Scientific Forecaster Discussion
fxus62 krah 231928
Area forecast discussion
National Weather Service Raleigh NC
328 PM EDT Mon Oct 23 2017
a potent upper level trough and accompanying surface front will surge
through the region this evening. A secondary dry cold front will follow
and cross our region Tuesday night. Cool high pressure will be in control
mid to late week.
Near term /through tonight/...
as of 328 PM Monday...
..Tornado Watch through 9 PM for the western Piedmont...
..Slight risk of severe storms late this afternoon and evening...
Radar indicated the line of convection was surging east-northeast across the NC
mountains and upstate SC, with warnings already being issued over
western NC and SC. The initial Tornado Watch has been issued for the
western zones through 900 PM, with additional watches likely
to the north and east. This is a traditional high shear, Low Cape
environment, with a low level boundary retreating northward through
the northwest Piedmont and foothills (where an in-situ cad developed today).
The approaching mid/upper trough has become strongly negatively
tilted over the Tennessee Valley with a parent low moving north through the
Ohio Valley. The trailing cold front was surging over the
Appalachians, with a meso-low and qlcs already pushing through the
The developing severe threat...
The extensive cloudiness and showers over the western Piedmont will
keep things more stable, lessing the severe threat over that region
by a category (more marginal than a slight risk for The Triad region
may be more appropriate). However, along and east of the in-situ
wedge there will be more instability - especially from Raleigh and
Southern Pines eastward where some sun will send temperatures well
into the 70s (even some lower 80s se). 500 to 1000 j/kg convective available potential energy are
expected by mid to late afternoon. Tornados parameters are favorable
in the western/southern Piedmont (especially from Charlotte and
Statesville east to near Fayetteville.
The current qlcs and meso-low upstream was racing NE and will
be affecting our western counties in the current Tornado Watch
beginning around 4-5 PM. This qlcs should continue to strengthen
as it moves into the more unstable environment over central NC
late afternoon and evening.
The previous timing still looks to be on target. We expect the line
to reach The Triad and southern Piedmont between 4 and 5 PM,
then the Triangle and Fayetteville areas around dinner time,
and the Interstate 95 corridor by mid to late evening.
A 3-4 hour period of significant weather is expected as the
lines of storms push through. The line itself should be accompanied by
a few damaging wind gusts and possibly isolated tornadoes. A few
supercell storms may develop out ahead of the line main line
with the threat of damaging wind gusts and isolated tornadoes.
Minor urban or small stream flooding will be possible during the
height of the event, with 2+ inch per hour rainfall rates generally
lasting 30 to 45 minutes at any given spot. Very dry antecedent
conditions and the quick movement of the system overall should keep
the flash flood threat low.
Clearing behind the convective line, with the passage of the cold
front, will spread west to east overnight. However, the latest models
have introduced higher probs of the development of low stratus
again between 4 and 8 am, with some patchy dense fog possible around
sunrise as well. This is due to the surface winds becoming light or
even calm very late tonight in the very moist near surface environment.
Lows have been raised into the 50s northwest to 60s down east.
Short term /Tuesday through Wednesday night/...
as of 325 PM Monday...
After low stratus early Tuesday, skies are expected to become partly sunny.
Highs will be cooler, 60s northwest to 70s southeast. A secondary cold front will bring
even drier and cooler air Tuesday night through Thursday. Still, readings
are expected to remain above the critical threshold of a killing frost
or freeze, even in the areas that fall into the mid to upper 30s (rural
Long term /Thursday through Sunday/...
as of 210 PM Monday...
Cooler and dry weather continues into the latter half of the week
with highs in the upper 50s to mid 60s Thursday and near 70 degrees
for the end of the week. Lows will remain in the 40s for the
majority of the week and then climb back into the upper 40s to lower
50s for the weekend.
The next chance for rain comes from a frontal system that will move
through the area next weekend. There is uncertainty with regards to
timing and also the evolution of a low pressure system riding up the
southeast coastline prior to the arrival of the cold front that
could cause a chance of precipitation earlier in the weekend.
Aviation /18z Monday through Saturday/...
as of 325 PM Monday...
An approaching frontal system will then result in the ewd
progression of a band of IFR-MVFR ceilings, heavy showers and
isolated storms with strong and gusty sly to sswly winds --including
some severe with isolated tornadoes and straight line wind gusts to
50 kts-- between 22z/23rd and 05z/24th. Southeast surface winds will
be strong and gusty ahead of the expected cold front. West to east
clearing will occur with the passage of the associated cold front
this evening-early tonight, followed by lessening winds and lingering
low level moisture late tonight resulting in the development of low IFR
stratus and MVFR fog between 08-123/24th.
Outlook: VFR conditions are anticipated 15z Tuesday through Friday.