511 FXUS63 KARX 201714 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1214 PM CDT Mon Sep 20 2021 .SHORT TERM...(Today and tonight) Issued at 336 AM CDT Mon Sep 20 2021 Today's main forecast concern is potential for some strong to severe storms this afternoon into early evening. A deep upper trough is slowly crossing the northern Plains early this morning. Meanwhile a shortwave trough to our south is lifting north-northeast and will move into eastern Wisconsin towards daybreak. At the surface, a strong cold front is positioned from western Nebraska to far northwest Minnesota. Ahead of this front, stout southerly low level winds (from surface to at least 700 mb per upstream soundings) continue to pump abundant moisture into the region, creating a ribbon of increasing PWATs of 1.5 to 1.8+ inches. Though it will weaken after daybreak, moisture transport will continue through the day out ahead of the front, bringing anomalous (near 99th percentile) ~2" PWATs into the area this afternoon- evening as the front approaches. While crops are largely cured, model consensus for dewpoints in the mid to upper 60s today does not seem far-fetched given we've had similar readings over the last couple weeks (even around 70 on the 11th). How much we warm today will be another key factor for building instability ahead of the front. The approaching southern shortwave will begin to spread widespread low clouds northward through our area around daybreak. These will merge with increasing mid/high clouds from ongoing convection over central/western Minnesota. HREF does show the low clouds starting to erode by early afternoon, but there may only be a fleeting sliver of partial clearing across our south given persistent higher clouds over most of the area. Despite the clouds, ongoing warm air/theta-e advection should be enough to push temperatures into the mid 70s to lower 80s. This will yield SBCAPE of 1000-2000 J/kg this afternoon, highest over western half of the forecast area. Plenty of support for developing thunderstorms this afternoon in a nearly uncapped environment, though tall/skinny CAPE profiles due to weak mid-level lapse rates will likely prove to be a limiting factor to overall updraft strength. With thunderstorm ingredients all looking to come together favorably later today, the only remaining questions are timing and severe potential. Should be first noted that the approaching southern shortwave may bring isolated to scattered showers, perhaps a storm, into our southeastern counties after daybreak through midday. The front is then poised to introduce shower/storm chances into our west toward late morning/midday, with the most widespread storm chances coming this afternoon into mid-evening as the front sweeps through. Severe potential will largely hinge on updraft strength and available shear. Bulk of deep layer shear will lag behind the front, but there may be a narrow region of overlap between instability and 0-6km bulk shear. Even ~30+ knots of 0-3km shear could favor some stronger, better organized segments within the line of storms, with shear vectors showing a small orthogonal component to the boundary. Damaging winds will be the primary severe threat, especially with any bowing segments. Isolated large hail possible and perhaps a small tornado threat given clockwise hodographs and respectable shear in the 0-1km layer. Certainly some heavy rainfall given the moist environment and deep warm cloud layers. Expect widespread rainfall of 1 to 1.5 inches, but not anticipating flooding issues given recent dry conditions and progressive nature of the front. Remaining storms will exit our east just after midnight. .LONG TERM...(Tuesday through Sunday) Issued at 336 AM CDT Mon Sep 20 2021 The unseasonably warm surface temperatures break on Tuesday as the upper-level trough amplifies while it passes over our forecast area Tuesday night. While dominant cold air advection occurs with the frontal passage Monday night, low-level (H850) northerly winds bring temperatures down another couple degrees to 1C at H850 overnight Tuesday. This is reflected in forecasted overnight low temperatures in the low 40s for most of our counties except for the northern-most in the mid to low 30s. Surface high pressure skirting just to our south will lead to clear conditions both Tuesday and Wednesday night with mild low-level winds leading to possible fog conditions, especially in the tributaries. The aforementioned upper-level trough becomes cutoff over the Ohio River Valley Wednesday into Thursday giving deterministic models a challenge of placement and timing of upper level flow. Depending on how/when this area of cutoff lower heights rejoins the parent flow, upstream dynamics vary regarding our next incoming disturbance. Currently, another incoming cutoff upper- level low occurs Friday, brisking by to the North of our forecast area. The GFS hints at earlier intensification than the ECMWF, momentarily placing us in a right-exit region of a cyclonically curved jet. The ECMWF has a more northern track with later deepening of the cutoff low and trough amplification, resulting in the favorable region for ascent missing our forecast area. Regardless, with a dry air mass in place (precipitable water <0.75"), there is limited instability and chances of precipitation are minimal at this time. As the week progresses and consensus on upper-level dynamics is ironed out, the strength and timing of any surface frontal boundaries and their associated precipitation will become clearer. Temperatures next weekend return to lows in the 40s after Friday's uncertain frontal passing. An upper-level ridge builds into Sunday, perhaps allowing temps back in to the 70s for some of the forecast area. Again, it will be important to iron out details for Friday's pattern to know more about timing of return flow. && .AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon) Issued at 1214 PM CDT Mon Sep 20 2021 Primary aviation concern is TSRA potential this afternoon/evening. Southerly winds gusting up to 20-25 kts and a few showers are possible through mid afternoon with lingering MVFR ceilings at KLSE expected to rise to VFR. A cold front will approach late afternoon/evening with a period of TSRA likely. Gusty winds/IFR conditions may accompany storms. A period of MVFR/IFR ceilings with NW winds gusting to 25 kts is expected behind the front with skies gradually clearing overnight/early Tuesday. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ SHORT TERM...Kurz LONG TERM...JAR AVIATION...JM