Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS61 KRNK 230452
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
1252 AM EDT Tue May 23 2017

.SYNOPSIS...
A weak stalled front over northern North Carolina will remain in
place today as ripples of low pressure slide northeast along 
the boundary resulting in added rainfall into tonight.  Yet 
another area of low pressure will arrive midweek with added 
showers likely for Wednesday and Thursday. Weak high pressure 
will follow this system for Friday.
&&

.NEAR TERM /THROUGH TODAY/...
As of 1132 PM EDT Monday...

Watching slow moving moderate to heavy showers in the NC
foothills/piedmont, along a weak convergence zone leftover by
the front. Models indicate that isolated to scattered coverage
will be the way to go over the next few hours per moisture
convergence off the high-res models. Appears best rainfall will
say just south of our CWA but looking for some increase
northward overnight as low level flow turns to the southeast.
The latest RAP indicates better deep moisture convergence
pooling toward the NC foothills after 06z, but only briefly.
Looks like rain showers will spread northeast along the Blue
Ridge through morning, while other are surges northeast to the
southside of VA. At the moment QPF seems low enough to not have
any watches at this time. Better threat arrives more toward
dawn. Will be watching slow moving cells for any local
advisories.

Previous discussion from early evening...

Enough of an instability gradient over the NC foothills/piedmont
to keep isolated to scattered convection around this evening.
The high-res (HRRR/RAP) favor showers/a few thunderstorms from
the NC/VA border in the foothills through southside VA through
this evening, then should be weakening somewhat, transitioning
to a more rain.

Previous discussion from this afternoon...

Water vapor loop showed a well defined short wave over Arkansas 
early this afternoon. Models track this feature into southern 
Virginia overnight. 

Surface and 850MB front cross the area this afternoon, but with 
little change in air mass behind it. 850MB winds back to the 
southeast by midnight brining the deep moisture north along the Blue 
Ridge and foothills. Best convergence will be from 12-18Z/8AM-2PM 
Tuesday. GFS may have some convective feedback and may be moving the 
wave too fast and too far northeast compared to other guidance. Some 
decent isentropic lift on the 3km NAM Tuesday morning. Leaned toward 
NAM and WPC for QPF .WIll have to monitor amounts since some 
locations have had lots of rain in the past 72 hours and have 
saturated soils as a result. 

Surface front reaches central North Carolina as high pressure moves 
from West Virginia into Pennsylvania. By Tuesday morning the in-situ 
wedge will be in place down the Appalachians aided by precipitation 
on the cool side of the front. Clouds and precipitation will keep 
temperatures down on Tuesday. Trending toward cooler guidance for 
maximum temperatures.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/...
As of 330 PM EDT Monday...

Large upper trof will be traversing the eastern US, keeping us in a wet 
pattern through the period.

Multiple waves along the front to our east will be pulling away from 
the region Tuesday night. However, a low over the Ohio valley will be 
captured by the upper trof as it closes off and the stacked system will 
wrap up a cold front a push it into our area from the west Wednesday 
morning. There will be a bit of an in-situ wedge ahead of the front to 
limit instability and convective development, but good isentropic lift 
and dynamic support will make for widespread rainfall and some possibly 
some embedded thunder from lift over the stable layer. Also, there may 
be a very narrow window just ahead of the front where the wedge has 
weakened that may allow for deeper instability to support elevated 
convection in a highly sheared environment. While the overall 
probability of severe weather is low, the situation bears watching 
until fropa occurs early Wednesday night. Expect some lessening of 
showers overnight Wednesday night in muddy dry slot of stacked low 
spinning over the Ohio valley.

Diurnal heating will pop showers and thunderstorms with steepening 
mid/upper lapse rates under the cold pool aloft as the upper trof 
swings through. Small hail/graupel in low topped convection look 
possible mainly west of the Blue Ridge.  Showers/storms taper quickly 
with loss of heating in the evening, leaving some lingering upslope 
precipitation west of the Ridge into Friday morning.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 330 PM EDT Monday...

Our unsettled pattern will continue through the weekend as a 
progressive upper ridge moves across the eastern US, followed by the 
development of another closed low moving through the upper midwest. 
This will allow a frontal boundary to sink into the region from the 
north and stall on Saturday, with a series of waves shearing off to our 
north dragging along weak, occluded fronts. While not likely a complete 
washout, we look to keep a good chance of showers and thunderstorms in 
the forecast through the weekend and into the first part of next week.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
As of 1250 AM EDT Tuesday...

Will see a gradual deterioration in flying conditions overnight
as scattered convection to the south combines with added
moisture heading north to bring a quick lowering of cigs through
daybreak. Rain should start spreading northward overnight and 
cigs will start to sink to MVFR or lower, especially along/east 
of the Blue Ridge. The rain will become steadier and possibly 
heavier late tonight into Tuesday from KBCB/KROA and points
east. Should also see areas of fog with mountain obscurations
likely as well into Tuesday.

This pattern will stick around through the end of the taf
period, but rain will be lighter over KBLF/KLWB.


Extended Aviation Discussion...
 
The chance of precipitation continues Wednesday and Thursday
with sub-VFR conditions associated with any of the
precipitation. Friday will be drier with a better chance of VFR
ceilings and visibilities. Another front reaches the area for
Saturday with more precipitation and sub-VFR conditions.

&&

.HYDROLOGY...
As of 345 PM EDT Monday...

An inch to inch and a half of rain is expected tonight and
Tuesday. Amounts will have to be monitored as some locations 
along the Virginia/North Carolina border, especially Carrol,
Patrick and Henry Counties in Virginia and Stokes, Rockingham,
and Caswell Counties in North Carolina. FFG along parts of the
southern Blue Ridge was in the 1.5 to 2.5 inch range. 

More rainfall is expected Wednesday and Thursday, which could 
lead to small stream and river flooding. Models continue to 
favor areas along/east of the Blue Ridge, possibly resulting in
another 1 to 3 inches of rain.

&&

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

&&

$$
SYNOPSIS...AMS/JH
NEAR TERM...AMS/WP

National Weather Service Glossary of Abbreviations