Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS61 KRNK 241430
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
1030 AM EDT Mon Apr 24 2017

.SYNOPSIS...
Wet weather will persist into Tuesday, as a slow moving area of
Low Pressure moves from the southern Appalachians to along the
South Carolina coast by Tuesday morning.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS of 1030 AM EDT Monday...

Flood watch remain in effect until 8 pm this evening for much of
the forecast area. 

Upper low over the southern Appalachians this morning 
is slowly moving to southeast and will push of the South 
Carolina coast by daybreak Tuesday.

Flooding continues in western portions of Flood watch this
morning, but the good news is that the rain has ended or is 
lighter. For this morning, the heaviest rain is located east of
the Blue ridge mountains. Watching on latest radars, convection
to our south lift north. The HRRR and NAM are pushing moisture 
northwestward this afternoon into tonight. With a deep southeast to 
easterly fetch of moisture into our forecast area, expected
periods of rain to continue in the east. The rain will be heavy
at times. While in the west, rain amounts will be lighter.
Adjusted temperatures with latest surface obs and trended the
late morning into this afternoon towards cooler GLAMP. With
clouds and rain, expected little temperatures changes today. 
Adjusted pops and weather for latest radar trends this morning.
Trimmed backed pops and intensities of rain in the west. More
changes later this morning...




As of 400 AM EDT Monday...
No changes to the flood watch, with still some model differences
in placement of highest rainfall threat.No change to Near Term
Forecast. Rain...Heavy at times pretty much sums it up. 

Upper low over the southern Appalachians has ground to a halt, 
showing only little movement the last 12 hours and is forecast 
to move very little today per being cut-off from the mean upper 
level flow. Overall projection is for this feature to drift ever
so slowly to the South Carolina coast by daybreak Tuesday. This
will maintain a deep southeast to easterly fetch of moisture 
into our forecast area resulting in a persistent rain with 
periods of heavy rain. As such will maintain the Flood Watch for
most of our forecast area along and east of the VA/WV border 
and into NC. 

As the rainfall continues to add up, expect streams to respond
with small stream flood warnings morphing into larger scale
river flood warnings. River Forecast Centers will be doing a lot
of number crunching this morning, so watch for revised river
forecasts and warning headlines. 

Rainfall today is expected to be the heaviest along and east of
the Blue Ridge where the southeast upslope wind component will
add orographic lift. Additional rainfall of 1 to 2 inches is
likely (in some cases on top of the 2 to 4 inches that has
already occured). Heaviest rain thus far has been along and just
west of I-77 from Tazewell and Marion VA, south into Sparta and
Wilkesboro NC. Focus today is expected to shift farther
northeast into the headwaters of the Dan and Roanoke River
basins.

Temperatures today and tonight will change very little, cool 
northeast to easterly low level wind persisting until the low 
drifts off the coast Tuesday.

&&

.SHORT TERM /TUESDAY THROUGH THURSDAY/...
As of 400 AM EDT Monday...

We'll have one more day of cloudy and rainy/showery conditions on 
Tuesday. However, trend will be toward above-normal, more springlike 
temperatures looking ahead into the midweek period.

Slow-moving, vertically-stacked cyclone should be roughly centered 
over the coastal NC/SC border based on general consensus reflected 
in 21z SREF mean/00z global model progs. Little significant change 
noted in the projected evolution of the cyclone, that being 
deamplificaton and gradual northeastward progress. Moisture plume on 
the eastern periphery of the cyclone, associated with PWAT values of 
up to 1.3" will continue to be drawn back westward, supporting a 
continued period of rain particularly for the central Virginia 
Piedmont and into the Southside of Virginia. Weak/nebulous forcing 
elsewhere across the CWA supports periods of light showers mainly 
during the first half of the day. As upper level low approaches the 
Chesapeake Bay and deeper moisture plume is shunted 
eastward/northeastward later in the day, PoPs then begin a taper to 
slight chance/lower Chance range areawide. An additional tenth to 
two-tenths of an inch of rain are possible in central Virginia 
associated with the most persistent rain band tapering to a few 
hundreths from the Blue Ridge foothills west. Thus a continued wet 
outlook but the steadiest and heaviest rains will have since ended. 
Aside from southeast West Virginia into western Smyth and Tazewell 
Counties in Virginia where at least some partial clearing is 
anticipated toward afternoon, for many it will continue to be 
another relatively dreary day with highs only in the 60s. Greater 
clearing anticipated on Tuesday evening with some radiational 
cooling under clearing skies in the upper 40s to mid 50s. 

Trend toward warmer temperatures remains on track starting Wednesday 
with building 500 mb heights and mostly sunny skies. Our 850 mb 
temperatures steadily rise Wednesday into Wednesday night to values 
around +14 to +16C. Much milder temperatures well into the 70s to 
low 80s anticipated for Wednesday, with lows Wednesday evening into 
the low 50s to around 60. These are some 10 to 15 degrees milder 
than 1980-2010 climatological normals. 

For Thursday, focus shifts to large-scale troughing evolving across 
the central Plains states and the ejection of a deamplifying mid-
level trough embedded in deep south-southwest flow, its associated 
surface cold front expected to move from the Ohio Valley to the 
foothills of the western Appalachians Thursday. I'm hesitant to 
introduce slight to low Chance PoPs any earlier than Thursday 
afternoon given the amplified southerly mid-level flow that may act 
to slow the cold front down than currently depicted in most model 
guidance, and we also are capped to any convection out ahead of 
this. Generally expect mostly sunny skies through much of the day, 
albeit with increasing cloudiness later in the day toward Thursday 
evening. Even warmer conditions anticipated on Thursday; aside from 
the higher elevations, many areas stand a good shot at reaching the 
low to mid 80s particularly east where clouds will be slower to 
advance. These are a couple degrees cooler than MEX MOS guidance 
which would argue for some upper 80s values in the Piedmont and 
Southside.

&&

.LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
As of 300 PM EDT Sunday...

500MB heights rise Friday over the southeast United States but with 
an overall deepening upper trough over the central United States. A 
deep closed low develops over the Rockies by the end of the week 
which will keep much of the east downstream in broad southwest flow. 

At the surface a low pressure system tracks into the Great Lakes. 
The front with this system stalls in the Ohio and Tennessee Valleys 
brushing our far western counties Thursday night and early Friday. 
Thereafter, differences become apparent between the 00z GFS and 
ECMWF on how far north and east a warm front associated with the 
next developing Plains system extends. Generally maintained 
continuity with prior forecast for the upcoming weekend until these 
details can be better agreed upon. In any event, temperatures 
appear to remain above normal.

&&

.AVIATION /14Z MONDAY THROUGH FRIDAY/...
As of 739 AM EDT Monday...

Continued poor flying conditions through the TAF period mainly 
due to low ceilings predominantly into the IFR to MVFR range.
Significant improvement in ceilings not anticipated until
later Tuesday morning. Periods of rain, heavy at times with 
MVFR visibilities, will continue to spiral north- northeast. 
Coverage and intensity of rain should begin to diminish to a VFR
visibility by tonight. Winds northeast 4-8 kts. By tonight, 
strong northeasterly low- level jet combined with inversion from
persistent wedge will lead to low-level wind shear at most 
terminals overnight. 

Extended Aviation Discussion...

Look for gradual improvement from west to east with better 
flying wx for by midweek, though another front may bring 
scattered MVFR showers/storms toward late Thursday in the 
mountains. Drier weather will return for Friday.

&&

.HYDROLOGY...
As of 300 PM EDT Sunday...

No changes to the flood watch, with still some model differences
in placement of highest rainfall threat. High-res models and
analogs along with pattern recognition favor the southern Blue
Ridge from Roanoke to Boone, east to the piedmont of VA/NC as
those areas which appear to be in line to receive 2-3" through 
early Tuesday. This amount of rain combined with the 1 to 2+ 
inches that has already fallen since Friday should lead to hydro
issues on rivers, as well as smaller creeks and streams.

Looking at the analogs this pattern favors April 5th, 1993, and
March 27th, 1993 which produced some minor to moderate flooding
along portions of the Dan and Roanoke River mainly downstream of
Danville and Altavista. Still will have to see how this sets up
as we had been dry prior to this event.

WPC has kept the risk of excessive rainfall along the Blue 
Ridge. Deep convection seems limited, thereby will have to rely
on upslope component to enhance rainfall rates. 

Saturated ground may also result in downed trees and rock or 
mud slides.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Flood Watch until 8 PM EDT this evening for VAZ007-009>018-
     022>024-032>035-043>046-058-059.
NC...Flood Watch until 8 PM EDT this evening for NCZ001>006-018>020.
WV...Flood Watch until 8 PM EDT this evening for WVZ042.

&&

$$
SYNOPSIS...PM
NEAR TERM...KK/PM

National Weather Service Glossary of Abbreviations