591
FXUS62 KRAH 050120
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
920 PM EDT Sat May 4 2024

.SYNOPSIS...
A series of mid and upper-level disturbances will overspread and
interact with a front that will remain quasi-stationary over central
NC through early Sunday, then weaken and move northward into VA
later Sunday and Sunday night.

&&

.NEAR TERM /TONIGHT/...
As of 920 PM Saturday...

The models and recent trends in water vapor satellite data indicate
an upr-level, srn stream shortwave perturbation may amplify slightly
as it moves east and across the srn Appalachians and wrn-cntl
Carolinas through early Sunday. Preceding this feature, a long-lived
MCV from convection that developed across LA Fri morning was evident
in regional radar data over the Upstate of SC this evening. This mid-
level feature is forecast to track generally newd, in ~15 kts of
mean swly 700-500 mb flow, and into the srn and wrn NC Piedmont
through early Sunday. It may be preceded by smaller-scale and weaker
MCVs from small convective clusters ongoing from near CLT to CAE to
AGS. The net result will be mid through upr-level forcing for ascent
that will edge newd and across the srn and wrn NC Piedmont (the wrn
half of cntl NC) late tonight-early Sunday.

At the surface, a quasi-stationary front extended at 01Z from near
ECG in nern NC wswwd to near PGV, where it intersected a nne to ssw-
oriented outflow boundary extending through the cntl and srn Coastal
Plain. To the west of that outflow boundary, the front, masked and
modulated by associated rain-cooled air over the rest of cntl NC,
extended across srn NC to a weak mesolow in the Upstate of SC near
GSP. The mesolow will follow a similar path as the parent MCV above
and reach the nw Piedmont/Triad region by 12Z Sun.

While a shallow frontal inversion and weakly stable layer, with a
surface temperature of 60 F, were evident in the lowest couple of
thousand ft AGL in the 00Z-observed GSO sounding, a deeply moist (PW
of 1.5"), saturated, and conditionally-unstable profile was
otherwise present throughout the troposphere. Even slight
modification of observed surface parcels with representative mid/upr
60s preceding the mesolow yields up to several hundred J/kg of
SBCAPE. Associated weak instability, and forcing for ascent as the
mid/upr-level features noted above overspread and interact with the
surface frontal zone, should cause showers and isolated storms to
become increasingly numerous over the srn and wrn NC Piedmont
overnight-Sunday, where a continued Marginal risk of excessive rain
(ie. isolated instances of flash flooding) will remain possible. It
will otherwise be unseasonably mild and muggy, with lows in the 60s.

&&

.SHORT TERM /SUNDAY AND SUNDAY NIGHT/...
As of 215 PM Saturday...

Aloft, the sub-tropical ridge will shift offshore and breakdown
Sun/Sun night as the increasingly sheared shortwave disturbance
moves through the mid-Atlantic and Northeast US. Another southern
stream s/w will strengthen over the southern Plains on Sun, then
move eastward into the lower/mid-MS Valley through Sun night. At the
surface, high pressure will move off the New England coast and
eastward over the northern Atlantic as a low moves through the Great
Lakes and into the Northeast Sun/Sun night. An attendant cold front
draped swwd from the low will become more W-E oriented through the
OH Valley by Sun night. Meanwhile over central NC, the quasi-
stationary front that had lingered over the area will lift northward
as a warm front, with warm, moist southerly return flow over the are
in its wake.

Precipitation/Convection: Showers and storms will likely still be
ongoing Sun morning, most prevalent over the Piedmont. PWATs will
remain anomalously high, in the 1.6 to 1.8 inch range, through the
afternoon. Surface-based instability should increase with daytime
heating, although the latter may be somewhat inhibited by the
ongoing convection and extensive cloud cover. Bulk shear is also
expected to be weak again on Sun. As a result, slow-moving, widely
scattered showers and storms will be possible through the aft/eve
Sun, with the main threat of some isolated heavy rainfall and
localized flooding, although most locations may only receive a few
hundredths of an inch of rain.

Temperatures: Widespread overcast skies and ongoing convection
should somewhat limit heating early, but temps could recover during
the afternoon. For now expect highs to range from the mid 70s along
the NC/VA border to around 80 degrees SE. Lows Sun night mainly in
the low to mid 60s expected.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
As of 415 PM Saturday...

The extended forecast will feature a continued chance of unsettled
weather to start the week Mon, followed by somewhat drier and hot
temperatures by the middle of the week. Energetic wswly flow aloft
will increase shower/storm chances again mid-late week, followed by
milder, stable, and less humid conditions by Sat.

The models remain in relatively good agreement regarding a srn
stream shortwave trough forecast to pivot across the TN Valley and
srn Appalachians Monday and srn Middle Atlantic and Carolinas Monday
night-early Tuesday, during which time above average chances of
showers/storms will result over cntl NC. Shortwave ridging will
follow and migrate across and offshore the South Atlantic with drier
conditions favored Tue-Wed. Associated subsidence, on the wrn
periphery of sub-tropical high pressure that will extend across the
swrn N. Atlantic, will also result in unseasonably hot and humid
conditions extending over cntl NC Wed-Thu. Increasingly-energetic
wswly flow, and at or above climatological chances of convection,
will otherwise prevail across the Southeast, within closely-spaced
srn and nrn stream branches of the westerlies, through early next
weekend. An associated synoptic cold front will move across cntl NC
Fri or Sat.

While the models are in relatively good agreement regarding the
pattern described above, they also depict the development of a
couple of rex blocks over n-cntl NOAM during the middle of the week
and over wrn NOAM late week into next weekend. With those blocking
developments in mind, model guidance may trend slower with the
progression of the cold front and following milder and more stable
conditions by early next weekend.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
As of 825 PM Saturday...

LIFR ceilings are expected at most sites through 15z/Sun. There is
also a good chance of MVFR to IFR vsbys with showers and fog.
Showers and thunderstorms are expected Sunday morning (south and
west) into the afternoon (east), with associated IFR to LIFR
conditions.

Outlook: MVFR to IFR ceilings are likely to sock back in Sunday and
Monday night with additional scattered showers/storms possible.

&&

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

&&

$$
SYNOPSIS...MWS
NEAR TERM...MWS
SHORT TERM...KC
LONG TERM...MWS/Kren
AVIATION...pwb/Luchetti