Forecast Discussion
Summary
SYNOPSIS
... Expect widespread showers and well-below normal temperatures this weekend as an upper low interacts with a coastal low that will move inland over the Carolinas on Sunday. The coastal low will linger over our area into early next week and keep our weather wet and breezy. The low is expected to eventually dissipate over the Atlantic Coast toward mid-week with temperatures warming to near normal.
NEAR TERM /THROUGH SUNDAY/... As of 730 PM Saturday: A widening band of rain continues to push west and northwestward across the forecast area early this evening with light to occasionally moderate rain invading the mtns and covering much of the foothills and Upstate. Looking to the east, a ragged back edge is seen, so eventually in the late evening the steadier rain should become more intermittent and light, perhaps filling in with some drizzle over a weak wedge. Winds should remain breezy well into the early morning hours with low pressure along the coast. Temps cooled quickly with the arrival of the rain, and from this point should fall only a few more degrees. Mins of 7 to 10 degrees below normal are still expected.
Otherwise...models depict a stronger frontogenetic QPF response as winds veer a bit more easterly and amplify late tonight, with the low moving north. The best precip rates generally still are depicted in that timeframe, similar to what was noted on model runs from Friday morning. However, the 850mb and 925mb wind fields take longer to veer than depicted at that time, and also remain generally easterly instead of gaining a southeasterly component. This reduces the influence of the Blue Ridge Escarpment overall, but especially along the southeast- or south-facing segments. Dynamic lift from DPVA furthermore appears less likely to overlap with the low-level forcing. As such, most model runs have trended down with QPF totals through 12z Sunday. Wouldn't say this warrants a complete write-off of the flood threat. The GFS, NAM, and GDPS all feature a band of higher QPF from the uppermost Savannah Valley to the Smokies, seemingly where the convergence maximizes on the periphery of the circulation around the low. Progged amounts aren't terribly high but the steady position of the convergence zone could anchor precip elements. The dry antecedent conditions still work in our favor, as far as mitigating flood threat. Although this also bodes well for the idea that heavy rain will be less likely to soak the ground and allow trees to fall via the gradient winds/gusts, with there being less rain the mixed layer may remain deeper, and we may more easily mix down stronger winds with the LLJ. So that aspect remains of some concern. On top of all this, the enhanced WAA will strengthen the warm nose above the wedge, and elevated instability will become possible, though not warranting more than a chance mention of thunder.
The low is expected to move onshore along the Grand Strand region Sunday morning, 12-15z, and continue on a northward track thru the remainder of the day. With the plume of best moisture/warm advection being on its northern flank, the low track should allow the primary impacts of these fluxes to shift north of the CWA during the day, but we will by no means see them completely shut off. The ECMWF and GDPS are slower to diminish them than the GFS/NAM, to boot. Easterly flow will continue at 850mb but taper off by midday; 925mb flow will remain northerly. The upper low, however, will wobble east and enhance lapse rates, with the warm nose basically as strong as it will be overnight. Models respond to this with higher MUCAPEs and more widespread convection above the wedge. Particularly over the northern half of the CWA where the easterly flow will be in play the longest, higher chance to low likely range thunder mention is advertised. The heavy rain threat will remain concentrated along the east-facing Escarpment; WPC Day 2 slight risk handles this well. Storm total QPF has fallen below an inch with this package over the southern half of the area, and less than 1.5 inches across much of the NC Piedmont. However, given the longer duration of rain and greater convective component developing Sunday, we retain 3 to 5 inch totals along some of the northern Escarpment. With much of that draining into the Catawba basin, elevated river levels could develop in the Catawba reservoir chain which could have lasting effects into Monday.
SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... As of 2:35 PM EDT Saturday: The short-term forecast picks up at 00z on Monday with a rex blocking pattern still in place over the Eastern CONUS and a large, closed upper low still centered over the Southeast. Over the next couple of days, the low will slowly drift eastward with the center of the system expected to be moving off the Carolina Coast and weakening by the end of the period late Tuesday. At the surface, the coastal low is expected to be centered over the Eastern Carolinas and gradually dissipating as the period begins late Sunday. Over the next 24 to 48 hrs, the remnants of the sfc low will further dissipate and become disorganized as another sfc high dives SE from Canada and settles just off the New England Coast. As for the sensible wx, sct showers and thunderstorms will be possible each day with more numerous showers and thunderstorms expected along and north of the I-40 corridor. Widespread excessive QPF is not anticipated across our area thru the period, but we could see some localized heavy rainfall in the vicinity of thunderstorms. Otherwise, temps will warm each day but are expected to remain a few degrees below climatology for late May.
LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As of 2:25 PM EDT Saturday: The extended forecast picks up at 00z on Wednesday with an embedded, closed upper low beginning to drift off the East Coast and weaken as very broad upper ridging remains centered over the Eastern CONUS. What's left of the upper low will likely linger just off the coast into Thursday as broad upper ridging remains over the Eastern CONUS. By the end of the period late next week, another broad upper trof will likely dig down over far Eastern Canada and begin to suppress the upper ridge. At the same time, the long-range guidance is suggesting that another embedded upper low may develop over the Southeast and drift right over our area. It's still too far out to know if this upper-lvl disturbance will actually form a closed low. At the sfc, weak wedging will be in place over our fcst area as the period begins late Tuesday. Most of the long-range guidance has any lingering wedging eroding by early Thursday as the sfc high moves further offshore and the low-level NELY flow weakens. At the same time, weak, broad sfc high pressure spreads back over much of the Eastern CONUS and lingers for the remainder of the period. As for the sensible wx, no major changes were needed to the fcst with primarily diurnally-driven sct showers and thunderstorms possible each day. Temperatures are expected to start out a few degrees below normal on Wednesday and warm to a few degrees above normal by the end of the week.