Area forecast discussion National Weather Service Huntsville Alabama 330 am CDT Tuesday may 21 2013 Synopsis... latest water vapor satellite imagery showing a large upper level vortex spinning over the northern plains (dakotas) with a smaller upper level circulation moving across New Mexico...along the base of the main vortex. Farther east...a narrow upper level ridge of high pressure resides over the Tennessee/Ohio valleys but is getting squeezed between the broad upper low over the northern plains and several weaker upper level disturbances along the East Coast...between Florida and Virginia. This primary upper level vortex which has been responsible for the devastating tornadoes across OK/Texas the past few days...will gradually migrate east over the next 24-48 hours...losing much of its punch in the process but still setting the stage for a few rounds of showers/thunderstorms across the Tennessee Valley late tonight through Thursday. Latest regional radar mosaic showing well-defined line of thunderstorms just crossing the MS river into western Tennessee/northwest MS. General movement of the storms is N/NE...with a few discrete storms seen developing ahead of the line over the past couple hours. Djn.83 && Discussion... warm and mainly conditions are expected today as the narrow upper ridge holds one more day across the region. Broad area of convection extending from East Texas to the upper Midwest/Great Lakes will continue to drift east through the morning. Aforementioned line of storms pushing into western Tennessee/NE MS has created a fairly pronounced rain-cooled outflow boundary which should end up affecting portions of northwest Alabama later this morning. This boundary could initiate new convection as airmass heats up by midday. How far east this potential convection GOES before interacting with upper ridge will be a challenge today. On the front side of the upper ridge...weak energy rotating counter- clockwise around East Coast disturbances could touch off a few rain showers/thunderstorms and rain across the southern Appalachians by early afternoon. Discounting the overly aggressive 00z/21 NAM for today. Overall...believe much of the area will remain dry today...however will mention threat of isolated afternoon (10-20% pop) rain showers/thunderstorms and rain across northwest Alabama and NE Alabama/southern middle Tennessee. If anything develops...not expecting convection to last long beneath upper ridge. Tonight into Wednesday...deeper moisture and energy arrive as longwave upper trough finally approaches. Convection will likely sustain itself along leading pre-frontal/850 mb boundary with maximum low level convg and low level wind energy existing across northwest Alabama by 12z/Wed. Probability of precipitation overnight will be highest over northwest Alabama...with lesser chances to the south and east until during the day Wednesday where increased low level forcing /albeit weak/ along with a Theta-E maximum will support scattered-numrs rain showers and a few thunderstorms and rain across the forecast area. Marginal low level shear/instability and relatively weak middle level lapse rates suggest a low potential for severe weather Wednesday at this time. However...a few heavier showers/storms embedded in perhaps a weak lewp could produce brief strong wind gusts late tonight through wedensday afternoon. Wednesday night/Thursday...shra/tsra are expected to diminish Wednesday night but moist/unstable airmass coupled with passing upper level waves could keep a few -shra going...especially across NE Alabama. Surface front/leading edge of much drier air will extend across northern Arkansas/northwest Tennessee/western Kentucky by 15z/Thursday. This boundary will acceleate southeast and cross the forecast area Thursday night. Before this happens...still influenced by broad longwave upper trough...additional upper level energy within northwest flow aloft will March across the area Thursday. This coupled with the approaching surface boundary could support a brief round of rain showers/thunderstorms and rain Thursday afternoon. The 00z/21 NAM is the most robust with this potential solution but even the medium range models show something coming together. Extended /Friday through Monday/... much drier and cooler conditions will prevail Friday into Saturday as high pressure builds across the eastern Continental U.S. In the wake of a departing longwave trough. After a cool start Friday morning /lows in the middle 50s/...temperatures should rebound to the middle to upper 70s beneath plenty of sunshine. The coldest morning is expected to be Saturday as lows could fall to the upper 40s in some valley locations. A "dirty" ridge of high pressure over the central Continental U.S. Will gradually move across the region by the end of the weekend into early next week. With the area in a northwest flow aloft...the 00z/21 European model (ecmwf) is slightly wetter and more aggressive with upper waves traversing the down the frontside of the upper ridge. This is a new solution and relative outlier with the other oper model/ensembles...so will lean toward a drier solution Sunday/Monday but show increasing clouds. Temperatures will rebound several degrees each day sun/Monday under the influence of upper ridge. Djn.83 && Aviation... /issued 1216 am CDT Tuesday may 21 2013/ for 06z tafs...will continue in the near term with VFR conditions at both hsv and mean sea level. Will keep in a tempo 5sm br group at mean sea level from 09z to 13z to account for some fog that may develop. VFR conditions are then expected through the rest of the forecast for hsv and mean sea level. Tt && Preliminary point temps/pops... Huntsville 88 68 81 62 / 10 20 50 30 Shoals 87 67 81 62 / 20 30 50 20 Vinemont 85 65 78 59 / 10 20 40 30 Fayetteville 87 68 77 62 / 10 20 50 30 Albertville 84 68 80 61 / 10 10 40 40 Fort Payne 85 64 78 60 / 20 10 40 40 && Hun watches/warnings/advisories... Alabama...none. Tennessee...none. && $$ For more information please visit our website at weather.Gov/Huntsville.