textproduct: Raleigh

SYNOPSIS

Continental Polar high pressure will migrate across the Southeast through early Thursday. A dry cold front will lead Arctic high pressure across and offshore the Middle Atlantic later Thursday through Friday, while weak waves of low pressure will track along a wavy frontal zone from the northern Gulf to offshore the Southeast coast.

NEAR TERM /THROUGH TONIGHT/

As of 330 AM Wednesday...

Strengthening, wswly flow will prevail from the Southwest to the Middle Atlantic, between a sub-tropical high over Cuba and an elongated, positive tilt trough that will extend from a strongly- amplifying shortwave over ON and the upr Great Lakes wswwd to another major shortwave over the srn Rockies and Southwest. Within that fast, quasi-zonal flow regime, strongly rising heights and strong and deep subsidence over cntl NC this morning will become neutral or tend toward weakly falling ones by Thu morning. It will also advect a plume of cirrus/cirrostratus that will stream from the Rio Grande this morning to the Carolinas this evening.

At the surface, an occluded front now approaching the Savannah Basin, the one which swept ssewd the in-situ CAD airmass that was over cntl NC on Tue, will stall and separate 1022 mb continental Polar high pressure over the Deep South from a separate high pressure cell over NC/VA.

While the associated sensible weather for cntl NC will be mainly sunny/clear and unseasonably chilly-cold, veering low-level flow may cause stratocumulus now wrapping clockwise from ern NC through SC to expand nwd across parts of the srn Piedmont and srn Sandhills through this evening, with associated partly cloudy skies. Calm and mainly clear tonight will favor strong radiational cooling atop saturated soil from yesterday's rain, supportive of at least patchy fog development Thu morning, with highest probability of occurrence and lowest visibility restrictions in an arc from the srn Piedmont to the cntl Coastal Plain.

SHORT TERM /THURSDAY AND THURSDAY NIGHT/

As of 425 AM Wednesday...

* Increasing clouds and milder/more-seasonable Thu

* Precipitation developing, including a rain/snow mix that will probably transition to all snow for several hours across the nrn Piedmont, late Thu night-Fri morning

The mid/upr-level height gradient will tighten across the lower mid- latitudes and contribute to the intensification of a powerful, 150- 175 kt upr-level jet across the OH Valley and Middle Atlantic, where u-wind anomalies are forecast to reach 3 sigma. It will do so in response to the pivot of a negatively-tilted, nrn branch polar shortwave trough across and offshore the Middle Atlantic and Northeast, and as the srn branch polar and sub-tropical streams merge and phase downstream of a couple of ejecting shortwave troughs now centered over the Southwest and near 25N/125W, respectively. A strengthening low/mid-level WAA regime, and also frontogenesis, will result within the elongated jet entrance region, equatorward of the jet core and across the Southeast.

At the surface, a dry cold front will lead Arctic high pressure across the OH Valley and Middle Atlantic Thu and swd into NC by Fri morning. The Arctic high will be favorably strong (~1028-1030 mb) and located (over PA) for wintry precipitation in cntl NC Thu night but then quickly weaken and progress offshore on Friday.

After a mostly sunny start to Thu, considerable mid and high-level moisture, and associated combination of altocumulus and cirrostratus ceilings, will overspread cntl NC through the afternoon and evening. Despite the increasing clouds, temperatures Thu will probably moderate into the 50s courtesy of both modification of the cP high and airmass over the Southeast, and also wly/downslope flow at 850 mb.

Lift, initially centered in the mid-levels between 700-500 mb, will cause a precipitation shield to blossom across the TN Valley and expand quickly newd and across the srn Appalachians and especially wrn Carolinas Thu night. Partial thickness and point forecast soundings (top-down) suggest that as the precipitation shield begins to overspread cntl NC Thu night, its nrn periphery will probably transition from a mix of rain and snow across the nrn Piedmont to all snow for several hours roughly along and north of I-85 Fri morning, as thermal profiles trend toward deep, near freezing isothermal, including into the near surface layer owing to diabatic contributions from both evaporational cooling and melting of snow. A very light coating may begin to accumulate over the nw Piedmont by sunrise, with some additional light accumulation through the morning, before thermal profiles trend above freezing amid continued mid-level WAA and a lack of continued surface cold/dry advection from the transitory and weakening surface high.

LONG TERM /FRIDAY THROUGH TUESDAY/

As of 130 AM Wednesday...

* Highest chances for precipitation still Friday into early Friday night, with lower confidence in chances Saturday through Monday.

* There may be some snow accumulation across the northern Piedmont and northern Coastal Plain Fri morning before mixing with and changing over to rain Fri afternoon.

Aloft, a s/w will track ewd across the region Fri and offshore Fri night as another s/w drops sewd into the cntl Plains. The second s/w may finally help the longwave trough progress ewd across the cntl and ern CONUS Fri night through Sun. Central NC should be under the influence of wswly to swly flow through at least Sun. The second s/w may move across the region Sun night/Mon, but model solutions vary wrt the timing, amplitude, and available moisture. Another nrn stream s/w may track across the Great Lakes/OH Valley and Northeast US Tue/Tue night. At the surface, Arctic high pressure over the Tri- State area Fri morning will shift ewd and off the New England coast on Fri, while continuing to ridge swwd across the region. Meanwhile, an area of low pressure will eject enewd across the Southeast US and lift newd off and along the Carolina coast, along a quasi-stationary front Fri/Fri night. There could be another wave/low that develops and tracks along the front Sat/Sat night, following a similar track as the previous low. Farther west, an area of low pressure may develop off the srn Rockies over the srn Plains Sat night in response to the s/w aloft. This low should track ewd across the lwr MS Valley and Deep South on Sun, however there are considerable model differences with this system that will influence its potential impacts, or lack thereof, for central NC Sun night/Mon.

Precipitation: Fri still looks like the wettest day/eve in the extended forecast. There is an increasing signal for another possible round of precipitation between late Sat and Sun aft, highest chances across the south and east. Sun night/Mon remains fairly uncertain, but there is at least a slight chance for some precipitation during that time.

P-types: While the majority of the precipitation from Fri to Mon is expected to be liquid, some wintry precipitation is possible with each of the rounds, mainly during the early morning timeframe and especially across the northern portions of central NC. For the Friday system, some snow may linger into the morning across the nrn Piedmont and nrn Coastal Plain, with up to 0.5 inch accumulation possible, before mixing with and becoming all rain during the afternoon. Based on current timing, this could impact the morning commute for locations along and north of the US-1 corridor. As the low moves away Fri night/early Sat, there could be a brief period of some fzra over the nrn Piedmont. Given the continued low confidence and model differences, will hold off on specifics for Sat night through Mon.

Temperatures: Highs will remain below normal Fri-Tue. Fri will the coldest day, with highs ranging from mid 30s NW to mid 40s SE, some 15-20 degrees below normal. Highs Sat and Sun ranging from mid 40s to around 50 degrees. Lows should generally remain near to slightly below normal, generally ranging from upper 20s to low/mid 30s, with the exception of Mon night, which could be several degrees lower, but uncertainty remains.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

As of 710 AM Wednesday...

While mainly VFR conditions are forecast for cntl NC through 12Z Thu, MVFR to low VFR stratocumulus now wrapping clockwise from sern NC into SC may expand in coverage with ceilings across srn NC this afternoon through early evening, including near and especially just southwest of FAY (ie. from LBT to MEB). Surface high pressure will otherwise build across the Southeast and favor light and variable surface winds in cntl NC. Strong radiational cooling tonight, atop saturated soil from yesterday's rain, will probably promote the development of at least patchy fog Thu morning, with highest probability of occurrence and lowest visibility restrictions in an arc from near CLT to FAY to OCW.

Outlook: An energetic southern stream jet will send a few waves of lift and precipitation --and flight restrictions-- into cntl NC late this week through early next week. The first will arrive and include a chance of snow over the Piedmont late Thu night-Fri.

RAH WATCHES/WARNINGS/ADVISORIES

None.