textproduct: Raleigh

WHAT HAS CHANGED

As of 655 PM Wednesday...

* The Winter Weather Advisory was expanded earlier to include Anson, Richmond and Scotland counties.

* The threat of snow has greatly diminished across the area. Any wintry precipitation is now expected to be mostly light freezing rain or freezing drizzle tonight.

* Increasing confidence for strong winds on Saturday.

KEY MESSAGES

As of 655 PM Wednesday...

Winter Weather Advisory started at 6 PM and is in effect through 10 AM Thursday.

1) Rain this evening will transition to drizzle and freezing drizzle overnight before coming to an end Thursday morning. Minor impacts are expected.

2) Light rain may mix with snow/sleet Friday evening, no accumulations are expected.

3) Strong winds should be expected on Saturday behind a cold frontal passage Friday night.

DISCUSSION

As of 655 PM Wednesday..

KEY MESSAGE 1...Rain this evening will transition to drizzle and freezing drizzle overnight before coming to an end Thursday morning. Minor impacts are expected.

With the evening update to the forecast, the primary change was to remove all mention of snow from the forecast. Looking at cloud-top temperatures, there will be enough warming aloft to prevent the generation of ice crystals - this usually requires a temperature of - 10 to -12 C. With cloud tops remaining warmer than this, it appears that all precipitation should fall as liquid. As the surface drops below freezing from north to south, this will also result in a transition of precipitation type from rain to freezing drizzle. With all locations forecast to fall into the 20s tonight, eventually everywhere should have precipitation change over to freezing drizzle. Precipitation chances fall under 60 percent after 4am Thursday, with precipitation chances confined to locations along and east of US-1 by 7am and ending everywhere by noon. The chance of precipitation and precipitation amounts are highest farther to the south and east, but it appears that the locations that will receive the most ice accumulation will generally be along US-64 to the east of the Triangle.

KEY MESSAGE 2... Light rain may mix with snow/sleet Friday evening, no accumulations are expected.

A low-amplitude clipper wave embedded within northwesterly flow is expected to shift across the Ohio Valley Fri morning then push through VA/NC Fri evening. This feature has trended south in the GFS/CMC deterministic runs and closer to the persistent ECMWF solution. Ensembles from their respective global models show higher probabilities of measurable precipitation across the northern Piedmont to the VA border, more in-line with the best DPVA associated with the clipper.

To determine p-type, a top-down method was applied. Point soundings suggest sufficient saturation within the ice-crystal growth region and moderate omega to support snow. However, most deterministic models depict a dry and warm sub-cloud layer (max wetbulb of 1-4C in lowest 500mb from 18-03z) that will likely result in partial or complete melting to sleet or rain. Surface temperatures are also expected to remain above freezing until after 00-03z, after which time precip should be moving out.

Light accumulations of rain will be possible, but poor diurnal timing and warm near-surface layer will likely prevent any frozen accumulations at this time.

KEY MESSAGE 3... Strong winds should be expected on Saturday behind a cold frontal passage Friday night.

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 evening.

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. Wind gusts should rapidly develop through the early morning hours as daytime heating mixes into the elevated mixed layer just above the surface from CAA behind the front. Widespread wind gusts of 25-30 kts should be expected with both NAM and GFS highlighting similar intensity averaged through the mixed layer. 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 30- 40kts, strongest from the mid-morning to early afternoon.

AVIATION /00Z THURSDAY THROUGH MONDAY/

As of 655 PM Wednesday...

Aviation conditions will remain poor over all of central NC through at least mid afternoon Thu, due to a slow-moving frontal system over the Carolinas, and the worst and longest-lasting adverse conditions will be over the S and SE (especially FAY). AT INT/GSO, MVFR cigs are expected to drop to IFR by 03z, with periods of MVFR vsbys overnight and spotty freezing drizzle from 03z to around 08z, at which time we should see MVFR cigs and VFR vsbys, lasting much of Thu until VFR conditions return around 21z. Elsewhere at RDU/FAY/RWI, the current IFR/LIFR cigs and MVFR/IFR vsbys will persist through mid morning Thu, with the current shield of more steady rain trending down to drizzle or freezing drizzle lasting from late evening through daybreak -- mainly 03z-08z NW (RDU) to 07z- 12z SE (FAY). A gradual trend to MVFR cigs and VFR vsbys is then expected after 11z at RDU and RWI, but not until after 19z at FAY. Accumulation of a light glaze is possible at all sites tonight (mainly late evening through daybreak Thu), but due to the light amounts and/or short duration at any given location, the impacts should be fairly minor, with a trace to a few hundredths of an inch of glazing as the most likely outcome. Surface winds will stay mainly from the NE or NNE sustained at 8-13 kts, and a few gusts to 15-20 kts are possible at FAY on Thu 13z-20z.

Looking beyond 00z Fri, any lingering sub-VFR conditions (most likely at FAY) after 00z should improve to VFR by mid evening Thu. VFR conditions should then persist through Mon, although a quick- moving system may bring brief sub-VFR cigs and a light wintry mix mainly NE (RDU/RWI) Fri night. Additionally, brisk and gusty winds are likely Sat, which may induce mechanical turbulence.

RAH WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from 6 PM this evening to 10 AM EST Thursday for NCZ007>011-021>028-038>043-073>078-083>086-088-089.