textproduct: Raleigh

WHAT HAS CHANGED

As of 330 PM Saturday...

* Liquid equivalent precipitation amounts for this weekend's storm have decreased slightly, but overall impact from sleet and freezing rain is still significant and little changed.

* No notable changes have otherwise been made to the ongoing forecast.

KEY MESSAGES

As of 330 PM Saturday...

1) Winter Weather headlines, and expected Moderate to Major Winter Storm Severity Impacts, remain unchanged for the winter storm that will affect central NC mainly tonight through Sunday night.

2) An extended period of well below normal temperatures is expected through the seven day forecast.

3) A period of light snow can't be ruled out on Wednesday night and early Thursday, with the best chance over northern and northeastern parts of the area.

DISCUSSION

As of 330 PM Saturday...

KEY MESSAGE 1... Winter Weather headlines, and expected Moderate to Major Winter Storm Severity Impacts, remain unchanged for the winter storm that will affect central NC mainly tonight through Sunday night.

Sleet and freezing rain should be the dominant precipitation types and account for the greatest and most significant accumulation. However, some snow is possible across the northwest and northern Piedmont this evening.

Discussion:

There has been little change to the overall synoptic pattern since the previous discussion. Most notable change in upper air analysis from GSO from 12Z to 18Z today is a decrease of 13 degC in the dewpoint depressions at 700 mb, indicating the moistening of the mid levels. However, there is still very dry air in place in the lower levels. As of 19Z, a 1040 mb surface high over the Great Lakes was ridging east and sewd into and across the mid-Atlantic and Carolinas. Across central NC, temperatures range from low 20s north to low 30s south, with some dewpoints of 0 to -10 degF spreading swd into the area.

Overview: Wswly flow aloft will become increasingly swly tonight through Sun night as the low over the Baja opens into a s/w trough and ejects ewd-enewd (moving across the Southeast and Carolinas Sun/Sun night) as it gets picked up by the amplifying longwave trough. At 850 mb, as the low strengthens and shifts east across the ARKLATEX and into the mid-MS Valley tonight, it will push the ridge axis out of the area, with WAA increasing from the southwest through Sun, such 850 mb temperatures are forecast to markedly increase by between 10-15 degC by Sun evening. The H85 low will subsequently lift newd from the mid-MS Valley to the OH Valley on Sun, while the jet strengthens east of the Appalachians before moving ewd across the are Sun night.

At the surface, the Arctic high will drifting ewd across the Northeast tonight, then lift nwd into and across Quebec and weaken Sun/Sun night. This high will be favorably strong and located and dammed east of the Appalachians for a significant (classical, diabatically-enhanced) cold air damming episode through much of Sun, before weakening and retreating poleward late Sun and especially Sun night. Despite its retreat and demise later this weekend, the magnitude of its related Arctic cold and dry, with associated sub- zero F surface dewpoints all the way into srn VA already this morning, may allow its influence to linger longer through the duration of the event than its increasingly unfavorable position and strength with time would suggest. Meanwhile, a pair of lows will develop along the srn and ern periphery of the wedge, one over the central Gulf Coast and the other off the Southeast US coast tonight/Sun. The leading low off the Southeast US coast will lift nwd along the Carolina and mid-Atlantic coasts Sun. A third low may develop north of or break off from the Gulf Coast low and lift nwd along and west of the Appalachians on Sunday, while the primary low tracks ewd across the Deep South and Southeast. It should then lift newd along the Carolina coast and meet up with the leading low off the Northeast US coast Sun night, while the nwrn low lifts into and ewd across the nrn mid-Atlantic. This surface pattern will favor corridors of predominant precipitation-types that will include and be highlighted by significant accumulation/accrual of sleet/freezing rain over cntl NC. The surface wet bulb freezing line roughly along the NC coast today will probably retreat nwwd across the NC Coastal Plain and ern Sandhills Sun afternoon-evening, then perhaps briefly into the Piedmont by midnight Mon morning.

Sensible Weather: This afternoon and evening, hydrometeors will have to overcome the very dry air to make it to the ground, and will initially serve to moisten the lower levels, falling as virga with perhaps some intermittent flurries across the northeast/north this aft/early eve. As the precipitation begins reaching the ground across the nw/nrn Piedmont this evening, partial thickness values and top-down both suggest snow would be the predominant precipitation type, for the first few hours or so. Most if any accumulating snow - perhaps an inch or so over the nrn Piedmont, depending upon how quickly it can reach the ground - will occur there during that limited time window. Expect a changeover to sleet as the aforementioned significant warming aloft (centered around 850 mb) noses rapidly newd and strong QG ascent (related to the mid- level warming via WAA) increases throughout cntl NC.

Overnight, precipitation types should transition to predominantly freezing rain and/or sleet and remain largely that into the day Sun. As the warm nose temperatures increase well above values to completely melt any snow/ice nuclei falling through it. Despite that complete melting, underlying cold nose temperatures are forecast to be atypically cold (~minus 10C) and sufficiently so to yield additional ice nucleation in the lowest couple of thousand ft AGL; and that may allow for periods of continued snow and sleet production, with crystal habits of the former that would favor columns/prisms/needles that would not accumulate very efficiently, particularly if accompanied by freezing rain as expected. The aforementioned nwwd retreat of the surface wet bulb freezing line into at least the Sandhills and srn-cntl Coastal Plain will favor a brief changeover to all rain there Sun aft/evening, and possibly into parts of the Piedmont overnight.

Accumulations: Forecast storm total liquid equivalent amounts have decreased a bit, now ranging from around 0.75-1.0" in the Sandhills and srn Coastal Plain, to around 1.0-1.25" across the Triangle, to 1.25-1.75" in the far nrn/nw Piedmont (Triad). We are still relatively confident there being corridors of precipitation types that will be mostly ice and a combination of sleet/freezing rain in cntl NC, but exact amounts remain somewhat in question given uncertainty with how long sleet over the Piedmont will be the predominant p-type before yielding to freezing rain; and accumulation of one will come at the expense of the other. Deterministic storm total frozen (mostly sleet) and freezing rain totals also remain similar to previous forecasts, but there will probably be a corridor where more sleet accumulates at the expense of freezing rain and may consequently reduce impacts, specifically those related to power outages. There is also still some uncertainty whether ice accrual across the southeast (srn Sandhills and srn-cntl Coastal Plain) will meet or exceed 0.25", where both a changeover to plain rain by Sun evening and also lesser liquid equivalent amounts are expected, thus have opted to keep them in the advisory.

Key Message 2... An extended period of well below normal temperatures is expected through the seven day forecast.

Temperatures will be well below normal from Monday onward. Some locations near the VA/NC border may not rise above freezing all week. Lows Monday night are expected to drop to the single digits to the north of US-64, with mainly lower-to-mid-teens elsewhere. Wind chills will get as low as below zero in the far north and single digits elsewhere. It appears likely that Cold Weather headlines will be needed Monday night, but forecast values aren't low enough at this point to support an Extreme Cold Watch. The climate section below includes records that may be in jeopardy. While normal highs in late January are in the low to mid 50s, expect widespread highs in the 30s on Tuesday. The Arctic air mass will modify slightly on Wednesday, but temperatures will still be well below normal with highs ranging from lower-30s in the far north to lower-40s in the far south. Lows Wednesday night will be mid-teens to lower-20s.

A shortwave trough moving through the southern Mid-Atlantic on Wednesday night will push another Arctic front through central NC, ushering in another bitterly cold air mass for late next week. Forecast highs on Thursday and Friday are only in the upper-20s to mid-30s. Breezy NW winds will make it feel even colder. Temperatures and wind chills on Thursday night will be especially cold, similar to Monday night. More Cold Weather headlines will likely be needed. Temperatures will start to modify a bit on Saturday but will still be well below normal.

KEY MESSAGE 3... A period of light snow can't be ruled out on Wednesday night and early Thursday, with the best chance over northern and northeastern parts of the area.

Guidance is consistent in showing the aforementioned shortwave (a "clipper" system diving SE out of Canada) swinging east across VA/NC on Wednesday night and early Thursday morning, inducing a surface low east of the coast, but confidence is much lower on whether it will still have enough moisture to squeeze out some precipitation across central NC. Temperatures both aloft and at the surface would certainly be cold enough to support all snow, and snow-to-liquid ratios would likely be quite high, but the question is whether it will be able to overcome the dry air mass in place or if precipitation will dissipate in the downslope flow as it crosses the Appalachians. The 12z GFS is dry, while the ECMWF has a small amount across the far NE and the CMC is wetter. A lot will depend on how much the trough is able to dig south into our area. Based on ensemble guidance and the track of the wave, the best chance looks to be across the northern Piedmont and northern Coastal Plain. The GFS and ECMWF ensembles backed off on their probabilities of measurable precipitation, so continue only slight chance POPs for now, but did expand them from the small area NBM had them.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

As of 650 PM Saturday...

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 KFAY: 30/2013

Record Low Temperatures:

January 26: KRDU: 10/1940

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

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.