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Forecast Discussion

Summary

SYNOPSIS

... A cold front extends from western Pennsylvania into central Illinois. This front is forecast to stall just north of the area today. An area of tropical low pressure along the Mississippi Gulf Coast will move northeast into the southern Appalachians by Sunday morning, then east across the Carolinas Sunday night. A strong cold front will cross the Mid-Atlantic region Tuesday.

Warm humid weather can be expected through Monday with the potential for unsettled weather associated with the stationary front today, the remnants of the tropical low to our south Sunday into Monday, and the cold frontal passage early Tuesday. Cooler drier weather is anticipated for the middle of the week.

NEAR TERM /THROUGH TONIGHT/... As of 935 AM EDT Saturday...

Breezy westerly winds today with very warm temperatures this afternoon. The thunderstorm threat will be primarily north of our region today, but can't totally rule an afternoon thunderstorm.

A cold front extends from western PA, into the western Ohio Valley. This front will be the focus for showers and thunderstorms across the Ohio Valley Today, debris cloudiness moving downstream across our forecast area in the form of higher level cloudiness. A robust westerly push of air was occurring in advance of the front and this wind flow pattern suggest we will experience breezy conditions today as the air passes over and down the east side of the mountains. Adiabatic warming should allow temperatures to spike well into the 80s across the mountains and lower to mid 90s for the piedmont. The only caveat is whether thicker cloud cover will limit full insolation.

As a general rule, downslope westerly flow tends to limit deep convection for our CWA. Thus, think potential for thunderstorms today will be muted by the subsidence from the adiabatic warming. That said, convective allowing models do allow for enough convergence along the southern periphery of the faster westerlies aloft to promote some isolated storms, and the forecast will reflect this today with token pops areawide even though expecting a mostly dry day. The primary thunderstorm focus is expected along the front to our northwest, and if there is an opportunity for the forecast to go awry, it would be from an MCS that could potentially move downstream and impact the forecast area from the Ohio Valley. For this reason will maintain highest pops across our northwestern CWA (WV counties and into the VA highlands) to account for this potential scenario. As for severe threat, due to the moderately strong westerly flow aloft, any storm that manages to develop would be at risk for producing wind damage. SPC has our WV/VA counties highlighted with a marginal risk.

For tonight, the expectation is for warmer humid conditions with lows in the 60s mountains to the lower 70s piedmont. Will maintain chance pop across our northern CWA where potential will exist for nocturnal thunderstorms along the front to our north. There is too much uncertainty with respect to timing of any MCS outflow features, thus keeping pops low for now until we see how today's activity plays out as to whether it will warrant any greater concern.

SHORT TERM /SUNDAY THROUGH MONDAY/... As of 410 AM EDT Saturday...

Weakening tropical system to our south may bring showers into North Carolina and far southern Virginia late Sunday into early Monday,with cold front approaching from the northwest by late Monday.

The tropical system making landfall along the Gulf Coast, which may may barely make it to a weak Tropical Storm, Claudette, as it makes landfall any time early this morning, will most likely track well to our south on Sunday and Sunday night and will also weaken to the point where it loses any surface circulation by the time it cross the southern Appalachians of nrn GA late in the day Sunday. With the most likely track of some weak remnant sfc convergence passing near Charlotte and toward RAH Sunday night into Monday morning, this could still bring some areas of showers up to about the NC/VA border during the day Sunday into the overnight and some parts of NW NC mtns with weak upslope southerly flow could see well over an inch. While there is a potential for a slightly more northern track that could conceivably result in enough rain in those upslope areas to cause at least a minor flooding concern, it appears at least equally possible that all but very light precip will remain entirely south of the forecast area if the track of the remnants remains south of Charlotte. Thus this remains a forecast with considerable uncertainty in terms of how much rainfall across the southern third of the fcst area, as well as the influence on high temps as well. High temp forecast will depict the influence across mainly the NW NC mtns and foothills, with highs near and north of US 460 northward generally 5 to 10 degrees warmer than farther south.

North of where any showers may reach associated more directly with this system, weak subsidence and some cloud cover should prevent any convective development, but will still carry a slight chance at this point for the northern half of the fcst area Sunday and Sunday night, mainly in case the system happens to track farther north.

By Monday morning any weak remnants of this system should be near the Carolina coast and not impacting our are with any rainfall any more, and subsidence with westerly winds of around 10mph should help things really warm up in most locations, especially east of Blue Ridge with downsloping, all in advance of an approaching front. Some guidance still suggesting some showers or storms forming well ahead of front but am leaning toward more likely being a dry day so continue to trend down on PoPs, with low chance to slight chance most locations, until very late in the day when front to the west approaches amd some showers and storms could move into western slopes late in the day. Can see a continued downward trend to the PoPs for most of the day Monday and we'll see if timing for the approaching front slow enough to lower late day PoPs in the west as well. Main story for Monday will be the warm temperatures.

Confidence relatively low for influence of tropical remnants Sunday across the south and moderate for any scattered to isolated precip on Monday and timing of front into the west late.

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 130 AM EDT Saturday...

Unsettled start of the workweek with passage of cold front, followed by cooler and drier midweek with moderating temperature into the end of the workweek.

A longwave trough digging over the Midwest and Great Lakes will push a cold front southeast through our region by Monday night into Tuesday. A wave of low pressure may develop along the frontal boundary early Tuesday to slow the front and provide some enhancement to associated rain, primarily in the far west it appears at this time. Then behind this wave the front should push completely through the forecast area more quickly some time on Tuesday and if by midday this could really limit any instability. Overall, much of the precip associated with this front could end up being just behind the surface front in the more stable air, and thus limit thunder potential. At this time based on majority of model solutions looks like best chances for more significant rainfall would be in the far west late Monday night into early Tuesday, and then perhaps smaller chance for some showers/storms developing in east Tuesday afternoon depending on eventual timing of front exiting to the east.

Confidence is higher now that by Tuesday night into Wednesday the front will be well east of the area with a drier cooler air mass and high pressure over the NE US controlling our weather, bringing conditions similar to earlier this past week with temperatures below normal, especially low temperatures where some locations in the mountains could get down into the 40s on Wed morning assuming good clearing. The high center will slide east Thursday and push into the Atlantic ocean Thursday night into Friday, allowing a slow return of milder temperatures and some moisture by the end of the week.

Moderate confidence in the forecast for the early part of the week with precip and thunder chances associated with the frontal passage, and then high confidence in the drier cooler air mass chance by Wednesday.

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