textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No major changes to forecast trends.

KEY MESSAGES

1. Mostly dry and hot Saturday. An approaching cold front may help produce an isolated shower over the mountains Saturday afternoon. Winds become gusty during the afternoon as well. 2. A cold front will bring only spotty beneficial rainfall to the mountains Saturday night into Sunday, with significantly cooler temperatures to follow early next week. Some frost is possible over the mountains Sunday and Monday nights, so precautions might be needed to protect sensitive vegetation. 3. Fire Weather concerns will continue through most of next week, and drought will continue to worsen, particularly as hot and dry conditions return during the latter half of the week.

DISCUSSION

Key message 1: Mostly dry and hot Saturday. An approaching cold front may help produce an isolated shower over the mountains Saturday afternoon. Winds become gusty during the afternoon as well.

High pressure slides eastward tonight as a cold front approaches the area from the west on Saturday. Can't rule out an isolated shower across the mountains late in the afternoon, but better precip chances hold off until evening.

Winds become light and variable overnight with mostly clear skies. Should see another round of mountain valley fog, but it could be more spotty than this morning. Lows will be well above normal.

Increasing thickness values and a developing southwesterly flow on Saturday as a shortwave ridge moves in will lead highs around 90 outside of the mountains. For now, GSP has the best chance of a record high, but it's possible at CLT. Gusty winds develop for the afternoon as well. Despite the higher temps, RH values dont fall as much as the previous few days with the southwest flow bringing in some moisture, although more muted than usual. Can't rule out the need for another Fire Danger Statement for NE GA, given the gusty afternoon winds, but it's uncertain at this time.

Key message 2: A cold front will bring only spotty beneficial rainfall to the mountains Saturday night into Sunday, with significantly cooler temperatures to follow early next week. Some frost is possible over the mountains Sunday and Monday nights, so precautions might be needed to protect sensitive vegetation.

A cold front, associated with low pressure tracking south of Hudson Bay and surrounding deep trough, will reach the upper Tennessee Valley Saturday evening. Dynamic lift will be lacking; though a jet streak will be amplifying aloft, it will be oriented unfavorably for upper divergence over the TN Valley. As noted previously, southwesterly winds preceding the front will be associated with some degree of Gulf moisture return, though relatively weak (20-25 kt at 850 mb), and mean PWATs from the ECMWF Ensemble are below normal over the source area near the Gulf Coast. Forecast vertical profiles do not look especially favorable for convection Saturday evening in East TN, though a couple hundred joules of "skinny CAPE" are seen. As a result of these conditions, CAMs mainly depict only a narrow band of convection to begin with, and any convection weakening as the front moves over the Appalachians in the middle of Saturday night. Severe threat looks minimal owing to the weak instability and likelihood of any overnight convection being elevated. Prog soundings saturate for a time overnight or early Sun morning in that area as the surface front itself moves in. Thus the chance for thunder ends up no better than about 25% along the TN/NC border, though PoPs rise to 60-80%. However, south/east of the Appalachians (below the Blue Ridge Escarpment) profiles remain drier and model QPF is less consistent. PoPs peak between 12-18z Sunday in that area and only at 30-40%. The TN/NC border areas still have potential for 0.25" or more of total precip, with isolated spots perhaps closer to a half inch, but only a few hundredths (if any) looks to result over the foothills/Piedmont. The 17/12z HiRes FV3 and 3km NAM go out thru 00z Monday and suggest precip could reorganize ahead of the front late Sunday morning or afternoon, but the bulk of the resultant QPF currently is shown east of our CWA. We will have a better handle on that possibility in tonight's 00z cycle when all the major CAMs incorporate that period.

The majority of guidance depicts temps on a somewhat nondiurnal trend owing to prefrontal cloud cover and cold advection setting in late in the night over the mountains, and in the early daytime hours Sunday to the east. For the Piedmont, min temps actually could be in the very late morning to midday--near the daily normal in the mountains but still several above normal over the Piedmont. Temps look to cool to near freezing (with wet-bulbs at or below 32F) in some higher elevations of the mountains Sunday morning. Most likely precip will have ended there by the time temps get that cool, and profiles are likely too warm just above the surface to permit any snow/sleet as a p-type, so our forecast reflects all rain. Afternoon max temps generally will be a few degrees below normal, especially over the mountains. Gusty NW winds will continue thru the day and into Sunday night over the higher elevations.

Temps should fall several degrees below normal and frost remains a possibility in sheltered valleys Sunday night, though the dry airmass may be a limiting factor. High pressure will continue to build across the area Monday maintaining breezy NW winds. Max temps rebound, especially across the Piedmont via downsloping. By Monday night the sfc high will center north of the area and winds abate, bringing another, arguably better chance for frost, but dry air again may be a limiting factor.

Key message 3: Fire Weather concerns will continue through most of next week, and drought will continue to worsen, particularly as hot and dry conditions return during the latter half of the week.

Dry high pressure will migrate across the Southeast Sunday afternoon through early Wednesday, while ridging generally builds across the Plains and Deep South from Monday onward. A weakening front in the Ohio Valley Wednesday-Thursday likely will stall or wash out before it reaches our area. The next Plains cyclogenesis event is not expected until Thursday or early Friday, and if the associated front even reaches our area it does not look possible until next weekend. Thus, the forecast for next week is dry, with modifying airmass leading temperatures to trend 7-10 degrees above climo by Wednesday.

The onset of drier air Sunday still appears likely to bring afternoon minimum RH critically low, below 30 percent across most of the CWA. Some areas of the mountains should see gusts of 25-30 mph that afternoon, though where such gusts are more likely the RH is less likely to be as low. Can't rule out some 25 mph gusts in a few spots across the Piedmont, however. Altogether some overlap of winds and low RH, meeting the objective criteria where we typically message increased fire danger in NC, SC and GA. Fuel moisture could be temporarily improved by rainfall with the front, though any improvement likely would be very short lived given that RH looks likely to dip below 30% Monday-Thursday as well.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR stratocu across the area this afternoon will dissipate early this evening with mainly cirrus overnight. Expect more mountain valley fog overnight, but should stay north of KAVL again. Mainly light and variable wind into the overnight, although wind may pick up from the NW from time to time this afternoon. Low VFR Cu or stratocu develops by Sat afternoon. An isolated shower may develop over the mountains, but likely to remain north and west of KAVL. Winds pick up from the S to SW after daybreak with gusts developing for the afternoon.

Outlook: A cold front may bring brief rain chances and associated restrictions Saturday night into Sunday, mainly across the mountains. Gusty winds also expected with and behind the front. Dry and VFR conditions are expected next week.

CLIMATE

RECORDS FOR 04-17

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 89 1896 45 1890 63 1927 26 1904 KCLT 94 1896 47 1890 66 1896 30 2018 1949 KGSP 92 2006 51 1904 63 1927 25 1905

RECORDS FOR 04-18

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 90 1896 44 1921 60 1927 28 1905 1891 KCLT 93 1896 45 1983 66 1896 32 2001 KGSP 89 2002 51 1983 66 1927 28 1905 1967

GSP WATCHES/WARNINGS/ADVISORIES

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