... The remnant low from Tropical Storm Bertha will move north tonight and Thursday across central North Carolina and into Virginia. Warming southerly flow will develop Thursday through Friday ahead of the next approaching cold front, but with continued moist conditions over the area. The cold front will move into the region from the northwest on Saturday, and settle southeast of the area through Sunday. Drier and cooler air will arrive from the northwest for the early part of next week.
NEAR TERM /THROUGH TONIGHT/... As of 1015 AM EDT Wednesday: The tropical system has been named Tropical Storm Bertha and is currently making its way onshore near Charleston. Showers will continue to expand across the area as the system tracks northward, however most of the more significant impacts are still expected to remain just to our south and east. The main concern for our fcst area remains the potential for flooding in the vicinity of the I-77 corridor.
Most of the guidance is in good agreement on the center of the low tracking thru eastern SC and central NC. This places a deformation zone on the NE side of the weak circulation that tracks right along and east of I-77. The 00z ECMWF comes in closer to this, but still a smidge west with its QPF. Going with the WPC preference, we're forecasting 1-2+" of rain in these areas, with a very sharp gradient just west of I-77. The rest of the forecast area looks to only get a couple tenths of an inch at most. The one exception that will bear watching is the Northern NC Foothills, where the NAM does produce a band of 1-2" of QPF. One saving grace is that the system will track pretty fast, which should limit really high rainfall totals. The rain should taper off south to north this evening. Based on this forecast and poor antecedent conditions and elevated stream levels across these areas, will issue a Flash Flood Watch for the NC counties along and east of I-77 from noon to 2 AM EDT. Otherwise, it will be another cool, cloudy day across the area with highs about 5- 10 deg below normal. Winds will pick up a little from last couple days as the low approaches.
Later tonight, there should be a lull in activity with some drier air on back side of the departing low into VA. There will be come convection across the TN Valley approaching the western edge of the forecast area. But only expecting a few weakening showers possibly getting in before daybreak Thursday. Lows will be about a category above normal under mostly cloudy skies.
SHORT TERM /THURSDAY THROUGH SATURDAY/... As of 245 am EDT Wednesday: A closed, upper-level low pressure system over the Mississippi River Valley will gradually fill starting Thursday, and then lift over the Ohio Valley on Friday to phase with deepening northern stream energy over the Great Lakes. East of this system, southerly flow and warming temperatures will return on Thursday and produce diurnally favored heavy rain rates in numerous showers and thunderstorms. There will, however, be reasonably good storm movement from the southwest to mitigate flooding problems a bit. In addition, SBCAPE should easily rise above 2000 J/kg, and sfc-6 km bulk shear should increase to 30 kt or better from the southwest during the mid to late afternoon. There will thus be an uptick in the strength of the convection, with a marginal threat for several thunderstorms across much of the area.
After some nocturnal waning of the coverage Thursday night, strong upper jetlet divergence will cross the region on Friday. Good warm sector instability will continue and sbCAPE values of 2000-3000 J/kg will be likely, especially in the Piedmont. Shear may be a touch better Friday afternoon, and mid-level lapse rates should be steeper as well, to yield another round of strong to severe thunderstorms for the afternoon and evening hours, with localized heavy rain issues.
The developing eastern trough will then push the associated surface cold front into the Appalachians Friday night and across the region on Saturday. Solid chance to low end likely PoP for showers and thunderstorms will be featured with the fropa. Anticipate temps remaining fairly near climatology through the period.
LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... As of 300 am EDT Wednesday: A deepening eastern U.S. trough axis will move to the Atlantic coast by Sunday. There is some potential for a trailing shortwave in the backside of the longwave trough to ripple down over the southern Appalachians on Sunday, with some associated low to mid-level moistures in the post-fropa cooler air. However, will keep any associated Sunday mountain PoPs on the very conservative side given the source region of the moisture and forcing.
Deep layer northwest flow will set up through Monday as a 590 dm ridge gradually builds over the central CONUS. Heights will steadily rise from the west through Tuesday, as the offshore Atlantic trough continues to deepen. A drier airmass will settle into the region, with temperatures some 5 degrees below climo.