textproduct: Raleigh

WHAT HAS CHANGED

As of 400 AM Saturday...

* There will probably be a corridor of more sleet accumulation at the expense of freezing rain accrual than currently forecast, probably over a portion of the nw Piedmont. This probability is not explicitly forecast at this time; and it may not be until the nowcasting phase, when observational trends indicate when and where it may occur.

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

KEY MESSAGES

As of 400 AM Saturday...

1) Incoming, Arctic cold and wind today will feel as cold as teens, to single digits this morning over the nrn Piedmont and nrn Coastal Plain.

2) 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.

3) The coldest temperatures of the season (so far) are expected from Monday night through the end of the week, with daytime highs and overnight lows 20 to 25 degrees below normal. Near zero or sub-zero wind chills are expected at times later this week.

DISCUSSION

As of 400 AM Saturday...

KEY MESSAGES 1 and 2...

A polar vortex centered in 00Z/24th upr air charts along the QC/NL border will progress across the Labrador Sea and nw N. Atlantic this weekend. Some degree of confluent flow, favorable for the maintenance of underlying high pressure and cold air damming east of the Appalachians, will remain its wake. However, backing of that flow and a broad area of height falls ahead of a complex and net, positively-tilted trough that will progress from the Rockies to the Great Lakes through mid MS Valley --and one which will be preceded by the deamplifying and accelerating remnants of a mid/upr-level cyclone now nearing the coast of the cntl Baja Peninsula-- will cause that underlying Arctic high to weaken and retreat newd/ poleward during the latter half of the weekend. That pattern and backing of the flow aloft, and to swly in the mid-levels, will also cause mid-level warmth to surge newd across the Carolinas and Middle Atlantic, such that at or below zero 850 mb temperatures over cntl NC this morning will be replaced by Sun evening, by 10-15 C ones observed from TX to the Gulf coast this past evening.

At the surface, a 1045 mb, Arctic high now centered over MN and with minus 30-40 F surface temperatures, and up to 10 F lower surface dewpoints, will progress ewd and across the Great Lakes, nrn Middle Atlantic, and Northeast at around 1040 mb through 12Z Sun, then retreat poleward and weaken considerably to sub-1030 mb across Atlantic Canada by 12Z Mon. It 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. Around the srn periphery of the Arctic high, a pattern of Miller Type "B" or Miller Type "A/B" cyclogenesis will result, with a couple of initial lows forecast to track across the TN and OH Valleys (inland) and along and offshore the srn Middle Atlantic coast (coastal), respectively, during which time a third low will follow and track across GA and the ern Carolinas. 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. An associated surface wet bulb freezing line that will become established along the NC coast today will probably retreat nwwd across the NC Coastal Plain and ern Sandhills by Sun evening, then *possibly* into the Piedmont later Sun night-Mon morning. How quickly and far nwwd it can retreat will have ramifications on how much additional icing may result with a sw to ne-oriented band of briefly heavy frontal precipitation forecast to sweep across the region Sun evening-early Sun night.

The sensible weather related to the synoptic pattern described above will feature an increasingly cloudy and cold, but dry Sat, with virga that will likely begin to reach the ground across the nrn Piedmont by this evening. It will do so during a time when partial thickness values and top-down both suggest snow would be the predominant precipitation type, for the first few or so hours, and when most any snow would be able to accumulate - perhaps an inch or so over the nrn Piedmont, depending upon how quickly it can reach the ground. The time window for snow will be limited, however, as the aforementioned significant warming aloft (centered around 850 mb) noses rapidly newd and promotes a changeover to sleet overnight, and as strong QG ascent (related to the mid-level warming via WAA) increases throughout cntl NC.

Further, significant warming aloft will ensue Sun-Sun night, when forecast 850-700 mb partial thickness values are off-the-nomogram high at around 1600+ meters, with an associated general sse to nnw mixing with or changeover to a predominance of freezing rain, as those 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 changeover to all rain there Sun evening-early Sun night, and possibly into parts of the Piedmont overnight.

Forecast storm total liquid equivalent amounts remain similar to the previous forecast: near one inch over the srn Coastal Plain, where a significant portion of that will probably come from the frontal precipitation band when above freezing surface temperatures may support just rain Sun night, to 1.75-2" in the far nrn/nw Piedmont (Triad).

We remain confident in the aforementioned corridors of precipitation types that will be mostly ice and a combination of sleet/freezing rain over most of cntl NC, except mostly freezing rain and then rain across the Sandhills and srn to cntl Coastal Plain, 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 relatively unchanged from 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; and that corridor appears most likely over portions of the nw Piedmont.

Key Message 3...The coldest temperatures of the season (so far) are expected from Monday night through the end of the week, with daytime highs and overnight lows 20 to 25 degrees below normal. Near zero or sub-zero wind chills are expected at times later this week.

In the wake of the departing winter storm, very cold air will overspread the entirety of the East Coast. Across NC, this will translate to steady or falling temperatures through the day on Monday with highs ranging from the upper 20s to mid 30s. Monday night's low is projected to be the coldest night of the season so far as a 1032mb surface high makes its way into western NC. The combination of clear skies and light winds should yield lows in the lower single digits in the NW Piedmont, and in the low to mid teens in the southern Coastal Plain. Cold Weather headlines will almost certainly be needed during this time period as wind chill readings will plummet below 0 along the NC/VA border and hover in the lower single digits elsewhere. Temperatures should slowly moderate Tuesday into Wednesday but highs and lows will remain well below normal with highs both days in the low to mid 30s. Overnight lows in the low to mid teens.

A fast moving shortwave trough embedded within northwesterly flow aloft is forecast to cross the mountains into the Mid Atlantic on Thursday. At this point nearly all ensemble cluster solutions agree that this trough will swing through the area, although precip amounts (if any) remain in doubt. Given downslope drying, it's hard to support more than a slight chance mention of precip late Wednesday into Thursday along the NC/VA border. If nothing else, this trough will usher in a secondary reinforcing shot of cold air.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

As of 636 AM Saturday...

12Z Update: The cold front has cleared through central NC, behind which we continue to see some enely gustiness especially at KRWI/KFAY. Still expecting this to subside some this morning and through the rest of the 24 hr TAF period. Timing of winter precipitation still appears similar to what's described in the previous discussion below. However, latest guidance is trending towards hanging onto sleet a little longer than previously thought (especially at KRDU/KFAY/KRWI). These sites will transition to more- so freezing rain later Sunday and continue through Sunday evening. Ceilings will also fall to IFR early Sunday morning.

Previous discussion:

A strong cold front continues to sag south from VA this morning. As it clears through central NC the next few hours, expect winds to turn more enely and for periodic gusts of 20 to 25 kts (especially at KRWI/KFAY) through sunrise. Gusts will mellow some from sunrise through the end of the 24 hr TAF period, but remain largely from the nne.

A winter storm with varying mixed precipitation will start to arrive at the end of the 24 hr TAF period. At KINT/KGSO, forecast soundings would support a few hours of pure snow (and associated typical LIFR visibilities) for a few hours tonight before transitioning to sleet by ~06Z. At KRDU, sleet will be possible at onset, but a quick transition to freezing rain seems likely. KFAY/KRWI will likely start off as freezing rain near the end of the 06Z TAF period. MVFR ceilings will also accompany the incoming precipitation later tonight.

Outlook: Ceilings will further tank Sunday as winter precipitation continues through late Sunday evening/night. Additionally, extreme LLWS is expected Sunday night. Conditions will improve by Monday and VFR is expected thereafter, although gusty northwest winds will continue Monday during the daylight hours.

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 from 1 PM Saturday to 1 PM EST Monday for NCZ007-021>024-038-039. Winter Storm Warning from 7 PM Saturday to 1 PM EST Monday for NCZ008>011-025>028-040>043-073>077-083>086. Winter Weather Advisory from 7 PM Saturday to 1 PM EST Monday for NCZ078-088-089.