Severe Thunderstorm Warning 121, NWS Raleigh, NC

AREA FORECAST DISCUSSION
National Weather Service Raleigh NC
803 AM EDT Fri May 27 2022

.SYNOPSIS...
A warm front and increasingly humid air will retreat north across
the southern Mid Atlantic this morning. A pre-frontal trough will
then become quasi-stationary over the Piedmont today, followed by
the passage of a cold front across the region tonight. High
pressure, and associated much drier and warm conditions, will build
across the region this weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 435 AM Friday...

A mid/upper-level cyclone centered over srn IL/IN this morning will
move ene to PA through 12Z Sat. A preceding, 50-55 kt sly jet streak
extending from the srn Appalachians through the OH Valley in 500 mb
upr air data last evening will strengthen to around 70 kts while
lifting northeast and across the Mid Atlantic today.

At the surface, a quasi-stationary wedge front and broad triple point
low over wrn NC, diabatically-reinforced from multiple rounds of
convection there during the past 12-18 hours, will continue to mix
and pivot north across VA this morning and up the Mid Atlantic coast
through the afternoon. An outflow boundary and confluence axis was
evident in surface observational data from the broad triple point
ssewd across the Upstate of SC and through the Savannah Basin, with
backed/ssely flow and richer maritime Tropical moisture
characterized by surface dewpoints in the lwr 70s to its east. That
boundary, beneath a plume of deeper moisture with PWs of 1.6-1.8",
will likely serve as a focus for bands of convection and multi-layer
overcast that will move slowly ne across the cntl and ern Carolinas
through midday-early afternoon. To the west of that moisture plume
and early day convection, a pre-frontal trough will develop and
strengthen swd across the Piedmont of the Carolinas and VA,
including over the srn and wrn Piedmont of cntl NC, this afternoon.
Stronger heating/deeper mixing in the drier air along and west of
the trough axis, where more sunshine will result, will serve to
sharpen a moisture and instability gradient over the Piedmont.
Additional convection will likely develop and move off this boundary
through the afternoon. Lastly, a cold front will move east and
across cntl NC tonight. While an additional band of showers and
isolated storms will likely accompany the front, the severe threat
will have ended by that time.

Regarding that severe threat, strengthening deep layer flow will
result in 35-50 kts of effective shear, characterized by generally
straight forecast hodographs. While these shear profiles will favor
bowing cells/segments capable of wind damage, the combination of
backed surface flow and the presence of a 35-45 kt low level jet
over the srn Mid Atlantic will maintain elongated and cyclonically-
curved hodographs through 1 km, with an associated mini-
supercell/tornado risk, both in discrete cells and embedded QLCS
segments.

In addition to that severe risk, a flash flooding threat will result
from training cells embedded in slowly-ewd progressing convective
bands and amidst the anomalously high PWs. This threat would be
greatest in urban areas from RDU to FAY.

Temperatures today will be modulated by convection and clouds, with
the highest temperatures in the lwr 80s likely over the srn and wrn
Piedmont this afternoon, and perhaps also in the Coastal Plain
before convection arrives, with upr 70s in between. Cooler and
drier/less humid air will filter across the region overnight-Sat
morning, with lows ranging from upr 50s to mid 60s.

&&

.SHORT TERM /SATURDAY AND SATURDAY NIGHT/...
As of 210 AM Friday...

The upper trough will progress through the region Sat, with the sub-
tropical ridge shifting eastward into the region Sat night. At the
surface, the cold front/surface trough will sit over the Coastal
Plain Sat, then continue eastward to along the NC coast Sat night,
with relatively cool high pressure settling over the Appalachians
and into the mid-Atlantic through Sat night. Some showers and storms
are possible along the lingering front/trough Sat, but should
largely be east of central NC. Otherwise, dry weather is expected
across the area through Sat night. Highs in the low to mid 80s, with
lows in the upper 50s NW to mid 60s SE expected.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
As of 225 AM Friday...

The sub-tropical ridge will remain over the Southeast and mid-
Atlantic through mid-week, with the center of the high generally
over the OH Valley/northern mid-Atlantic. An area of low pressure
off the Southeast US coast Mon/Tue, should move westward and inland
over the Southeast by mid week. While rain cannot be completely
ruled out, the high just to the north should keep it suppressed to
the south. High pressure will dominate at the surface Mon, with a
Piedmont trough possible Tue and Wed. A northern stream wave will
move through the northern Plains and upper MS Valley through Wed and
into the Great Lakes/OH Valley Wed night/Thu, pushing the ridge
south and east. The best chance for rain in the extended forecast
will be with this wave and associated cold front and pre-frontal
trough, as they move into the mid-Atlantic/Northeast US Thu/Thu
night. Temperatures will remain above average, gradually moderating
with time, through at least mid-week.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
As of 800 AM Friday...

Mostly MVFR ceilings will generally lift and scatter to VFR by early
to mid afternoon, while being prolonged and briefly lowered by
showers and thunderstorms. Those showers and storms will come in the
form of a band/line that will progress across the RDU and FAY
vicinity between 14-16Z, then at RWI between 16-18Z. Additional,
more-scattered showers and storms will then likely redevelop and
affect the same three sites through 20-22Z. Lastly, a broken line of
showers will accompany a cold front as it crosses cntl NC tonight,
mainly north of FAY. Drier and stable air will filter into the
region behind the front overnight-Sat morning.

Outlook: Warm and dry high pressure, and associated VFR conditions,
will extend across the region this weekend through early next week.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

SYNOPSIS...MWS
NEAR TERM...MWS
SHORT TERM...KC
LONG TERM...KC
AVIATION...MWS

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