textproduct: Raleigh

WHAT HAS CHANGED

As of 400 AM Tuesday...

* Confidence is increasing for coastal cyclogenesis favorable for a period of snow and bitter cold in the Carolinas some time between Fri night into Sun morning, although amounts and impacts remain uncertain.

KEY MESSAGES

As of 400 AM Tuesday...

1) A Cold Weather Advisory is in effect through 10 AM. A prolonged period of Arctic cold and well below average temperatures, with departures of mostly 10-20F (except around 30 F on Sat), will otherwise continue through the forecast period, and likely beyond into the first week of Feb.

2) Confidence is increasing for coastal cyclogenesis favorable for a period of snow and bitter cold in the Carolinas some time between Fri night into Sun morning, although amounts and impacts remain uncertain.

DISCUSSION

As of 400 AM Tuesday...

KEY MESSAGE 1...A Cold Weather Advisory is in effect through 10 AM. A prolonged period of Arctic cold and well below average temperatures, with departures of mostly 10-20F (except around 30 F on Sat) will otherwise continue through through the forecast period, and likely beyond into the first week of Feb.

A polar vortex initially over Hudson 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 and offshore the srn Middle Atlantic this weekend. Associated broadly cyclonic flow and anomalously low heights, characterized by standardized 500 mb height anomalies of -1-3 sigma, will encompass much of the ern US through Fri. During that time and beneath that cyclonic flow, a couple of Arctic highs centered this morning over the lwr MS Valley and SK, respectively, will progress across the Southeast. Related high and low temperatures through Fri in cntl NC will mostly be in the 30s to lwr 40s and teens to lwr 20s, respectively, with morning wind chill values in the single digits and teens.

KEY MESSAGE 2...Confidence is increasing for coastal cyclogenesis favorable for a period of snow and bitter cold in the Carolinas some time between Fri night into Sun morning, although amounts and impacts remain uncertain.

An elongated mid/upper level trough axis stretching across the International border Fri morning will pivot south across the Mississippi and Tennesse Vallie's before the embedded basal shortwave pinches off and occludes over the southern Appalachian and Southern Mid-Atlantic by Sun. The progression will develop an initially weak surface low over the central Gulf states, skip across the FL peninsula, and rapidly deepen off the Carolina coast Saturday. The trend in the latest suite of 00z guidance has been a farther west occlusion of the mid/upper level low which has resulted in an increase in the liquid equivalent over the inland areas of the Carolinas. Based on the 1000-850 and 850-700mb thicknesses, precipitation would fall as all snow.

Miller-A cyclogenesis patterns are known for predominantly producing rain or snow with a very narrow transition in between. Despite the lack of a cold Arctic high to the north, which is typically more favorable for wintry precipitation in Miller B tracks, Miller A cyclogenesis will lock in the cold Arctic airmass already in place over the Mid-Atlantic with the strengthening northerly winds from rapidly deepening low pressure off shore; so cold air should be in abundant supply to produce all snow, expect for perhaps towards the Carolina coast. This synoptic setup is the most favorable pattern to produce significant snowfall totals for the Carolinas, but are not a guarantee that we would see impactful snow accumulations everywhere/anywhere. The finer details within the mesoscale still need to be resolved, and the placement of the mid/upper low and surface cyclone need to remain in close proximity to the Carolinas to increase confidence in an impactful event. For now, greatest confidence to see at least minor impacts from this system would be from the eastern Piedmont towards the Carolina coast where greater and more prolonged rates within the deformation band are most probable.

AVIATION /11Z TUESDAY THROUGH SATURDAY/

As of 600 AM Tuesday...

Arctic high pressure will extend across the Southeast and favor VFR conditions over cntl NC through the 12Z TAF period, and beyond through most of this week. A little gustiness will be possible with daytime heating each late morning-afternoon, including from a swly direction today.

Outlook: Coastal low pressure will develop along the South and Middle Atlantic coasts and favor a good chance of flight restrictions and snow, potentially significant, over cntl NC Fri night through at least the first half of the weekend.

CLIMATE

Record Low Maximum Temperatures:

January 31: KGSO: 22/1936 KRDU: 27/1948 KFAY: 22/1966

February 1: KGSO: 27/1971 KRDU: 28/1900 KFAY: 32/1981

Record Low Temperatures:

January 27: KRDU: 8/1940 KFAY: 11/1940

February 1: KRDU: 8/1981

February 2: KFAY: 15/1971

RAH WATCHES/WARNINGS/ADVISORIES

Cold Weather Advisory from 7 PM this evening to 10 AM EST Tuesday for NCZ007>011-021>028-038>041-043. Cold Weather Advisory from 9 PM this evening to 10 AM EST Tuesday for NCZ042-073>078-083>086-088-089.