textproduct: Raleigh

SYNOPSIS

A series of upper level disturbances will track east and southeast through the Mid Atlantic region tonight into Sunday, as a backdoor cold front approaches from the north.

NEAR TERM /THIS AFTERNOON THROUGH TONIGHT/

As of 220 PM Friday...

Flash Flood Watch for the northern and much of eastern NC through 400 AM Saturday. Flash flooding is possible, especially along and north of the Triad and Triangle to Rocky Mount this afternoon and evening.

Look for the likelihood of at least locally severe storms this afternoon into the evening, especially over the northern portions of central NC.

Heat Advisory through 800 PM.

Strong heating (temperatures in the lower to mid 90s) of the very moist environment (dew points in the mid to upper 70s) will continue ahead of the advancing short wave that will move east across the mid- Atlantic states later this afternoon into tonight. There were a few lingering clouds at early afternoon over central NC; otherwise, it was nearly full sunshine across the region. Scattered thunderstorms were already developing at 17z over the Foothills of the northern and central NC mountains in response to the approaching trough and strong heating. As the scattered thunderstorms move east into the Piedmont where the very moist, heated, and unstable atmosphere exists, expect strong to severe storms to develop with the main hazard of damaging wind gusts. These storms will likely become clusters across the Triad between 200 and 400 PM, then shift into the northern and central Piedmont by late afternoon and the Coastal Plain this evening. Some of the storms may produce damaging wind gusts.

There is also a flood risk this afternoon and evening, especially over north-central and NE portions of the region where it has been exceptionally wet in the past few weeks. We issued a Flood Watch through 400 AM to match our neighbors to the north and east and match with the latest slight risk (level 2 of 4) for flash flooding, and with areas that are nearly saturated or heavily urban. If repeated areas are hit, especially over saturated areas of north- central NC and urban areas, isolated to scattered flash flooding could result. Rainfall rates in excess of 2 inches per hour are likely.

The Heat Advisory will continue into the evening or may be trimmed as thunderstorms overturn the boundary layer.

The convection may persist longer into the evening with some upper support and on outflows. POP will trend much lower after midnight. Lows in the lower to mid 70s.

SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/

As of 200 PM Friday...

* Heat Advisory in effect for the Eastern Piedmont, Sandhills, and Coastal Plain Saturday from 11 AM to 8 PM. Heat index values between 105 and 109 are expected.

* Slight (level 2 of 4) risk of flash flooding possible for the northern Piedmont and northern and central coastal plain where scattered to numerous slow moving storms are likely.

A similar pattern of slow moving afternoon showers and storms will persist on Saturday. Convection is expected to initiate over the mountains Saturday afternoon and move eastward into the region as well as forming on residual outflow boundaries left from convection that occurs Friday afternoon. Shear values look to be less than 20 kts, limiting severe potential, however a few stronger to severe storms cannot be ruled out in the afternoon and evening hours. The main hazard with these storms should be the flooding potential, as weak cloud-layer flow as well as plenty of moisture are expected. If storms stall or train over urban areas or areas with already moist soils, flash flooding will be possible.

High temperatures Saturday afternoon will peak in the low to mid 90s, with the warmest temperatures in the south and east. This will combine with high humidity values to raise heat indices into the upper 90s to mid 100s. Thus, a Heat Advisory has been issued for the Eastern Piedmont, Sandhills, and Coastal Plain from 11 AM to 8 PM.

LONG TERM /SUNDAY THROUGH FRIDAY/

As of 305 PM Friday...

A broad mid/upper trough over the Northeast US and Mid-Atlantic associated with a low over eastern Quebec will push east into the Atlantic on Sunday and Sunday night. Weak subsidence and mid-level height rises behind a departing shortwave rounding the base of the trough will result in lower shower and storm chances on Sunday across central NC. But 2000-3000 J/kg of MLCAPE and a moist air mass (PW values 2+ inches) mean isolated convection will still be possible. The highest POPs (still only around 30%) are focused in the NW with terrain convection spilling into the Piedmont and the SE with the sea breeze. The bigger story will be temperatures, as good sunshine and lower precip chances will help them rise even a bit higher than Saturday, with highs ranging from lower-90s in the far north to upper-90s in the far south. With dew points in the 70s, heat indices are likely to reach as high as the 105 to 109 range from the Triangle south and east, and another Heat Advisory may be needed. Any convection will quickly diminish after dark with lows Sunday night mostly mid-to-upper-70s.

Monday will feature the best precipitation chances of the period, as a series of upper impulses moves across the area in the NW flow between the trough just east of the Mid-Atlantic coast and building ridge from the Deep South into the Central Plains. At the surface, low pressure over the Canadian Maritimes will send down a backdoor cold front that reaches the southern Mid-Atlantic on Monday night. Lift from these features combined with moderate to strong destabilization will result in more widespread showers and storms on Monday afternoon and evening, with perhaps some lingering into Monday night as the front moves through our region. Isolated flash flooding can't be ruled out where storms train and in urban areas. We also get into the SW periphery of the belt of stronger NW mid- level flow, perhaps up to 25-30 kts, so there could be a threat of severe storms. Monday's highs will be in the upper-80s to mid-90s with lows in the upper-60s to mid-70s.

The ridge will build farther north and east into the OH Valley and Upper Great Lakes on Tuesday, weakening the mid-level flow across central NC. Meanwhile drier air and below-normal PW values will start to filter in behind the backdoor front which settles SW of central NC. There is still some uncertainty on where the front will be, but guidance has been trending faster with its passage, so wouldn't be surprising if Tuesday ends up largely dry. The best chance for any convection will be in the SW in closest proximity to the front. Precipitation chances are very little if any on Wednesday and Thursday with cooler more stable NE flow from surface high pressure east of New England and the mid/upper high centered over the TN Valley. The best chance would be in the NW with terrain convection and the SE with the sea breeze. Moisture may begin to increase again on Friday as a Bermuda surface high begins to set up with southerly flow returning. Temperatures will slowly increase from Tuesday through Friday as cooler air from the high gradually erodes. The "coolest" day looks to be Tuesday with highs in the mid- to-upper-80s and lows in the mid-60s to lower-70s, increasing back to highs in the lower-to-mid-90s and lows in the lower-to-mid-70s by Friday.

AVIATION /23Z FRIDAY THROUGH WEDNESDAY/

As of 659 PM Friday...

Prevailing VFR conditions are ongoing outside of scattered convection resulting in MVFR to IFR vsbys elsewhere from the TAF sites. Outflow boundaries may collide near the northern 4 TAF sites with scattered showers/storms possible at any of them. Handled initially with PROB30s given low confidence and struggling model support, but may need to be increased to TEMPOs shortly as it becomes clear where storms will develop. MVFR to IFR stratus will be possible at RWI and RDU with less confidence at FAY. The Triad terminals may see some shallow fog and/or very low stratus around daybreak, but confidence again remains low since they have missed out on the rainfall thus far this evening. Afternoon showers/storms will again be possible Sat afternoon with perhaps greatest coverage from the Triad terminals to RDU.

Looking beyond 00z Sun, The threat for diurnal showers/storms and early morning sub-VFR fog and stratus will continue through early next week. A frontal passage Mon into Tues will bring a wind shift out of the north and brief relief from this pattern with less convective coverage and relatively drier air filtering in from the north expected into the midweek.

RAH WATCHES/WARNINGS/ADVISORIES

Flood Watch through late tonight for NCZ007>011-021>028-038>043- 076>078. Heat Advisory until 8 PM EDT this evening for NCZ008>011-024>028- 040>043-073>078-083>086-088-089. Heat Advisory from 11 AM to 8 PM EDT Saturday for NCZ008>011-024>028- 040>043-073>078-083>086-088-089.