979 FXUS64 KHUN 071203 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 603 AM CST Thu Nov 7 2019 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 500 AM CST Thu Nov 7 2019 Mid-level flow will continue to strengthen and back to the west- southwest across the TN Valley this morning, in the contracting height gradient between the base of an amplifying longwave trough to our north and a subtropical ridge retrograding across the southern Gulf of Mexico. A small but well-defined disturbance embedded within the flow aloft is currently lifting east-northeastward across AR and has contributed to the development of widespread precipitation on both sides of an Arctic cold front that is surging southward through the Ozarks and Red River Valley of OK/TX. We still expect the coverage of warm sector showers to increase across the local area beginning shortly after sunrise, as strengthening deep-layer ascent begins to overspread the increasingly moist airmass characterized by precipitable water values in the 1.2-1.4 inch range. A consensus of near term guidance still indicates that the Arctic cold front will enter the northwestern portion of the CWFA around 18Z, and should spread rapidly southeastward -- exiting our region around 08/00Z. Synoptic scale ascent will remain rather strong for several hours in the wake of the frontal wind shift axis, and should even increase a bit late this afternoon as another mid-level disturbance lifts east-northeastward into northern MS. Thus, we expect a fairly widespread coverage of light stratiform rainfall to occur even as low-level flow veers to the north and strengthens in the shallow cold airmass. Due to an early onset to thicker/lower clouds and precipitation this morning, highs should only reach the l-m 60s, before CAA results in falling temperatures this afternoon. .SHORT TERM...(Tonight through Saturday night) Issued at 500 AM CST Thu Nov 7 2019 Widespread light (but measurable) stratiform precipitation will likely still be in progress across much of our region at 00Z, but should end fairly rapidly from NW-to-SE between 00-03Z, as the deepening longwave trough to our north shifts into the eastern Great Lakes/western New England veering deep-layer flow to the west- northwest and inducing stronger descending vertical motions. However, forecast soundings suggest that low postfrontal stratus clouds will be much slower to dissipate, and we would not be surprised to even see a few pockets of snow flurries during the early morning hours, especially in the higher terrain of southern TN/northeast AL. Due to a fairly high coverage of stratus clouds and strong north- northeasterly flow in the boundary layer, we have raised lows a bit above model blends. However, even with air temps expected to only reach freezing across the northern third of the forecast area, apparent temps will still fall into the l-m 20s around sunrise, and conditions will flirt with Wind Advisory criteria in our higher terrain locations, where sustained winds around 25 MPH and gusts up to 35 MPH are expected overnight. Seasonably strong west-northwest flow aloft will continue on Friday, with an increase in cirroform clouds anticipated during the afternoon as a trailing vort max shifts southeastward into the OH Valley. Mid/high-level flow will veer to the northwest in the wake of this disturbance, as it tracks further southeastward into the southern Appalachians Friday evening, and this may allow for additional high clouds to complicate an otherwise favorable radiational cooling event Saturday morning. Regardless of this, we have still predicted a hard freeze for much of the region, which may be sufficient to terminate the Frost/Freeze program for locations west of the I-65 corridor in AL. Very pleasant conditions are expected on Saturday, as a deepening surface low tracking through the northern Great Lakes will induce a light southwesterly flow and allow highs to rebound into the m-u 50s. Lows will fall into the l-m 30s once again on Sunday, in an environment more favorable for radiational cooling. .LONG TERM...(Sunday through Wednesday) Issued at 500 AM CST Thu Nov 7 2019 High pressure shifting to the east will bring southerly flow and moisture back into the area on Sunday. This will help warm temperatures up into the low 60s on Sunday with sunshine. However, an approaching front will be looming on the horizon. During the day on Monday we will see increasing clouds ahead of the approaching front. Recent model runs have now trended into agreement that we will have precipitation with this front, so started increasing PoPs starting Monday afternoon. Most of the precipitation with this front will occur right along and behind the front, so as soon as rain starts, expect temperatures to start to drop. Models have been in good agreement and have consistently showed this arctic air mass moving into the Tennessee Valley behind the front Monday night into Tuesday morning. With the addition of precipitation, the big question becomes will we see snow? Right now, we just don't know at this time. Right now the models are showing a good possibility of a changeover to at least a rain/snow mix as precipitation is coming to an end Tuesday morning. A lot can still change as we get closer to Tuesday, and any shift in the ending time of this precipitation will increase or decrease the chance of any snow. Right now, went with a full changeover to snow in far NW Alabama and south-central Tennessee, and a rain-snow mix elsewhere generally east of I-65. Little-to-no accumulation expected as the ground should be too warm for snow to accumulate very easily, but those that wake up early on Tuesday could see snowflakes in the air and snow trying to accumulate on grassy and elevated surfaces. Precipitation should be coming to an end between 12Z-18Z, though perhaps a few rain/snow showers could linger into Tuesday afternoon over the Cumberland Plateau in NE AL. Temperatures on Tuesday will only get up into the 35-40 degree range, and cold northerly winds will not help. A strong pressure gradient behind the front as high pressure builds in will bring 10-20 mph winds that will continue through most of the day, which will make temperatures feel like they stay in the 20s. Skies will be clearing Tuesday night and winds will be weakening, allowing temperatures to plummet into the upper teens to around 20 degrees by Wednesday morning. Wind chill will make it feel like 10 to 15 degrees. With a cold start, temperatures are only expected to get up to around 40 degrees on Wednesday even though it should be a sunny day. && .AVIATION...(For the 12Z TAFS through 12Z Friday morning) Issued at 603 AM CST Thu Nov 7 2019 VFR cigs consisting of bkn stratocu beneath an ovc altostratus deck will fall rapidly this morning, as atmospheric moisture increases ahead of an approaching cold front. Prefrontal SHRA will also expand in coverage during this period, with sufficient coverage anticipated by 16Z to include this as the prevailing weather condition. The passage of a strong cold front this aftn will result in an abrupt wind shift to NNW around 19Z/MSL and 20Z/HSV, with a 4-5 hour period of lgt-mod stratiform rainfall expected in the wake of the wind shift axis -- ending at the terminals between 08/00-01Z. Ceilings will descend to IFR levels as coverage/intensity of SHRA increase later this morning, with MVFR vsby reductions anticipated as well. Although cloud bases will slowly rise early Friday morning, complete dissipation of the stratus deck may not occur until beyond the end of the valid TAF period. Sfc winds will veer from ESE-SSW this morning, before shifting to NNE and increasing to 10G20 knots just after frontal passage. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...70/DD SHORT TERM...70/DD LONG TERM...McCoy AVIATION...70/DD For more information please visit our website at weather.gov/huntsville.