textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 12Z TAF issuance.

KEY MESSAGES

1. Near record high temperatures linger through Friday before a strong cold front crosses the region Friday night. Gusty winds are expected across the higher elevations along and behind the front, but confidence in Advisory level wind gusts is low. 2. Behind the cold front, temperatures will drop below normal through the weekend. Low temperatures across the mountains will be cold enough for frost and could drop below freezing in some places early Sunday morning.

DISCUSSION

Key message 1: Near record high temperatures linger through Friday before a strong cold front crosses the region Friday night. Gusty winds are expected across the higher elevations along and behind the front, but confidence in Advisory level wind gusts is low.

Not much change to earlier thought regarding the slow migration of a mid/upper anticyclone from east TX/LA today to the Carolina Coast Thursday, which will keep a warm sfc high in control of our weather through the end of the work week. The upper ridge will keep a sfc front at bay strung out to our north through Friday, with a dry air mass and no chance of rain through at least Thursday night. Once the upper anticyclone moves offshore Friday, that will change and we may see enough moisture return to support some pre-frontal showers over the mtns by late in the day. The main story before the front approaches will be the well-above normal temperatures. As it stands, the NBM has highs at all three climate sites within a degree or two of record highs for the next three days, and the NBM ran a little below actual on Tuesday. A few broken records are a decent bet.

The upper pattern will be progressive late in the week, with a strong short wave coming down out of the Canadian Plains and phasing with some energy lifting out of a trof over the Rockies Thursday night. This helps to amplify the mid/upper trof as it drops down over the upper Midwest/Great Lakes on Friday, which drives a strong cold front across the region mainly Friday night. Overall, moisture return looks meager ahead of the front, so most of the shower activity will be confined to the western half of the mtns and closer to the TN border. The pattern has very little support for any meaningful shower activity making it over the mtns. However, the pressure gradient with and behind the front will be supportive of decent wind gusts across the mtns and foothills Friday night and early Saturday morning. Peaks above 6k feet may see some advisory-level gusts, but locations below 5000 feet have a low likelihood of getting wind gusts above 45 mph. Confidence in needing an Advisory is low as a result.

Key message 2: Behind the cold front, temperatures will drop below normal through the weekend. Low temperatures across the mountains will be cold enough for frost and could drop below freezing in some places early Sunday morning.

The strong cold front is expected to bring in the first true cool continental air mass for the weekend. The center of the sfc high will quickly reach the mtns by the end of the day Saturday. Temps will be very noticeably cooler compared to Friday...on the order of 20 degrees cooler...under a bright sunny sky. Model guidance shows that the center of the sfc high will settle over the mtns overnight and be located over western NC at 12Z Sunday, suggesting excellent radiational cooling conditions across the region. The NBM shows the chances of getting down to 32F or less to be a coin flip across the northern mountains, while even the mtn valleys would have an excellent chance of dropping down low enough to allow for frost. Knowing that we often underestimate the frost/freeze potential in the first cool airmass of autumn, it would not be surprising if we ultimately need some collection of frost advisory/freeze warning for early Sunday morning. Monday morning would also be well below normal.

Eventually, the sfc high moves off early next week and temperatures slowly climb while remaining on the cool side of normal. Some of the guidance shows a short wave moving through mid-week, but little stock is put in that for now and the forecast will remain dry.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Fog did make it to KAND. LIFR should become VFR by 13Z or so. Mountain valley fog should stay away from KAVL this morning. Otherwise, just a few VFR stratocu at KAVL/KHKY this afternoon. Light and variable wind becomes light SW for most. KAVL sees light NNW wind. KHKY sees light NW wind becoming SW. Light and variable to calm wind overnight. Expect another round of mountain valley fog as well. Can't rule out patchy fog elsewhere.

Outlook: Mainly VFR conditions are expected thru early next week, outside of the potential for mtn valley fog and low stratus each morning. A cold front could bring scattered showers and associated restrictions to the mountains Friday afternoon and evening. Gusty winds expected all areas ahead of the front Friday and behind the front Saturday.

CLIMATE

RECORDS FOR 10-14

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 82 1928 51 1907 64 1954 25 1988 KCLT 87 2021 56 1922 68 1883 32 2006 1940 1881 KGSP 86 1985 57 1907 67 1954 30 1988 1928

RECORDS FOR 10-15

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 86 1897 52 2002 60 2017 27 1907 1970 1928 KCLT 88 1985 53 2009 66 1985 30 2006 KGSP 86 1985 53 2002 65 1985 31 1907 1941 1970

RECORDS FOR 10-16

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 87 1897 49 1954 63 1928 29 1939 KCLT 87 1925 58 1937 68 1947 35 1939 1925 KGSP 85 1935 53 1937 67 1925 32 1907 1911 1892

GSP WATCHES/WARNINGS/ADVISORIES

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