textproduct: Raleigh

WHAT HAS CHANGED

As of 345 AM Friday....

A Winter Storm Warning has been issued for all of central NC in anticipation of a major winter storm for the Carolinas.

A Cold Weather Advisory has been issued for all of central NC and will be in effect tonight, through Saturday, and into Saturday night.

An Extreme Cold Watch remains in effect for Saturday night. A Cold Weather Advisory and/or an Extreme Cold Warning will likely be needed in subsequent forecast updates.

KEY MESSAGES

As of 345 AM Friday...

1) A Winter Storm Warning will be effect this afternoon/evening until Sunday morning for all of central NC. Accumulations range from 3-7 inches with the possibility of localized amounts of 10+ inches.

2) A prolonged Arctic air mass in place will result in well below normal temperatures across central NC through much of next week.

3) There is a chance for precipitation mid-week, but the timing, temperatures, and overall amounts remain uncertain at this time.

DISCUSSION

As of 345 AM Friday...

KEY MESSAGE 1... A Winter Storm Warning will be effect this afternoon/evening until Sunday morning for all of central NC. Accumulations range from 3-7 inches with the possibility of localized amounts of 10+ inches.

A major winter storm is likely for the Carolinas beginning later this evening and lasting through Saturday night into Sunday morning. The main driver in this winter storm is seen on satellite imagery this morning as an elongated northern-stream trough axis stretching across the International border from the Northern Atlantic into the northern Plains. The basal portion of this trough is forecast with a high degree of certainty to pivot across the Mid-Mississippi and Ohio Valleys tonight before closing off over the southern Appalachians Sat and swing through the Carolinas into Sun.

Snow is expected to start falling as early as this afternoon in the Piedmont of VA and slowly fill in southward into the NC Piedmont this evening. This will be driven by low-lvl mass response from the approaching trough resulting in WAA, weak FGEN, and moist isentropic ascent creating lift within the dendritic growth zone. This beginning phase of the winter storm will be mostly light and patchy snow, but high snow-liquid-ratios (SLR) and temperatures dropping into the 20s and teens, will allow snow will accumulate quickly.

Conditions should rapidly deteriorate Sat morning as the anomalous deep trough for this latitude begins to close over the southern Appalachians. Moist isentropic ascent will become deeper/stronger through the morning and into the evening resulting in more widespread light snowfall. Areas of moderate to heavy snowfall appear likely to develop where an H850 FGEN band strengthens over the Carolinas. Where this develops will likely be the first of two potential significant areas of precipitation, but even at this time range, models are struggling with its placement/magnitude. The general consensus appears to be somewhere near the NC/SC border and arcing generally northeast into portions of central NC. Where this initially develops may be very slow moving and produce an area of 8-12 inches of snowfall through Sat evening within a larger area of 3-7 inches.

By Sat evening, intense lift is expected to reach the Gulf Stream and rapidly deepen a broad area of low pressure just off the NC coast in a instant occlusion surface low pattern. This is when the H850 FGEN band is expected to shift more rapidly eastward and pivot into a north-south orientation within the H850 cold conveyor belt. Where this sets up, probably along and-or-east of the I-95 corridor towards the Carolina coast, remains uncertain. This area will likely result in another maximum of snow amounts where 10-12+ inches will be most probable. Whether this occurs in our area or not is difficult to say and depends on how rapidly the low off the coast deepens, slow development would keep this axis farther inland. Also during this time, sustained winds of 20 mph and gusts 25 to 35 mph will likely create blowing and drifting snow, especially along and east of I-95 corridor, and may reduce visibility to less than 1 mile. Brief blizzard conditions can not be ruled out. Sporadic power outages may also occur, but high SLR should limit accumulations in trees and prevent this from being a larger concern.

When all is said and done as precipitation moves out of the area Sun morning, storm total amounts should range from 3-7 inches in most locations and still result in moderate travel impacts, even in the absence of the higher end amounts due to the exceptional cold temperatures and efficient accumulations. Major impacts to travel and infrastructure should be expected where the significant snowfall occurs and travel will become dangerous or even impossible Sat into Sun morning. Blowing and drifting snow may limit accumulations towards the low-end scenarios in spots.

KEY MESSAGE 2... A prolonged Arctic air mass in place will result in well below normal temperatures across central NC from today through much of next week.

Cold weather headlines span from tonight into Sunday morning. The Extreme Cold Watch will likely need to be upgraded to a Cold Weather Advisory and/or Extreme Cold Warning for Saturday night with subsequent forecast updates.

A strong Arctic high tracking south from south-central Canada across the Plains today and Saturday will continue ridging ewd into the mid- Atlantic/Northeast US, with persistent nely flow and CAA across central NC. The parent 1044 mb Arctic high will weaken to about 1035 mb as it drifts esewd across the ARKLATEX Sat/Sat night, while the ridge axis will continue to extend nnewd across the Great Lakes and Quebec. The high will continue to weaken and modify as it moves ewd along the cntl Gulf Coast Sun/Sun night. With the high over the srn Plains/lwr MS Valley trying to build ewd toward the East Coast and an area of low pressure rapidly deepening off the NC coast Fri night through Sun, the pressure gradient across central NC will increase, resulting in strong, gusty winds, even overnight. Sat/Sat night will be especially blustery, with gusts of 25-35 mph possible under continued CAA. Winds will back to more nly then nwly on the back side of the low as it begins to move newd away from the coast on Sun, while still deepening. The pressure gradient should gradually relax Sun night through Tue as the high ridges in from the south.

There is still some uncertainty wrt exactly how cold it will be, as some guidance indicates the coldest air may get hung up west of the Appalachians, and thus the cold air will have to spill over the nrn Appalachians and advect swd into the area. Regardless, the temperatures and wind chill values will still be dangerously low Sat and Sun nights.

Forecast highs today range from upr 20s/low 30s near the VA border to mid 40s south, which is 8 (SW) to 19 (NE) degrees below normal. Lows tonight will be in the low/mid-teens to low 20s. If the winds do indeed pick up late tonight as expected, wind chill values could drop into the low single digits to low teens around daybreak Sat, which would meet cold weather advisory criteria. Expect a noticeable drop in temperatures for Sat/Sat night, with highs ranging from upper teens to mid 20s and lows dropping into the 11-16 degree range. The blustery winds will make the highs feel more like 4-12 degrees and the lows feel like 2 to -5 degrees. Sunday should still be blustery early, with clearing skies and winds gradually decreasing through the aft/eve hours. Temperatures Sun/Sun night could be impacted by the snowfall. For now, expect highs in the upper 20s to mid 30s. Expect lows Sun night in the 7-13 degree range. With an expected continued light breeze, wind chill values of -2 to 10 above zero are possible. Temperatures will moderate through mid-week, but remain largely below normal for the remainder of the extended forecast period.

KEY MESSAGE 3... There is a chance for precipitation mid-week, but the timing, temperatures, and overall amounts remain uncertain at this time.

Some of the medium-range model solutions show a srn stream s/w and nrn stream s/w interacting with one another over the MS Valley then tracking ewd across the Appalachians an East Coast between Tue night and Thu night. However, there are still significant differences wrt timing, location, and strength of the waves, which then impact the precipitation types and amounts. It is still too early for specifics at this time given all the uncertainties above.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

As of 1240 AM Friday...

There is reasonably high confidence in VFR conditions to start the TAF period. Clouds in the mid to high levels will gradually lower this afternoon and evening with the approach of the well advertised winter storm. As low pressure develops to our south and the pressure gradient tightens, we should see winds shift from ENE to NE tonight, with some gusts toward the end of the period to around 18-20 kt. Guidance depicts light snow developing late in the TAF period, mainly between 00z-06z, perhaps earliest at GSO and INT. Confidence was high enough to include IFR light snow at these terminals, but overall confidence was not high enough at the other terminals given disagreement in the predicted snow shield. The better chance of sub- VFR visibilities in snow at all terminals will be after 06z.

Outlook: Widespread light to at times moderate snow is expected by early Saturday morning and continuing until tapering off early Sunday. LIFR-MVFR restrictions are expected at all terminals, along with NNE gusts up to 30 kt. Winds subside Sun aftn/eve, along with a return to VFR.

CLIMATE

Record Low Temperatures:

January 31: KGSO: 3/1966, KRDU: 3/1966, KFAY: 9/1934 February 1: KGSO:-4/1936, KRDU: 8/1981, KFAY: 1/1936

Record Low Maximum Temperatures:

February 1: KGSO: 27/1971, KRDU: 28/1900, KFAY: 32/1981 February 2: KGSO: 30/1951, KRDU: 31/1994, KFAY: 31/1948 February 3: KGSO: 23/1996, KRDU: 24/1996, KFAY: 30/1961

RAH WATCHES/WARNINGS/ADVISORIES

Winter Storm Warning from 4 PM this afternoon to 7 AM EST Sunday for NCZ007>010-021>026-038>041-073>076-083-084. Cold Weather Advisory from 1 AM Saturday to 1 AM EST Sunday for NCZ007>011-021>028-038>043-073>078-083>086-088-089. Extreme Cold Watch from late Saturday night through Sunday morning for NCZ007>011-021>028-038>043-073>078-083>086-088-089. Winter Storm Warning from 10 PM this evening to 1 PM EST Sunday for NCZ011-027-028-042-043-077-078-085-086-088-089.