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FXUS62 KRAH 030826 AFDRAH

Area Forecast Discussion National Weather Service Raleigh NC 425 AM EDT Wed Jul 3 2024

SYNOPSIS
Canadian high pressure centered just offshore the Middle Atlantic coast will linger over the Carolinas through tonight. A subtropical ridge will otherwise extend across the South Atlantic states through the weekend.

NEAR TERM /THROUGH TONIGHT/
As of 350 AM Wednesday...

Around an anomalously-strong (+3 sigma), 500 mb anticyclone centered at 596 dam over the Deep South, a subtropical, mid-level ridge will continue to extend from the srn Plains to the South Atlantic coast.
In wswly flow poleward of the expansive subtropical ridge, convectively-generated disturbances will develop and move from the cntl Plains and mid MS Valley ewd and across the OH Valley and lwr Great Lakes/nrn Middle Atlantic. While associated height falls will be maximized across the OH Valley and nrn Middle Atlantic, 10-20 meter/12 hr falls will support some gradual weakening, as far south as cntl NC later tonight, of a preceding strong subsidence inversion evident in regional RAOB data the past couple of days. Some mid/high- level moistening and cloudiness may result especially across nrn NC tonight, after high-based, diurnally-enhanced stratocumulus become scattered to broken this afternoon (with greatest coverage over the wrn Piedmont) and dissipate through the evening. Otherwise, temperatures will moderate to slightly above average, with highs in the upr 80s to lwr 90s and lows in the mid 60s to lwr 70s, as Canadian surface high pressure, which will extend from just offshore the Middle Atlantic coast southwestward into the Carolinas, becomes increasingly-modified and gradually loses influence.

SHORT TERM /THURSDAY AND THURSDAY NIGHT/
As of 420 AM Wednesday..

Much hotter, but with seasonable humidity characterized by mixed/afternoon surface dewpoints in the upr 60s to around 70 F.

A subtropical ridge over the Deep South will weaken and retreat slightly, as a mid-level cyclone develops and deepens from the nrn Plains to the upr MS Valley. In the intervening wswly flow between the two, a corridor of convectively-amplified disturbances will sag sewd and extend from the Middle Atlantic to the srn Plains by 12Z Fri. Weak mid-level height falls and associated adiabatic cooling will cause initially weak mid-level lapse rates to steepen to 5.5 to 6 C/km over the srn Middle Atlantic by Thu afternoon.

At the surface, very warm-hot sswly flow will expand throughout the Southeast, equatorward of a convective outflow-modulated frontal zone that will settle to a similar position as the corridor of mid- level disturbances noted above (ie. from the Middle Atlantic to the srn Plains). Meanwhile, an Appalachian-lee trough will become established across the Foothills and wrn Piedmont of the srn Middle Atlantic/Carolinas.

The influence of the subtropical ridge and sswly low-level flow will allow heat to expand newd and across cntl NC once again, with forecast high temperatures in the mid 90s to around 100 F and around daily records at all three climate sites. Mixed/afternoon surface dewpoints mostly in the upr 60s will somewhat temper associated heat index values that are expected to peak in the upr 90s to near 105.
Thu night will be very mild and muggy, with lows centered in the middle 70s.

While the intersection of the lee trough and convectively-modulated frontal zone will likely focus the greatest concentration of diurnally-maximized convection across n-cntl VA, isolated cells will be possible swwd in the vicinity of the lee trough over the wrn NC Piedmont during the afternoon. Additionally, some outflow from upstream convection may outpace the main baroclinic zone and move into the nrn NC Piedmont during the evening, during which time scattered, multi-cell development will be possible. An isolated strong to damaging wind gust will be possible with any afternoon cells owing to strong diabatic cooling potential in the strongly- heated boundary layer characterized by 9.5 C/km lapse rates and 25- 30 degree dewpoint depressions, after which time nocturnal cooling and stabilization will limit convective intensity.

LONG TERM /FRIDAY THROUGH TUESDAY/
As of 225 AM Wednesday...

Aloft, a potent northern stream s/w will move through the Great Lakes Fri/Fri night, as the longwave trough amplifies southward through the Plains and MS Valley. As the trough amplifies, the high over the Southeast US will be gradually pushed southeastward off the Southeast US coast Fri night/Sat. Another northern stream s/w will drop into the northern Plains and track eastward along the US/Canada border Sat/Sat night as the initial s/w lifts newd into Canada. As this second s/w moves through the Great Lakes Sun/Sun night, another disturbance will quickly drop ssewd through the northern Rockies and into the central Plains, helping further amplify the longwave trough through the southern Plains/lower MS Valley by Mon morning. All the while, central NC will remain situated between the trough to the west and the high to the east, with deep southwesterly flow prevailing. The fly in the ointment is the medium-range guidance picking up on a possible tropical low approaching the Carolina coast over the weekend. There is some uncertainty wrt this feature for Mon and Tue. Also on Sun/Sun night, Beryl is expected to make landfall along the western Gulf Coast, however exactly where that happens and when will determine what happens to the system as/where it moves inland early next week. There is still a lot of uncertainty in the forecast for Mon/Tue, but for now, it appears the longwave trough axis will generally remain west of the Appalachians through Tue, while s/w disturbances move through it. At the surface, a lee trough will remain in place through Sat, when a cold front will approach from the west. As of the latest model solutions, the front still appears to get hung up and generally wash out along the Appalachians Sat night, as Bermuda high pressure ridges westward. An area of low pressure will sit off the Southeast US coast Fri/Fri night, then weaken on Sat before the remnants of the low lift newd off the Carolina coast Sat night/Sun. possibly laying W-E through WV/VA Sun as high pressure lifts newd through the OH Valley and Northeast, but the front appears to stay north of central NC. Expect swly flow and the advection of warm, moist air into the area to take over once the ridge moves out Thu morning and prevail through at least Sat. The lee trough should once again strengthen over the area early next week.

Temperatures will be above to well above normal through Tue. Latest forecast heat index values of 100-106 are expected across much of central NC for Fri and Sat. Sun and Mon heat index values max out around 100, mainly across the south, but may creep upward again on Tue.

As for rainfall, the weather seems to be turning a bit wetter. As the mid-level ridge gradually gets suppressed south and the lee trough strengthens, expect more diurnally driven convection Fri through Tue, with higher chances/amounts as s/w disturbances clip the area.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/
As of 210 AM Wednesday...

The presence and influence of Canadian high pressure that will extend from just offshore the Middle Atlantic coast southwestward into the Carolinas will favor mostly VFR conditions in cntl NC through Thursday morning. While a brief period of visibility restrictions cannot be entirely ruled out at RWI around daybreak, the likelihood seems relatively low given the foregoing Canadian/continental air that would favor more dew than fog, should surface temperatures even radiationally cool sufficiently amid areas of ~ 6 thousand ft AGL stratocumulus that are apt to persist there.

Outlook: A chance of showers/storms will exist over the wrn NC Piedmont (INT/GSO) Thu afternoon-evening and throughout cntl NC Fri through the weekend.

CLIMATE
Record High Temperatures:

July 4: KGSO: 98/1970 KRDU: 101/2012 KFAY: 98/2019

July 5: KGSO: 98/2012 KRDU: 102/1999 KFAY: 101/2002

July 6: KRDU: 102/2022 KFAY: 99/1990

Record High Minimum Temperatures:

July 4: KGSO: 73/2018

July 5: KGSO: 74/1999 KRDU: 78/1902 KFAY: 77/2005

July 6: KGSO: 76/1999 KRDU: 79/1900 KFAY: 76/2017

July 7: KGSO: 73/2019 KRDU: 77/1900 KFAY: 78/2017

RAH WATCHES/WARNINGS/ADVISORIES
None.

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