textproduct: Greenville-Spartanburg

SYNOPSIS

A warm upper ridge over the lower Mississippi Valley will keep our region warm and dry for Christmas and the start of the weekend. The warm weather will end dramatically early next week as cold air from Canada drops the temperature about 30 degrees from weekend temperatures.

NEAR TERM /THROUGH TONIGHT/

As of 1250 AM EDT Wednesday:

Key Message #1: Warm temperatures with near-record highs today.

An upper ridge will persist across the Plains, with a series of shortwaves riding over the axis across southern Canada and diving into New England. The first wave is already about to enter New England, and is helping to veer the flow across the region from westerly to more northwesterly. While there is initially some CAA with this flow, it will become neutral by this aftn, and with downslope effect taking over, this will allow temps to warm into the 70s east of the mountains and 60s in the mountain valleys. These temps will be 15-20 deg above normal, and likely to tie or break record highs at KGSP and KCLT (71 and 73, respectively). KAVL will not likely reach theirs, due to persistent cool up-valley flow and periods of cloud cover. Speaking of cloud cover, plenty of moisture will continue to bank up against the NC mountains near the TN border thru most of the day. Spotty light showers will continue to affect those areas thru about daybreak, then a lull is expected with dry conditions likely from mid-morning thru the aftn. Mostly sunny skies expected elsewhere.

Key Message #2: Warm temperatures tonight with spotty light rain showers mainly in the mountains near the TN border.

Tonight, a second wave approaches New England, and will be accompanied by a secondary jet streak to the south, tracking from the Upper Midwest to the Mid-Atlantic. This wave will help activate a subtle frontal boundary draped roughly NW-SE across the forecast area. Guidance agrees on increasing clouds along the frontal zone. The 00z CAMs have come in with some spotty light showers breaking out across mainly the eastern half of the forecast area overnight, quickly tracking east of the area early Thursday morning as the warm front exits to the NE. The deterministic/global models don't show any QPF response that corresponds to these showers. Perhaps, the shallow moisture is not being resolved well and the CAMs are on to something. For now, I think impacts would be very low if anything develops, and confidence remains too low to add mentionable PoPs beyond what's in the latest NBM. The TN border counties, however, will likely see scattered light showers develop again, given better moisture and lift with backing westerly flow. Lows will be quite elevated tonight, mainly in the upper 40s to lower 50s (not far from our normal highs).

SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/

As of 1200 AM EST Wednesday:

Key Message: Near-record warmth expected Christmas Day...and for some locations on Friday.

A strong upper ridge over the central Conus will begin to break down during the latter half of the short term, as the pattern will remain quite active across southern Canada into the northeast Conus...with one strong speed max digging across New England on Thursday. This will support a warm frontal activation/a moist warm advection across the region...although the greater forcing is forecast to remain north of our CWA. Low level flow veering to the WNW throughout the day will result in increasing upslope lift near the TN/NC border...and this will actually be the primary impetus for precip potential Christmas Day through the early part of the 26th. PoPs (mainly in the 30-50%) range are therefore primarily confined to the counties bordering TN during that time frame. Very warm thickness values combined with increasing downslope flow outside the mountains are expected to result in max temps around 20 degrees above normal. Forecast maxes fall a category shy of the upper 70s daily records at KCLT/KGSP, but will be within the ballpark of the record of 71 at KAVL. The unseasonably warm conditions continue into Christmas night...when min temps will be close to where normal highs should be for the time of year.

Meanwhile, in the wake of Thursday's speed max, 1035-ish mb surface high pressure is forecast to progress from eastern Ontario into Quebec, with inverted ridging developing down the eastern slopes of the Appalachians Thu night into Friday. This will likely send a backdoor cold front into the northern part of the forecast area early Friday. How far south the front pushes before stalling is the subject of much controversy...in light of the fact that the next round of height falls will already be approaching the East from the northern Great Plains during this time. This makes for a very tricky max temp forecast for Friday...with major bust potential on both the warm and cold side of the equation. For now...have opted to ride the National Blend of Models guidance for maxes...which features near-normal temps across the northwest NC Piedmont...increasing toward the SW such that forecast temps across far southwest NC and the upper Savannah River Valley are 10-15 degrees above normal.

PoPs for mostly light showers increase once again across the mountains later Friday into Friday night as the next frontal zone/warm advection regime associated with aforementioned height falls progresses across the region, with chances peaking at 50-60% across the TN border counties. Unseasonably warm (and humid) conditions continue through Fri night.

LONG TERM /SATURDAY THROUGH TUESDAY/

As of 1215 AM EST Wednesday:

Key Messages: Well-above-normal temperatures are expected through the weekend. A major pattern change involving the arrival of an arctic front is expected early next week.

Very warm weather will return (or continue for some areas) through the weekend, as an upper ridge reasserts itself closer to home...downstream of major height falls overspreading the central Conus. Forecast temps are 15-20 degrees above normal Saturday through Sunday. Early next week...an arctic cold front associated with strong upper low responsible for aforementioned height falls is expected to impact our region. While this will be another frontal system that is unlikely to bring appreciable rainfall (probabilities mainly in the 30-50% range Sunday/Sunday night), temperatures are forecast to plummet Sunday night/Monday such that max temps are forecast to be several degrees below normal Monday...and 10 or more degrees below climo Mon night/Tuesday. In other words, Tuesday's forecast highs will be 30-35 degrees below this weekend's maxes.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: VFR conditions expected thru the period, except for possible MVFR cigs working up the French Broad Valley and reaching KAVL in NW flow for part of the morning. Guidance continues to back off the low cigs, and KAVL is still VFR at time of 12z TAF issuance. Quiet everywhere else today, with light wind. Mainly just some cirrus streaming thru today, then VFR-level stratocu is expected to develop this evening. Spotty light showers or sprinkles may develop and cross mainly the NC foothills and Piedmont, but should be very spotty and not worth a mention in the current TAFs. Cigs may lower somewhat overnight, and get to MVFR at KHKY before 12z Thu.

Outlook: Low cigs should scatter out Thursday, except for along/near the TN border. A potential backdoor front may slip into the area Thursday night and set up some sort of cold-air damming Friday. Confidence remains low, but there is potential for MVFR or lower cigs developing Friday and persisting into Friday night across most of the area.

CLIMATE

RECORDS FOR 12-24

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 72 1955 17 1906 59 2015 -5 1983 KCLT 73 2015 29 2022 63 2015 6 1983 1906 KGSP 71 1964 28 2022 61 2015 7 2022

RECORDS FOR 12-25

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 71 2021 14 1983 59 2015 -7 1983 KCLT 77 1955 22 1983 63 2015 4 1983 KGSP 78 1955 22 1983 61 2015 6 1983

RECORDS FOR 12-26

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 78 1971 17 1902 55 2015 0 1983 KCLT 77 2021 27 1983 58 2015 6 1983 1964 KGSP 76 2021 28 1983 62 1987 5 1985 2015 1980

RECORDS FOR 12-27

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 72 1971 19 1902 59 2015 3 1925 KCLT 72 2021 22 1892 65 2015 15 1970 2015 1948 1971 1925 KGSP 75 1971 29 1925 64 2015 12 1925

RECORDS FOR 12-28

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 73 1889 19 1894 56 2015 0 1925 KCLT 77 1971 30 1950 58 2015 10 1925 1935 1925 KGSP 75 1971 31 1950 64 2015 9 1925 1925

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.