Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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687 FXUS63 KDMX 121747 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1247 PM CDT Wed Jun 12 2024 ...Updated for the 18z Aviation Discussion... .KEY MESSAGES... - Several opportunities for thunderstorms through tomorrow. Most noted this afternoon and evening north, and again Thursday afternoon and evening south. Slight Risk (2/5) of severe weather for both of those windows. - Elevated heat and humidity approaching advisory criteria south half Thursday. Additional heat concerns next week Sun-Tue. - Dry Friday, then intermittent chances for showers & storm Fri night into early next week && .DISCUSSION... Issued at 400 AM CDT Wed Jun 12 2024 A fairly typical summer-like pattern will be in place through this forecast period (into early next week) as IA remains between an active northern CONUS jet and progressive upper level ridging. This will result in seasonal to above normal warmth and humidity and varied thunderstorm chances for an extended period. Our weather was largely inactive early this morning, although our attention will turn toward slowly expanding convection from MT into the Dakotas. This will continue through the day and expand eastward driven by lift and theta-e advection ahead of the US/Canadian border upper trough. Increasing instability across the High Plains is expected to expand into MN ahead of an approaching frontal boundary. The 03Z extended RAP suggests 2-3K J/kg MLCAPEs into MN, but lesser values +/- 1000 J/kg farther south into northern IA. 00Z HREF members are in fairly good agreement with generally elevated convection advecting and/or expanding into northern IA late morning through the afternoon and early evening, with additional storms ignited by low level convergence along the front possibly following close behind. Forecast soundings in the airmass across northern IA depict very summer-like and warm inverted-V soundings with fairly steep low level lapse rates. 0-6km shear of 40+kts with favorable NW-SE orientation, elongated hodographs, and the steep lapse rates with 1000-1400 J/kg DCAPEs all support a risk of damaging wind and large hail in the Slight Risk area, especially if storm become rooted lower and more surface based with time. The heavy rain potential appears to have diminished somewhat from this time yesterday with 00Z HREF 2"+ probabilities much less, now <30%. Temp wise, the NBM was around ~4F too cool yesterday so boosted temps further to higher percentiles today and tomorrow, at or approaching 90th percentile of 00z MOS and deterministic guidance, and there is some concern that may even not be sufficient. There should be a relative lull overnight into early Thursday morning, but lingering mid level forcing in the left entrance region of the jet segment approaching the Great Lake will keep chances for weaker convection into the morning north. By afternoon the associated frontal boundary should be sagging southward through the forecast area with our attention turning to southern IA. The warm sector ahead of the front should see highs in the mid 90s, and with 65-70F dewpoints conditions approaching Heat Advisory criteria will be in place as heat indices reach 100F in a few spots, and Wet Bulb Globe Temperature (WBGT) Risk reaches High and isolated Extreme levels, 3 and 4 of 4 risk levels respectively. 00Z HREF ARW core HRW ARW and HRRR members are quite aggressive developing a strong MCS across southern IA during peak heating tomorrow afternoon. Recent RAP and HRRR extended runs both have been consistent depicting 2-3K J/kg, low inhibition MLCAPEs driven by 65-70F surface dewpoints: higher than today. With 40+kts of deep shear and four figure DCAPEs large hail and damaging winds will again be possible. Although the low level kinematics are weak, an isolated more thermodynamically driven tornado cannot be ruled out considering the degree of instability. 00Z HREF strong wind probabilities are fairly high across southern IA, and 00Z and 06Z HRRR runs have both depicted a small cluster of storms with significant wind potential (gusts 60+kts) somewhere in the vicinity of our far SE corner into DVN`s eastern IA counties. Would not be surprised to see SPC raise severe weather probabilities for those areas later today or heading into tomorrow. Any precip potential should exit south and east overnight into Fri morning as the front exits, leading to fair weather and seasonal temperatures for a brief break. It won`t last long however with off and on thunderstorm chances starting Friday night and lasting through the remainder of the period into early next week. Lift ahead of a Rockies short wave will start by then, and continue through its passage later in the weekend, and then persistent southwest flow will follow ahead of a northern Plains trough into Tuesday. Iowa should be into the warm sector starting Sunday with highs and heat indices often in the 90s. The WBGT Risk categories seem a bit more tempered in this range however, often no worse than Moderate (2/4) south half, likely due to breezy winds at times. Seasonal to occasional plus climatology moisture parameter space suggests locally heavy rains may be possible at times from the weekend into early next week, with windows of concern over the weekend along and ahead of the aforementioned drifting short wave, and along a slow moving frontal boundary which appears to be late Tuesday at this lead time. There will be some severe weather potential considering the instability driven by a seasonally warm and humid airmass, but weak shear in upper ridging may temper that a bit. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1247 PM CDT Wed Jun 12 2024 Scattered showers and thunderstorms will pass across the area early this afternoon in northern Iowa with impacts to KFOD/KMCW/KALO. Showers are producing stronger gusts around 40-45 kts in additional to background breezy winds gusting 20-30 kts. A secondary line of storms is possible north late tonight, though confidence is much lower. Any impacts would mainly develop at KMCW if this line develops. More robust storms possible tomorrow evening near KOTM. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Small AVIATION...Hagenhoff