textproduct: Raleigh

WHAT HAS CHANGED

* Air Quality Alert was issued earlier this morning for the southern Piedmont and a portion of the Sandhills.

* Temperatures trending cooler for Monday with brief CAD signal behind the cold frontal passage.

KEY MESSAGES

As of 145 PM Thursday...

1) An Air Quality Alert will remain in effect for the southern Piedmont and a portion of the Sandhills until 8 PM this evening.

2) Continued above normal temperatures through Saturday.

3) Measurable precipitation chances and frequency of events increase starting this weekend through the end of April.

DISCUSSION

As of 145 PM Thursday...

KEY MESSAGE 1... An Air Quality Alert will remain in effect for the southern Piedmont and a portion of the Sandhills until 8 PM this evening.

In collaboration with the NC Department of Environmental Quality, an Air Quality Alert has been issued for the southern Piedmont and portion of the Sandhills through this evening. Warm and dry conditions across the Carolinas, abundant sunshine and weak surface winds are all favorable for ozone formation at the surface. Additionally, regional satellite imagery this morning showed smoke and haze from wildfires in in GA/FL as far north as the NC/SC border. These factors combined will contribute to Code Orange Air Quality, which indicates these levels of ozone will be unhealthy for sensitive groups. Further Air Quality Alerts may be needed for portions of the area Friday and Saturday as the forecast pattern remains relatively unchanged with perhaps greater influence from wildfire smoke from the southwest.

KEY MESSAGE 2... Continued above normal temperatures through Saturday.

A high-latitude omega blocking pattern is expected to develop over Canada as early as Thurs and persist into the weekend. This will allow for weak southwesterly surface flow to persist over the area and steadily raise low-level thicknesses by 2-5m each day. These thicknesses and deep, boundary-layer mixing will likely support unseasonably warm temperatures --10-15 degrees above normal-- beginning this afternoon through Sat. A weak area of low pressure is expected to track along a stalled boundary over the Mid-Atlantic late Sat into Sat night and push a weak cold front through our area by Sun morning; effectively settling daytime temperatures back to near normal.

Based on these temperatures, Heat Risk is highlighting Minor to Moderate levels (Level 1 and Level 2 respectively) through Sat. This level of heat will primarily affect those individuals who are especially sensitive to heat and without access to adequate cooling and/or hydration. One positive is that the air mass will remain very dry with 30-40 degree dew point depressions during peak heating, and result in minimum RH values in the 20-30s.

KEY MESSAGE 3... Measurable precipitation chances and frequency of events increase starting this weekend through the end of April.

Guidance continues to suggest two main chances for measurable rainfall through the forecast period; Saturday afternoon into Sunday and Tuesday, although no significant rainfall is expected.

Saturday afternoon into Sun: A convectively modified shortwave ejecting over the Ohio Valley Sat afternoon is expected to shift across the Mid-Atlantic Sat evening into Sun morning. This feature, along with eastward advection of seasonably anomalous deep-layer moisture, will help direct convectively perturbed westerly flow over the Carolinas. Forcing for ascent will be overall quite weak and localized along the track of MCV's from upstream convection so significant precipitation is not expect. Expect primarily trace amounts to around 0.3" in most locations. Limited hi-res guidance suggests 250 to around 500 J/kg of SBCAPE may develop during the afternoon hours and may support deeper updrafts and locally higher rainfall totals. Additionally, steep low-lvl lapse rates and modest DCAPE environment could support a localized wind risk if deeper convection and stronger cold-pools can develop.

Tuesday: A more significant trough is forecast to pivot from the Four Corners Region to the Great Lakes from Sun to early Tues around the parent closed-low over Saskatchewan/Manitoba. Once again, the Carolinas will only see glancing influence from the trough and mainly directs another plume of seasonably anomalous deep-layer moisture and perturbed westerly flow over the Carolinas. The timing of this first round of most-likely stratiform rain, given the stable preceding environment, is expected Tues morning into the early afternoon hours. This will make the second round, featuring scattered showers/storms, less certain and dependent on destabilization in the wake of the departing mid/upper level moisture axis.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

As of 105 PM Thursday...

VFR conditions will prevail through the 24 hour TAF period. Overall light winds are expected this afternoon, with 5-10kt northwesterly winds, with a few occasional gusts up to around 16-18kts. Winds should lessen overnight and back to southwesterly overnight, before veering back to northwesterly by morning. Skies look to be mostly clear this afternoon, with a few clouds around 10kft possible this evening.

Outlook: VFR conditions should dominate through Sat morning. The chance for sub-VFR conditions will increase as showers and a few storms sweep through the area from west to east from late Sat afternoon through Sun morning. Another round of showers and storms with possible sub-VFR conditions will occur late Mon night through Tue evening.

RAH WATCHES/WARNINGS/ADVISORIES

None.