000 FXUS63 KLMK 100518 AFDLMK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LOUISVILLE KY 1217 AM EST FRI FEB 10 2012 .Update... Issued at 930 PM EST Feb 9 2012 Clouds are eroding a little quicker from the south than forecast...so issued a quick update to account for this trend. This change is noted with 850 mb RH fields in the latest RUC, which also shows some subsidence occurring in that layer. Still should see enough mid/upper clouds streaming in from the southwest to keep at least partly cloudy wording in the forecast. With this change though and with light winds as a result of high pressure over the region, did drop temperatures some over south central Kentucky to midnight. After midnight, temperatures should level off as those clouds thicken up a bit. The updated temperature forecast has lows pretty much between 26 and 30 areawide. .Short Term (Tonight through Friday Night)... Updated at 325 PM EST Feb 9 2012 ...Wintry Mix of Weather Expect Friday with Snow Accumulations Likely Friday Night... Afternoon satellite and surface observations reveal mostly cloudy skies across the region this afternoon. Low level moisture has failed to scour out despite the numerical modeling suggesting that it would. In addition, the clouds have also kept temperatures well below forecast. In the very near term, have updated the forecast to go with mostly cloudy skies and temperatures topping out in the lower to mid 30s in the north with upper 30s to the lower 40s in the south. Temperatures are likely at the current highs for the day so a slow fall is expected through the evening. For tonight, mostly cloudy skies are likely to persist through the overnight hours as low level moisture will remain entrenched across the region. In addition, additional high level clouds ahead of the next weather system will add yet another layer of clouds across the region. Since we're already below guidance...I've trended the ongoing forecast slightly below the new guidance with overnight lows in the upper 20s to around 30 in the north with lower 30s in the south. Main forecast challenge is in the Friday and Friday night period as a complicated weather pattern is expected to unfold across the region. Two mid-level waves, one in the southern stream and one in the northern stream are forecast to impact the region in the Fri-Fri night. The models continue to maintain separation between the systems and a phase is not anticipated. The southern stream will be the first to arrive early in the day on Friday. This system will spread a swath of moisture northeastward into central Kentucky and southern Indiana during the day. As the wave approaches, some warm air advection will bring a slightly warmer layer of air aloft into the region. This results in thermal profiles being very close to critical thresholds for liquid vs. frozen precip...despite that surface temperatures will likely be above freezing through the day. Based on the variety of deterministic and ensemble thermal soundings feel that the bulk of the precip on Friday will be in the form of a rain/snow mix across southern Indiana and the northern half of Kentucky with just plain rain across the south. The secondary wave in the northern stream will arrive Friday night and push a strong cold front through the region. Colder air will push in behind this front and change the rain/snow mix over to snow during the evening hours. The exact change over time is somewhat in question since the models have been too warm with the vertical profiles of late. Taken verbatim, the vertical soundings would suggest a change over by late evening across the north with a change over in the south during the overnight hours. Given the warm bias, we've sided on the colder side of the guidance envelope and will start changing things over Friday evening with full change over by midnight across the south. As far as accumulations go, there is a bit of spread between the GFS and NAM guidance on snowfall totals. The GFS has a more smeared out QPF which paints 1-2 inches of snowfall in the north with as little as an inch across southern KY. The NAM on the other hand has two snowfall maximas...one across southern Indiana and another across southern KY. Given that we're still over 24 hours away from the event, I expect a bit more forecast convergence with the future runs. For now, it appears that an inch of snow will be possible across southern KY with 1-2 inches of snow being possible across southern Indiana and northern KY...mainly north of I-64. Much of this accumulation will not take place during the daylight hours...but will occur Friday evening as the colder air pushes in. It should be noted that these amounts are subject to change and may be adjusted with subsequent forecast updates. With the colder air pushing in Friday night, temperatures are expected to fall into the lower-mid 20s. These temperatures combined with the snow and wet roads will likely result in hazardous driving conditions Friday night. Therefore, after coordinating with surrounding offices, a Winter Weather Advisory will be issued with this forecast issuance. It will run from 6PM Friday night through 12 Noon Saturday. .Long Term (Saturday - Thursday)... Updated at 309 PM EST Feb 9 2012 By Saturday, the bulk of the precipitation from the system on Friday night will have moved east of the forecast area. However, with a northerly fetch off of Lake Michigan, it looks like lower levels may stay saturated during the morning and possibly into the afternoon. A stratocu deck lingering over the region in the morning may bring a few flurries, especially to the northern half of the forecast area. Skies should slowly clear through the afternoon and into the evening. Winds Saturday will be out of the north around 10-15 mph with strong cold air advection. Combine all this and Saturday will looks to be cold and raw with temperatures struggling to make it out of the 20s in the north and reaching only around 30 in the south. With mostly clear skies expected on Sunday morning and high pressure over the area, lows will drop into the low to mid teens. The high pressure will slide east Sunday into Monday. Temperatures will slowly moderate a bit as winds shift to southerly again. Moisture will stream into the middle of the country with precipitation moving into west central Kentucky and southeastern Indiana Monday evening. This system bears watching as thermal profiles are coming in a bit colder over the last few runs. We will have either a mix of rain and snow or possibly all snow overnight. This should then transition back to all rain on Tuesday as temperatures rise into the lower to mid 40s. Wednesday will be dry as high pressure once again quickly slides across the region, with the next shot at rain Wednesday night into Thursday. Temperatures will be warmer Wednesday and Thursday with highs in the lower 50s. Lows will be in the upper 30s to lower 40s. && .Aviation (06Z TAF Issuance)... Updated at 1215 AM EST Feb 10 2012 Low-end VFR ceilings persist at SDF and LEX, but BWG has scattered out. Expect these conditions to persist through late Friday morning, with the exception of the ceiling getting back into BWG around daybreak. Winds will remain light and variable, but start to pick up out of the SW mid-morning. Southern stream system will phase with a cold front approaching from the NW, and precip will develop over Kentucky around midday Friday. Low to moderate confidence regarding precip type, but our best guess is starting out as rain in BWG and a rain/snow mix at SDF and LEX. Main bust potential would involve SDF and LEX as all snow from the onset. Initially high-end MVFR conditions until the snow gets going in earnest. Look for all snow at SDF and LEX by late afternoon/early evening as the cold front makes its way into the immediate area. Conditions will deteriorate to IFR for most of the evening. Cold front will move through SDF just before 06Z Saturday, with fuel-alternate MVFR ceilings and increasing NW winds behind it. && .LMK WATCHES/WARNINGS/ADVISORIES... IN...WINTER WEATHER ADVISORY FROM 6 PM EST /5 PM CST/ THIS EVENING TO NOON EST /11 AM CST/ SATURDAY FOR INZ076>079-083-084- 089>092. KY...WINTER WEATHER ADVISORY FROM 6 PM EST /5 PM CST/ THIS EVENING TO NOON EST /11 AM CST/ SATURDAY FOR KYZ023>043-045>049- 053>057-061>067-070>078-081-082. $$ Update...........RJS Short Term.......MJ Long Term........EER Aviation.........RAS