737 FXUS63 KMPX 152236 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 536 PM CDT Wed Jul 15 2020 .UPDATE...For 00Z Aviation discussion below && .SHORT TERM...(Tonight through Thursday night) Issued at 249 PM CDT Wed Jul 15 2020 High clouds continue to stream across the eastern half of the forecast area this afternoon. Further to the north, some cumulus are forming in response to a shortwave moving across northern Minnesota. This upper level disturbance could spark a few showers across our northern counties late tonight into early Thursday as it continues to push eastward. However, rainfall amounts are expected to remain light. The only other thing of note in the short term is the possibility of fog again tomorrow morning across the river valleys. Winds will be light and variable and lows are expected to dip into the upper 50s to low 60s across the region. Any cloud and shower activity that does develop across central Minnesota should dissipate fairly quickly along with the fog Thursday morning. As a surface high passes to our southeast, we'll see an uptick in southwesterly winds. This will allow more humid air to slowly begin to creep back up from the southwest, with dew points reaching into the 60s. Highs will be a few degrees warmer than today, with most places making a run at the mid to upper 80s under mostly sunny skies. .LONG TERM...(Friday through Wednesday) Issued at 249 PM CDT Wed Jul 15 2020 A high impact pattern will evolve Friday and Saturday with both excessive heat and severe weather possible. Two rounds of convection are expected nearby Friday night/early Saturday and late Saturday afternoon into Saturday night. A sprawling area of surface high pressure encompassing the eastern third of the CONUS will slide to the east Friday. A deep southerly tropical flow, unimpeded from a lack of convection in the Plains Friday, will allow for strong moisture advection by Friday night. A cold front trailing a large low over Manitoba will track across North Dakota Friday afternoon. Significant CAPE will allow thunderstorm development in this area which will quickly become linear and severe by Friday evening. Fast zonal flow with the expansive flat mid level ridge across the southern CONUS, warming 700 mb temps to +13 to +14C over the southern half of Minnesota, and an impressive instability gradient over central Minnesota should keep this complex over the northern half of Minnesota and northern Wisconsin. In addition, the degree of instability, moisture advection, and mid level flow in excess of 50 kts suggests the potential development of a progressive derecho. Models are in pretty decent agreement with respect to timing and track of this complex, so confidence is increasing. As is always the case, it's difficult to know how far south the line will build before it actually develops. However, the aforementioned 700 mb temps above +12C serves as a decent proxy for the approximate southern extent of the line. This only grazes the northern counties of our CWA. Even if the complex tracks a little farther south than currently forecast, its rapid movement east should still bring it to the U.P. of Michigan, Lake Michigan, and/or Lake Superior by Saturday morning, leaving little concern for lingering convection in this area. Two significant stories could evolve Saturday - the heat and another severe threat. Southerly surface flow should resume Saturday regardless how far south an outflow boundary would reach. A cold front or quasi-stationary front, however, will be sagging south Saturday afternoon into central Minnesota while a secondary low pressure center develops over Nebraska and southeastern South Dakota. Continued moisture advection and increasing convergence from the surface to 850 mb would result in very limited mixing of surface dew points Saturday afternoon. In fact, the NAM, GFS, and ECMWF have raw 2M surface dew points in the 75-82 deg range in the warm sector. This is not completely unreasonable given the set up. In addition, as one could recall, July 19th last year featured widespread dew points in the lower 80s and a heat index of 115 at MSP. A bit of skepticism is always warranted for extreme events, but models seem pretty bullish with this potential so it will be monitored. Temperatures in the upper 80s to lower 90s are also expected, making for warning criteria heat indices in some areas when combined with the humidity. WPC daily max heat index probs at MSP are 60% for 100F, 30% for 105F, and 10% for 110F. A very robust elevated mixed layer will overspread the warm sector Saturday afternoon. Mid level lapse rates of 8.5-9.0C/km atop surface dew points in at least the mid 70s would result in an extremely unstable airmass characterized by MLCAPE in excess of 5000 J/kg. A strong cap appears to remain per forecast soundings thanks to very warm 850-700 mb temps, even with the likely slightly overdone dew points in the lower 80s per global guidance. This is a highly conditional threat and confidence is pretty low in any development at this point. Forcing does not appear particularly strong due to the positively tilted trough and a veered 850 mb jet well to the south across Iowa and northern Illinois. The front is also likely not deep enough to help erode the cap on its own either. If enough forcing exists along the front and the cap can break in some areas, however, then explosive thunderstorm development could occur during the evening. Activity would grow upscale into a complex and track southeast into Wisconsin before weakening overnight. Cooler and drier air will follow for Sunday, Monday, and Tuesday with surface high pressure largely in control. Most of the convective chances will be tied to the front draped across the central Plains and mid Mississippi Valley. A short wave with poor consistency may impact the area midweek and increase thunderstorm chances some. && .AVIATION...(For the 00Z TAFS through 00Z Thursday evening) Issued at 537 PM CDT Wed Jul 15 2020 Fleeting high clouds will move across the area throughout the period. VFR conditions are anticipated overall, with the exception of the potential for light/patchy fog development again in eastern areas overnight. The other possibility for visibility reductions is over the northwest (KAXN), where a waning complex of showers and thunderstorms could roll in from the Dakotas toward daybreak Thursday. Some models have it dissipating by the time it reaches our area, while others bring waning showers as far east as I-35. For now, have only included a Prob30 mention at AXN. Light and variable winds overnight increase from the southwest on Thursday ahead of a developing Dakotas surface trough. Gusts into the 15-25 knot range look to develop by Thursday afternoon. KMSP...No concerns. /OUTLOOK FOR KMSP/ Fri...Mainly VFR. Chc SHRA/TSRA/MVFR late. Wind S 10 kts. Sat...Mainly VFR. Chc SHRA/TSRA/MVFR. Wind SW 5 kts. Sun...Mainly VFR. Chc SHRA/TSRA/MVFR. Wind NW 5-10 kts. && .MPX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. && $$ SHORT TERM...Dye LONG TERM...Borghoff AVIATION...LS