371 FXUS63 KLMK 260446 AFDLMK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LOUISVILLE KY 1245 AM EDT FRI APR 26 2013 ...Updated Aviation Discussion... .Short Term (Now through Friday Night)... Issued at 300 PM EDT Thu Apr 25 2013 Latest surface analysis shows a weak cold front stretching northeast to southwest from Ohio, Indiana and back into west-central Kentucky. Some high-res guidance continues to show a shower or two sparking along this front, but due to the lack of low-level moisture (dewpoints in the lower 30s), think that conditions will remain dry for the remainder of the afternoon. Focus then shifts to frost potential tonight. An area of surface high pressure will shift east across the Ohio Valley overnight, settling across southern Ohio by 12Z Friday morning. This feature will cause all winds to go light for most of the overnight hours. However, as the high continues to shift east, some light return flow may develop across western Kentucky towards dawn. Additionally, some upper-level clouds will be pushing in ahead of the next approaching system, which will quickly limit radiational cooling across the west. For this reason, will hold off on a frost advisory for the southwestern FA, where the combination of clouds and light return flow should limit frost potential. Confidence is low on the western periphery of the advisory, as the clouds and/or winds coming in quicker/slower than expected may affect temperatures. However, do have high confidence in another widespread frost event across the Northern Bluegrass, where clouds and winds will be less of a factor. Lows will range from the lower 40s southwest near Bowling Green to the low and mid 30s northeast across the Northern Bluegrass. The next upper level system will remain south of the stronger westerlies, meaning it will slowly meander into the Ohio Valley late Friday into Saturday morning. The latest trends in guidance continue to exhibit a northern shift, which seems logical given the system is now being sampled by the upper-air network. Therefore, have raised pops across the region Saturday morning. Still think it may take awhile for the column to saturate, but isentropic ascent ahead of this upper-level low looks like it'll be enough to pull moisture far enough north to warrant at least slight chances across the whole area by 12Z Saturday. However, the bulk of the precipitation looks to come after 12Z, which is detailed in the long term section below. Given the expected mid and upper level clouds on Friday and Friday night, look for highs in the mid 60s, with lows Friday night in the mid to upper 40s. .Long Term (Saturday - Thursday)... Issued at 330 PM EDT Thu Apr 25 2013 Saturday through Monday Night... Latest suite of forecast data continues to be in fairly good agreement with the synoptic weather pattern in this portion of the forecast period. The upper low ejecting out of the southwestern US will move slowly across the Plains over the next 24-48 hours and then roll through the Ohio Valley just in time for the weekend. This feature will bring plenty of clouds, periods of showers, and much cooler than normal temperatures to the region for Saturday and into Sunday. At this time, the best coverage of precipitation still looks to be mainly across Kentucky...and in particular the southern half of the state. The models have been fluctuating a bit with their QPF fields but recently have come into better agreement in the last 12 hours. Still plan on running chance PoPs across southern Indiana and northern Kentucky for Saturday with likely to categorical PoPs (70-80%) across the southern half of the state. The high PoPs will continue into Saturday night as well. Forecast model soundings still do not look all that impressive with respect to instability. Plenty of moisture and upper level support look to produce plenty of rain showers with perhaps an embedded thunderstorm or two possible...mainly across southern KY. With all the cloudiness and rainfall, have lowered temperatures a bit with this forecast and will go with highs in the upper 50s in the north with upper 50s to near 60 in the south. Lows Saturday night will cool back into the lower-middle 50s. Plenty of cloudiness and shower activity will likely persist into Sunday as well as the upper low and associated surface low take a slower track through the Ohio Valley. Thus, have increased cloudiness and rain chances into Sunday as well. Overall, it appears that rainfall will probably not be as extensive on Sunday with better chances in the east and lesser chances in the west. Highs may be a touch warmer than Saturday with readings in the lower to middle 60s in the north and middle to upper 60s in the south. The upper low is forecast to pull out Sunday night, so we should see a diminishing trend in the showers by then with lows cooling into the lower 50s. A rather zonal flow pattern will set up for Monday and we should see a weak ridge of high pressure build into the region. Looks like we'll see a partly to mostly sunny day on Monday with temperatures moderating into the lower 70s and lows Monday night cooling into the lower 50s. Tuesday through Thursday... The long term guidance continues to suggest that the upper level pattern will become quite amplified through much of next week. Early in the period, a new upper trough is forecast to develop across the western US and then push into the Plains. High level blocking pattern looks to set up with more of an omega type pattern setting up over the eastern US. The GFS ensembles show 500 hPa positive height anomalies of over 2.5 standard deviations over Ontario by Wednesday and much of the Ohio Valley will be centered under the omega ridge axis. This should result in continued dry conditions with temperatures warming to above normal levels. Highs Tuesday-Wednesday look to warm into the mid-upper 70s with a shot of 80 degrees across the far south. Overnight lows will generally be in the mid-upper 50s. Model spread rapidly starts to increase by Thursday as the guidance starts to diverge a bit on how the upper air pattern will evolve. The strong positive height anomaly over Ontario and Quebec looks to continue and this will cause quite a bit of blocking over the eastern third of the US. As the upper trough across the Plains attempts to barrel eastward, the block will likely result in the upper trough closing off and turning into a large 500 hPa gyre across the northern Plains. It does appear that some sort of cold front or possibly a large occluded front will push through late Thursday and into Friday bringing a round of showers and storms to the region. Temperatures look to remain mild with readings in the low 70s and overnight lows in the 50s. Beyond the period, the general model consensus is for the upper level blocking to continue resulting in a large cut off low developing and dropping southward into the Midwest and possibly down into the central Plains. Several vorticity maxima rotating around the cut off could bring episodic rounds of convection to the area, but timing these disturbances is impossible at this point. With the region being on the east side of the gyre, temperatures look to remain below seasonal normals through next weekend. && .Aviation (06Z TAF Issuance)... Issued at 1245 AM EDT Fri Apr 26 2013 High pressure moving from Ohio to New York today will keep winds light and variable with well VFR flying conditions. An upper level disturbance approaching from the west will spread mid clouds into the region late this afternoon into tonight. Some light rain may begin to affect BWG by the end of the TAF period, but should hold off until after this TAF period at SDF/LEX. Conditions will still be well VFR with light winds. && .LMK WATCHES/WARNINGS/ADVISORIES... IN...FROST ADVISORY UNTIL 9 AM EDT THIS MORNING FOR INZ076>079-084- 090>092. KY...FROST ADVISORY UNTIL 9 AM EDT /8 AM CDT/ THIS MORNING FOR KYZ025- 028>043-045>049-053>057-064>067-077-078-082. $$ Short Term.......KJD Long Term........MJ Aviation.........13