textproduct: Raleigh

WHAT HAS CHANGED

* Confidence is increasing that highs today will remain just under most guidance, given expected slow clearing of clouds through the day.

* Shower/storm chances have increased for Tue afternoon into Tue night.

KEY MESSAGES

As of 255 AM Sunday...

1) Areas of early morning fog will improve by mid morning. Isolated shower or storm possible far SW through late afternoon, and over the NE tonight.

2) A cold front passage late Tuesday will bring the next chance for showers/tstms with isolated severe storms possible.

DISCUSSION

As of 255 AM Sunday...

KEY MESSAGE 1... Areas of early morning fog will improve by mid morning. Isolated shower or storm possible far SW through late afternoon, and over the NE tonight.

Areas of fog continue to form early this morning, mostly through the central Piedmont into the N Sandhills, some of it locally dense. Given the nearly calm air and high moisture content in the low levels early this morning (via obs and 00z regional 925 mb analysis), we should continue to see low clouds and areas of fog through much of the morning, as supported by GLAMP guidance and most HREF/REFS members. With light winds expected to hold today from the surface through the first few thousand ft AGL today, diurnal mixing will be hampered, leading to a slow lifting and breakup of low clouds and delayed heating. Will continue to lean just under NBM and most statistical guidance today, with highs generally in the low-mid 80s.

The surface front remains just S and SW of the CWA according to the latest analysis, from W NC across NE SC, with a large cluster of convection currently just S of the CLT area, just SW of our CWA. As this front wavers today with additional convection expected to ride along the front/CAPE gradient, isold to sctd showers and storms may edge into our far W and SW CWA, from mid morning through late afternoon, as a subtle shortwave trough evident on WV imagery tracks from the OH Valley SE through W and S NC, into the mean longwave trough base. Another shortwave is expected to track from MI this morning SE into the Mid Atlantic region tonight, which is expected to bring another chance of isold-sctd showers/storms across NE sections from late evening through the overnight hours. The degree of destabilization with both potential rounds of convection is highly uncertain given the abundant low level moisture and limitations in surface heating, along with the lower PW as compared to past days. But our mid level flow will become stronger and more cyclonic late today through tonight, supporting more organized convection, so an isolated strong storm can't be ruled out. But overall coverage in our area is expected to be limited.

KEY MESSAGE 2... A cold front passage late Tuesday will bring the next chance for showers/tstms with isolated severe storms possible.

A short wave trough currently crossing the Pacific NW will spend the next two days moving around the northern periphery of the ridge that's centered over the Plains and Rockies. By late Tuesday it will drop down the east side of the ridge and into the back side of the l/w trough that's just off the East Coast. In doing so, it will move across the Mid Atlantic region, deepening the l/w trough and pushing a cold front across our area late Tuesday or Tuesday night. As the front approaches, low level S to SW prefrontal winds will advect a warmer, more moist and unstable airmass into our area. Meanwhile, with the approach of the s/w, look for mid and upper winds to increase from the NW as the day progresses. Pre-existing prefrontal showers/tstms from the prior day will cross the mountains Tue morning, with new cells forming downstream across our area along the Piedmont trough and outflow boundary. So look for the chance for shower/tstm activity to increase across central NC as the afternoon progresses. The combination of increasing instability and deep layer shear will offer the opportunity for any of the taller longer lasting tstm clusters to produce localized damaging winds gusts.

Given the prefrontal warming airmass, high temps on Tuesday should be the warmest of the week with readings climbing into the low-mid 90s. The showers/tstms, then cold front, are all expected to exit central NC to our east by 06Z Wednesday.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

As of 625 AM Sunday...

Areas of IFR/LIFR conditions in fog and stratus will persist over portions of the NC Piedmont through mid morning, particularly affecting RDU and GSO. At other sites, the fog/stratus will be more patchy, resulting in wide intrahour variability in flight rules for the next few hours. Vsbys at all sites should improve to MVFR then VFR 13z-15z, and cigs should improve gradually to MVFR then VFR between 15z and 19z as cigs slowly lift and break up. A few showers are possible near INT/GSO between 15z and 20z, with a very low chance of thunder. RDU/RWI/FAY will have a chance for a few showers and storms late tonight, after 03z, with a slow development of patchy MVFR vsbys and cigs expected after 05z. Surface winds at all sites will remain light, under 8 kts.

Looking beyond 12z Mon, areas of sub-VFR conditions are likely to last until around 13z-14z. Scattered showers and storms are possible in the E (RDU/RWI/FAY) Mon afternoon into early evening, then a higher chance of scattered to numerous storms (some potentially strong to severe) is expected Tue, mainly afternoon through the evening, associated with an approaching cold front.

RAH WATCHES/WARNINGS/ADVISORIES

None.