textproduct: Raleigh

WHAT HAS CHANGED

As of 400 AM Wednesday...

* Confidence continues to increase for at least light measurable snow in central NC Fri night into Sun morning; and the potential exists for significant snow. However, considerable uncertainty remains regarding amounts and related impacts.

KEY MESSAGES

As of 400 AM Wednesday...

1) Confidence continues to grow in at least measurable snowfall in central NC Fri night into Sun morning, but considerable uncertainty remains with an incredibly wide range of potential snowfall amounts and related impacts.

2) A prolonged period of Arctic cold and well below average temperatures will turn even colder during and following a winter storm that appears increasingly likely to impact cntl NC this weekend.

DISCUSSION

As of 400 AM Wednesday...

KEY MESSAGE 1...Confidence continues to grow in at least measurable snowfall in central NC Fri night into Sun morning, but considerable uncertainty remains with an incredibly wide range of potential snowfall amounts and related impacts.

A polar vortex initially over James Bay will be drawn swd and across ern ON and the Great Lakes this week, while probably elongating and splitting, with one piece likely to progress across Atlantic Canada and another forecast to pivot across the Southeast and along and offshore the coasts of the Middle Atlantic and New England through the weekend.

At the surface, cyclogenesis appears likely just off the Southeast coast, where a weaker but still prevalent baroclinic zone is forecast over the Gulf Stream. The primary front is expected to be still draped across the Bahamas, setting up a probable instant occlusion low surface pattern. The low off the Carolina coast is expected to rapidly deepen Sat into Sat night, as stronger synoptic ascent overspreads the Gulf stream and the main synoptic front is pulled northward. On the backside of the low, an Arctic airmass will become locked in over the Mid-Atlantic as the low deepens off the coast, resulting in potentially record breaking low max temperatures during the event.

This pattern is favorable for at least light snow with a high snow/liquid ratio within central NC, but also brings an incredibly difficult forecast challenge related to a deformation band on the north and west side of the deepening low. The likelihood of band formation, let alone its timing and placement, remains a point of considerable uncertainty and may not be ironed out until 1-2 days before the event begins. However, the top analogs and latest suite of 00z model guidance highlights at least the potential for significant snowfall totals somewhere from the Carolinas into the Mid-Atlantic.

There are a few failure modes for this setup which would result in less precipitation over central NC. The first is the placement and the latitude the mid/upper level low closes; minor adjustments will have potentially significant ramifications on if/when/where the deformation band sets up. The second is that some degree of cyclogenesis will occur along the primary baroclinic zone over the Bahamas and may draw stronger ascent, moisture, and related liquid equivalent and snow amounts away from the instant occlusion-type, nearer-shore coastal low.

KEY MESSAGE 2...A prolonged period of Arctic cold and well below average temperatures will turn even colder during and following a winter storm that appears increasingly likely to impact cntl NC this weekend.

Forecast confidence is higher in an Arctic airmass in place, one continually replenished from the Arctic, which will become further locked in as the cyclone off the coast deepens. The result will be bitterly cold temperatures and dangerous wind chills as early as Fri night, but especially Saturday through Sunday night. Blustery winds from the tight pressure gradient will result in multiple nights of wind chills in the single digits to even below zero possible Sat night into Sun morning. Daytime wind chills won't be much of a relief, as maximum apparent temps only reach into the teens Sat and Sun.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

As of 145 AM Wednesday...

Arctic high pressure will extend across the Southeast and favor VFR conditions over cntl NC through the 06Z TAF period, and beyond through most of this week. Initially light, nwly surface winds this morning will back through wly today, while strengthening and becoming at least occasionally gusty mainly at Piedmont sites and RWI this afternoon.

Outlook: An unusually strong mid and upper-level low, and related coastal low pressure, will favor a high probability of flight restrictions and snow, potentially significant, over cntl NC Fri night through Sun morning.

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

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

RAH WATCHES/WARNINGS/ADVISORIES

None.