Turner Downs Forecast Discussion

FXUS62 KRAH 180718

National Weather Service Raleigh NC
318 AM EDT Tue Jun 18 2019

Beneath occasionally and weakly perturbed west southwesterly mean
flow aloft, and on the western periphery of Bermuda high pressure,
an Appalachian-lee trough will linger over Virginia and the
Carolinas through Wednesday night. A much stronger trough aloft and
cold front will cross the region Thursday and Thursday night.


As of 10 PM Monday...

Convection has largely died off this evening with nocturnal
stabilization. However, showers and storms continue to fire across
portions of the northwest Piedmont, along a surface trough/outflow
boundary. Given the loss of heating and lack of any good deep layer
shear any isolated severe threat is generally over. However, with
the mean storm motion oriented somewhat along the surface
trough/outflow across the northwest Piedmont and potential for a
weak disturbance to move into the area, we may have a small window
for training showers/storms and the potential for flash flooding
across the NW Piedmont (generally from now until around 06Z or so).
Otherwise, expect dry conditions overnight, with patchy stratus
and/or fog possible in areas that received some showers/storms.
Expect lows to generally be in the around 70 to lower 70s.


As of 214 PM Monday...

Heights will begin to fall on Tuesday as the aforementioned upper
trough over the Central Plains gradually moves east.  Meanwhile, the
low level pattern will largely remain the same, featuring plenty of
moisture and instability.  The combination of the Piedmont trough,
lingering outflow boundaries, and improved forcing aloft with the
approach of the short wave from the west, will lead to increased
shower/tstm chances with increased coverage compared to today. Given
the approach of the trough from the west, deep layer shear will
increase some, thus the risk for isold severe storms looks better
for Tuesday compared to today.   With 20-30kt shear, tstms may
organize into a couple of multi-cell clusters with damaging wind the
main hazard concern with any of the stronger cells.  Highs on
Tuesday will range from the upper 80s NW to lower 90s S/SE.

Showers and storms should gradually diminish in coverage Tuesday
evening with loss of heating, but a few may linger well into the
early overnight given the available upper forcing with the short
wave passing across the region.  Lows Tuesday night around 70.


As of 330 PM Monday...

The summer-like pattern looks to continue during the long term
period with strengthening sub tropical ridging aloft and Bermuda
High pressure at the surface. This will allow temperatures to rise
into the upper-80s to low 90s (near/slightly above normal for this
time of year) each afternoon, limited only locally by developing
cumulus towers and eventually thunderstorms most afternoons.
Overnight lows Wednesday and Thursday will settle into the low to
mid 70s, with a few 60 degree readings possible Friday night and
Saturday morning as drier air and lower dewpoints settles overhead
behind a cold front, likely to pass through early Friday. Moisture
will recover quickly over the weekend, once again returning the
higher moisture contents within the boundary layer by Sunday and
Monday of next week.

Precipitation wise, expect daily thunderstorm chances to persist
through the work-week with a brief break Friday afternoon and the
first half of Saturday. Wednesday morning, some leftover morning
showers and thunderstorms will be possible as the period begins with
the remnant mid level vort max continuing its journey east through
central NC, likely to push out to sea by lunchtime. Should see at
least some weak subsidence behind this wave, which could help to
limit overall instability and thus storm coverage Wednesday
afternoon and evening. Still thinking daytime heating will be enough
to get a few isolated showers and storms popping though, especially
along the lee side/Piedmont trough between 1pm - 7pm, with some
additional non-indigenous convective clusters plausible. Severe
threat, however, should remain isolated at best due to reduced
effective CAPE/Shear values present.

By Thursday afternoon, mid-level troughing will become increasingly
more amplified to our west with shortwave energy expected to help
usher through a dewpoint front late Thursday into pre-dawn Friday.
Will need to monitor this time period for the development of a
strong storm or two, however, with displaced TOA and best height
falls remaining north of our area, not expecting a significant
outbreak by any means. Have trended POPs to LKLYs west of US-1
before 00z Thursday night with CHC POPs prevailing across the entire
CWA during the overnight period when the FROPA should occur.

The Bermuda high becomes split, transitioning further west across
the FL Panhandle region to end the weekend. This will allow for a
brief stint of surface subsidence returning behind the front on
Friday, further persisting into the first half of Saturday with warm
yet slightly drier conditions filtering into the region. Plenty of
uncertainty later in the period, with a more amplified GFS returning
showers and storm chances to the forecast as early as Saturday
evening while the ECMWF keeps subsidence a bit stronger, secluding
the bulk of the shower/storm activity to our north and west until
early next work-week. For now, have attempted to split the
difference, siding a bit more with climatology, keeping low-end POPs
in the forecast until we can get better feel for model
forecast consensus.


As of 220 AM Tuesday...

Ongoing convection over the nw NC Piedmont, including INT and GSO,
may persist for another couple of hours, as a couple of associated
mid-level disturbances move newd and interact with a surface
boundary there. Otherwise, the presence of Bermuda high pressure
will maintain a 6-12 kt swly surface wind, strongest with diurnal
heating, along with seasonably moist low levels that will present a
risk of early morning stratus mainly at FAY around 12Z this morning.
Additional showers and storms are likely to develop with diurnal
heating/by early this afternoon and persist through early tonight,
with the largest and longest coverage expected over the Piedmont,
particularly INT and GSO. An isolated to widely scattered cell will
be possible throughout cntl NC, however.

Outlook: A persistence forecast of late night-early morning stratus
and mainly afternoon-evening showers and storms will exist through
Thu night, with the former (stratus) maximized Wed morning and the
latter (convection) maximized Thu-Thu night, followed by a
drier air mass and mainly VFR conditions Fri-Sat.





NWS RAH Office Area Forecast Discussion