792 FXUS64 KBMX 220539 AFDBMX Area Forecast Discussion National Weather Service Birmingham AL 1139 PM CST Mon Jan 21 2019 .UPDATE... For 06Z Aviation. && .SHORT TERM... Surface high pressure shifts across the Appalachians and over the Mid Atlantic tonight. This will allow low level winds to become southeasterly, and slowly bring moisture back to the area. With a moderating airmass, temperatures will be warmer tonight despite mostly clear skies. Lows will be in the upper 20s northeast to lower 30s elsewhere. Did not make many changes to the extended forecast, in order to maintain forecast consistency. The pattern has the potential to be active after the mid week system leaves behind large troughing over the conus. Expect model solutions to bounce around while trying to get a handle on the timing of clipper systems and just how cold the airmass will be that settles into the area late in the forecast period. Will lean toward maintaining forecast consistency instead of significant changes based on one model run. 14 .LONG TERM... Tuesday through Monday. A highly amplified synoptic pattern begins to set up early on Tuesday as a deep trough digs through the Western US and into the Central Plains. The surface high pressure in the Eastern CONUS continues to slide eastward, and we end up with more of a cold air damming setup with cooler easterly flow. A surface low lifts northeastward from the Central Plain into the Upper Midwest, stretching a cold front down through the MS River Valley by Tuesday afternoon. Ahead of this cold front, synoptic wedging between the approaching trough and a ridge to our east sets up moist southerly flow and isentropic lift. Therefore, by Tuesday afternoon/evening, I do expect some light rain to begin moving into our western-most area. Rain chances continue to trend up after midnight into Wednesday morning as the cold front approaches and pushes through the area from west to east. Due to better consistency in timing of guidance, I've increase PoPs to 90%+ for most all of Central AL Wednesday into Wednesday night as the front sweeps through. Some guidance continues to hint that very low amounts of elevated instability could work in Wednesday afternoon, so I've kept a slight chance for thunderstorms mentioned, but I don't think this will be widespread. Guidance has also come into better agreement with a very weak secondary low pressure developing over the Gulf States and lifting northeastward as the trough moves through. This will help wrap around the colder air quicker behind the departing cold front. I can't rule out a brief change over to some winter precip (mainly flurries) as the system exits late Wednesday evening into the overnight hours. Forecast soundings show a shallow saturated layer generally around - 7 to -8 degrees which could squeeze out a few flurries, but shouldn't be enough to produce significant accumulations or impacts. With the passage of this trough early Thursday morning, the synoptic pattern shifts to become a very large scale trough stretching through most of the CONUS. Within this broader trough, several shortwave clipper systems swing through for the second half of the week and into the weekend. This makes the forecast challenging for Thursday through Monday as any slight change in timing of any single clipper shortwave could lead to significant changes in the forecast. Essentially, it will be difficult to rely on any single deterministic model guidance, and we'll have to lean more towards the range of solutions in ensembles to come up with a forecast (even more than we already do). I've edged daily high and low temps away from blends that include MOS guidance and closer to blended raw guidance because the synoptic pattern becomes more anomalous. The NAEFs, GEFs, and ENS all highlight a below average temperatures into the early part of next week. Thursday will remain dry as surface high pressure moves through the Gulf States. Meanwhile the first clipper system slides through the Central Plains. Guidance varies substantially on how much moisture is available as this next cold front moves through after midnight Thursday night into Friday morning. Enough ensemble members show the possibility for light winter precip during this time, so I've added 15% chance of light snow showers for northern portions of Central AL. Confidence is low at this time, and any snowfall will be very light and shouldn't accumulate, so again, I don't expect any major impacts with this. I am more confident in the cold temperatures on Friday, where some locations in the far north may not get above freezing due to cold air advection behind the cold front. Another thermally-induced surface high pressure slides through the region Friday afternoon through Saturday as the next clipper system dips through the Central Plains. Current timing of this system brings the increased PoPs in as early as Saturday evening into the night. This will be a system that we'll have to watch for any winter precip as most guidance does suggest some transition line in the precipitation. However, there's not too much consistency in the precise timing, location, or orientation of the transition line. Plus, this is 6-7 days out and I have little confidence in the fine details of model guidance this far out. For now, I do have a chance for rain and/or a slight chance of winter precip mentioned for roughly the northern half of Central AL Saturday night through early Sunday morning, but I do expect this forecast to change. After the clipper system on Saturday/Sunday, slight synoptic height rises are expected for Sunday evening through Monday, so we should clear out any precip. However, the cooler air mass remains in place. 25/Owen && .AVIATION... 06Z TAF Discussion. A sfc ridge will slide east of Alabama before 12z and sfc winds will become east-southeast. The low level pressure gradient has already increased since sunset and winds have become easterly at 5-7 kts. Sfc wind speeds will continue to increase overnight and into Tuesday with sustained winds 10-13 kts with higher gusts. The air mass is very dry and it will take awhile for cigs to lower ahead an approaching short wave trof. MVFR cigs will likely develop across west Alabama arnd 21z and spread eastward. Showers will develop across west Alabama late in the period, and included VCSH at northern TAF sites after 00z. 58/rose && .FIRE WEATHER... Winds shift to the southeast tonight, allowing moisture to slowly increase across Central Alabama. Next rain system approaches the area Tuesday evening, with widespread rain expected Wednesday. Overall, no fire weather concerns. && .PRELIMINARY POINT TEMPS/POPS... Gadsden 27 49 43 56 31 / 0 10 60 100 100 Anniston 29 49 44 59 32 / 0 10 50 90 100 Birmingham 32 51 47 58 31 / 0 10 60 100 100 Tuscaloosa 33 52 49 57 31 / 0 20 60 100 80 Calera 32 50 47 59 32 / 0 10 60 100 100 Auburn 32 50 45 61 38 / 0 10 30 80 100 Montgomery 32 54 49 65 36 / 0 10 40 90 100 Troy 33 54 51 65 38 / 0 10 20 80 100 && .BMX WATCHES/WARNINGS/ADVISORIES/... None. && $$