textproduct: Raleigh

WHAT HAS CHANGED

As of 952 AM Friday...

* Forecast confidence has increased and a Winter Storm Warning is now in effect for the Northwest Piedmont, including the Triad/areas along and north of Interstate 85. Additional Watch to Warning upgrades are possible later today.

* The remainder of the forecast remains on track. Mild temperatures today ahead of a significant winter storm this weekend. Turning substantially colder early next week.

KEY MESSAGES

As of 952 AM Friday...

1) This is the time to prepare for the winter storm and very cold temperatures this weekend. Today will be relatively mild and mostly dry. The arctic front will move through the region tonight introducing very cold air for the weekend.

2) A combination of Winter Storm Warnings and Winter Storm Watches remain in effect for Saturday afternoon into early Monday for all of central NC with continued increasing confidence significant wintry precipitation from Saturday afternoon through early Monday morning. The focus will be on sleet and freezing rain. It will be dangerously cold with readings in the upper teens and 20s through much of the winter storm over the Piedmont.

3) Dangerously cold temperatures and wind chill values expected Monday night/Tuesday morning following the storm.

4) Very cold temperatures expected through much of the week.

DISCUSSION

As of 400 AM Friday...

KEY MESSAGE 1... Today is the time to prepare for the winter storm and very cold temperatures this weekend. Today will be relatively mild and mostly dry. A brief light period of rain is possible, mainly south and east this afternoon. Highs will be mostly in the 45- 52 range N to S.

The arctic front will dive south through the region tonight. Temperatures are expected to fall into the teens NE and 20s elsewhere by daybreak Saturday. Wind chill readings will fall into the single digits and teens late tonight through mid day Saturday. High temperatures Saturday should remain in the 20s north ranging into the mid 30s south. The winter precipitation should arrive in the west and north in the very late afternoon into the evening with snow and sleet at the onset. More on the storm below.

KEY MESSAGE 2... A combination of Winter Storm Warnings and Winter Storm Watches remain in effect for Saturday afternoon into early Monday for all of central NC with continued increasing confidence significant wintry precipitation from Saturday afternoon through early Monday morning. The focus will be on sleet and freezing rain. It will be dangerously cold with readings in the upper teens and 20s through much of the winter storm over the Piedmont.

A polar vortex will progress across Atlantic Canada Sat and wobble over the Labrador Sea through the remainder of the weekend. At its base, initially quasi-zonal flow across the lwr mid-latitudes, and a strong west-east-oriented upr jet across the OH Valley and nrn Middle Atlantic, will undergo amplification and back to swly, as a complex and net, positively-tilted trough progresses from the Rockies to the Great Lakes through mid MS Valley. That trough will be preceded by the deamplifying remnants of a mid/upr-level cyclone now paralleling the srn CA and Baja coast, which the models indicate will eject rapidly from the Baja Peninsula at 12Z Sat, to the srn Plains by 12Z Sun, then off the New England and nrn Middle Atlantic coast by 12Z Mon. The progression of the polar vortex, and amplification/backing of flow at its base and in its wake, will advect warm air centered around 850 mb rapidly newd and across the Southeast and srn Middle Atlantic during the latter half of the weekend, such that sub-zero 850 mb temperatures over cntl NC at 12Z Sat are forecast to markedly increase to between 10-15 C by Sun evening.

At the surface, a nearly 1050 mb, Arctic high, now centered along the MT/SK border with minus 30 F surface temperatures and dewpoints, will progress sewd to the upr MS Valley around 1045 mb by 12Z Sat, then ewd and across the Great Lakes and Northeast 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. Around its srn periphery, 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 the models indicate a third low will track across GA and the ern Carolinas. An associated surface wet bulb freezing line roughly along the NC coast on Sat will probably retreat nwwd across the NC Coastal Plain and ern Sandhills Sun afternoon-evening, then perhaps briefly into the Piedmont by or shortly after midnight Mon morning. This surface pattern will favor corridors of predominant precipitation-types that will likely include significant accumulation/accrual of sleet/freezing rain over cntl NC.

The sensible weather related to the synoptic pattern described above will feature an increasingly cloudy and cold, but dry most of Sat, with virga that will likely begin to reach the ground across the nrn Piedmont late in the afternoon or by evening. It will do so during a time when partial thickness values and top-down both suggest snow would, for the first few or so hours, be the predominant precipitation type, 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 brief changeover to all rain there Sun evening-early Sun night, and possibly into parts of the Piedmont including Raleigh, however briefly around midnight.

Forecast storm total liquid equivalent amounts are slightly lower than previously forecast and range from around 0.75" in the srn Coastal Plain, to around 1-1.25" across the Triangle, to 1.75-2" in the far nrn/nw Piedmont (Triad).

We are relatively confident in the aforementioned 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 the duration of when each will be predominant, and what precipitation rates are during each. The best estimate at the moment would be for 2-4" of frozen (ie. snow and mostly sleet across the far nrn/nw Piedmont, to around one inch frozen around the Triangle, to Trace frozen over the srn tier. Freezing rain accrual, which would be mostly on top of frozen, or mixed with any that may linger given the atypically cold and possibly ice nucleating cold nose, are forecast to maximize between 0.5-0.75" over the srn through ne Piedmont, just to the south and southeast of the max frozen totals over the nw Piedmont. That maximum will be surrounded by somewhat lighter accrual, but still mostly one quarter to one half inch, relatively lightest to the southeast and across the Sandhills and srn-cntl Coastal Plain, where both a changeover to plain rain by Sun evening and also lesser liquid equivalent amounts will likely result. An upgrade to Winter Storm Warning will likely be warranted for all of cntl NC later today, or tonight.

KEY MESSAGE 3... Dangerously cold temperatures and wind chill values expected Monday night/Tuesday morning following the storm.

A Cold Weather Advisory or Extreme Cold Watch or Warning may be needed.

The bitter cold is expected to arrive Monday and peak on Monday night and Tuesday. Actual lows may fall into the single digits, with wind chill readings Monday night and Tuesday morning likely from 5 below to 5 above zero. Actual highs Tuesday will only be in the upper 20s to mid 30s. This will be followed by lows of 10 to 18 on Tuesday night.

These temperatures may approach daily records, and daily highs may come close to record low daily maxima as well, especially if we have a persistent layer of snow/sleet/ice on the ground.

KEY MESSAGE 4... Bitter and possibly some record cold temperatures are expected through much of the week.

Current projections indicate extremely cold temperatures all week, with some moderation around mid-week, followed by another arctic surge into the region later in the week.

Some areas may generally remain below freezing all week, even during the daytime. The mildest day may be Wednesday when some areas may only push near 40. Overnight lows will be in the teens for most locations all week, with some readings in the single digits Monday night and again Friday night.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

As of 628 AM Friday...

12Z update: Light radar echo returns continue to stream across southern areas this morning, but very little has reached the sfc. Will continue to highlight -RA at KFAY in the near term and through the mid-afternoon. Light rain may also make it up to KRDU/KRWI for a few hours this afternoon before collapsing south of central NC this evening. Otherwise, have added nnely post-frontal gust potential at all sites later this evening. Periods of gustiness should continue through the end of the 24 hr TAF period.

Some MVFR stratus may make it up to KFAY this afternoon, but confidence isn't the highest as latest guidance has trended more towards keeping MVFR ceilings further south.

Outlook: Confidence continues to increase in a potential winter storm this weekend, which will likely bring a wintry mix of sleet and ice Sat night through Sun. Periods of LLWS may also be possible during this time with a strong low-level jet atop a surface stable layer. VFR conditions should return early next week.

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 Watch from Saturday afternoon through Monday afternoon for NCZ008>011-025>028-040>043-073>078-083>086-088- 089.