Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
517 FXUS64 KHUN 151114 AAA AFDHUN Area Forecast Discussion...UPDATED National Weather Service Huntsville AL 514 AM CST Fri Dec 15 2017 .UPDATE... For 12Z TAFS. && .SHORT TERM...(Today through Saturday Night) Issued at 202 AM CST Fri Dec 15 2017 Satellite imagery indicates that the northern extent of a large veil of CI/CS associated with the 160kt 250mb jet continues to stream eastward across the TN valley. Model indications are that this will will exit through the day as the jet core shifts east and NVA develops behind the passage of a positive tilt trough. The mid level ceilings to our north in central and northern TN are showing no signs of making it this far south either. Cold advection will become neutralized rather quickly by this afternoon per 850 mb temp progs. Nevertheless, highs will be several degrees colder today in the 40s. Lows tonight will be in the 20s again with the surface high translating east across the area. Then, as has been advertised for several days, mid to upper level ridging and a turn to south-southwest flow in low levels will bring a rebound in temperatures on Saturday. With 850 mb temps in the 5-6C range, highs in the lower to middle 50s are expected. As a strong shortwave begins to eject northeast through TX, a broad fetch of southerly flow off the Gulf of Mexico will develop over the central and eastern Gulf Coast Saturday afternoon and evening. Although trending a bit slower now, isentropic lift and moisture transport will be significant over the lower MS valley into the mid South region Saturday night. This should spread rain showers and elevated high based convection across areas just to our west through 12Z. Despite the shortwave dampening as it arrives, deep southwesterly flow will bring a steady feed of moisture (PW of greater than 1 inch) and isentropic ascent. Our eastern counties may need a bit more time to saturate due to the very dry low level dew points. But expect this to be overcome quickly by the strong UVV and large scale precipitation area that will be spreading eastward. This first "wave" of rainfall will diminish and/or end from west to east late in the day into Sunday evening. If the GFS and NAM are correct, the synoptic pattern will become quite favorable for heavy rainfall again Sunday night with a weak upper ridge position and ample south- southwesterly Gulf inflow. Meanwhile, the ECMWF is remaining the outlier keeping a slower approach and dry weather Sunday Night into Monday. Will side closer to the wetter solutions for now given so many GFES plumes are suggesting this. The low level boundary will be fortified with Sunday rainfall, producing a good thermal/convergence zone for renewed development Sunday night with the aid of a nocturnal increase in the low level jet (40-50kt). The best chances of heavy rainfall will occur from central MS into north central AL, so have gone with our higher PoP south of highway 72. .LONG TERM...(Sunday night through Thursday) Issued at 202 AM CST Fri Dec 15 2017 A progressive prevailing westerlies pattern shift will be well underway by Monday as a ridge axis centered over the Bahamas continues to bring southerly flow across the TN Valley while successive shortwave troughs impact the ridge. The differences in ridge strength between the medium range models make the Monday/Tuesday timeframe particularly uncertain. So, additional refinements to this forecast are likely. A couple of things that do seem to be consistent: 1) Temperatures are definitely warmer due to the warm air advection in place with daytime highs in the low 60s on Mon/Tues with some slight fluctuations on Wed/Thu in the 50s/near 60 degree highs. 2) Ensembles (GEFS) are showing a wetter pattern this next week with some QPF amounts as high as 2-4 inches during the extended forecast. With the caveats in mind, the strong ridge stretching from the Bahamas to the Southeast may cause an oncoming shortwave trough to deamplify and with the resulting weaker lift result in lighter rainfall on Monday. However, the Canadian/ECMWF are showing weak undulations moving across the MS River Outlet of LA to central AL/GA which may concentrate rainfall over the SE portions of the forecast area. Have kept the overall trend in higher POPs over central and northeast Alabama. An additional noticeable trend is steepening lapse rates aloft from the Gulf of Mexico to just south of the TN River. Though the instability is rather weak (100-300 J/kg), there may be enough to support thunderstorms on both Monday and Monday night ahead of a shortwave trough moving across the Southern Plains/Ozarks. The thunderstorm threat may spread north over the rest of the forecast area on Monday night as warm air advection strengthens. This oncoming shortwave trough's progression definitely differs among the medium range models with the ECMWF showing a much weaker and further north solution due to the stronger ridge solution. However, will keep chance of rain in the forecast on Tuesday before a modified cold front arrives ahead of this shortwave trough and brings slightly cooler and dry air for Wed. Another broader polar jet trough arrives on Thursday with a cold front. This trough and accompanying front may provide additional showers and have kept the Superblend POPs in the forecast. && .AVIATION...(For the 12Z TAFS through 12Z Saturday morning) Issued at 515 AM CST Fri Dec 15 2017 VFR flight weather conditions are expected during the next 24 hours. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ SHORT TERM...17 LONG TERM...SL.77 AVIATION...17 For more information please visit our website at weather.gov/huntsville.