textproduct: Raleigh

WHAT HAS CHANGED

* The threat of severe weather, including isolated tornadoes, has increased across the region, especially for Sunday.

KEY MESSAGES

As of 304 AM Saturday...

1) The remnants of Isaias will bring periods of showers through Sunday. A threat of severe thunderstorms capable of tornadoes will also exist Sunday.

2) Much of next week will be unseasonably warm, with very little to no chance of rain.

DISCUSSION

As of 304 AM Saturday...

KEY MESSAGE 1... The remnants of Isaias will bring periods of showers through Sunday. A threat of severe thunderstorms capable of tornadoes will also exist Sunday.

The remnants of Isaias are presently located some 50 miles south- southwest of Montgomery AL. Over the next 24-36 hours, most model and ensemble guidance is in good agreement in tracking the remnants of Isaias from east-central AL today into the eastern TN valley by early Sunday. From Sunday onward, the extratropical system will occlude and weaken as it tracks ENE south of New England by early Monday.

Rainfall expectations: There has not been much change with the latest guidance with respect to rainfall totals. The latest 00z HREF/REFS are in relatively good agreement on precipitation totals, ranging from 0.75 inches to 2 inches. Amounts are expected to be highest across the far NW Piedmont/Triad, where upslope may combine with isentropic ascent, as well as over the Coastal Plain where instability will come into play. A pocket of lower rainfall totals may exist over the Triangle and central portions of the area, with some spots potentially seeing a half inch or less. Of course, embedded higher rainfall totals are possible anywhere in central NC, especially Sunday when convective chances greatly increase within the very moist tropical-like airmass. Periods of showers will exist today and tonight, but there will certainly be times of dry weather, especially the early part of the day when dry air will exist in the low-levels.

Wind gusts: An increasing pressure gradient will develop today with a retreating high to our northeast and the remnants of Isaias moving northeastward. Wind gusts out of the ENE will range from 25-30 mph, with infrequent gusts upwards of 35 mph across the west and southern Piedmont. These gusts should weaken later tonight in the 15-25 mph range as the gradient gradually relaxes. Under a developing CAD wedge today, highs will range from the low/mid 60s NW to low 70s SE.

Severe threat: Probably the most concerning aspect to this weekend system is the threat of severe weather. The latest update from SPC indicates a category increase in the severe weather risk, especially for Sunday. The Marginal Risk has expanded for late tonight and early Sunday across the southern Piedmont, Sandhills, Central Coastal Plain, and far southern areas of the Triangle. On Sunday, there is now a Slight Risk in place for all but the far western Piedmont of central NC.

Based on model fields, it would appear the chance of severe storms will be rather limited for most of Saturday. The better chance of some isolated severe storms will exist after midnight tonight and prior to daybreak Sun, when lower 70s dewpoints spread northward within an increasingly sheared environment. Confidence is not great on coverage for this early Sunday period given a lack of storm coverage shown in high-res guidance.

The better chance of severe weather should be from mid-morning Sunday into early Sunday evening. During this time, a secondary low will form across the NW Piedmont along the CAD boundary/warm front. As the low tracks ENE into VA during the day, low to mid 70s dewpoints will spread over most of the region. A southwesterly low- level jet of 40-45 kts will be in place, along with dewpoints in the low/mid 70s in an increasing warm sector. The 00z HREF/REFS indicate a corridor of strong instability of 1200-1500 J/kg as a dry slot steepens lapse rates across central and eastern areas of the Slight Risk zone. A strengthening low and deep-layer shear profile will support transient supercells with the threat for a brief tornado or two and damaging wind gusts. The 00z HREF/REFS and HRRR show a signal for a broken band of convection moving through the early afternoon to early evening along and east of the Triad. While uncertainty remains, things are certainly concerning for Sunday.

KEY MESSAGE 2... Much of next week will be unseasonably warm, with very little to no chance of rain.

Broad mid/upper ridging over the Southeastern US will bring mostly sunny and dry weather to central NC from Monday through Friday of next week. Any troughing will be confined to the Western and Northeastern US. One of those shortwaves (and associated area of surface low pressure) will move across the Great Lakes and New England on Tuesday into early Wednesday, dropping a backdoor cold front south into the Mid-Atlantic. This front will be moisture- starved so there will most likely be no precipitation with it, and if there is any it would be very light, with even the 90th percentile of ensemble guidance less than a tenth of an inch. A northern stream shortwave will dive down from Canada late in the week and potentially interact with the Western US trough, which should eventually help suppress the ridge or shunt it to our east over the weekend. But significant differences still exist in the guidance, with the phasing solution on the GFS resulting in a much deeper and slower trough compared to the ECMWF, so forecast confidence is lower by this time.

There is high confidence in above-normal temperatures early next week, with highs in the upper-70s/lower-80s and lows in the mid-50s to 60 on Monday. Tuesday will be even warmer, with highs in the lower-to-mid-80s (within a few degrees of record highs) and lows in the upper-50s to mid-60s. After this, ensemble spread in temperatures greatly increases as a lot will depend on the timing of the backdoor cold front, if it even makes it here. For now the most likely timing of the passage is late Wednesday, resulting in continued warmth on Wednesday followed by a cooler Thursday and potentially Friday. However, the GFS lifts it back north as a warm front which would bring temperatures back well above normal on Friday.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

As of 152 AM Saturday...

VFR conditions are expected to start the TAF period. The remnants of Isaias will continue to move northward over the next 24 hours, tracking from central AL into the eastern TN valley and southern Appalachians. Radar is presently indicating pockets of light rain moving north from SC, though there is still a lot of dry air to overcome before sub-VFR conditions can develop. The challenge with the forecast will be the timing of IFR/MVFR conditions. The latest forecast may be too pessimistic, but decided to go closer to the median trend in the guidance, with onset of IFR/MVFR in the 14-17z time frame as rain overspreads central NC. Conditions may deteriorate faster than anticipated at GSO/INT with upslope aiding moderate rain later this morning. Conditions may be slower to deteriorate at RWI as this terminal will be on the eastern fringe of the precipitation shield most of Sat. Overall, expect a gradual lowering of ceilings Sat night, in the LIFR/IFR range, lowest at GSO/INT. Some patchy fog may develop across the GSO/INT terminals into early Sun but confidence on this is low at the moment. Wind gusts Sat will range from 25-30 kt out of the ENE, weakening after 00z Sun in the 15-20 kt range.

Outlook: Areas of showers with LIFR/IFR to some MVFR conditions will continue most of Sun, with patchy fog possible in the morning. A band of showers and embedded thunderstorms will be possible from late morning Sun into early Sun evening, most favored at RDU/FAY/RWI. Some of the storms could be severe. A return to VFR conditions is expected Mon, although some gusty NW winds will exist. VFR should prevail into midweek.

RAH WATCHES/WARNINGS/ADVISORIES

None.