textproduct: Raleigh

WHAT HAS CHANGED

As of 150 PM Friday...

* Snow may briefly mix with mostly rain this afternoon and evening, but little to no impacts from accumulations are expected.

* Wind Advisory has been issued for a majority of central NC and will be in effect from 4 AM until 7 PM on Saturday.

* Confidence is low on bitter cold wind chills tonight, but may result in impactful or dangerous cold early Sat morning.

KEY MESSAGES

As of 150 PM Friday...

1) A vigorous trough will bring a period of precipitation late this afternoon and evening, where a 1-2 hour period could mix or change to snow along/north of US-64 and I-85

2) A Wind Advisory will be in effect for most of central NC Saturday as strong/impactful winds are expected behind a cold frontal passage.

3) Unseasonably cold wind chills are expected tonight into Sunday morning, but confidence is low on duration/magnitude for a Cold Weather Advisory at this time.

DISCUSSION

As of 150 PM Friday...

KEY MESSAGE 1... A vigorous trough will bring a period of precipitation late this afternoon and evening, where a 1-2 hour period could mix or change to snow along/north of US-64 and I-85

A vigorous upper-level trough presently over the lower OH valley will dive into the central NC region late this afternoon and evening. Impressive height falls will favor strong lift over the region this evening. Upstream radar returns are showing a nice blossoming of precipitation across KY/VA and this precipitation is expected to reach our NW areas such as the Triad around 3 pm. Light snow has even been reported upstream in southwest VA. The precipitation band will mainly move across areas along and north of US-64, but some light rain may reach as far south as Fayetteville and the Sandhills. The main time frame for some light precipitation of a tenth of an inch or less is between 3 pm and 9 pm, tracking from NW to ESE from the late afternoon into the mid-evening hours for portions of the Triad, Triangle, and central to northern Coastal Plain. For areas along and north of US-64 and I-85, a rain/snow mix or brief changeover to some heavy wet snowflakes will be possible over a 1-2 hour window. Forecast soundings indicate a narrow time period where strong forcing in the dendritic growth zone and diabatic cooling could favor this potential. Snow chances will largely be driven by precipitation rates. A light dusting or less of snow accumulation will be possible in these northern areas, mainly on grass/elevated surfaces given temperatures in the 40s this afternoon. Some brief low visibilities are possible on area roadways for the evening commute, but overall impacts will be limited. Once the precipitation comes to an end this evening, a strong Arctic cold front will move through tonight and overnight. Wind gusts will start to pick up early Sat morning, producing wind chills in the single digits to lower teens by early Sat morning.

KEY MESSAGE 2... A Wind Advisory will be in effect for most of central NC Saturday as strong/impactful winds are expected behind a cold frontal passage.

Behind the clipper wave, a highly amplified mid/upper level pattern is expected from the trough over and off the East Coast and the anomalous ridge over the Great Plains. At the surface, an area of broad low pressure is expected to rapidly deepen to a sub-990mb low just north of Bermuda, while at the same time strong > 1030mb high pressure over the Upper Great Lakes shifts into the Ohio Valley by Sat afternoon.

Strong subsidence on the backside of the slowly departing trough axis will result in mostly clear skies during the daylight hours and favor efficient momentum transfer through the boundary layer. Although the leading edge of the cold front is expected to shift across the region Fri night, point soundings suggest a very shallow stable layer will keep the stronger winds just above the surface until after 4 AM.

After 4 AM, strong pressure rises, on the order of 4-6mb per 3hrs, are expected to shift into the NW Piedmont and erode the very shallow stable layer at the surface before sunrise, producing strong winds of around 35 kts. Strong wind gusts should rapidly develop across the rest central NC through the early morning hours as daytime heating mixes into the elevated mixed layer just above the surface. Widespread wind gusts of 25-35 kts should be expected. Clear skies and abundant insolation, should result in efficient momentum transfer from the top of the mixed layer and may result in infrequent gusts of 40-45 kts, strongest from the mid-morning to early afternoon.

KEY MESSAGE 3... Unseasonably cold wind chills are expected tonight into Sunday morning, but confidence is low on duration/magnitude for a Cold Weather Advisory at this time.

A ~1034 mb Arctic high will track SSE from the upper MS Valley to the OH Valley, then continue ewd across the Great Lakes/OH Valley Sat/Sat night. The high will ridge esewd across the Appalachians and into the mid-Atlantic/Carolinas as it moves across the OH Valley, with nwly flow advecting Arctic air into the area.

Strong winds behind the fropa tonight may couple with bitter cold temperatures in the teens and 20s to produce wind chills in the single digits to near zero across the Piedmont and northern/central Coastal Plain. Confidence is low in this scenario since cold air typically takes noticeably longer to progress across the southern/central Appalachians than hi-res guidance indicates, and results in warmer wind chills. Current forecast indicates wind chills in the low teens to 20s will be most common, but briefly may drop below 10 to around 5 degrees.

Sat night into Sun morning will likely see air temperatures in the teens to near 20 degrees and with some continued light stirring may result in wind chills in the teens to as low as 5 degrees. Confidence is higher that a Cold Weather Advisory may be needed during this time for the Coastal Plain (where advisory criteria is 10 degrees).

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

As of 1240 PM Friday...

VFR conditions continue this afternoon ahead of an approaching clipper system. Upstream snow/rain showers continue to spread east across the Blue Ridge mountains. Guidance still brings showers into our Triad around ~20Z, KRDU around ~22Z, and KRWI by ~23Z. Not entirely confident any precipitation will make it as far as KFAY, but added a Prob30 to handle for some potential for rain showers later this afternoon/evening. Think the dominant ptype at KINT/KGSO/KRDU/KFAY should remain all liquid rain. However, there is still a chance we could see a rain/snow mix at KRWI (and a lesser chance at KINT/KGSO) for a few hours. Overall though, other than brief reductions in visibility, impacts to airports should be minimal this afternoon and evening.

The line of showers should dissipate through early tonight ahead of a strong arctic front. Wind gusts are still expected to pick up overnight from the northwest and peak in the mid 30s to around 40 kts Saturday morning (highest chances for high end wind gusts at KINT/KGSO/KRWI). Wind gusts will subside later Saturday afternoon, but still mix some even into the evening period.

While traditional LLWS is not likely, mechanical low level turbulence may be possible Saturday morning especially at KINT/KGSO.

Outlook after 00z/Sun: Winds will diminish Saturday night and Sunday. VFR conditions expected Sunday into early next week.

RAH WATCHES/WARNINGS/ADVISORIES

Wind Advisory from 4 AM to 7 PM EST Saturday for NCZ007>011- 021>028-038>043-073>077.