Scientific Forecaster Discussion
000 FXUS64 KBMX 212321 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 521 PM CST Thu Nov 21 2019 .UPDATE... For 00Z Aviation. && .SHORT TERM... /Updated at 0321 PM CST Thu Nov 21 2019/ Through Tonight. Upper level flow across the area today has remained zonal ahead of an approaching trough and surface front. In response to a weak impulse within that zonal flow, light rain showers are moving eastward across northern MS and AL. Stronger returns are making it to ground, overcoming the still relatively dry air below 700mb. PW values are now around 1 inch across far northwest Central Alabama, and drop to 0.6 inches near and south of the I-20 corridor. For the remainder of this evening, light showers extend back to the Mississippi River and expect this activity to continue across northern areas through early evening. The weak impulse will move east of the area and rain activity will pull north of Central Alabama, closer to better upper level dynamics ahead of the approaching front. Expect cloud cover and low level moisture to continue to increase overnight. This will keep overnight lows in the mid 40s east to low 50s west. 14 .LONG TERM... Friday through Saturday. /Updated at 0321 PM CST Thu Nov 21 2019/ No wholesale or major changes were made to the extended forecast, though some PoP adjustments were for the weekend and mid next week to become more aligned with trends in recent guidance prognostics. In particular, PoPs were advanced slightly more progressive for the weekend system. This remains a widespread chance of rain for the forecast area with totals generally ~0.50" or less in the southeast, to as high as ~1.75" in the northwest through Saturday night. A slight chance of embedded thunderstorms was maintained Friday evening through Saturday afternoon as MUCAPE <500 J/kg is possible along the frontal boundary and primary area of rain as it swings through Central Alabama. Rain is expected to quickly advance out of the area with dry conditions expected by early Sunday morning, though cooler air is on its heels. The next period of rainy & unsettled weather is forecast for the Tue/Wed timeframe. Inconsistencies with how this system evolves has yielded low forecast confidence, thus PoPs were adjusted slightly more conservative with this forecast update. These changes aside, the previous long-term discussion is a good analysis for the forecast across Central Alabama in the coming days including the varying scenarios for next week. 40/Sizemore /Updated at 0353 AM CST Thu Nov 21 2019/ A broad area of moist deep layer southwesterly flow will be present along and ahead of the front on Friday, with several embedded shortwaves. PWATs will increase to around 1.5 to 1.6 inches Friday/Friday night. Moist isentropic lift will result in several waves of showers moving through Friday/Friday evening across the northwest half of the area as the column saturates with the front also providing some forcing. This will also result in a tricky high temperature forecast with increased cloud cover. Southerly flow and rain-free conditions will result in highs continuing to be above guidance in the south, while highs across the north will be dependent on how quickly the rain moves in, and a secondary warm front may set up in between. An additional area of showers will set up along the cold front which moves in late Friday night and Saturday. An isolated embedded thunderstorm is possible late Friday night in the west with some weak, primarily elevated instability. Dew points only in the 60-62F range and lack of stronger theta-e advection, combined with expected cloudcover and precipitation should prevent any strong or severe surface-based activity late Friday night. Dew points may creep up to around 65F on Saturday in our far southern counties, which may allow some weak surface-based instability to develop, though the consensus of guidance keeps CAPE values still only around 200 J/kg or less. Forecast soundings also suggest hodographs will straighten as low-level flow veers with time even as instability begins to increase, and profiles remain saturated. If an overlap between enough instability and curved hodographs could develop, then there would be a non-zero threat of a brief tornado on Saturday, but this seems unlikely at this time, though some gusty winds will be possible. Rainfall amounts look to be around 0.5 to 1.5 inches, with locally higher amounts in the far northwest, so with a lack of saturated conditions any minor flooding should be limited to poor drainage areas. Conditions mainly dry out Saturday night, but can't rule out some drizzle across the far north as some wrap-around low-level moisture rotates through. Sunday through Thursday. Rain-free and cooler conditions are expected Sunday, as a few moisture-starved waves move through in northwest flow aloft. Southerly winds on Monday will signal a warming trend. Meanwhile a strong North Pacific jet will carve out broad troughing from the northwestern to the north-central CONUS. Models have been trending less amplified with a shortwave and strong mid-level speed max rounding the base of this trough Tuesday/Tuesday night, but vary regarding how much less amplified. There is still ensemble support for a surface low lifting northeast from the Southern Plains to the Great Lakes, with high rain chances and possible breezy conditions for Central Alabama with the cold front passage Tuesday night. Moisture return and instability within the narrow warm sector is expected to be somewhat limited, and heights remain neutral/rising with time. Therefore any potential severe threat remains very marginal at this time. Strengthening ridging over the Gulf of Mexico is expected through the end of the period downstream of West Coast troughing. A very early look at the Thanksgiving forecast suggests a warm front lifting northward across West Alabama while a CAD wedge tries to build in from the east, suggesting varied temperatures across the area and some light rain chances especially the further north you go, but details remain murky at this time. 32/Davis && .AVIATION... 00Z TAF Discussion. Sct light showers will continue north of I-20 thru 03z, but not likely have any significant impact on TAF sites as vsbys should stay above 5 miles. Mostly mid to high level clouds will continue across central Alabama thru 09z. Isentropic lift will increase across north Alabama between 09z and 12z, and cigs 2500-3000 feet agl will likely develop along and north of I-20. Showers will increase across north Alabama after 18z, but coverage uncertain for northern TAF sites, and handled with VCSH. MVFR cigs likely to remain across northern TAF sites thru the end of the period. 58/rose && .FIRE WEATHER... Most of the area will be dry tonight, with only scattered light rain showers across the far north. The chance for wetting rains returns increase Friday and lasts through Saturday thanks to a frontal system that will move across Central Alabama. Minimum relative humidity values will be elevated through the weekend. Drier conditions return by Sunday through early next week. && .PRELIMINARY POINT TEMPS/POPS... Gadsden 46 67 59 65 37 / 30 80 90 90 10 Anniston 48 70 59 66 40 / 20 60 80 90 10 Birmingham 51 69 60 65 39 / 20 70 90 90 0 Tuscaloosa 51 71 61 65 38 / 20 70 90 80 0 Calera 49 71 60 65 39 / 10 50 80 90 10 Auburn 48 69 58 67 43 / 0 10 50 80 20 Montgomery 49 75 60 70 43 / 0 20 60 90 10 Troy 48 74 60 72 44 / 0 10 30 80 20 && .BMX WATCHES/WARNINGS/ADVISORIES/... None.