textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

A Fire Weather Watch has been issued for the South Carolina Upstate and northeast Georgia for Saturday.

The aviation discussion was updated to reflect the recent 12Z TAF issuance.

KEY MESSAGES

1. Anomalously warm conditions again today threaten to break record heat for late March. Showers and a couple of thunderstorms develop along an advancing cold front this afternoon and evening, but should not be impactful. 2. A cold front ushers in gusty winds and extremely dry air for Saturday and Sunday. Fire Danger is expected to be elevated for most of the forecast area, and a Fire Weather Watch has been issued for the South Carolina Upstate and northeast Georgia for Saturday. 3. Temperatures and humidity tick upward through next week, with afternoon shower and thunderstorm chances slowly increasing.

DISCUSSION

Key message 1: Anomalously warm conditions again today threaten to break record heat for late March. Showers and a couple of thunderstorms develop along an advancing cold front this afternoon and evening, but should not be impactful.

As of around 130 AM, skies were generally clear over the forecast area. A baroclinic zone was draped across the upper Ohio Valley, and was making southward progress at a snail's pace...under the influence of generally zonal flow aloft on the periphery of deep troughing over Quebec. WPC analyzed a surface front extending from central Illinois to the Allegheneys.

Over the next 24 hours, weakly confluent flow aloft will push the front gently south. Ahead of it, an anomalously warm air mass will remain in place for most of Friday, with westerly flow and extremely weak WAA and moisture flux in the low levels. Some measure of downsloping seems likely across the NC Piedmont...and accordingly, afternoon highs should creep into the mid- to upper-80s...some 20 degrees or more above normal and flirting with daily records at all three climate sites.

Most of the CAMs depict destabilization to the tune of 300-600 J/kg sbCAPE in the immediate vicinity of the front, and resultant showers...perhaps with some embedded thunder over the I-77 corridor where the unstable layer looks deepest...during the late afternoon into this evening. Whatever shear is present will be more or less boundary parallel...and this will significantly curtail prospects for severe weather, especially in an environment with such little synoptic forcing. Won't completely rule out an isolated low-end damaging wind gust should any semi-organized line segments develop, especially since there does look to be a shallow dry layer aloft in deterministic soundings, which could help amplify any downdrafts that get going. But...generally think the severe risk is a barely step above nil...and agree with SPC's latest Severe Outlook, which has only a General Thunder area for our NC and eastern SC Upstate zones.

Given the lack of incipient moisture and overall weak forcing, QPF response unfortunately also looks to be quite paltry...with the 00z HREF 3hr PMMs suggesting only a couple hundredths of an inch of rain along the I-77 corridor this evening. So, this will barely qualify as a wetting rain...and will do little to offset D2 to D3 drought across most of our area.

Key message 2: A cold front ushers in gusty winds and extremely dry air for Saturday and Sunday. Fire Danger is expected to be elevated for most of the forecast area, and a Fire Weather Watch has been issued for the South Carolina Upstate and northeast Georgia for Saturday.

A strong surface cold front will settle to our south on Friday night. Strong CAA will quickly develop behind it, ushering in gusty NNW winds throughout Friday night and the first part of Saturday morning. The core of strongest CAA would appear to cross the area in most ensemble guidance by around 12z Saturday or shortly thereafter, such that winds will steadily ramp down through most of the day.

From late morning through the afternoon Saturday, the setup looks conducive to a so-called "dewpoint bomb" in which afternoon dewpoints crash below what most guidance depicts. To wit, forecast profiles depict a mass of extremely dry air just atop the PBL at around 850mb...featuring dewpoints as low as -50 degrees. Under normal circumstances, one would expect the postfrontal subsidence inversion to squander our chances of tapping into that air; in this case, however, NNE winds of 25-35kts are depicted within the inversion layer, which should be enough for at least some turbulent mixing of that excessive dry air into the PBL. Most of the statistical guidance as well as the experimental NBMv5, which has been calibrated to effectively key on these big fire weather days, both depict afternoon dewpoint mixing much more substantial than the standard NBM. Forecast RHs get as low as the low- to mid-teens Saturday afternoon...and even this could be optimistic.

The South Carolina Forestry Commission and Georgia Forestry Commission have both indicated that existing dry fuels combined with these meteorological conditions pose a dire threat for wildfire activity to develop and spread on Saturday. The North Carolina Forest Service indicated that fuels across the mountains and Piedmont were not as dangerously dry. So, in coordination with these land managers as well as neighboring NWS offices...a Fire Weather Watch has been issued for the SC Upstate and northeast Georgia...while the decision was made to hold off on a product for our NC zones. For NC, while concern remains for fire weather conditions, it appears more likely that a Fire Danger Statement will be issued either Friday or Friday night.

Key message 3: Temperatures and humidity tick upward through next week, with afternoon shower and thunderstorm chances slowly increasing.

The southeast remains under a steady upper ridge from Sunday onward, with gradual height rises throughout the week. Translating to the surface, a more typical summer-like pattern emerges as high pressure shifts offshore and amplifies. The CWA remains under the influence of the western fringe of this persistent area of high pressure. As surface winds turn and prevail out of the south, a continued fetch of moisture is expected. This means that humidity increases along with the air temperature. Current guidance has daytime highs well above normal toward midweek. Additionally, as the heat and moisture tick up, so does the chance for pop-up convection, mainly over the mountains. For now, only a slight chance PoP (15-30%) starting Tuesday and slowly increasing through the remainder of the forecast period.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Predominantly VFR conditions are expected across the terminal forecast area for the 12z TAF cycle. Circa 10z, cirrus had begun building into the northern portions of the terminal forecast area along I-40, while the remainder of the area was still mostly clear. Steadily increasing high- and mid-level cloud cover is expected through the day today ahead of an advancing cold front. Gusts of 15-20kts are possible within a steady W/WSW wind this afternoon. Late this afternoon, most of the hi-res models depict a loose line of showers, perhaps with some embedded thunder. Probability of seeing any precipitation looks highest across the NC Piedmont, and lowest in the SC Upstate...and PROB30s/VCSH have been assigned accordingly. More or less concurrently with the passage of these showers, winds are expected to quickly toggle from the WSW to almost due N late this evening, and to become very, very gusty through tonight and Saturday morning. By early afternoon, gusts should begin to taper off...but winds will remain elevated through the end of the TAF period.

Outlook: Gusts taper off by Saturday evening. VFR continues through the rest of the weekend and into next week.

CLIMATE

RECORDS FOR 03-27

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 85 2020 32 1955 60 1921 11 1955 1894 KCLT 85 1950 40 1894 62 1949 19 1955 1944 KGSP 86 2020 45 2011 60 1921 15 1894 1947

RECORDS FOR 04-01

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 83 1974 40 1919 60 1927 20 1923 KCLT 86 1986 40 1887 64 1998 24 1923 KGSP 85 2010 45 1965 64 2024 24 1900 1986 1910

GSP WATCHES/WARNINGS/ADVISORIES

GA...Fire Weather Watch from Saturday morning through Saturday evening for GAZ010-017-018-026-028-029. NC...None. SC...Fire Weather Watch from Saturday morning through Saturday evening for SCZ008>014-019-101>109.