703 FXUS63 KLMK 300741 AFDLMK Area Forecast Discussion National Weather Service Louisville KY 341 AM EDT Thu Jul 30 2020 .Short Term...(Today and tonight) Issued at 340 AM EDT Thu Jul 30 2020 The big change from hot and humid to less hot but wet starts today. Lots of internal discussions with neighboring offices and WPC with respect to issuance of a Flash Flood Watch for today, and consensus for areas east of the PAH CWA was to hold off for now for a number of reasons, including dry antecedent soils in all but isolated locations across our CWA, as well as expected progressive movement of storms as opposed to the stalled nature of the isolated heavy rainers of the past few weeks. All of that being said, with PWATs between 2.0"-2.5" there is definitely the potential for tropical rainfall rates today, but still uncertainty on how long these can be sustained with only limited instability that could be hindered by cloud cover. There is greater certainty that what falls today will set up the stage for additional rounds of heavy rain that will occur due to the upper level pattern developing with a positively tilted trof extending from eastern Canada to the the southern plains. The interaction of short waves ejecting from this feature, in combination with deepening moisture over the area is the cause for the heavy rainers. After the first round today, a smaller area of showers and storms are expected tonight, but the more significant ones fall in the extended time frame discussed below. The plus side of all this rain and clouds will be daytime temperatures noticeably cooler than the rest of July, with highs today in the upper 70s to lower 80s. Lows tonight will still be on the warm side though - in the upper 60s to lower 70s. && .Long Term...(Friday through Wednesday) Issued at 330 AM EDT Thu Jul 30 2020 The large ridge over the western US will not budge for the next few days, which will continue to force this established trough over the central/eastern US to be responsible for a rather active period for the Ohio Valley through next week. WPC highlights the mid-Mississippi Valley across the TN/OH Valley and into the mid-Atlantic states with a Slight Risk of excessive rainfall for Friday. Their Slight Risk appears to follow along the east-west oriented frontal boundary that will continue to sit overhead. While this wavy boundary fluctuates north and south over our region, there is some uncertainty regarding the amount of moisture and instability available for heavy rain-producing thunderstorms. PWATs are generally in the 1.8-2.0 inch range, with higher moisture content south of our CWA. Instability is also much higher for areas south of KY, but models suggest 1000-2000 J/kg of MUCAPE for south-central Kentucky. Further north, CAPE values are less than 500 J/kg for the most part. However, if the frontal boundary does not sag as far south as model guidance is currently suggesting, then moisture pooling along the boundary and higher CAPE values could filter further north into central KY. In this saturated environment, clusters of thunderstorms will be capable of producing heavy rain and localized flash flooding hazards. On Saturday, a sfc low will slowly propagate from the Ozark Plateau into the Ohio Valley. With moisture advection from a SW 850mb LLJ helping to increase dew points to the low to mid 70s and instability in the 1000-2000 J/kg range, afternoon thunderstorm development is possible. Due to this, SPC has placed our entire CWA in a Day 3 Marginal Risk for severe weather Saturday afternoon and evening. In addition to possible strong thunderstorms, areal soundings show K- Index values of 33-36, which suggests that thunderstorms can produce heavy rain as well. By Sunday afternoon, the slow moving sfc low will eventually move NE into Ohio. Looking ahead on Monday, a 500mb vorticity lobe could swing off the parent vort max over Canada and track southward, eventually generating a stacked low over IL/IN. Precip chances will remain a possibility for our CWA for the first half of next week. This rainy pattern will be responsible for below normal max temps for early August across KY. Normal highs are around 90F, but we should stay in the low to mid 80s. && .Aviation...(06Z TAF Issuance) Issued at 116 AM EDT Fri Jul 30 2020 Currently playing the waiting game on deteriorating flight conditions for all of the TAF sites as a large, potent system develops just to our west and sends multiple rounds of tropical-like thunderstorms into the area. Current VFR conditions will give way to MVFR and occasional IFR CIGS/VSBYS at HNB, SDF and LEX between 13Z and 17Z and from 18Z-20Z at BWG as the first wave of TSRA arrives. These will saturate the lower atmosphere and while the rain will not be continuous, the stratus will be persistent through the end of the valid period. With an east-west quasi-stationary front running from HNB to SDF and LEX serving as the assumed focus for the heaviest rains, MVFR to IFR CIGS/VSBYs will dominate here, while BWG will fluctuate between MVFR and VFR conditions. Winds will be 3-7kts - southerly at BWG, but variable in direction in the vicinity of the front at the other sites. . && .LMK WATCHES/WARNINGS/ADVISORIES... IN...None. KY...None. && $$ Short Term...JBS Long Term...CJP Aviation...JBS