textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated for the 00z aviation discussion.

A Freeze Warning has been issued for Avery County NC. A Frost Advisory has been issued for the remainder of the NC mountains, the northern NC foothills, and Rabun County GA. Both the Freeze Warning and Frost Advisory are in effect from midnight tonight until 9 AM Sunday morning.

For the system expected during the middle of next week, the threat for severe weather and excessive rainfall remains low, but guidance shows an upward trend.

KEY MESSAGES

1. Cold temperatures across the mountains early Sunday morning will likely result in patchy frost. Precautions should be taken to protect sensitive plants. 2. Another cold front will bring rain to the area during the middle part of the upcoming week. The threat for severe weather with this system, while appearing low at this time, has started to trend upward in the new guidance. The forecast trend should be monitored through the early part of next week.

DISCUSSION

Key message 1: Cold temperatures across the mountains early Sunday morning will likely result in patchy frost. Precautions should be taken to protect sensitive plants.

Broad upper trofing will translate over our area tonight and then lift off the Atlantic Coast on Sunday. At the sfc, low pressure will lift NE off the NC Coast today/tonight as broad high pressure spreads over the Southeast in its wake. NW winds will bring colder air to our area tonight into Sunday morning, as the sfc high shifts east over TN/KY. Modest wind gusts of 15 to 25 kt are expected later this afternoon and into the evening along the Blue Ridge Escarpment and over higher elevations along the NC/TN border. We could see some amount of upslope cloud development, as shallow instability over eastern TN may produce some cumulus and/or stratocu along the crest of the Appalachians which could move into western NC thru the late evening. In addition, enough low-level moisture may be present in the upslope layer to produce some stratus later tonight. The cold- air advection will likely be capable of lowering temps into the mid- 30s over a good portion of the NC mtns overnight. As winds subside and cloud potential decreases, further radiational cooling could result in elevations above 3500 ft dropping below freezing. As such, we have issued a Frost Advisory for all of our NC mtn zones and for Caldwell, Burke, McDowell, and Rabun counties. With a good portion of Avery now forecast to drop just below freezing, we have issued a Freeze Warning for Avery County. Both the Advisory and Warning are valid from midnight tonight until 9 AM Sunday morning. Outside of these areas, temperatures will likely remain too warm for any frost development. Nonetheless, low temperatures are expected to bottom- out roughly 10 to 15 degrees below normal Sunday morning across most of our CWA.

Key message 2: Another cold front will bring rain to the area during the middle part of the upcoming week. The threat for severe weather with this system, while appearing low at this time, has started to trend upward in the new guidance. The forecast trend should be monitored through the early part of next week.

The first few days of the upcoming week continue to look relatively benign as flat upper ridging and then confluent flow from an upstream split in the pattern moves sfc high pressure off the Southeast Coast and then keeps it there into mid-week. The new guidance shows a bit better phasing of nrn and srn stream waves Tuesday into Wednesday, which is reflected in a deeper mid/upper trof with an axis farther west by Wednesday morning. The downstream affect will be to open up the Gulf a bit earlier and more effectively, if this model development comes to pass. For now, the ensemble downplays what is seen in the operational GFS, so it would be best to manage expectations for getting a good shot of rain. That being said, the chance of getting at least an inch of rain in the ensemble has climbed from under 30 percent yesterday into the 30-50 percent range over the mountains in the new run, so that upward trend is appreciated. The Excessive Rainfall Outlook for Wednesday has a Marginal to our west, so we shall see if this is moved eastward with the new Day 5 forecast on the mid shift tonight. As to the severe thunderstorm chances, a similar upward trend is noted, which tracks based on pattern recognition and time of year. The SPC outlook for Day 5/Wed has a large area of risk that extends as far east as AL, and the NCAR/AI guidance shows some possibilities over the eastern Carolinas for Day 6/Thu. We would be left out mainly because of time of day when the front moves through, but it wouldn't take much to speed up/slow down the timing of the front and improve our chances. The joint probs for getting sfc-based CAPE above 500 J/kg and deep layer shear above 40 kt top out around 30-40 percent over the southeastern part of metro CLT Thursday afternoon. This is an improvement compared to yesterday's guidance and worth monitoring. We've done more with less, as I always say. For the time being, our forecast will have the highest probs of rain on Thursday. The passage of the front will knock temps back to ten degrees below normal for Friday, but a quick recovery to normal can be expected for next weekend.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: VFR conditions are expected to prevail at all terminals through the TAF period. A frontal boundary has slid across the area with a few trailing showers across the Upstate. These may briefly pass over KAND/KGMU/KGSP before pushing east later this evening. Otherwise, cloud cover will gradually clear through the overnight with clear skies by Sunday morning. Winds will be generally light out of the north and/or variable overnight with a north wind through the morning hours. Thereafter, winds are expected to shift to back out of the southwest during the afternoon.

Outlook: Expect dry/VFR conditions to linger through the first half of the work week. Restrictions are possible with next frontal system Wednesday night into Thursday.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Frost Advisory from midnight tonight to 9 AM EDT Sunday for GAZ010. NC...Frost Advisory from midnight tonight to 9 AM EDT Sunday for NCZ048>053-058-059-062>065-501>506. Freeze Warning from midnight tonight to 9 AM EDT Sunday for NCZ033. SC...None.