Area Forecast Discussion
Issued by NWS Huntsville, AL

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000 FXUS64 KHUN 211857 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 1257 PM CST Thu Nov 21 2019 .UPDATE... For Near Term and 18Z TAFS. && .NEAR TERM...(Remainder of Today) Issued at 1257 PM CST Thu Nov 21 2019 A deamplifying 500-mb ridge will continue to shift eastward and away from the TN Valley today, with deep-layer west-southwesterly flow becoming more established to the north of a subtropical ridge over the southern Gulf of Mexico and to the east of a cutoff upper low moving slowly eastward across the southwestern deserts. At the surface, light south-southeasterly winds will slowly veer to the south-southwest and strengthen as a low-level ridge spreads southeastward across NC/SC and a cold front approaches the region from the northwest. Adequate lift in the resulting lower-tropospheric WAA regime has been sufficient to sustain light rain moving into the region from the west-southwest, and we expect this trend to persist throughout the afternoon. POPs have been adjusted to reflect a higher coverage of measurable precipitation, but rainfall amounts should be very light. Otherwise, we made minor downward adjustments to the max temperature grid to reflect the earlier onset of thicker clouds and precipitation. .SHORT TERM...(Tonight through Saturday Night) Issued at 227 AM CST Thu Nov 21 2019 Expect an increase in shower activity through the overnight hours, as the aforementioned frontal boundary tracks closer to the Valley. Guidance seems to be in fairly good agreement about a frontal wave forming near the ArkLaTex region around 18z, stalling the frontal boundary near southern TN, keeping us soggy on the warm side of the front. High temperatures will remain close to seasonal norms, ranging from the low to middle 60s. Expect the heaviest rain to come Friday night and Saturday morning, as the frontal wave tracks near the northwest corner of AL and eventually pushes a cold front east of the Valley by late Saturday. Looking at thermodynamic profiles, have continued a slight chance of thunder Saturday morning as some marginal elevated instability looks to be present, especially across northwest AL. With that said, do not expect thunderstorms to be widespread but an occasional rumble or two of thunder cannot be ruled out. Expect shower activity to clear northeast AL by sunset Saturday, with rainfall amounts totaling between 1.5-2.0". While widespread flooding issues are not anticipated at this time, some ponding of water on roadways will be possible during periods of heavier showers, especially Friday night and Saturday morning. Exercise caution especially if traveling during this time. After another mild day of highs in the low to mid 60s, temperatures will fall steadily behind the front as cooler, drier air is ushered in and high pressure moves into the area. Overnight lows will fall back into the mid to upper 30s Saturday night/Sunday morning. .LONG TERM...(Sunday through Wednesday) Issued at 227 AM CST Thu Nov 21 2019 High pressure will quickly move into the area Sunday and Monday in wake of the frontal system, resulting in a brief period of fair weather and dry conditions early next week. On Sunday, northerly flow will reinforce a cool, post frontal air mass. Thus, despite the abundant sunshine, highs will struggle to climb above the lower 50s. As high pressure slides off to the east of the region on Monday, we`ll see some air mass recovery as southwest flow helps push temperatures into the 60s. Subsidence from the ridge will prevent anything in the way of cloud cover developing, so expect another sunny and dry day. This bout of quiet weather will be short-lived as we transition back into a more active pattern as a developing low pressure system shifts from the Southern Plains into the Mississippi Valley on Tuesday, eventually moving into the Great Lakes Tuesday night into Wednesday morning. A surface cold front will track eastward into the Mississippi Valley on Tuesday and push into the Tennessee Valley region Tuesday night, clearing the area by Wednesday morning. Models have changed their thinking regarding the evolution of this system. Earlier runs indicated the northern stream trough merging with a closed low across Baja California and the Desert Southwest. It now appears that this northern wave may be a little too far removed to accomplish this, and thus, expect this next system to move through the Ohio/Tennessee Valley region as more of an open wave, rather than a dynamic, deep upper cyclone. Moisture recovery ahead of this frontal system will be very slow significantly limiting instability and the onset of precipitation somewhat on Tuesday. For this reason, think that the thunderstorm threat during this period is quite low and have maintained a forecast of rain showers. However, we should get a good slug of moisture right along the front as it moves in Tuesday afternoon/evening and Tuesday night. Thus, we should get another appreciable round of moderate to locally heavy rainfall during the window of the frontal passage Tuesday night. Thereafter, the boundary will shift east of the area on Wednesday as high pressure moves in from the west, bringing an end to the rainfall by midday Wednesday. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) Issued at 1257 PM CST Thu Nov 21 2019 VFR altostratus cigs arnd 15 kft will continue to slowly descend through the aftn, as lift strengthens ahead of an approaching cold front and light rain spreads northeastward into the TN Valley. However, model consensus indicates that a more rapid deterioration in conditions will occur at the terminals arnd 22/09Z as cigs become established in the 1500-2500 ft layer. Both coverage and intensity of rainfall will increase around or shortly after 12Z as the cold front drifts southeastward into the region and stalls. Thus, we have indicated both MVFR vsby reductions in heavier precipitation and IFR cigs. Sfc winds will oscillate btwn SSE-SSW at speeds of 5-10 knots through 12-14Z, before becoming lgt/vrbl as the pressure gradient diminishes with the approach of the cold front. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...70/DD SHORT TERM...25 LONG TERM...AMP.24 AVIATION...70/DD For more information please visit our website at is the U.S. government's official web portal to all federal, state and local government web resources and services.