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Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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144 FXUS63 KDMX 042256 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 556 PM CDT Thu Jul 4 2024 ...Updated for the 00z Aviation Discussion... .KEY MESSAGES... - Scattered thunderstorms north and northeast this afternoon and evening. Severe weather possible, primarily in the form of hail, along with locally heavy rainfall. - Relatively cool and cloudy on Friday with scattered light showers and isolated thunderstorms. - Steady temperatures from this weekend through the first half of next week, with periodic thunderstorm chances beginning around Saturday night. && .DISCUSSION... Issued at 218 PM CDT Thu Jul 4 2024 Well-anticipated thunderstorms are tracking across northern Iowa early this afternoon, within a region of enhanced instability ahead of an approaching cold front. Storm intensity has been held in check by a relative lack of low-level shear, but should slowly increase over the next few hours leading to a threat of severe weather primarily in the form of hail as the front and storms move eastward across the area. Once they clear out later this evening we will then see several hours of dry and quiet weather. However, the 500 mb low over Minnesota will drift slowly southeastward into Wisconsin by Friday morning, spreading a shield of clouds and light showers/isolated thunderstorms across parts of Iowa from later tonight in the north and expanding through the day Friday. Rain will be most likely across the northeastern half of the state, closer to the low, but there is enough signal further southwest that low POPs have been included for virtually the entire forecast area on Friday afternoon. The clouds will keep temperatures unseasonably low during the day, and have continued to nudge high temps down with low to mid 70s forecast across much of the area. This will also limit instability and severe weather is not expected with the isolated storms on Friday. From Friday night into Saturday weak ridging will briefly move across the area providing a respite, but by Saturday evening, as a broad mid-level trough begins to carve out over the north central U.S., the leading shortwave impulse within that trough will round the base near the South Dakota/Nebraska border and generate late day thunderstorms over Nebraska that will move eastward toward Iowa on Saturday night. Current indications are that decent flow aloft and broad forcing concurrent with the approaching mid-level shortwave could support some threat of strong to severe storms in our area, however, this is mitigated by only modest instability with such a short airmass recovery time behind the Friday coolness. Even so, with decent warming and moistening on Saturday afternoon, Saturday night will be a period to watch for potential severe weather. During the first half of next week, from Sunday through around Wednesday, the slowly deepening northern U.S. trough will dominate our region and prevent any significant warming, with daily highs generally in the upper 70s to mid 80s during that time. Subtle impulses moving through the trough will also provided opportunities for periodic showers and storms, however, the timing and placement of any such precipitation is highly uncertain at this range. There are now increasing signals that a large heat dome building over the southwestern U.S. by the middle of next week may finally expand eastward and push away the trough by the following weekend, supporting generally hotter and drier weather by that time. However, that is beyond the scope of the 7-day forecast so we will have to wait to see if it pans out over the next few days. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 556 PM CDT Thu Jul 4 2024 First aviation concern will be storms impacting ALO at the start of the period with sub-VFR restrictions likely and gusty winds possible. As the cold front moves through the rest of the state this evening, winds will become from the west-northwest and northwest. Gustier winds are possible through the day Friday and have included gusts at MCW, ALO, and OTM. MVFR clouds will rotate back into the state from northwest to southeast after midnight tonight staying northeast of a roughly SLB to AMW to OOA line through early afternoon Friday. Light showers remain possible with these clouds despite VCSH mentions being removed as they are likely too generous. Will reevaluate at 6z issuance if appropriate to reintroduce VCSH or SHRA. && .HYDROLOGY... Issued at 218 PM CDT Thu Jul 4 2024 Most area rivers continue to gradually fall, excepting a few areas mainly in the Cedar River basin where modest secondary rises are occurring. Saylorville Reservoir also continues to slowly rise, with a peak level still forecast near 878 feet on July 10-11. With scattered thunderstorms moving across the area today and seasonally high PWATs around 1.5 inches there is some risk for excessive rainfall in our northern and northeastern counties through this evening. However, the storms are expected to be fairly progressive in their movement which mitigates the threat somewhat. There is a possibility of two or more rounds of storms impacting a few areas, but even so, recovery time since last weeks heavy rains and the state of vegetation across the area also support a relatively low threat of renewed/flash flooding, at least outside of urban areas. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Lee AVIATION...Ansorge HYDROLOGY...Lee