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FXUS62 KRAH 231319 AFDRAH

Area Forecast Discussion National Weather Service Raleigh NC 920 AM EDT Thu May 23 2024

SYNOPSIS
A warm and humid air mass over our region, combined with the passage of a series of upper level disturbances over the area, will produce unsettled weather through Monday, with daily chances of showers and storms. A drier air mass will arrive by the middle of next week.

NEAR TERM /THROUGH TONIGHT/
As of 920 AM Thursday...

Marginal Risk of severe thunderstorms this afternoon and evening.

Satellite and radar data indicate several MCV's that may affect portions of the region this afternoon and tonight. An initial MCV was producing thunderstorms over portions of NE Tennessee. Another was located in AL/GA and still another approaching the Tennessee Valley region. The CAMS have backed off a bit on convection in central NC this afternoon, holding off until late day and tonight.
We are not totally biting off on this scenario due to the forecast CAPE values of 1500 to 2000 j/kg, but we will delay timing slightly and lower initial POP. It appears the best chance of thunderstorms will be in the NW associated with the initial wave in the afternoon.
This will be followed by the additions waves and potential convection later in the and tonight. Highs in the mid to upper 80s NW and upper 80s to lower 90s elsewhere.

Previous discussion as of 400 AM Thursday...

Regional radar imagery shows an MCS traversing through the lower MS Valley with multiple embedded convectively induced MCVs. These localized areas of ascent will track generally ENE along the cyclonic side of the mid-level ridge and should initiate convection along the NC mountains where a lee-trough will help focus showers/storms with 500-1500 J/kg of SBCAPE over the Piedmont into the Sandhills during the afternoon/evening. Slight strengthening of the 500mb flow will support 30-40 kts of bulk shear and will favor some organization of stronger storms. The big question mark today is the subtle forcing from the MCVs and inconsistencies in timing any MCVS that emerge from convection upstream over the lower OH Valley.
This is leading to wildly different solutions from run-to-run, making a deterministic forecast difficult.

An ensemble approach from the HRRR suggests initially cellular convection over the mountains and foothills transitions into a messy convective cluster as it approaches the Piedmont as deep shear vectors align parallel to the lee trough and eventual outflow and cold pool from merging clusters. Any discrete cells will bring the risk of hail and locally damaging winds while congealing clusters will primary bring a straight-line wind threat. However, an overall moist thermodynamic profile and only modest instability may struggle to produce severe wind gusts, but localized strong to severe gusts within organized cold pool clustered convection can not be ruled out. Afternoon temperatures will be similar to yesterday with mid/upper 80s, although isolated locations of around 90 will be possible especially in the Coastal Plain and portions of the Piedmont and Sandhills.

Showers and storms will likely weaken as they transition eastward with less instability to work with and diurnally unfavorable timing based on current forecast. Dependent on the rainfall footprint, patchy fog may be possible late tonight into Fri morning, but given the already myriad of uncertainties with the evolution of afternoon/evening convection, confidence is low on fog potential. No major airmass change is expected behind the afternoon showers/storms and lows will remain in the mid/upper 60s.

SHORT TERM /FRIDAY AND FRIDAY NIGHT/
As of 145 AM Thursday...

Tough to have a lot of confidence in the forecast for Friday, given that the timing and evolution of convection in our area will be greatly dependent on what happens today in NC (including any lingering outflows or differential heating boundaries) and upstream in the S Plains into the lower Miss Valley, as any resulting MCVs from this area (riding on long-fetch SW flow from off Baja California) could help drive our Friday storm chances. This uncertainty is reflected in the later hours of the CAMs, which show little agreement.

The most likely scenario involves some combination of the larger scale mid level trough now over the Desert Southwest, a residual MCV from today's S Plains convection, and perhaps a weak boundary left over our region from today's storms, all working with high PW in the 90th percentile, marginal to mdt SBCAPE, and warm surface temps and dewpoints in the warm sector within weak lee surface troughing to promote scattered to numerous multicell storms from mid to late afternoon through the evening. Such storms would be most likely across our S and E, as some models suggest that an earlier mid level wave at 700 mb pushing off the Mid Atlantic coast in the early afternoon would introduce some brief WNW flow and drying across our N and W. Then the GFS/ECMWF depict a trailing shortwave trough (perhaps an MCV) moving into the NC mountains Fri evening/night, which, if it occurs, could support additional late-day mountain convection poised to push into the NC Piedmont overnight. Will start with slight shower chances through lunchtime, then ramp up pops to good chance everywhere except likely far S and SE, where any lingering boundary from today is apt to settle. Will keep chance pops going overnight for showers and isolated storm, but again, confidence in all of this is low, considering how much is predicated on today's activity here and well upstream. Under partly to mostly cloudy skies, and starting the day with thicknesses just a bit above normal, expect highs in the 80s (low NW ranging to upper SE), around 5 degrees above normal, followed by muggy lows in the mid 60s to lower 70s, 8-10 degrees above normal. -GIH

LONG TERM /SATURDAY THROUGH WEDNESDAY/
As of 310 AM Thursday...

Daily chances for showers and storms are expected Sat through Mon, with pops decreasing by Tue/Wed with a shift in the longwave pattern.

The aforementioned Fri night shortwave trough is expected to be over central or eastern VA/NC Sat morning. If this feature is a bit slower, then the cooler mid levels and DVPA may prompt high coverage of storms with heating across our east. If this wave is faster, then that will bring rearward shortwave ridging into our area Sat, reducing our pops. With our area firmly in warm/moist advection, PW still above normal, and surface troughing still in place over the area, will lean on the high side of climo pops, esp E, with 40% NW ranging to 55-60% SE Sat. Expect a brief downturn in pops late Sat night through much of Sun as the shortwave ridging passes overhead coincident with a dip in PW, but with upstream convection in the Plains and Miss Valley continuing to eject perturbations through the Mid Atlantic region with no real pattern change for us at the surface, we should see convection chances quickly return, and will retain chance pops Sun afternoon and evening. We start to see signs of a longwave pattern change starting Memorial Day, as a deep northern stream low/trough moves into the Midwest/Great Lakes. This will set up faster and more cyclonic mid level flow with steepening mid level lapse rates, suggesting high pops, esp late in the day and into Mon night. Will have good chance to likely pops, highest Mon afternoon/evening. As longwave troughing sets up over E NOAM, Tue/Wed should be fairly dry with below-climo chances for showers and storms.

Thicknesses will hold above normal Sat, favoring highs in the 80s to near 90, then highs will reach peaks of mid 80s to lower 90s Sun and Mon, pushing heat indices well into the 90s all but the far N. By Tue, as the upper trough axis and associated cold front move through, temps should drop back close to normal. -GIH

AVIATION /12Z THURSDAY THROUGH MONDAY/
As of 700 AM Thursday...

VFR conditions continue this morning with a low predictability for showers and storms this afternoon through the evening. The forecast has trended slower and with less coverage in the afternoon compared to 00z guidance with a better chance for a cluster of showers and isolated storms moving through the area overnight. The 12z TAFs have trended towards this threat given the sparse guidance that has trickled in since 06z, but further updates will likely be needed as observations and model trends continue to narrow in on the wide variety of forecast solutions. Behind the showers, patchy fog and low stratus may be possible near FAY tonight into Fri morning, but given inconsistencies in prior convection this threat also includes low predictability.

Looking ahead: Primarily diurnally driven showers/storms will be possible at all terminals Fri through Mon.

RAH WATCHES/WARNINGS/ADVISORIES
None.

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