textproduct: Raleigh

WHAT HAS CHANGED

As of 122 PM Saturday...

* Precipitation amounts tonight through Sun have increased from a tenth to three tenths of an inch over the NW/Triad, to one half to three quarters of an inch along/east of US-1 * Probabilities for a higher end/worse case snow event of 1-1.5 inches are low, but have increased slightly in the high-res HREF to about 30-40 percent across the northern to northeast Piedmont to northern Coastal Plain

KEY MESSAGES

As of 122 PM Saturday...

1) A strong mid-level system will move across the region tonight through early evening Sun, where a band of rain changing to rain/snow and then snow on the back end will favor a slushy accumulation of snow over northern and northeast sections of central NC

2) Cold conditions expected behind a reinforcing Arctic cold front Monday night and persisting into Wednesday.

DISCUSSION

As of 122 PM Saturday...

KEY MESSAGE 1... A strong mid-level system will move across the region tonight through early evening Sun, where a band of rain changing to rain/snow and then snow on the back end will favor a slushy accumulation of snow over northern and northeast sections of central NC.

A rather vigorous upper-level shortwave, presently on satellite over northern TX, will round the base of a larger longwave trough set to move into much of the eastern US Sun. As the shortwave rounds the base of the trough over the Carolinas Sun aftn/eve, it will become negatively tilted before exiting off the Mid-Atlantic coast Sun night into Mon. An impressive 150-180 kt jet streak will be positioned from SW to NE from the southern Appalachians into the Mid- Atlantic, putting central NC in the favored right-entrance region for enhanced lift at mid/upper levels, primarily from overnight tonight into early evening Sun.

At the surface, we will see a cold front slowly sag south and east from the west on Sun, crossing over the mountains and oozing into eastern sections of the state Sun/Sun night, as an area of low pressure develops off the coast and tracks off the NE US Sun night.

Precipitation amounts have increased since the last forecast issuance. This is driven by strong isentropic ascent and some frontogenesis driven by the jet/shortwave and attendant PW-values 120-130 percent of normal. QPF amounts could range from a a tenth to three tenths of an inch in the Triad, to one half to three quarters of along/east of US-1 to the Coastal Plain. Rain should overspread the region as early as late this evening, but should really get going after midnight into early Sun morning, when the forcing really blossoms with the jet. A changeover from rain to rain/snow is favored for northern areas Sun morning to early Sun afternoon, becoming briefly all snow in the mid to late afternoon across the north before exiting early Sun evening.

The main challenge with this system continues to be how fast the cold air can arrive before the precipitation shield exits early Sun evening. The other question is whether mesoscale bands within the precip shield can favor diabatic cooling for a quicker changeover to snow. If one takes a look at the mesoscale and global models from a forecast sounding standpoint, there is actual decent agreement on thermal profiles. Thicknesses would support a narrow 1-3 hour window when precipitation would changeover to all snow, which is roughly 9a- 12pm in the NW, 1-4pm in the Triangle and points N and NE, and 3p- 6pm in the central/northern Coastal Plain. Little if any snow is expected in the Sandhills and southern Coastal Plain. Overall, expected snow amounts have not changed much, with a slushy trace to half inch or less of snow on mainly elevated/grassy surfaces.

Where could this forecast go off the rails and reach the high end/worse case amounts of 1-1.5 inches? This scenario could happen if we see some isolated mesoscale precip bands that enhance diabatic cooling of the column. There could also be a narrow 1-3 hr window when mid-level lift/fgen could coincide with saturation in the dendritic growth zone to favor a narrow zone of heavier snow rates and support this higher worse case scenario. If this were to occur, it would roughly be from just east of Burlington to Roxboro to NE of Raleigh to Henderson to Roanoke Rapids, where HREF/LREF snow probabilities for this worse case are highest. Confidence in this scenario remains low but HREF probabilities for 1-1.5 inches do show 30-40 percent values in this region, a little higher than the global LREF of 15-percent. For now, will continue to message the most favored outcome noted earlier.

Precipitation wraps up early Sun evening and temperatures fall into the 20s overnight into Mon, which may result in some refreezing and icy patches on roadways Sun night to Mon morning.

KEY MESSAGE 2... Cold conditions expected behind a reinforcing Arctic cold front Monday night and persisting into Wednesday.

Highly amplified mid/upper level flow is expected across the CONUS by Mon morning, potentially record breaking >580dam H5 ridge over the eastern Pacific off the British Columbia coast and deep/broad troughing over the central/eastern NOAM. In between these upper level features, strong surface high pressure will ooze down the eastern Rockies into the northern/central Plains before settling over the Southeast and Mid-Atlantic by Wed morning.

After some modification of the Arctic air mass in place on Mon within the southwesterly winds, CAA behind the fropa Mon evening will return temperatures to well-below normal Mon night, Tues, and Wed morning when highs/lows will be 10-15 degrees below normal. The coldest air temperatures will likely occur Wed morning due to the optimal radiational cooling setup. Apparent temperatures are not expected to reach Cold Weather Advisory criteria, but hypothermia could still occur with any prolonged exposure and lack of proper clothing.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

As of 122 PM Saturday...

TAF period: INT/GSO currently have MVFR ceilings which should lift as clearing occurs this afternoon. VFR cloud cover should persist through the afternoon at RDU/RWI/FAY, with the possibility of MVFR ceilings at RDU. Several hours of VFR conditions are expected at all terminals this evening into the overnight hours, but restrictions will develop once again as precipitation moves back into the region. Rain will be the dominate precipitation type, although snow could mix in at INT, GSO, and RDU. It appears that INT/GSO will likely remain MVFR tonight into Sunday morning, although there is high confidence that RDU/RWI/FAY will all drop to IFR ceilings, if not LIFR ceilings. The wind will be out of the southwest this afternoon, with gusts up to 20 kt at FAY/RWI, veering to the northwest after midnight as precipitation begins.

Outlook: Widespread restrictions will continue Sunday afternoon as precipitation moves to the east, but dry VFR conditions should return by Sunday evening. The dry VFR conditions should continue through Thursday, when a slight chance of rain will return to the forecast. There could be gusts out of the west as high as 20 kt on Monday.

RAH WATCHES/WARNINGS/ADVISORIES

None.