685 FXUS63 KLSX 130537 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1137 PM CST Thu Jan 12 2017 .UPDATE... Issued at 950 PM CST Thu Jan 12 2017 The 00z NAM continues to support the current timing of onset of what will be predominantly a freezing rain event for our region. There is some indication for support of the initial onset from approximately I-70 northward could briefly be sleet or a freezing rain and sleet mix. QPF has come in higher for Friday afternoon, particularly for STL metro, possibly in response to an atmosphere more convectively favorable. But the NAM on the last snow event had QPF that was much too high and something to take into account before a knee-jerk bumping up of this factor. The 00z GFS has just arrived and has quickened its northward pace a bit (it was slower than the NAM) and now matches the NAM almost exactly for onset times. This also is reflected well with the current forecast. The 00z GFS likewise has boosted its QPF for late morning and afternoon Friday, particularly around the I-70 corridor: to 0.20 to 0.35" for STL metro and up to 0.2" for central Missouri. Not as high as the NAM, but also not showing an atmosphere as convectively favorable. This would tend to give a support to the NAM trend but not quite on actual magnitude. The GFS also supports the idea of an initial pcpn-type as sleet or a sleet/freezing rain mix briefly before switching to all freezing rain. Surface temps continue to be advertised as sub-freezing thru the day on Friday and for many areas into at least Friday evening. The main change for the evening update will be to incorporate some sleet into the onset pcpn-types for around I-70 northward, otherwise the next shift will hammer out any QPF increases. TES && .SYNOPSIS... Issued at 319 PM CST Thu Jan 12 2017 Storm highlights - 1) All areas converted to an ice storm warning due to forecast of 0.25 ice accumulations and greater. Warning timing starts late tonight in the south, and Friday evening in the far north. Ending in the far south by daybreak Sunday and ending in the north late Sunday afternoon. 2) Overall looking at ice accumulations from around .25 to .50 inches, perhaps a little higher in central MO. 3) Impacts will be largely confined to elevated surfaces - icing on bridges, trees, and power lines, cars... Untreated roads may have some issues Friday-Friday night when the coldest surfaces temps are expected. Given this amount of ice and light winds, power outages are typically isolated to scattered. Glass/Tilly && .SHORT TERM... (Through Late Friday Afternoon) Issued at 319 PM CST Thu Jan 12 2017 Focus thru this period is the impending icing event. A large, arctic sfc ridge, stretching nwd well into nrn Canada, will continue to build into the area this evening. As it builds ewd into the Great Lakes region, sfc winds across the area will continue to veer to become ely. Just off the sfc thru low levels, WAA will return and begin the moisture transport nwd tonight. Still expect precip to arrive in srn portions of the CWA around 09z tonight. However, have low PoPs beginning in this area at Midnight to show some uncertainty. This area of precip will slowly move nwd late tonight and thru Fri. Precip shud begin late tonight as more of a shower activity and become more stratiform on Fri with better forcing. Precip may begin briefly as sleet, but shud transition to FZRA fairly quickly and remain FZRA thru this period. There is still some uncertainty regarding ice accumulations. Precip tonight shud be rather light QPF, and therefore ice amounts are probably on the higher end. With temps in the upper 20s to around 30 where precip is occurring, wud not expect a lot of ice. With the recent warm temps the past couple of days, the ground temps remain rather warm according to MO Mesonet sites. This shud help prevent problems with roads and much of the accumulation limited to elevated sfcs. Even sites thru mid sections of the CWA show 2 inch soil temps of 34 degrees. However, across nrn portions of the CWA, these temps are already aob freezing, which shud continue to cool tonight with temps in mid teens expected. Temps on Fri are going to be tricky. Temps at onset of precip in the morning will likely remain steady until the precip comes to an end. Temps may warm slightly north of the area of precip, but shud cool again as precip begins with wetbulb effects. Regardless, it appears that temps will max out right at the freezing mark, or perhaps a degree or perhaps two warmer. However, those warmer temps shud be brief. Fri still appears to be the worst time for this event overall. Given the recent warm temps, light winds and precip falling during largely marginal temps, do not currently anticipate a major ice event. The going ice accumulations shud be considered a higher end forecast for now. Tilly .LONG TERM... (Friday Night through Next Thursday) Issued at 319 PM CST Thu Jan 12 2017 Freezing rain will continue to overspread the remainder of northeast MO and west central IL early Friday evening. Like other areas earlier in the day on Friday, there could be a brief period of sleet at the onset, but this will be short-lived as strong low-level WAA continues above the shallow cold dome. There are also suggestions that the precipitation intensity/rate and hence overall ice accumulations will diminish on Friday night into Saturday. Reasons for this appear to be two-fold: 1) the initial impulse aloft scoots by early Friday evening and the LLJ veers to more westerly diminishing the isentropic lift; 2) we get some warming of surface temperatures above freezing across southern portions of the CWA, and this is where the majority of the precipitation may occur late Friday through Saturday. Another wave of more significant precipitation will impact the area late Saturday night into Sunday with high precipitation rates as an impulse aloft moves east- northeast across the area and backing mid-upper flow once again results in increasing isentropic ascent. The thermal characteristics of the atmosphere remain the biggest challenge. The elevated warm layer is stout, well above freezing, as is the depth of the warm layer aloft. So surface temperatures are the main issue. A broad corridor centered along I-70 has the most uncertainty. There will be impacts of latent heat release from the freezing rain acting to warm surface temperatures, and then late Saturday into Saturday night the surface high builds in a bit from the north and winds veer to more n/nely bringing weak CAA. Thus this region could see a change from ZR to R on Saturday and then a change back from R to ZR on Saturday night, lasting into Sunday morning. On Sunday and into Sunday night, low level warming should progress from south to north as surface winds become more southeasterly and winds above the boundary layer strenghen from the south, driving all surface temps above 32F. This will occur attendant with the main surface low moving from the TX Panhandle into south- central KS. The continued north-northeast movement of the surface low from the central Plains into the Upper MS Valley Monday into early Tuesday will bringing an expanding warm sector, mild temps, and strictly liquid precipitation. In fact the models show enough instability Monday afternoon-night to suggest some thunder potential along and in advance of the progessing cold front. The cold front with the northeast moving surface low should exit the area by Tuesday evening, and present thoughts are pops may be too high the remainder of the week. Temperatures Monday and beyond look to be above normal thru the week. Glass && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1137 PM CST Thu Jan 12 2017 Low clouds have moved off to the southeast--for now--with VFR conditions prevailing at the TAF sites. Clouds will return Friday morning with lowering CIGs into MVFR range by late morning or midday Friday for COU and the STL metro sites with pcpn entering the picture almost at the same time. Pcpn-types continue to look overwhelmingly FZRA thru the valid period with increasing evidence of PL mixing in for the first 2-3 hours of intial onset. Conditions should continue to deteriorate sufficiently to IFR as we head into Friday night. TES && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Ice Storm Warning from 3 AM Friday to 6 AM CST Sunday for Crawford MO-Iron MO-Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Washington MO. Ice Storm Warning from 3 PM Friday to 3 PM CST Sunday for Audrain MO-Lincoln MO-Monroe MO-Pike MO-Ralls MO. Ice Storm Warning from 9 AM Friday to noon CST Sunday for Boone MO-Callaway MO-Cole MO-Franklin MO-Gasconade MO-Jefferson MO-Moniteau MO-Montgomery MO-Osage MO-Saint Charles MO- Saint Louis City MO-Saint Louis MO-Warren MO. Ice Storm Warning from 6 PM Friday to 6 PM CST Sunday for Knox MO-Lewis MO-Marion MO-Shelby MO. IL...Ice Storm Warning from 3 AM Friday to 6 AM CST Sunday for Randolph IL. Ice Storm Warning from 3 PM Friday to 3 PM CST Sunday for Calhoun IL-Greene IL-Jersey IL-Macoupin IL-Montgomery IL- Pike IL. Ice Storm Warning from 9 AM Friday to noon CST Sunday for Bond IL-Clinton IL-Fayette IL-Madison IL-Marion IL-Monroe IL- Saint Clair IL-Washington IL. Ice Storm Warning from 6 PM Friday to 6 PM CST Sunday for Adams IL-Brown IL. && $$ WFO LSX