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FXUS62 KRAH 080801 AFDRAH

Area Forecast Discussion National Weather Service Raleigh NC 400 AM EDT Wed May 8 2024

SYNOPSIS
High pressure across the western Atlantic will extend into the Southeast through Thursday as a series of mid to upper-level disturbances move across the region on Wednesday and Thursday. A cold front will move across the region late Thursday into Friday.

NEAR TERM /THROUGH TONIGHT/
As of 330 AM Wednesday...

* Slight/Level 2 threat for multiple rounds of severe storms from with the primary threat during the late afternoon/evening, followed by a secondary threat late tonight/early Thursday.
* Summerlike heat and humidity is expected with highs in the upper 80s to lower 90s and heat index values in the lower to mid 90s in many locations.

Upper ridge centered over the SE US this morning will shift offshore early in the day, leaving the Carolinas under the influence of perturbed quasi-zonal flow aloft until the arrival of the strong Northern Plains/Upper Midwest trough into the region on Friday. At the surface, a lee sfc trough will sharpen over western NC.

A complex and multi-episode severe threat exists today through Thursday. Model guidance continues to exhibit considerable spread and high run-to-run variability wrt convective evolution (timing/location). A majority of this uncertainty can be attributed to the fact that the forcing mechanisms are upstream convective artifacts that currently developing or have yet to develop. As such confidence is lower than normal. However, there is high confidence that the near-storm environment over central NC will be very conducive for severe storms that starts first with the summer-like heat and humidity.

Under the influence of SWLY flow, low-level thicknesses will be more comparable to late June than early May. Afternoon highs are expected to warm into the upper 80s across the north to lower 90s over the remainder of the forecast area. When coupled with the humid BL dewpoints of 65-70, heat indices over the central and eastern portions will be in the mid 90s. The resultant low-level lapse rates will foster moderate to strong destabilization with MLCAPE of 2500- 3500 forecast across the area. Convective initiation could be triggered by storms or associated outflow moving off the higher terrain and/or just develop along the lee side trough or differential heating boundary. This first round of convection should propagate east through the afternoon and early evening. Effective shear of 30-40kts and another high DCAPE environment of 1000-1500 Joules will support a threat for supercells with damaging winds and large hail, given the robust/fat CAPE and initially straight hodographs.

An evening/early night lull in convection/rain chances is possible before a potential upstream convective complex moves across mtns and spreads east into the area during the overnight and morning hours.
While a nocturnal curtailment of instability will occur, moderate instability will likely survive with hodographs becoming strongly cyclonically curved, resulting in added threat for an isolated tornado as well. Lows in the 65-70.

SHORT TERM /THURSDAY AND THURSDAY NIGHT/
As of 330 AM Wednesday...

* Slight/Level 2 Threat continues into Thursday...

Lower than normal forecast confidence carries over into Thursday with looming uncertainty as to whether or not organized convection will be ongoing across central NC during the first half of the diurnal cycle and the potential negative consequences of associated widespread clouds, rain, and convective overturning that would greatly limit destablization. It's quite possible that the severe threat may peak during the morning and early afternoon, decreasing thereafter as the convection moves south and east of the area.

Alternatively, if the morning/early day convection is less organized or is not realized across the area, additional upstream convectively enhanced disturbances/MCC, with an attendant effective front moving through the area, could potentially spur another round of strong to severe storms Thursday afternoon and evening. Weak to moderate instability with deep-layer shear progged to increase will pose a continuing threat for damaging winds and hail. Severe threat should wind down with the passage of the effective front late Thursday evening/night.

Not as warm given the clouds and rain chances. Highs ranging from lower/mid 80s north to near 90 SE. Lows near 60 north to upper 60s SE.

LONG TERM /FRIDAY THROUGH TUESDAY/
As of 400 AM Wednesday...

A pair of shortwaves will move across central NC on Friday, one around midday and the other in the late afternoon/early evening. At the surface, the cold front will begin to cross the area as a wave of low pressure rides along it, but exactly how far it gets is a bit uncertain. Regardless, guidance is in good agreement that moisture and instability will be much less impressive than today and Thursday, and the highest will be focused across the south and east.
CAPE looks to be more in the 500-1000 J/kg range, so severe weather looks like less of a threat, but isolated storms still can't be ruled out. After the deterministic 12z ECMWF was largely dry, the 00z run came in wetter and more in line with most of its ensembles as well as the GFS/GEFS and NAM. So keep likely POPs across the SE and chance elsewhere. Forecast highs range from mid-70s far north to lower-80s far south. Any precipitation will come to an end Friday night as the cold front sweeps off the coast and lows turn slightly below normal (upper-40s to mid-50s).

NW flow will dominate this weekend as surface high pressure and associated mid-level ridging approach from the west. A mid-level trough will move across the OH Valley on Saturday and the northern Mid-Atlantic and New England on Saturday night/Sunday. This could bring glancing weak forcing for ascent on Saturday afternoon and evening, but considering how dry the airmass will be from downsloping flow, any precipitation would be light and confidence is low we will see anything at all. The forecast only has slight chance POPs across the far north. Otherwise it will be a very pleasant weekend with temperatures remaining slightly below normal (highs lower-70s to upper-70s on Saturday and mid-70s to 80 on Sunday). Dew points will only be in the 40s.

The next shortwave will move into the southern Plains on Monday, but models differ on how quickly it will move east with the GFS much faster and the ECMWF and Canadian holding it back. This affects when we would begin to see any precipitation from this system. For now have gradually increasing POPs early next week, with enough wet ensemble members to justify chance areawide by Tuesday. The surface high will move offshore and turn the flow southerly, which should help increase temperatures a bit, but they will also depend on the timing of the aforementioned system which is still highly uncertain.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/
As of 158 AM Wednesday...

Patchy/isolated pockets of fog is possible overnight. Otherwise, expect predominately VFR conditions across the area. Some minor gustiness will develop in the afternoon, with SWLY gusts of 15 to 20 kts possible during the peak afternoon heating/mixing.

A moist/humid and very unstable airmass will support the development of scattered showers and storms as early as late morning and continuing into the afternoon and evening. Forecast spread in the exact timing and location is high. Thus, given the low confidence, will handle with a PROB30 group during the climatological peaked diurnal hours. There is the potential for a second round of showers and storms to move into the region during the overnight hours.

Outlook: Scattered to possibly numerous showers and storms are expected through Thursday with associated CIG and VSBY restrictions.
Some storms may be strong to severe, especially on Thursday.
Somewhat more limited convective coverage is expected on Friday and much quieter conditions with generally VFR conditions expected into the weekend.

RAH WATCHES/WARNINGS/ADVISORIES
None.

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