textproduct: Raleigh

WHAT HAS CHANGED

As of 215 PM Wednesday...

* A Cold Weather Advisory has been issued tonight for three counties. * Increased confidence for higher snow amounts across the region this weekend.

KEY MESSAGES

As of 215 PM Wednesday...

1) A Cold Weather Advisory has been issued for three northeastern counties tonight, although wind chill values will be low across the area.

2) Confidence continues to increase in the potential for a coastal low to bring a period of snow to central NC from Friday night through Sunday. While confidence in occurrence is rising, exact timing and snowfall amounts remain uncertain.

3) A prolonged Arctic air mass will result in well below normal temperatures across central NC from Thursday through at least the middle of next week.

DISCUSSION

As of 215 PM Wednesday...

KEY MESSAGE 1...A Cold Weather Advisory has been issued for three northeastern counties tonight, although wind chill values will be low across the area.

With clear skies tonight, overnight lows will drop a few degrees compared to last nights values, particularly across northern counties. Lows will range from 10 near the VA/NC border to the low 20s across the south. The wind will increase to 5-10 kt across northeastern counties after midnight, dropping the wind chill below 10 degrees in Halifax, Nash, and Edgecombe counties. Wind chills will range between 5 and 25 degrees across the area.

KEY MESSAGE 2...Confidence continues to increase in the potential for a coastal low to bring a period of snow to central NC from Friday night through Sunday. While confidence in occurrence is rising, exact timing and snowfall amounts remain uncertain.

Medium-range deterministic and ensemble guidance continue to signal a highly favorable synoptic pattern for rapid coastal cyclogenesis late Friday into Saturday. An upper-level trough is forecast to dive southward across the Mississippi Valley, cutting off as it crosses the Tennessee Valley and move into the southern Mid-Atlantic. As the system becomes negatively tilted over the Carolinas, a surface low is expected to rapidly deepen offshore bringing the precip to central NC.

With the upper low positioned to the southwest and the surface low just offshore, a deformation band may develop across portions of central NC, where snowfall accumulations could be significant. Current guidance suggests snow may begin as early as Friday afternoon across the northwest Piedmont, spreading into the Triangle and points south and east Friday night. Snow, potentially heavy at times, will persist through Saturday, with the greatest risk for heavier snowfall Saturday afternoon as the offshore low deepens. Snow should taper from west to east by early Sunday morning, with precipitation exiting the eastern areas by early Sunday afternoon. Impacts will vary across the region, but the synoptic setup supports the potential for a high-impact winter storm. Winter Storm Watches were deferred for now pending additional high-resolution guidance and improved sampling of the parent upper low. Additional messaging and potential headlines may be needed late tonight or Thursday.

Forecast confidence remains sensitive to the track of the upper low. If the upper low tracks farther north over central NC rather than swinging south of the region, snowfall and overall precipitation amounts would likely be lower, with surface low development displaced farther northeast. Given that the parent upper low is currently over northern Canada, it still has a long track before reaching the Mid-Atlantic, and subtle changes in its evolution could result in meaningful forecast adjustments.

KEY MESSAGE 3... A prolonged Arctic air mass will result in well below normal temperatures across central NC from Thursday through at least the middle of next week.

A strong Arctic high building down from south-central Canada into the Northern Plains and stretching into the Eastern US will result in continued very cold temperatures across central NC on Thursday and Friday. Forecast highs are mainly in the lower-30s to lower-40s, with lows Thursday and Friday nights in the mid-teens to lower-20s.

Temperatures will turn even colder this weekend as the coastal low that is increasingly likely to affect our region reinforces the Arctic air mass with strong northerly and eventually northwesterly flow. Forecast highs on Saturday are only in the lower-20s to upper- 20s, with mid-20s to lower-30s on Sunday. Our current forecast has these highs getting close to or breaking the record daily low maximum temperatures on both dates at all three climate sites. See the climate section below for more details. Winds gusting as high as 20-30 mph on Saturday and 25-35 mph on Sunday will keep wind chills from getting above the teens even during the day. On Saturday and Sunday nights, lows in the lower-to-mid-teens will result in wind chills getting below zero in many areas on Saturday night and single digits on Sunday night. Additional Cold Weather Advisories, and perhaps even Extreme Cold Watches/Warnings, will likely be needed.

Very cold temperatures will continue behind the storm from Monday through Wednesday, which will slow the melting process of whatever snow ends up accumulating this weekend, prolonging the impacts. Forecast highs Monday are in the mid-to-upper-30s with lows in the mid-teens. A slight modification is expected through midweek but temperatures are still only expected to be in the lower-to-mid-40s for highs and 20s for lows by Wednesday. There is some disagreement in guidance with the 12z ECMWF bringing down another Arctic high into the Great Lakes region, keeping us colder, while the GFS holds back the high farther NW and thus is slightly milder (but still well below normal).

AVIATION /00Z THURSDAY THROUGH MONDAY/

As of 643 PM Wednesday...

TAF period: High confidence in VFR conditions through the next 24 hours.

Outlook: VFR conditions are expected to continue through Friday afternoon. By Friday night, widespread restrictions and snow is expected to develop late and last into Saturday and Saturday night.

CLIMATE

Record Low Temperatures:

January 28: KGSO: -2/1940, KRDU: 1/2000, KFAY: 10/1935 January 29: KGSO: -7/1940, KRDU: 7/2000, KFAY: 11/1986 January 30: KGSO: 4/1966, KRDU: 7/2014, KFAY: 8/2014 January 31: KGSO: 3/1966, KRDU: 3/1966, KFAY: 9/1934 February 1: KGSO: -4/1936, KRDU: 8/1981, KFAY: 1/1936 February 2: KGSO: 6/1971, KRDU: 5/1971, KFAY: 15/1971 February 3: KGSO: 7/1917, KRDU: 7/1980, KFAY: 10/1917

Record Low Maximum Temperatures:

January 28: KGSO: 23/1986, KRDU: 23/1897, KFAY: 32/1961 January 29: KGSO: 23/1966, KRDU: 27/2014, KFAY: 28/2014 January 30: KGSO: 13/1966, KRDU: 19/1966, KFAY: 29/1966 January 31: KGSO: 22/1936, KRDU: 27/1948, KFAY: 22/1966 February 1: KGSO: 27/1971, KRDU: 28/1900, KFAY: 32/1981 February 2: KGSO: 30/1951, KRDU: 31/1994, KFAY: 31/1948 February 3: KGSO: 23/1996, KRDU: 24/1996, KFAY: 30/1961

RAH WATCHES/WARNINGS/ADVISORIES

None.