textproduct: Raleigh

WHAT HAS CHANGED

As of 400 PM Tuesday...

* Confidence in a significant weekend winter storm, and bitterly cold weather from Saturday into Tuesday, continues to increase.

KEY MESSAGES

As of 400 PM Tuesday...

1) Cold and dry weather prevails through Wednesday, with temperatures moderating through Thursday ahead of the next system.

2) A major winter storm is likely this weekend across all of central NC, with snow, sleet, and freezing rain all possible precipitation types. There is still uncertainty on exact timing and snow and ice amounts, so stay tuned.

3) Extremely cold temperatures are likely Saturday through Monday.

DISCUSSION

As of 400 PM Tuesday...

KEY MESSAGE 1... Cold and dry weather prevails through Wednesday, with temperatures moderating through Thursday ahead of the next system.

Cold Canadian high pressure anchored across the Mid-Atlantic region today and tomorrow will result in dry weather and cooler temperatures. High temperatures today will be in the upper 30s to low 40s with one more night of lows in the mid/upper teens to low 20s. After the surface high shifts offshore Wednesday afternoon expect temperatures to moderate with the warmest day of the forecast being on Thursday with highs in the 50s to near 60 degrees.

KEY MESSAGE 2... A major winter storm is likely this weekend across all of central NC, with snow, sleet, and freezing rain all possible precipitation types. There is still uncertainty on exact timing and snow and ice amounts, so stay tuned.

A strong Arctic cold front will move through on Friday afternoon and evening. Moisture and isentropic lift along and ahead of the front will result in some light rain across the area from early morning into the evening. Amounts are expected to be light, around a tenth to quarter inch. A remarkably strong ~1050 mb Arctic surface high moving SE over the Northern Plains and Upper Midwest will then push very cold and dry air into the region behind the front. Temperatures are likely to remain steady or even begin dropping on Friday afternoon with highs in the mid-40s to lower-50s, then drop into the upper-teens to mid-20s by Saturday morning as dew points crash into the single digits and teens. A lull in precipitation appears likely on Friday night and early Saturday morning as drier air briefly moves in behind the front.

A vigorous mid-level shortwave or cutoff low will move onshore Baja California on Saturday, drawing plenty of Pacific moisture into the Southern US. There will also be a strong influx of Gulf moisture, and PW values 150-250% of normal are expected across central NC. Meanwhile a polar stream trough will move across the Northern Plains and Upper Midwest. Strong phasing of the Baja low and southern with the northern stream trough along with plenty of low-level isentropic lift will set the stage for plenty of forcing for precipitation, though differences in guidance remain on the timing and degree of phasing. A weak surface low will initially develop over the north- central Gulf, before a secondary low develops off the GA coast on Saturday night, deepening as it moves NE offshore the coast of the Carolinas on Sunday. There also remains some uncertainty on the track of this low, which will affect how much snow vs ice occurs across the area.

All of the above factors combined with bitterly cold and dry air continuously being drawn south into the Carolinas from the Arctic high moving east from the Great Lakes into Upstate NY are likely to result in a moisture-laden and potentially long lasting winter storm across central NC from Saturday into Sunday. Precipitation will move in from the west on Saturday morning and afternoon, potentially on the lighter side initially, with the heaviest likely on Saturday night and early Sunday, tapering off from west to east sometime Sunday afternoon into Sunday night. There is still uncertainty on timing with the 12z ECMWF a bit slower compared to the GFS. GFS and ECMWF ensemble mean QPF is generally in the 1.5 to 2 inch range, and with very cold air firmly entrenched at the surface, all of it is likely to be frozen across central NC, whether that be snow or ice. The main question that has to be answered is how strong the push of warmer air is several thousand feet above the surface, which almost always occurs in these setups across central NC. The latest GFS is farther south with the 850 mb front and tracks the surface low farther offshore, thus depicting more snow compared to the ECMWF, but this has been going back and forth, and their respective ensemble guidance has generally been a bit warmer/less bullish with snowfall overall. The CMC is warmer than either model and would result in a significant ice storm over more of the area. So there is likely to be a sharp gradient in snowfall amounts somewhere over central NC, and an all snow event everywhere is very unlikely, but the most favored area for that climatologically would be along and north of the I-85 corridor. This is where current NBM probabilities for 6+ inches of snowfall are in the 60-80% range. Meanwhile NBM probabilities of 0.25+ inches of freezing rain are in the 60-80% range across the southern Piedmont, Sandhills, and central/southern Coastal Plain. Significant accumulations of sleet will also be possible especially between where the highest freezing rain and snow amounts are. In addition to major travel issues, significant tree damage and power outages may occur especially where the highest ice accumulations take place. While details on specific amounts are still uncertain, confidence is increasing in a major winter storm across central NC, and it is imperative to continue staying tuned for the latest updates as the forecast is likely to change.

KEY MESSAGE 3... Extremely cold temperatures are likely Saturday through Tuesday.

A very cold Arctic high will result in record or near record low maximum temperatures on Saturday and Sunday as highs struggle to get out of the 20s in many spots, with wind chills not getting out of the teens. See the climate section below. Widespread lows in the teens are likely on Saturday and Sunday nights, followed by lows in the single digits on Monday and Tuesday nights, especially if there is a snowpack. Highs on Monday and Tuesday are likely to only reach the lower-to-mid-30s, which would result in minimal melting. This is likely to keep impacts from the snow and ice lasting into at least into early next week. A Cold Weather Advisory or Extreme Cold Watch or Warning may be needed.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

As of 613 PM Tuesday...

VFR conditions with calm to variable winds will persist through the 24 hour TAF period. Some mid-high level cloudiness will stream in from the southwest tomorrow, but VFR conditions will prevail through the end of the TAF period. Expect generally light swly sfc flow tomorrow.

Outlook: Marginal LLWS may develop at northern TAF sites after 00Z Thursday as a strong low-level jet develops across the mid-Atlantic. Otherwise, expect generally VFR conditions to prevail through Thursday night (light rain may be possible Thursday afternoon, but generally should be dry for much of this period). This weekend, confidence is increasing in a potential winter storm may bring wintry precipitation and associated sub-VFR conditions Saturday and Sunday.

CLIMATE

Record Low Max Temperatures:

January 24: KGSO: 25/2014 KRDU: 27/2014 KFAY: 28/1970

January 25: KGSO: 23/1940 KRDU: 28/2013 KFAY: 30/2013

Record Low Temperatures:

January 26: KGSO: 3/1940 KRDU: 10/1940 KFAY: 14/1963

January 27: KGSO: -7/1940 KRDU: 8/1940 KFAY: 11/1940

RAH WATCHES/WARNINGS/ADVISORIES

None.