Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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291 FXUS63 KDMX 110852 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 352 AM CDT Thu Jul 11 2024 .KEY MESSAGES... - Pleasant summer day today with light winds. - Heat arrives for this weekend. Heat index values near to above 100 possible Saturday and Sunday afternoons. The heat may persist over central and southern Iowa on Monday. && .DISCUSSION... Issued at 346 AM CDT Thu Jul 11 2024 Upper level low pressure is over east central Iowa early this morning. A few showers and storms are still ongoing across parts of east central and into southeast Iowa in response to the system. The precipitation is co-located with the system`s upper level cold core evident around 600 mb and higher. This activity will be diminishing over the next several hours as the upper level system begins to depart to the east. A few areas of fog have also developed early this morning with the fog is located along and south of Highway 34 over far southern Iowa and also over parts of far northwest Iowa. Additional expansion of the fog is possible this morning in areas with the clear skies. Otherwise for today, surface high pressure will control the weather with light winds today. Some shallow and flat cumulus development is possible by this afternoon though the cumulus over northeast Iowa could have more vertical extent than central Iowa though no precipitation is expected. Warm advection will begin to lift into southwest Iowa tonight but the moisture advection will be lagging and a very dry air mass will remain over much of Iowa and any precipitation should remain south into Missouri. The moisture advection will begin to pivot into Iowa on Friday but precipitation is not expected as it will take time for profiles to saturate and the Elevated Mixed Layer (EML) will become established which will cap off any surface based convection. The low level jet will tilt into southern Iowa Friday night and will bring some elevated instability to that part of the state. A few elevated storms are possible if enough saturation can occur aloft and near the forcing to release some of that instability. The EML is expected to become more pronounced on Saturday and Sunday as more warm advection aloft occurs and a portion of the thermal bubble to the west arrives. This brings the million dollar question that we have been discussing the past several days, how warm will it get this weekend? All points discussed during these past several days remain valid. One, the boundary layer should mix well below the EML but it is capped due to the EML somewhere below 850 mb. Two, surface dew points will be on the increase due to moisture advection and a very big second this time of year, we are entering peak evapotranspiration of Iowa crops. What impact will the mixing have on the dew points. The most like scenario is dew points will be in the low to mid 70s by mid afternoon. Three, the NBM temperatures remain near the 90th percentile or higher and are likely overdone. That said, the NBM spreads are fairly low and the 925 mb temperatures at 12z each morning are in the mid 20s C which would support a launch into the 90s by the afternoon. Finally, Four, does it matter meaning higher temperatures with lower dew points or lower temperatures with higher dew points will result in similar heat index values. Expecting heat index values of 95 to low 100s on Saturday and roughly 100 to 105 on Sunday. Heat will be the key messaging this forecast period. A buckle in the western upper high is still on track sometime Monday or Monday night which will drop a boundary into the state. The exact timing will play a role on if the heat remains Monday. At this time, some relief into northern Iowa is expected but the heat may linger central and south. A more active period in the weather looks possible by mid next week as a stronger upper level system passes through the region. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1048 PM CDT Wed Jul 10 2024 A few showers linger in eastern Iowa, otherwise activity has ended across the area. Overnight patchy fog remain possible, however data is split on where this will be impactful at TAF sites. Have included MVFR mention at KMCW/KFOD where fog is more likely. Amendments may be necessary as conditions change through the early morning. Otherwise winds shift to out of the east/southeast with VFR conditions to finish out the period. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Donavon AVIATION...Hagenhoff