Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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787 FXUS63 KARX 151711 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1211 PM CDT Sun Sep 15 2024 .KEY MESSAGES... - Warm temperatures persist through much of this week with high temperatures in the 80s for much of the region. - Relatively dry conditions are expected through the first half of the week. Precipitation chances return starting Wednesday with increasing chances as you head further west. && .DISCUSSION... Issued at 325 AM CDT Sun Sep 15 2024 Today - Tuesday: Patchy Fog This Morning, Otherwise Above Normal Temperatures With Minimal Precipitation Chances Patchy fog will be possible this morning in portions of the area with dewpoint depressions lowering throughout the overnight. While surface winds are generally on the lighter side in recent RAP/NAM soundings, winds just off the deck are slightly stronger to around 10-15 kts by 09z which should work as a mitigating factor for fog. That being said, have noted some under 1 mile visibilities at Black River Falls, Winona, and La Crosse early this morning so certainly could be locally dense early this morning, particularly in areas that more routinely see fog (cranberry bogs/river valley locations). As we head later into the day, some moisture remains in place with precipitable waters of around 1.25" per the 15.03z RAP and instability reaching around 1000 J/kg of MLCAPE. However, with no clear forcing mechanism to provide any lift to get showers/storms going, any convective development would be very isolated in nature (less than a 15% chance at any given location) and likely would result in minimal QPF with the recent HREF having very low probabilities (0-30%) for measurable precipitation today. Otherwise, upper-level ridging will be the name of the game over the next few days with the general pattern featuring an upper-level high sitting east of the region and a developing trough moving into portions of the Rockies. In this setup, current deterministic guidance (15.06z GFS/EC/NAM) keeps our region subjected to upper-level ridging with southwesterly flow developing to our west. Consequently, expecting generally warm conditions to persist as the grand ensemble (GEFS/EC ensemble/Canadian ensemble) keeps median high temperatures generally in the 80s with minimal inter-quartile spread (around 1 to 3 degrees of spread) into mid-week. Wednesday - Sunday: Precipitation Chances Return While Remaining Seasonably Warm Looking later into the week, the aforementioned upper-level ridge initially holds across our region with a large trough situated over the Rockies keeping our temperatures fairly steady-state. However, some pieces of shortwave energy try to sneak their way west of the region along the eastern periphery of this upper-level trough. As a result, noting some increasing precipitation chances in ensemble guidance for western portions of the local area (southeast MN/northeast IA) late week. While both the 15.00z GEFS and EC ensemble have differing probabilities into Thursday/Friday, both groups increase probabilities for measurable precipitation. The EC ensemble is generally the more aggressive solution with higher probabilities (50-80% as opposed to 20-40% in the GEFS) for measurable precipitation on Thursday. A bit early to evaluate exactly how thermodynamics will look late week but an initial look at the deterministic 15.00z GFS would suggest minimal instability building into our region with the stronger axis of 850mb moisture transport remaining further west in addition to minimal shear as we are still fairly removed from the stronger upper-level flow. Therefore, when also considering that the grand ensemble joint probabilities for 500 J/kg of SBCAPE and 30 kts of bulk shear being very low (0-15%), the potential for severe weather currently appears on the lower side late this week. As we head towards the weekend, early indications show some potential for the ridging pattern to finally break down as a larger trough gets ejected towards the local area in the 15.06z GFS/EC. A large degree of disagreement exist amongst various deterministic models and ensemble groups on timing, position and strength of the trough as it swings through. This can be highlighted with the 500mb geopotential height inter-quartile range in the grand ensemble which show roughly 10 decameters of spread for Sunday across the region, a fairly substantial range of outcomes. Consequently, the previously mentioned low inter-quartile spread in the grand ensemble for surface temperatures observed through Friday gets tossed out the window by the weekend. This can be noted at La Crosse where the median high temperature for Sunday is 72 but the inter-quartile range is anywhere from 67 to 79 degrees. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 1211 PM CDT Sun Sep 15 2024 VFR conditions are expected to prevail at both RST/LSE taf sites through the taf period...as surface ridge over eastern Wisconsin remains the dominant weather feature. Fog will form in river valleys tonight. However...winds just above the surface will inhibit any widespread fog in the Mississippi River Valley and limit any impacts to the LSE taf site. If fog does impact the LSE taf site...the reduced visibility will be very brief. Wind speeds will be around 10 knots or less at both RST/LSE taf sites through taf period. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Naylor AVIATION...DTJ