Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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955 FXUS63 KARX 071803 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 103 PM CDT Fri Jun 7 2024 .KEY MESSAGES... - Rain chances move in for parts of southeast MN/northeast IA later this afternoon, but more widespread overnight/Sat morning. Only a low end thunder threat (30%) with rain amounts mostly 1/4" or less. - Relatively cool weekend into the start of new work week (highs 60s to low 70s for most) but turning warmer for the latter half. && .UPDATE... Issued at 100 PM CDT Fri Jun 7 2024 Current surface analysis shows low pressure centered over the pan handle of Nebraska with southerly flow advecting moisture north in a somewhat narrow ribbon 3/4ers the width of the state. The 60F dewpoint line has rapidly shot north to near I-80 with radar showing elevated /10ft/ showers and storms across SD into nwrn IA early this afternoon. This area is associated with a strong area of 600-700mb frontogenesis within the MuCAPE gradient of 250-750 J/Kg per SPC, with the heart of the MUCAPE pool further southwest over wrn NE/CO. Per latest RAP guidance, this elevated forcing /fgen/ translates east today in swift zonal flow, grazing northeast IA closer to I-35 early this evening, however without the elevated instability /which spreads in late tonight/. With the low-level ridging and dry air mass holding firm across the Upper Miss Valley, would think this will translate into thick clouds at 10kft and scattered very light showers or sprinkles. GOES water vapor showing a well-formed shortwave trough in northwest flow dropping southeast through srn Saska/Alberta at 18Z. Short term model consensus has moderate 300-500mb QVector convergence along and north of I-90 Saturday morning with moderate isentropic lift through the low-levels as well with this wave. As the low-level southwest flow increases late tonight as the ridge axis pushes east, low MUCAPE instability /100-300 J/Kg/ does shift in near and south of I-90, and is speed convergent, isentropically upgliding. Have a hard time believing showers would not be present south of I-90 with isolated thunder too after about 3 am. At that same time, a cold front with the northwest flow trough is heading through MN, moving through the area Saturday morning into very early Saturday afternoon. This will act to prolong the period of showers Saturday and there is some timing spread among the CAMS. So, forecast will paint a little bit bigger window than may actually occur and hopefully timing can be refined in the next 12 hours. So, continued very high rain chances near and I-90 and north late tonight into Saturday, and increased them southward into northeast IA and southwest WI /60%/...with clearing trying to work in post-frontal in the afternoon from NW->SE. && .DISCUSSION... Issued at 200 AM CDT Fri Jun 7 2024 * OVERVIEW: Upper level longwave trough over the great lakes/new england states will persist as various shortwaves drop into the trough, re- enforcing/carving it out. This setup will continue to funnel relatively cool air southward from Canada while the shortwaves and afternoon instability work in concert to spark a few showers from time to time. Thunder chances relatively low moving into next week. The long range guidance remains steadfast in breaking down the blocking pattern as we move into the middle part of the new week, with broad upper level ridging/zonal flow taking over. Heights climb as does low level warming. How strong the ridge becomes, and where it moves/centers itself is not clear. WPC cluster analysis suggest a few different ways this may play out - which could result in steamy temps locally, or just more moderate warming. The EPS and GEFS members don`t have much of a preference at this time, dropping members in all 4 clusters. Will let the model members details temps for the time being. * WEEKEND RAIN CHANCES - most likely late tonight/Sat (60-80%). Only a low end thunder threat (10-20%). A bit of upper level energy is progged by the short/medium range range guidance to slide east out of the southern rockies today, shifting across IA by late afternoon/evening. Some low level warm air advection and mid level frontogenetic forcing with the feature. Good fetch of low level moisture return into the shortwave as 30 kt 850mb jet noses into northcentral IA. Pool of instability holds southward of the systems associated sfc warm front. CAMS models develop a northwest-southeast running bkn line of showers in response to the forcing/moisture interaction, but mostly mid level variety. Higher convective threat lies south/southwest of this band. Moving into the overnight another shortwave trough looks to drop out of southern Canada and move across the upper mississippi river valley. Weak low level thermodynamics but some upper level QG convergence to work with. Sensible weather impacts come in the form of widespread showers, move favored across parts of MN and WI. Other, small bits of upper level energy and weak afternoon instability could continue spotty shower chances from time to time Saturday afternoon through Sunday, but not a lot of consensus in the models on how this will play out. * RAIN CHANCES FOR NEW WORK WEEK? mixed messages in the models, lower confidence How the rain potential plays out for next week is unclear as the upper level ridging works in. Some of the GEFS and EPS bring rain chances to the region while other members favor dry conditions. Not a clear signal either way in movement/track of any upper level disturbances, and confidence isn`t high in any particular period over another. Will hold with the blend for the details for now. * COOL THIS WEEKEND INTO EARLY NEXT WEEK - but looking warmer by week`s end The cool low level air that settled across the region Thu looks to hang around through the upcoming weekend and into the early part of the new week. Still looking at highs approx 5 to 10 degrees below the early June normals - mostly topping out in the upper 60s to lower 70s. With broad ridging building in as we move into the middle part of the new week, increasing heights with corresponding bump in temps promises a return to the seasonable normals, if not exceeding them by 5 to 10 degrees. Consistent signal in the long range guidance to warm back into the 80s while the GEFS and EPS suggest a 5 to 10% chance to push 90 for a few locations by the new weekend. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 100 PM CDT Fri Jun 7 2024 Moisture will be on the increase overnight with probabilities will increase toward sunrise for some flight restrictions. A weather system moving in from the northwest will provide widespread rain showers later tonight and Saturday morning. It looks like visibilities should remain VFR except small periods of time where more intense rainfall cores may hit the airfields. However, the ceiling probabilities increase rapidly for either MVFR or IFR ceilings after 11Z. NBM guidance suggests IFR would be reached at RST by 14Z, with MVFR guaranteed. Probabilities are more favorable for lower MVFR at LSE after 11Z per the NBM guidance. For this TAF cycle, have decided to play a bit more conservative with MVFR ceilings at both sites during the morning. Probabilities for IFR will be monitored in future forecasts as there is some chance of that occurring /25%/ at KRST. Moisture is increasing across the area versus the dry air in place, however confidence is a bit lower on the degree and depth to the saturation. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ UPDATE...Baumgardt DISCUSSION...Naylor/Zapotocny AVIATION...Baumgardt