textproduct: Raleigh

WHAT HAS CHANGED

As of 400 AM Sunday...

* Mixed frozen precipitation (snow and sleet) will be more-prolonged and over a larger area over the Piedmont this morning, but gradually retreat nwwd with time; and this will at least initially delay ice accrual and risk of power outages.

* Storm total liquid equivalent precipitation amounts continue to trend lower, but overall winter storm impacts will still be significant.

* Forecast confidence continues to increase regarding rising temperatures to above freezing over all but probably the northwest through far northeast Piedmont and nrn Coastal Plain tonight, and probably by late afternoon over the Advisory area.

KEY MESSAGES

As of 400 AM Sunday...

1) Winter weather headlines --and expected Moderate to Major Winter Storm Severity Impacts in the Warning area and Minor ones in the Advisory area-- remain unchanged for the winter storm underway over cntl NC.

2) An extended period of bitter cold temperatures into next weekend

3) A few periods where light snow could occur, but confidence remains low

DISCUSSION

As of 400 AM Sunday...

KEY MESSAGE 1...Winter weather headlines --and expected Moderate to Major Winter Storm Severity Impacts in the Warning area and Minor ones in the Advisory area-- remain unchanged for the winter storm underway over cntl NC.

Strong low-mid-level WAA will continue to progress from the Carolinas through the Middle Atlantic today, most intense and influential over the Carolinas at 925 mb, and accompanied by equally intense Fgen at that level early today.

Classical, diabatically-enhanced cold air damming, related to a 1040 mb, Arctic high now centered along the NY/ON border, will continue to mature over the Carolinas this morning, then weaken through tonight. It will do so as a coastal low develops along a coastal front off coast of the Carolinas early today and then deepens while tracking along and offshore the Middle Atlantic and srn New England coasts through tonight (a cold air damming erosion scenario), and as the Arctic high weakens to around 1025 mb and retreats poleward across Atlantic Canada by 12Z Mon. Nely flow, with an associated feed of cold and dry advection, will gradually weaken, with increasingly divergent flow in the CAD layer this afternoon-evening, between the coastal low and an inland low forecast to develop and track along an inverted trough west of the Appalachians.

The aforementioned strong low/mid-level WAA regime has caused maximum warm nose temperatures at KGSO to increase from minus 2C at 18Z Sat, to near freezing at 00Z Sun, to most recently 5.2C between 816-810 mb at 06Z Sun. That warming was sufficient to completely melt snowflakes falling into-through it and favor only super-cooled water droplets devoid of ice nuclei, with a consequent changeover to FZRA earlier this evening. However, top-down saturation into a substantial cold nose, observed at 06Z at minus 14.5C and a few degrees colder than forecast, yielded ice nucleation (and Bergeron precipitation production) supportive of the redevelopment of ice crystals/snow that have been reported by broadcast media to be a combination of needles and small dendrites. These broadcast media reports have been corroborated by numerous mPING ones, webcams, and also surface observational data over most of the Piedmont. Although those crystal habits will not accumulate as readily as larger and less dense dendrites, the continuation of frozen precipitation will delay the changeover to freezing rain and at least initially delay power outage impacts. This frozen precipitation will likely gradually retreat nwwd across the Piedmont, per forecast 0-1 km min wet-bulb temperatures used as a proxy for minimum cold nose temps, and whereby the recommended operational cut-off of minus 10C for the presence of ice has aligned relatively well with observations. If the cold nose maintains ice nucleation, even as cold nose temperatures warm to minus 6-8C this afternoon, then additional sleet and less complex crystal habits may further delay a changeover to freezing rain and ultimately yield additional, light frozen accumulation at the expense of freezing rain.

Otherwise and elsewhere, freezing rain will become increasingly- predominant and continue the likelihood for significant icing, power outages, and hazardous travel.

The demise of the CAD layer will also very likely allow the surface wet bulb freezing line to retreat nwwd over all but probably the northwest through far northeast Piedmont and nrn Coastal Plain tonight, and probably by late afternoon over the Advisory area. This will both allow for melting of existing ice and cause a secondary, frontal band of precipitation early tonight to fall as plain rain in those areas. Travel conditions will consequently improve in those areas overnight and especially with sunshine, and further warming temperatures during the day Mon.

KEY MESSAGE 2...An extended period of bitter cold temperatures into next weekend

An extended period of bitter cold temperatures will dominate much of the upcoming week and into next weekend. Aloft, a consistent WNW will lead to several bouts of Arctic air that will continue to build southward into the region. The CAD boundary gets eroded early Mon ahead of another strong cold front that moves through Mon aftn. This will create gusty NW winds Mon aftn to early eve in the 20-30 mph range, which could cause additional impacts of power outages due to ice potential on trees and powerlines. An Extreme Cold Watch or Cold Weather Advisory may likely be needed Mon night into Tue morning with lows in the single digits to teens and apparent temps below zero to the low single digits. Highs Mon to Wed only hover in the low 30s to low 40s.

Another Arctic cold front moves through Wed-Wed night and this air looks even colder than what we presently have, with highs only in the upper 20s to mid 30s from north to south and lows in the single digits to teens. This period may also warrant an Extreme Cold Warning or Cold Weather Advisory.

KEY MESSAGE 3...A few chances for light snow are possible Wed and Sat, but confidence remains low

Within the WNW flow aloft and tied to the front Wed night, there is the potential for some light snow into early Thu. As it stands now, the trough has most of its energy to our northeast. However, there are a good number of cluster solutions that indicate a deeper and more westward trough, which would favor a better chance area-wide. For now, the best chance is over the northern Coastal Plain. Confidence is low at this point. Ensembles are also hinting at another chance on Sat, this time with a possible northern and southern stream wave. Confidence is quite low at this stage so for right now the main messaging is for the bitter cold over the upcoming weekend.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

As of 202 AM Sunday...

A mixed bag of winter ptype's at all terminals early this morning. KINT/KGSO have seen some snow mix in with sleet (and some freezing rain). KRDU/KFAY have largely reported freezing rain, although sleet has been observed nearby at the NWS Raleigh location. Expect sleet and snow to continue at KINT/KGSO for a few hours before likely changing over to all sleet and then eventually freezing rain later this afternoon. KRDU/KFAY/KRWI will likely maintain freezing rain through this afternoon, although a lull looks likely at these sites sometime between 17 and 20Z. However, a final west to east burst of freezing rain (with rain likely at KFAY) looks likely later this afternoon before pushing east of central NC this evening. This burst may be accompanied with elevated instability and perhaps some elevated wind gusts at the sfc.

Otherwise, LLWS will develop at all sites later this afternoon and evening. Lingering moisture may produce -FZDZ at all TAF sites for a few hours after sunset. Additionally, dense fog may be possible for a few hours near the end of the 24 hour TAF period.

Outlook: Precipitation should depart all sites by Monday morning, and the rest of the outlook period is dry. IFR/LIFR ceilings are expected to continue with precipitation Sunday night before returning to VFR Monday morning.

CLIMATE

Record Low Maximum Temperatures:

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

January 29: KRDU: 27/2014

Record Low Temperatures:

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

January 30: KGSO: 4/1966 KRDU: 7/2014

RAH WATCHES/WARNINGS/ADVISORIES

Winter Storm Warning until 1 PM EST Monday for NCZ007-021>024- 038-039. Winter Storm Warning until 1 PM EST Monday for NCZ008>011- 025>028-040>043-073>077-083>086. Winter Weather Advisory until 1 PM EST Monday for NCZ078-088- 089.