forecast area.

As we progress Wednesday night through Thursday night, our region 
will remain within a progressive pattern, influenced by both the 
northern and southern stream jets. First there is the question as to 
how quickly the NY/PA upper low will head northeast, taking its 
influence with it. While there are some minor differences within the 
deterministic models, a clear consensus is that by daybreak Thursday 
all should be dry, at least for a little bit of time.

As early as Thursday afternoon, we will need to start examining our 
next potential weather maker. This one is another shortwave trough, 
but within the southern stream this time. Guidance sweeps the system 
from the Lower Mississippi River Valley on Thursday afternoon, to a 
location near GA/SC by daybreak Friday. The path and timing vary 
such that either this feature will have no influence on our weather, 
or the southern half or third of the region will experience showers 
on its northern flank Thursday night, potentially starting as late 
Thursday afternoon across the far southwest section of the forecast 
area. Our forecast will reflect a toned down version in terms of 
POPs that the wetter solutions have for our region.

During this portion of the forecast, high temperatures will trend 
milder to readings near normal for this time of the year. Low 
temperatures will trend slightly cooler to readings slightly below 


As of 335 PM EDT Monday...

On Friday, we will still have the question as to the degree of 
influence the southern stream shortwave trough has for our area. A 
lot will depend upon how quickly yet another northern stream system 
progresses eastward, inviting a turning northward of the southern 
system. If in fact it does turn north Thursday night, scattered 
showers will continue to impact the area on Friday.

The aforementioned northern stream system is progged to move through 
the area Friday night into Saturday morning. Models differ as to 
whether this will be a quick moving clipper, or be a bit beefier 
with an actually associated closed or open low that crosses our 
region. Either way, the the forecast detail this far out in time 
will be the same, one of forecast scattered showers. 

Once this system clears the area by Saturday night, the overall 
synoptic pattern changes. For Sunday into Monday, expect an upper 
level ridge to approach, amplify, and then be centered over the 
area. This will result in a forecast that has two consecutive days 
of no precipitation.

Temperatures will start this portion of the forecast a few degrees 
below normal, but trend milder, especially for Sunday and Monday. By 
Monday temperatures are forecast to average about five degrees above 


As of 207 AM EDT Tuesday...

Poor flying conditions expected during the taf period with rain
and low clouds. The combination of a wedge east of the 
mountains and increasing isentropic lift will result in 
rain spreading north into our region today. 

Cigs will lower to IFR conditions this morning. With slow 
motion of the closed low moving out of the Mississippi valley 
expect rain and IFR conditions to linger into This afternoon.

Believe the wedge barrier jet will keep winds a bit gusty at
KLYH and KDAN through the period. While the southeasterly low
level jet running over the top of the wedge is quite robust,
very moist and stable boundary layer conditions will inhibit
winds mixing down. The most likely locations for stronger wind
gusts will be at the higher elevations in the west to include
the airfield at KBLF.

Medium to low confidence in ceilings, visibilities and winds
during the taf period. 

Extended Aviation Discussion...

Tuesday afternoon the low will be situated overhead so rainfall
may start to fade in the southwest around BLF after 18z Tuesday
then shifting northeast through Tuesday evening. Expect sub-VFR
through the afternoon, and into Tuesday night/early Wednesday.
Winds turn to the northwest, by late Tuesday night into
Wednesday and allow for the precipitation to trend showery in 
the west. 

The weather pattern will remain unsettled Thursday into Friday
as a series of disturbances crosses the area, each with the
potential for providing showery precipitation and localized
sub-VFR conditions.


As of 240 PM EDT Monday...

No changes need to the flood watch, which remains in effect 
through 400 PM Tuesday. Forecast amounts from this morning 
through Tuesday showing 2-4" along the southern Blue Ridge from 
near Meadows of Dan to Blowing Rock. Looks likely that the areas
around the Watauga River in NW NC as well as streams flowing 
off the Blue Ridge are going to need to watched. Ensembles for 
the rivers are still lagging though the SREF is a bit robust in 
bringing flooding threats later on to the Dan River, as well as 
portions of the Greenbrier and New. Other ensembles as well as 
the RFC are keeping all rives at or below flood stage. Something
to watch once we see how much falls, but rises along the main 
stems, including the New and Dan Rivers are likely, as well as 
portions of the Roanoke.

Given lack of deep convection, flash flooding appears less likely.


VA...Flood Watch through this afternoon for VAZ015>017-032.
NC...Flood Watch through this afternoon for NCZ001>003-018-019.



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