Turner Downs Forecast Discussion

FXUS62 KRAH 181401

National Weather Service Raleigh NC
1000 AM EDT Sun Apr 18 2021

A weak surface low pressure center will hold over coastal South
Carolina today through tonight. An upper level disturbance will
cross our region late tonight through Monday, then push off the
coast late Monday. Weak high pressure will settle over the area
Monday night, then move off the coast early Tuesday, setting up a
warm southerly flow for Tuesday through early Wednesday. A cold
front will cross the area Wednesday afternoon, bringing cooler
weather starting Wednesday night.


As of 1000 AM Sunday...

Minimal changes to forecast this morning. Satellite imagery shows a
few mid level clouds still over the area from the early-morning weak
wave passage, but these are dwindling. The 12z GSO sounding shows
the potential for deep mixing, as we are warming sufficiently this
morning to be breaking through the low level inversion. While the
scattered mid level clouds are decreasing with the arrival of drier
air at 700 mb, there may be enough lingering shallow moisture near
the top of the mixed depth (below the subsidence inversion aloft)
for scattered flat high-based clouds this afternoon. The batch of
high clouds now over E TN and far W NC will spread overhead today as
well. All in all, looks like lots of sun today, but with a period of
milky sunshine and scattered mid level clouds. With more sunshine
today but with 12z observed thicknesses very close to what they were
yesterday morning, expect highs a bit above yesterday`s, in the
upper 60s to mid 70s. -GIH

Earlier discussion from 300 AM: Water vapor satellite showed a weak
shortwave moving across central NC early this morning, and an upper-
trough across the Midwest and central Plains. Surface obs showed low
pressure across northern FL and high pressure across the Rockies.
Radar was picking up a few light returns across the region, however
dry air near the surface was preventing any precipitation from being
observed. The shortwave will exit the area by late morning, then SW
flow aloft will develop over central NC throughout the day as the
trough to our west approaches the region. North to northwest surface
winds are expected today, then flow will shift easterly tonight as
surface low pressure approaches the coast from the southwest. Skies
will become partly cloudy today, then cloudy again late this evening
as the trough approaches. There should be sufficient moisture and
instability to generate light, isolated to scattered showers across
the northern Piedmont overnight as the weak trough pushes into the
western half of the state. Highs: low to mid-70s. Lows: upper 40s to
mid 50s.


As of 300 AM Sunday...

The upper trough will continue across the region Monday morning, and
combined with a surface low moving northeast along the NC coast,
create higher chances for showers through early afternoon. Highest
precip chances will be along the NE Piedmont and Coastal Plain in
the morning and early afternoon, then conditions will become dry by
late afternoon as upper-level ridging builds in aloft and the
surface low continues northeast. Marginal instability from weak CAPE
will create a possibility of isolated thunderstorms in the late
morning/early afternoon, mainly for the Coastal Plain. Storms are
not expected to be strong at this time. Skies will become mostly
clear Monday night as surface high pressure build in from the west.
Highs: low to mid-70s. Lows: mid-40s to low 50s.


As of 355 AM Sunday...

A wave of surface low pressure will develop along the stalled
frontal boundary over FL and off the Southeast US coast Tuesday -
Tuesday night. However, both the GFS and ECWMF continue to be in
good agreement that it will stay flat enough and far enough off the
coast to only warrant a slight chance of showers for central NC. The
best chance is in the far SE. Skies will be partly to mostly cloudy,
but it still looks to be the warmest day for the region as a whole
with highs in the lower-70s north to upper-70s south. Lows Tuesday
night will be in the lower-to-mid-50s.

A mid/upper trough will move from east from the Great Lakes into New
England and strengthen into a closed low on Wednesday and Wednesday
night, dragging a cold front to its south. The best forcing and
moisture will remain across the northern Mid-Atlantic and New
England, with only a glancing blow this far south. However, the
height falls and associated cold front are still strong enough to
warrant slight to low chance POPs (highest north). While instability
appears relatively minimal at this time, the GFS continues to show
just enough CAPE (a few hundred J/kg) to warrant a slight chance of
thunder in eastern zones. Confidence in the temperature forecast on
Wednesday is still low as it depends on the exact timing of the
frontal passage, but there will likely be a sharp gradient in highs
across the region, with the current forecast ranging from the mid-
60s in the far NW to near 80 in the far SE. W/NW winds behind the
front will gust as high as 20-30 mph.

W/NW flow and high pressure building in from the west will bring dry
and cooler conditions for Thursday and Friday. Thicknesses will be
lowest on Thursday morning and skies will be clear, so leaned toward
the colder side of model blends and close to GFS/ECMWF ensembles for
temperatures, with forecast lows in the mid-to-upper-30s. Thus some
patchy frost will be possible, especially in the usual cooler spots.
Forecast highs Thursday are only in the lower-to-mid-60s, which is
10-15 degrees below normal for this time of year. While thicknesses
will be higher by Friday morning, winds will have died down as the
center of the high approaches the region. Thus better radiational
cooling may allow for similar lows as Thursday morning (mid-to-upper-
30s), with patchy frost again possible. Temperatures will warm on
Friday and Saturday as the high pressure system moves offshore. Both
the GFS and ECMWF show the next mid/upper trough approaching from
the west on Saturday. Details are uncertain this far out, but this
looks to be the best chance for precipitation in the entire period.


As of 630 AM Sunday...

12Z TAF Period: SCT/BKN clouds at 8-11 kft will continue through
this morning, then become FEW/SCT by late morning. A mid-level cloud
deck will redevelop again by late this evening ahead of an upper-
level wave, then -SHRA will move into the Triad overnight pushing
east towards the Triangle and Coastal Plain by early Mon morning.
Variable winds less than 10 kt today, with VFR conditions through
the period.

Outlook: -SHRA and MVFR conditions are possible Monday as an upper-
level wave moves by. An approaching cold front will create a chance
of RA, breezy west winds and sub-VFR conditions Wednesday, then VFR
conditions return Thursday.




NEAR TERM...Hartfield/JJT

NWS RAH Office Area Forecast Discussion