Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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455 FXUS63 KFSD 202355 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 655 PM CDT Mon May 20 2024 .KEY MESSAGES... - Isolated strong to severe storms remain possible this evening with threat increasing overnight. - Latest WPC guidance has cut rainfall amounts for tonight through Tuesday. Even so, it will still be possible for areas to see 1 to 2 inches of rainfall through this period - with locally higher amounts possible in heavier storms. - Isolated to scattered strong to severe storms remain possible overnight with the greatest threat from 3 AM to 9 AM Tuesday morning and again 12 PM to 6 PM Tuesday afternoon, mainly near and east of a Marshall to Sioux Falls to Yankton line. All threats are possible, though large hail up to ping pong ball size, isolated flash flooding, and damaging wind gusts up to 70 mph are the main threats - though isolated tornadoes will be possible over northwestern IA on Tuesday afternoon. && .DISCUSSION... Issued at 320 PM CDT Mon May 20 2024 Current radar this afternoon indicates very light showers oriented from south central SD into northeastern SD - this associated with mid level frontogenesis and lift provided by an upper level jet residing through northwestern SD and ND. In any event, little precipitation is expected with this band with dry air residing below about 700 mb. Otherwise, our attention turns to the convection potential overnight into Tuesday. Latest hi-res models indicate isolated to scattered showers and thunderstorms developing early this evening (some time after 22Z), with the earlier development occurring over through east central SD and southwestern MN as warm air advection begin to increase out ahead of shortwave lifting into western NE. Latest guidance would suggest MUCAPE of 1500 J/KG from the Interstate 29 corridor in the the evening, though with relatively weak mid/upper level winds, effective shear is on the lower side at 30-35 kts. Even so, there will be some potential for a stronger to severe storm through late evening with hail to the size of ping pong balls and winds to 60 mph. There may be somewhat of a break in convection for the early night time hours, before invigoration late in the night as an upper level shortwave begins to lift through NE while a surface low deepens over northern KS and southern NE. With increasing midlevel warm air advection, showers and thunderstorm coverage will increase after 06Z - most notably in the lower MO River Valley into northwestern IA after 09Z as a low level jet strengthens over eastern NE into that area. The better elevated instability at that point will reside over northwest IA, and with increasing effective shear as winds aloft increase, there will again be the potential for isolated severe storms. With that will come an increasing chance of heavy rainfall as PW values over that area climb to 1.5" plus, with some locales receiving an inch or two in heavier storms (see hydro section below). CAMs would suggest a possible break in the shower and thunderstorm activity by mid to late Tuesday morning, before additional thunderstorms develop on Tuesday afternoon - centered on the lower MO River Valley and northwestern IA as the upper level low tracks directly over our area and the surface low pushes through the eastern half of our CWA. If morning convection does not have an impact on later day storms, afternoon CAPE will average 1500 J/kg + in the warm sector, with 0-6 km bulk shear, so there will be a good chance of at least a few severe storms. While large hail and damaging winds will again be threats, there will also be a higher risk of tornadoes in our far east in the presence of the surface low. In addition to the convective potential, it will be a windy day with the tight surface pressure gradient in place - and it will be possible that winds approach advisory criteria in some locales. There will be a quite a temperature gradient across the area on Tuesday, with highs only in the 50s in our western CWA, to upper 60s/lower 70s over northwestern IA. Latest models indicate that shower and thunderstorm activity will exit our CWA close to 00Z on Tuesday evening. Tuesday night will see clearing skies and cooler temperatures with lows in the 40s. Less active weather over our area for midweek as the upper level/surface low lift into east central Canada on Wednesday and Thursday. After a brief cool down on Wednesday (upper 60s to lower 70s), temperatures warm back into the mid and upper 70s for Thursday. Our next best chance of showers and storms will come on Thursday night into Friday as another upper level/surface low track into the Northern Plains. Latest ensembles would indicate moderate probabilities (40-60%) of receiving a quarter of an inch of rain with this system, and lower probs (30-40%) of receiving a half an inch. Models then trend toward a drier period into next weekend, with temperatures falling back closer to normal. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 650 PM CDT Mon May 20 2024 Scattered strong convection will develop this evening over Northwest Iowa, bringing a risk for large hail and locally strong winds. Further west, we should see an increase in elevated showers. Greater convection chances arrives after 05Z with a mix of showers and thunderstorms moving northward out of Iowa and Nebraska. Brief period of reduced visibility is expected into the overnight hours. As the upper begins to move towards the area in the morning, MVFR to IFR ceilings will begin to expand across the CWA, mostly along and northwest of a line from Windom to Yankton initially. Intermittent period of drizzle may be possible at times. A third round of convection is likely by late morning through early afternoon, posing a greater risk for severe weather in portions of northern Iowa into early afternoon. && .HYDROLOGY... Issued at 320 PM CDT Mon May 20 2024 A couple of different rounds of rainfall will spark some hydrology concerns over the next 36 hours. The first of which will come tonight on the convergence of the 850mb LLJ. This could result in a west to east area of training thunderstorms with overall system propagation Corfidi vectors dropping near or below 10 kts. At the same time, PWATs AOA 1.5 inches and warm cloud layers AOA 10 kft should drive efficient rainfall rates. Should any of this more robust rainfall occur over urban or more flood prone areas, flash flooding could result. The next round of rain will approach quickly by midday as the main vort lobe ejects. While this activity should be more progressive, parameters are still favorable for efficient rain rates near or above 1 inch per hour, some of which will be falling on already saturated soil from tonight`s activity. From a river perspective, HEFS guidance may be suffering from a low QPF bias compared to that of WPC and other deterministic values (such as HREF PMM/LPMM) and thus its hydrologic response may also be too muted. Utilizing a mix of expected and high-end (95th percentile) WPC QPF would suggest rises near or into flood stage along portions of the Little Sioux, Ocheyedan and perhaps lower Big Sioux and Redwood. Even some smaller potential of flooding on other basins. With all of the above in mind, and factoring in the preexisting saturated conditions from early month flooding in NW IA, have issued a Flood Watch for a handful of Iowa counties. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...Flood Watch from 1 AM CDT Tuesday through Tuesday evening for MNZ081-090. IA...Flood Watch from 1 AM CDT Tuesday through Tuesday evening for IAZ003-013-014-021-022-031-032. NE...Flood Watch from 1 AM CDT Tuesday through Tuesday evening for NEZ014. && $$ DISCUSSION...JM AVIATION...Dux HYDROLOGY...Kalin