Forecast Discussion
Summary
SYNOPSIS
... Moisture will return atop another high pressure center ridging down from the north today and into Wednesday with cloudy, damp, and cool conditions lingering through Thursday. Expect a slow warm up and drier conditions beginning Friday.
NEAR TERM /THROUGH TONIGHT/... As of 324 am EDT...still quiet on radar at this hour, but low clouds continue to develop steadily to the SSW on the east side of the mtns, indicative of some weak low level forcing. At this point, think it unlikely that any measurable precip will occur before daybreak, so precip probs have been dropped until the start of the Tuesday daytime period. Temp/dewpt trend looks good.
Not much change to earlier thinking with regard to the synoptic situation across the region today and tonight. A mid/upper blocking pattern over northeastern North America today was supporting a large sfc high over Quebec and part of that sfc ridge was steadily nosing down east of the Appalachians, perhaps aided by some light precip in eastern PA. A shallow and relatively stable air mass should continue to ooze into our region this morning, although it remains to be seen how far it will make it once the sun comes up and warms the boundary layer. It would appear that for most of the day, the situation will only be somewhat wedge-like, as the static stability in the boundary layer will be too low to keep the low level flow from running up the Blue Ridge Escarpment. That alone should allow for some light precip to develop through the middle part of the day near the Escarpment and then expand from there. Meanwhile, this afternoon, some locations might get unstable enough to allow for a few showers to develop, mainly over the Piedmont. Precip probs were kept in the slight chance to chance range because of disagreement among the CAMs as to the coverage and the overall lack of decent forcing away from the weak upslope. The HRRR is particularly uninterested in precipitating. This may play into temps getting a bit warmer than expected, especially over the lower Piedmont. After sunset, the low level S/E upslope flow will improve a bit and some of the guidance shows the development of weak low level isentropic upglide. Precip chances improve as a result, but the forecast does not yet climb above 50 pct at this time. The light precip, if it develops, should at least temporarily establish a cool pool in the usual wedge region. A low diurnal range in temperature is expected.
SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 3:15 AM EDT Tuesday: The short-term fcst picks up at 12z on Wednesday with a closed upper low slowly tracking eastward across the lower Midwest and Ohio Valley while gradually weakening and opening back up. By the end of the period early Friday, the low will have morphed into a weak, embedded upper trof centered just to our north while stout upper ridging amplifies over the central CONUS. At the sfc, broad Canadian high pressure will continue to sink southward, nosing into our fcst area by the start of the period early Wednesday. The southern periphery of the Canadian high will remain over our area thru the rest of the period keeping NELY low-level flow in place. Overall, precip coverage continues to trend downward. Thus, I kept PoPs capped at high-end chance thru the period. The latest guidance continues to suggest that the higher terrain will have the best chance of seeing showers with embedded thunderstorms thanks to weak upslope flow. Confidence is lower east of the mtns, hence PoPs remain lower. Wednesday still appears to be the coolest day with bkn to ovc cloud cover keeping temps roughly 5 to 10 degrees below climatology. Cloud cover is still expected to gradually diminish from west to east throughout the day on Thurs allowing high temps to rebound somewhat. However, they will still remain about a category below climo.
LONG TERM /FRIDAY THROUGH MONDAY/... As of 255 AM EDT Tuesday: The extended fcst picks up at 12z on Friday with what's left of an embedded upper trof lifting over the Eastern Seaboard while stout upper ridging builds over the central CONUS and broad upper trofing amplifies over the Pacific Northwest. Over the next few days, this pattern will amplify and an Omega block will establish itself. As far as Omega blocks go, this one appears to be fairly short-lived with most of the long- range guidance breaking it down towards the end of the period early next week. Nonetheless, steep upper ridging is expected to remain over our region beyond day 7. At the sfc, broad high pressure will slowly migrate southeastward out of eastern Canada and into the northeastern CONUS. Shower and embedded thunderstorm chances will linger thru Fri, with the higher PoPs mainly across the NC mtns and foothills. I kept PoPs capped at slight chance across these areas as the long-range guidance has been trending drier overall. Otherwise, the rest of the period is expected to be dry as both the upper high and sfc high build over the SE. Temps should remain near climatology if not slightly above.