textproduct: Raleigh

WHAT HAS CHANGED

As of 240 PM Thursday...

* Only a chance of light rain today into Friday mainly in the south.

* Confidence in a significant weekend winter storm continues to increase, with a greater focus on icing and sleet potential.

* Continued high confidence in bitterly cold temperatures from early Saturday through the middle of next week.

KEY MESSAGES

As of 240 PM Thursday...

1) Temperatures will be mild through tonight, then slightly cooler tomorrow, ahead of the Arctic front on Friday night. There is a chance of light rain from today into Friday, mainly across the south. This is the time to prepare for the winter storm expected this weekend.

2) A Winter Storm Watch including all of central NC, with increasing confidence in significant wintry precipitation from Saturday afternoon through early Monday morning.

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

4) Well below normal temperatures expected Monday night through Thursday.

DISCUSSION

As of 335 AM Thursday...

KEY MESSAGE 1...Temperatures will be mild through tonight, then slightly cooler tomorrow, ahead of the Arctic front on Friday night. There is a chance of light rain from today into Friday, mainly across the south. This is the time to prepare for the winter storm expected this weekend.

A baroclinic zone will settle near the NC/SC border through Friday, with periods of light rain possible, mainly across southern parts of central NC. Rainfall amounts are expected to be less than a tenth of an inch. After one day of above normal temperatures with highs today in the mid-50s to mid-60s and lows tonight in the lower-30s to lower- 40s, it will turn a bit cooler on Friday (highs mid-40s to 50). Skies are turning overcast across all of central NC as mid and high clouds move in from the SW, and this will continue through Friday. An Arctic front will then move through the area on Friday night as lows drop into the mid-teens to mid-20s and wind chills drop into the single digits and teens. Rain chances will come to an end in the SE on Friday night as we get a brief dry period in advance of the upcoming winter storm. Today and tomorrow are the time to prepare!

KEY MESSAGE 2... A Winter Storm Watch including all of central NC, with increasing confidence in significant wintry precipitation from Saturday afternoon through early Monday morning.

Overview: Latest models and ensemble members continue to support passage of an Arctic cold front N to S Fri night, introducing a frigid air mass as polar high pressure spans the Great Lakes region. Just aloft, the 850 mb front will dip SSE into the area Fri night and briefly stall across the Carolinas before slowly nudging back NNW, a key mechanism affecting precip types. As the surface front settles well to our S across GA/SC Sat, inverted troughing will develop from low pressure over the central Gulf coast NNE through E TN, the precursor to what will likely be a Miller B cyclogenesis pattern as the secondary low develop off the Carolina coast. Despite increasing PW spreading in from the SW Sat, precip during the first half of the day should be light and spotty, as the subcloud layer remains rather dry (including initially high surface dewpoint depressions) and the forcing for ascent modest. By late Sat and esp Sat evening, PW ramps up to 225% to nearly 300% of normal, while overrunning over NC strengthens with the arrival of the nose of a 35- 50 kt 850 mb SW jet. Upper level forcing for ascent will also strengthen, with increasing mid level height falls as a phased deep trough shifts into the central CONUS Sat night/Sun, along with a couple rounds of jet-induced upper divergence peaking Sat night and late Sun. Precip will finally wind down late Sun night into early Mon morning as the low level jet shifts to coastal NC and the coastal low takes over off the Northeast coast.

Precip timing: A prolonged period of mixed wintry precipitation is expected across central NC this weekend, with regional differences. In the northern Piedmont and Triad, precipitation is expected to begin Saturday afternoon as snow and sleet, transitioning to a mix of sleet and freezing rain by Saturday evening, with sleet occasionally heavy. Ice accumulations of one-quarter inch to just over one-half inch are possible through Sunday, with sleet and snow totals of 2 to 3 inches along and north of the I-85 corridor. Freezing rain will be the dominant precipitation type Saturday night through Sunday, resulting in moderate to potentially major impacts across the region. Precipitation should exit early Monday morning, though a brief burst of snow is possible on the back edge as dry air erodes the warm nose and a few flakes reach the surface. Across the Triangle, precipitation types will be especially challenging as two waves move through the region. Precipitation will begin Saturday afternoon as sleet, mixing with snow, then quickly transition to freezing rain and sleet Saturday night, with periods of heavy sleet producing minor snow/sleet accumulations. Freezing rain is expected to be the predominant precipitation type from Saturday night through Sunday night. A second surge of warm, moist air from the southeast on Sunday may bring periods of moderate to heavy freezing rain, with intermittent transitions to just rain during the afternoon and evening. As colder air pushes in from the west Sunday night into early Monday, precipitation should transition back to freezing rain and sleet. On the back edge of the system early Monday morning, a brief burst of snow mixed with freezing rain is possible before precipitation ends. Ice accretion totals remain a little uncertain given the mixed precipitation, but a quarter to three-quarters of an inch appears possible. Snow and sleet totals may range from around a half inch near I-95 to up to 1.5 inches near the I-85 corridor. Hazardous travel conditions are expected late Saturday night through Sunday evening, lingering into Monday morning. Across the Sandhills and Coastal Plain, precipitation is expected to begin Saturday night as freezing rain, with occasional sleet, and remain primarily freezing rain through Sunday afternoon. As warmer air advances northward with the second wave, precipitation may transition to mainly rain Sunday afternoon and evening. However, as colder air pushes in from the west late Sunday night into Monday morning, rain may change back to freezing rain before exiting the region. A few snowflakes may mix in across the northern Coastal Plain Monday morning, though no additional snow accumulation is expected. Ice accretion will be sensitive to precipitation type transitions, but around a quarter inch of ice is possible across much of the Sandhills and Coastal Plain. Sleet and snow accumulations should remain minimal, generally a dusting to a half inch especially on grassy and elevated surfaces. Hazardous impacts will depend on the extent of ice accretion prior to the arrival of warmer air and the intensity of freezing rain. Total liquid QPF is expected to range from 1.0 to 1.75 inches across the region. Surface temperatures Saturday will range from the upper 20s north to low 30s south. Saturday night temperatures will be upper teens to low 20s. Sunday will be a tad warmer with upper 20s north to low 40s creeping into the far southeast. By Monday morning, temperatures will be in the upper 20s NW to low 30s (below freezing) in the SE.

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

As the parent surface high tracks ewd across the lower MS Valley, a smaller, secondary area of high pressure in the northward extending ridge may break off and race newd across the TN Valley and into the central Appalachians Mon night. Low-level thicknesses will drop into the 1250-1260 meter range, nwly winds of 5-10 kts. There is still some uncertainty wrt wintry precip accumulations, which could impact temperatures Mon night, however for now, expect temperatures to fall into the single digits across much of the area by daybreak. However, lows of 10-15 degrees are more likely across the southeast portions of central NC. Additionally, with the continued breeze expected overnight, wind chill values as low as -5 degrees will be possible, especially across the north. Overnight lows may approach or exceed daily record values. A Cold Weather Advisory or Extreme Cold Watch or Warning may be needed.

KEY MESSAGE 4... Well below normal temperatures expected through Thursday.

Aloft, a s/w trough will move across the region Mon night, with the broad longwave trough generally remaining over the area thereafter. At the surface, aside from a lee trough in place on Tue, high pressure will generally dominate through the period. Not only will any lingering wintry accumulation have a tough time melting on Tue (with highs ranging from around 30 degrees north to mid 30s south) it could also help keep the temperatures muted despite expected clear skies and light swly breeze.

Unchanged from the previous discussion: Overnight lows may approach daily records, and daily highs may come close to record low daily maxima as well, especially if we have a persistent thick glaze of snow/sleet or ice. This extended cold weather will mean that whatever wintry precip accumulates will stick around through at least the middle of next week, with very slow melting or sublimation.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

As of 1250 PM Thursday...

24 hour TAF period: VFR conditions should prevail through this evening and into tonight. A band of mid-level clouds is overspreading the area from the WSW. As precipitation within the band falls, cloud bases may lower a couple/few thousand feet, with light rain eventually reaching the ground (most likely at KFAY beginning this aft/eve). May need to add some light rain into the KINT and KGSO TAFs this afternoon, but for now its a bit too dry in the lower levels. Expect light rain possible off and at KFAY through mid-day Fri, with cigs gradually lowering to MVFR Fri morning. There is a slight chance light rain could occur at the other terminals, however confidence is lower so have left mention out for now. Do not expect sub-VFR conditions outside of where rain occurs. Swly winds this afternoon will become light this eve/tonight and veer around to more nly. Continued veering to nnely/nely is expected Fri, increasing to around 5 kts by the afternoon.

Outlook: Confidence continues to increase in a potential winter storm this weekend, which will likely bring wintry precipitation and associated sub-VFR conditions from Sat eve through Sun night. 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 Watch from Saturday afternoon through Monday afternoon for NCZ007>011-021>028-038>043-073>078-083>086-088- 089.