textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 00z TAF issuance.

KEY MESSAGES

1. A few severe thunderstorms will be possible Monday and Tuesday, mainly in the late afternoon and early evening. The main concern will be damaging wind gusts and excessive rainfall. 2. A cool-down will finally happen during the middle of the week that should persist into the next weekend, with temperatures switching to 5-10 degrees below normal.

DISCUSSION

Key message 1: A few severe thunderstorms will be possible Monday and Tuesday, mainly in the late afternoon and early evening. The main concern will be damaging wind gusts and excessive rainfall.

Over the next 48 to 72 hrs, our main weather player will be a weak mid/upper low, currently over the eastern Gulf and FL peninsula, that is expected to move slowly north/northeast through the weakness in the flow between the old upper anticyclone over the Arklatex region and the Atlantic Subtropical Ridge. The environment will continue to have high precipitable water that will fuel the development of deep convection during the afternoons and evenings with associated risks for wet microbursts and excessive rainfall.

For today, the risk for a severe storm is relatively low because of poor lapse rates and a lack of dCAPE in low levels, but that risk goes up marginally for Monday with better sfc-based CAPE expected. The 15Z RAP shows most-unstable CAPE climbing above 3000 J/kg across parts of the region Monday afternoon, for example, along with sufficient dCAPE for strong downdrafts, but weak shear. Some of the CAMs suggest storm clustering near the Blue Ridge Escarpment in the afternoon once the weak cap is breached, then storms moving out to the S and E on cold pools. Tuesday is more uncertain because it will depend on the amount of cloud cover across the area in the morning and how quickly and strong an easterly low level flow develops. A wide discrepancy is seen in fcst soundings by afternoon in terms of cap strength, basically whether or not we already have a cold air damming wedge developing east of the mtns by Tuesday afternoon. The GFS says no. The threat for excessive rain will climb upward as the threat for severe storms decreases on Tuesday. Better forcing and improving low level easterly flow could force the development of slow moving precip near the Blue Ridge Escarpment starting on Tuesday.

As for temps, Monday looks like a continuation of seasonally hot, but Tuesday is really up in the air. It could be another hot one with temps climbing above normal once again like the GFS shows, or it could be the start of a cool wedge like the NAM shows, with temps remaining five degrees on the cool side of normal. There's a 15 degree difference between the two models. Unfortunately, the model blend tends to split the difference, meaning it looks equally likely to be five degrees too warm or too cold, and not likely to be correct. Needless to say, confidence is low.

Key message 2: A cool-down will finally happen during the middle of the week that should persist into the next weekend, with temperatures switching to 5-10 degrees below normal.

Meanwhile...by Tuesday...a blocking pattern will form across the northern Plains/upper Midwest/upper Great Lakes that is expected to drift eastward through Wednesday and Thursday. Naturally, when this sort of pattern change happens, there is a lot of uncertainty regarding the details of the transition and where the upper and sfc lows develop, because of poor run-to-run consistency. At any rate, it appears likely that an upper low will be located somewhere over the Midwest/OH Valley region Tuesday night into Wednesday that would support a sfc ridge coming down the east side of the mtns that may or may not be aided by low pressure off the Mid-Atlantic coast. Either way, improving moist east northeast flow is expected to lock in a cold air damming wedge by daybreak Wednesday that could drop temps 10-20 degrees potentially for highs Tuesday to Wednesday. If enough light precip develops, temps might not get out of the 60s on Wednesday. Once the wedge develops, it should strengthen Thursday and then linger through Friday. Eventually, we may get into drier northerly flow aloft as the mid/upper low wobbles off the Northeast Coast over the weekend. If that can happen, it would set up our region for classic early autumn weather with a dry cool northerly breeze and bright sun with temps in the low to mid 70s. How refreshing would that be?

AVIATION /00Z MONDAY THROUGH FRIDAY/

At KCLT and elsewhere: Convection should wane within the first couple hours of the 00z TAFs, with just VCSH around KCLT to start the TAFs. Guidance has backed off any widespread low stratus or fog development tonight, except for the usual mountain valley fog. Cannot rule out some MVFR to IFR cigs forming around sunrise and lingering thru the morning, but confidence is too low to put anything more than sct030 in the TAFs for Monday morning. Confidence remains high in LIFR restrictions in the mtn valleys, so will keep that in KAVL. Another round of scattered to numerous showers and thunderstorms is expected Monday aftn into early evening, with perhaps slightly greater coverage than we had today. Have PROB30s at all sites, highlighting the most likely time storms will be nearby.

Outlook: Another round of afternoon/evening showers and thunderstorms expected again Tuesday. Mountain valley fog and low stratus possible again Monday night thru Tuesday morning. A cold air damming wedge is expected to develop by early Wednesday morning, likely resulting in widespread ceiling/vis restrictions that may persist thru Thursday. Conditions should improve on Friday.

GSP WATCHES/WARNINGS/ADVISORIES

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