textproduct: Raleigh

SYNOPSIS

A backdoor cold front will hold just to our south and west through much of today, then lift back north as a warm front tonight. A strong cold front will sweep across the state on Monday, bringing dry and cool weather through Friday.

NEAR TERM /THROUGH TONIGHT/

As of 140 AM Sunday...

* Cloudy, dreary, and drizzly in the Triad today within the developing cold-air-damming regime.

* Patchy fog, potentially dense at times, will be possible along and north of the I-85 corridor late this evening into the early overnight hours.

An in-situ CAD regime is beginning to take shape this morning and will likely persist throughout the much of the day. Surface winds are predominately out of the NE-E with a stalled boundary draped from western NC arcing back towards coastal SC. The parent 1025mb surface high is located over the Chesapeake Bay and is forecast to continue to shift SSE off the Carolina coast this afternoon and towards Bermuda tonight. Despite the unfavorable surface high placement, the in-situ CAD is expected to be locked in by low overcast and drizzle/light-rain, especially over the Piedmont, developing early this morning and persist through tonight. High temperatures with the latest forecast have leaned heavily on the 10th percentile of the NBM and blend of statistical guidance resulting in a range from upper 40s to upper 50s. Depending on the intensity and onset of the steady drizzle and light rain over the Piedmont, hi-res guidance suggest highs may be around 5 degrees cooler compared to the current forecast.

Light rain and drizzle is expected to shift east and weaken through the evening hours. At the same time, the front to our south will begin to lift northward into the overnight hours. Moisture advection ahead of the warm front will likely result in rapidly lowering cloud bases after 03z and may result in patchy to areas of fog, potentially dense, as this cloud layer lowers to the ground. The most likely area will be the climatologically favored areas along and north of the I-85 corridor, but could be as far south as the Triangle. A non-diurnal temperature curve is expected tonight as the warm front gradually mixes northward into Mon morning, although this warming may be slower than forecast guidance suggest which typically mixes out the CAD regime too quickly. Forecast refinements will likely be needed once the strength of CAD develops.

SHORT TERM /MONDAY AND MONDAY NIGHT/

As of 230 AM Sunday...

* Pattern change with the passage of a strong cold front, which will bring gusty winds, mostly light precipitation, and return temperatures to below normal.

* Southwest wind gusts of 20-30 mph will be possible ahead of the cold front with frequent 30-35 mph northwest winds after its passage.

* Low relative humidity and strong, veering wind gusts may result in increased fire danger to near Red Flag criteria behind the cold front Mon afternoon. Please see Fire Weather section below for more information.

The wedge-front is expected to slowly mix northward into VA Mon morning, with even the slowest guidance suggesting it will clear the Triad around 12-15z, perhaps only hanging on in far northwest Forsyth. A deep sub-990mb low over the Upper Great Lakes Mon morning will shift into western Quebec by Mon evening and will drive a strong cold front east of the Appalachians Mon morning and traversing central NC by early afternoon. Since central NC should be within the warm sector at this point, as the boundary layer begins to heat, southwesterly winds ahead of the front (eastern Piedmont, Sandhills, and Coastal Plain) are expected to begin gusting 20-30 mph. Persistent cloud cover ahead of the front should limit the stronger wind gusts highlighted in the hi-res guidance. The strongest wind gusts will most likely be behind the front and when clearing of the low-overcast begins (16z northwest, 20z east). At this time, frequent wind gusts of 30 to 35 mph will be possible; infrequent gusts of around 40 mph should be expected as well. Pressure rises and CAA should continue well into Mon night and result in periodic gusts of around 25 mph becoming less frequent after 2 AM.

Precipitation is expected to spread east immediately along and behind the cold front, driven by strong 925mb FGEN overlapping intense H5 height falls of 100-150 dam in 12 hours. This intense forcing may provide a favorable setup for a narrow cold-frontal rainband to develop, which would be capable of locally heavier rainfall and briefly strong wind gusts. Latest hi-res guidance suggest timing of this feature, and the cold front forcing it, would be 12-18z from west to east across central NC.

LONG TERM /TUESDAY THROUGH SATURDAY/

As of 100 AM Sunday...

* Gusty winds and low humidity Tuesday may bring a concern for adverse fire weather conditions, which could last into Wed.

* Chilly Tue with much below normal temperatures, then moderating to more seasonable readings Wed-Sat.

* Patchy light rain possible Sat, but confidence is low.

Tue will be our coldest day of this stretch, as the Arctic high builds in post-front, supported by large mid level low over the Canadian Maritimes and a trough covering much of eastern NOAM. Despite ample sunshine in our area with NW flow and deep subsidence, strong CAA and thicknesses 30-40 m below normal support highs in the upper 30s to upper 40s, with wind chills in the 20s and 30s for much of the day. Gusts of 20-25 mph should be common, and these winds plus min RH values of 20-30 percent will bring about some fire weather concerns, although the chilly temps should mitigate this risk somewhat. Lows Tue night will be mostly in the 20s.

The rest of the week will lean closer to seasonable temperatures and appears generally dry, although low rain chances will arrive Fri into Sat. Surface winds will gust up to 20 mph Wed, with continued- low dewpoints with milder temps resulting in minimum humidities down into the upper teens to mid 20s, leading to another day with potential fire weather concerns. The mid level trough axis pushes off the East Coast Wed, but the longwave trough will maintain a presence over eastern NOAM through the rest of the week with northwesterly and gently-cyclonic mid level flow, leading to bouts of fair to partly cloudy skies, including across the N Tue night/Wed morning and Wed night through Thu. The surface high will modify and settle over TX then into the Gulf, giving us rebounding thicknesses closer to normal, lasting through the week. By Thu into Fri, polar stream energy dives into the base of the mean low over James Bay, over the Great Lakes and Northeast, pushing the Arctic front south toward NC and possibly causing a brief cooldown especially across our N on Fri. As that wave pushes offshore and out over the NW Atlantic, we get into a still-wavy but overall flatter flow across the CONUS, with greater influence from Pacific-source waves. Both the predictability of these waves at this range and how much moisture return we are able to tap into are both questionable, so while we are bringing in low chance pops from the W Sat, the confidence in the details is low. Temps should be within a category of normal Wed-Fri, then most likely a bit above normal Sat with partly to mostly cloudy skies. -GIH

AVIATION /06Z SUNDAY THROUGH THURSDAY/

As of 100 AM Sunday...

Satellite imagery shows an expanding layer of 1500-2500 cigs blossoming just south of the Triad terminals, west of RDU, and FAY. This layer will move over the Triad within the next hour and steadily lower towards 1000 ft through the pre-dawn hours. A period of IFR is possible, most probable at INT, between 08-11z, but confidence is too low to include even a TEMPO at this time. A secondary area of MVFR cigs over the Coastal Plain may move into RWI and RDU and create brief cigs, but prevailing MVFR should hold off until towards daybreak at RDU and this afternoon at RWI.

After daybreak, light rain and mostly drizzle will become increasingly possible through the daylight hours as cigs lower to IFR by mid-morning. As the very light precipitation moves east and weakens this evening, expect a rapid deterioration in aviation conditions as cigs lower to LIFR to VLIFR at the Piedmont terminals (INT, GSO and RDU) and MVFR to IFR at FAY and RWI. A very strong low- level jet and and gradually veering winds between the surface and 2000 ft will result in LLWS of 45-50 kts at all of the northern terminals after 02z (after 05z at FAY) and persisting into Mon morning.

Outlook: Low level wind shear conditions are likely to persist through much of the overnight, along with a high chance for continued sub-VFR cigs and vsbys. Sub-VFR cigs will prevail through much of Mon morning, exiting from western terminals in the late morning and out of eastern terminals in the early to mid afternoon as the Arctic cold front crosses the area. A chance of a narrow band of light showers is expected with the front, in the morning until mid afternoon. Blustery winds are likely just ahead of and behind the front, with gusts up to 25-35 kts from ~09z Mon until early evening. Winds will lessen somewhat but remain gusty up to 20-25 kts Mon night through much of Tue. Otherwise, VFR conditions will dominate from Mon evening through Thu under high pressure. -GIH

FIRE WEATHER

As of 240 AM Sunday...

A strong cold front is forecast to shift across central NC Monday morning through the early afternoon hours. Behind the front, and especially as skies begin to clear, strong downsloping winds, rapidly dropping relative humidity, and gradually veering winds, will pose an elevated concern for increased fire behavior. By 18z, sustained winds of 20 mph with gusts of 30 to 35 mph will become increasingly possible as relative humidity values rapidly drop below 30%. These parameters will likely overlap for 3-5 hours over a given area, first over the western Piedmont, and shift eastward through the eastern Piedmont, Sandhills, and Coastal Plain between 18z and 00z.

Marginally hazardous fire weather conditions will also be possible on Tues and Wed when relative humidity values will be exceptionally dry, in the low 20s and teens, during peak heating. Wind gusts and soil conditions will likely be the driving factors on whether any headlines will be needed each day. Gusts on Tues will be slightly stronger with frequent gusts around 20 to 25 mph expected.

RAH WATCHES/WARNINGS/ADVISORIES

None.