Area forecast discussion National Weather Service Des Moines Iowa 314 am CDT Thursday may 23 2013 Short term.../today /... issued at 313 am CDT Thursday may 23 2013 Upper low will move out of the area today as surface trough extending from northern Illinois into northwestern Iowa rotates south across the state this morning. Isolated to scattered showers over the east half of the County Warning Area should come to an end early this morning as drier air begins to push south behind the trough. High pressure centered over northern Manitoba will push south today. This will bring gradual clearing from the north today. Cool 800 mb air for this time of the year will put a limit on the high temperatures today with forecasts of +3 to +7 across the state this afternoon. Sunshine should overcome the cold air advection and give highs in the 60s across the County Warning Area this afternoon. Long term.../tonight through Tuesday/... issued at 313 am CDT Thursday may 23 2013 Little has changed in overall forecast thinking this shift. Tonight and Friday will remain dry and uneventful with a large surface high pressure center sliding from The Arrowhead of Minnesota across Wisconsin and into the Great Lakes region. This will result in northeast winds this evening becoming calm or light and variable...then turning around to south southeast on Friday. With calm/light winds and dry air filtering out of the high pressure area temperatures will drop to well below seasonal levels tonight especially across our northeastern counties where Friday morning lows in the upper 30s are forecast in some areas. However...temperatures are not expected to reach levels sufficient for frost formation. By Friday night the axis of the central U.S. 500 mb ridge will be located over Iowa...while a large 500 mb low centered over the Pacific northwest will have carved out a deep but broad trough down the West Coast. This general pattern will then remain in place through the entire remainder of the seven day forecast. This will lead to an overall wet and active forecast for the area this weekend and the first half of next week as a train of middle-level impulses move up the southwesterly flow on the western flank of the ridge and over the midwestern states. Unfortunately any attempt to pin down the precise timing and location of these impulses...much less the precipitation they produce which will also be dependent on the location of subtle surface boundaries and other difficult to predict features...is futile at this point. Therefore the forecast remains peppered with probability of precipitation for the foreseeable future. From Friday night through Sunday have maintained probability of precipitation generally in the 40 to 60 range...which then decrease steadily into next week as confidence decreases further. The good news is that the overall synoptic set up does not lend itself well to large severe weather events...however at this time of year such a prolonged period of thunderstorm chances implies that some degree of severe weather will probably be achieved at some point. In addition...heavy rain could result in localized flash flooding and possibly larger scale areal flooding depending on whether thunderstorms form over the same areas day after day or shift around the region a bit. && Aviation...23/06z issued at 1055 PM CDT Wednesday may 22 2013 Main concerns continue through 12z with MVFR/IFR ceilings and dz/-shra crossing the area with last upper level trough and surface trough now heading southeast across central Iowa. Upper level system over kmke but still has wrap around moisture extending back into southern Minnesota and northern Iowa affecting most of the eastern taf sites including kmcw...Kalo...kotm and to a lesser extent kfod and kdsm though both of those sites will see passing -dz or -shra through 12z. Finally the low drifts east and VFR conditions expected to return to area after 12-15z as weak ridge of high pressure builds southeast. Some concern with extent of clearing...however as subjective h850 analysis shows trough axis back to next upstream system over Utah. With brief return to northwest flow aloft...clearing still more likely to occur than not due to late may solar influence. && Dmx watches/warnings/advisories...none. && $$ Short term...MS long term...Lee aviation...rev