Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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834 FXUS63 KFSD 061100 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 600 AM CDT Sat Jul 6 2024 .KEY MESSAGES... - Patchy fog this morning and again tonight-early Sunday. Widespread dense fog is not expected, but isolated visibility below a mile is possible. - Increasing coverage of showers and thunderstorms through this afternoon to early evening, diminishing again by midnight tonight. A few strong to severe storms are possible with large hail and damaging winds the primary threats. - Unsettled weather continues early next week with spotty afternoon to early evening showers/storms Sunday-Tuesday. Severe weather is not expected. - Cooler than normal temperatures for early July through the early part of next week, but trending closer to normal by the end of the week. && .DISCUSSION... Issued at 359 AM CDT Sat Jul 6 2024 TODAY-TONIGHT: Light winds, mostly clear nighttime skies and plenty of surface moisture from recent rains will favor at least patchy fog development this morning and again tonight into early Sunday. Seeing some of this already on satellite and in some surface obs as of 08Z so have expanded mention to most areas near/east of the James River Valley through 7-8am this morning. As the fog is burning off shortly after sunrise, attention will turn to a well-defined shortwave currently spinning east-southeast out of far eastern Montana. This wave will slide east toward the eastern Dakotas by this afternoon, providing some upper level support during our diurnal peak of instability. Models continue to show broad area of MUCAPE 1500-2000 J/kg ahead of the wave, though model consensus indicates 700-500mb lapse rates remain on the weak side (mainly 6-7 C/km, perhaps as high as 7.5 near the Missouri River and slightly steeper in a more shallow layer). At the same time, modest deep layer shear of 30-40kt builds into southern parts of our forecast area as the wave slides through. While stronger shear and steeper lapse rates are focused south of our forecast area in Nebraska, this combination could support a few organized storms capable of large hail up to half dollar size (1.25 inches) and damaging wind gusts to 60 mph. Tornadoes appear unlikely given high LCLs seen in the soundings. Stronger storms could also produce locally heavy rain, but given progressive storm movement and PWAT (precipitable water) values near to slightly above average for this time of year, widespread heavy rainfall is not expected. Although CAMs do not show great agreement on timing/location details, there is modest consensus that greater storm coverage/persistence will be focused from east central SD into southwest MN, which can also be seen in moderate (40-60%) HREF probabilities of rainfall exceeding 0.50" and low (10-30%) probs of exceeding 1 inch. Timing of stronger storms should be focused from mid afternoon through shortly after sunset, with the wave sliding east and instability quickly waning thereafter. A weak surface pressure gradient and decoupling after sunset will allow for light winds again tonight, which could again lead to patchy fog after midnight into early Sunday. SUNDAY-TUESDAY: The broad mid-upper level trough remains in place across the northern Plains/Upper Midwest early next week, slowly drifting east by later Tuesday. Modest afternoon instability along with optimally timed shortwaves sliding through the base of the trough could support scattered showers/storms, mainly during the afternoon to early evening aided by the diurnal heating. However, weak mid-level flow leads to minimal shear across our area, so do not expect severe storms during this period. WEDNESDAY-FRIDAY: The early week trough tries to slide east by midweek, but appears a little slower than seen in previous runs. This could allow for another day of spotty afternoon showers on Wednesday, though confidence is too low to include at this time. As we progress through the latter part of next week, the upper ridge makes a stronger push eastward by the end of the work week. This should allow for dry conditions and gradually warming temperatures for the end of this forecast period. && .AVIATION /12Z TAFS THROUGH 12Z SUNDAY/... Issued at 600 AM CDT Sat Jul 6 2024 Areas of reduced visibility in fog as we start this TAF period, but expect this to burn off rather quickly by 13-14Z. Main concern through this evening will be increasing potential for scattered thunderstorms, especially as we progress through the afternoon and evening. Timing consensus among various high- resolution models is poor, so will try to target a relatively narrow 2-4 hour window when TS is more favored at TAF sites. Convective threat begins to wane after 07/03Z-05Z, with patchy fog once again possible by late in the period. Confidence is on the lower side regarding extent, though, so will not include in TAFs at this time. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...JH AVIATION...JH