Subj : HVYRAIN: Excessive Rainfa To : ALL From : Mike Powell Date : Tue Sep 19 2023 20:46:00 FOUS30 KWBC 200108 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 907 PM EDT Tue Sep 19 2023 Day 1 Valid 01Z Wed Sep 20 2023 - 12Z Wed Sep 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF THE SOUTHERN PLAINS AND WESTERN OZARKS... ....Southern Plains/Missouri Valley... 0100 UTC Update -- Made modifications to both the Slight and Marginal Risk areas based on the latest observational/mesoanalysis trends along with (especially) the latest short range CAM guidance/trends. Not much of a change with the Marginal Risk area -- outside of paring the northern periphery a bit, along with a slight extension south of the Red River in North TX. However we were able to cut a large area of the Slight Risk, outside of the MOKSAROK 4-state area where the guidance (both deterministic and probabilistic) is in a bit more agreement. This is also where the 18Z HREF exceedance probabilities are most elevated, particularly after 03-04Z with the uptick in the LLJ. ....South Florida... As is typical, latest SPC mesoanalysis indicates a general negative dCAPE/dt across S FL following sunset. However given the current radar (trends), along with residual modest deep-layer instability, PWs ~2.25", and sufficient large-scale forcing ahead of the upper trough/south of the surface front (upper divergence with plenty of kinematic support for more organized clusters, with 0-6km bulk shear values of 25-30 kts along with good 925-850mb moisture transport), will maintain a Marginal Risk in the ERO given the potential for localized/isolated flash flooding, mainly over urban areas. Hurley Day 2 Valid 12Z Wed Sep 20 2023 - 12Z Thu Sep 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OZARKS, MISSISSIPPI VALLEY, INTER-MOUNTAIN WEST, AND THE FLORIDA PENINSULA... ....20Z Update... ....Intermountain West/Great Basin... No big changes were made to either the QPF footprint for the large Marginal Risk area across this region. The area of biggest concern remains northeast NV and northwest UT north and west of the Great Salt Lake, for potential upgrades, but since the rainfall footprint is largely unchanged, saw no reason to deviate from inherited. ....Northern Plains... Removed the inherited Marginal Risk for the Dakotas with this update. In addition to a very weak (and decreasing) rainfall forecast in the guidance, much of the eastern Dakotas are very difficult to flood even with more rain. The antecedent dry conditions even further diminish the flash flooding threat. ....Ozarks of MO/AR/OK... A Marginal Risk was maintained with this update, with a bit of trimming on the western side. The ongoing MCS at the start of the period will continue into the morning hours for eastern OK, though with limited instability, the likelihood of heavy rainfall is low. Much of the rain should be over early on across eastern OK, so think most of the impactful rain for that area will be in Day 1/Tonight. Much of the associated rain will shift north into MO for much of the day, though little in the way of heavy rain is expected. A second round of convection may form into a line across eastern OK overnight Wednesday night, which may require further adjustments or potentially an upgrade due to multiple days of heavy rainfall. ....Florida Peninsula... A stalled out frontal boundary with a developing low off the Atlantic coast will provide plenty of forcing for another round of widespread showers and thunderstorms across much of the peninsula again on Wednesday afternoon and evening. PWATs will exceed 2 inches for much of the southern half of the peninsula, which in turn will provide ample moisture for the thunderstorms. The greatest chance of flash flooding will be in urban and flood-sensitive areas. A Marginal Risk area was introduced for much of the Peninsula outside of the Everglades with this update. Wegman ....Previous Discussion... ....Eastern ID, Western WY, northern UT/NV... A deepening upper trough over the western CONUS will create an enhanced area of difluence and attendant vorticity maxima rotating around the general upper circulation positioned over the PAC Northwest. Modest low to mid-level moisture advection within the confines of the Snake River and adjacent terrain will create a pocket of better convective potential Wednesday afternoon and evening as a stronger vorticity maximum pivots through the region providing deep layer ascent during peak diurnal destabilization. Despite the limit on potential due to only a modest moisture profile, enough instability (~500-1000 J/kg SBCAPE) and focused ascent over the topography should provide some heavy rain potential within a zone that is highly prone to flooding due to steep terrain and already elevated soil moisture content as pinpointed by the NASA SPoRT data set indicating areal coverage of 70-90th percentile of climatological norms within the confines of the outlined area. Only minor adjustments were made to the MRGL risk, mainly attributed to the QPF reflection within the NBM and bias-corrected ensemble forecast over the area of best positive vorticity advection (PVA). ....Upper Midwest... MRGL risk from previous forecast was maintained with a slight expansion over the Dakotas and northwest MN to reflect the uncertainty of model guidance where the best convection will develop Wednesday evening. The ingredients for locally heavy rainfall with a zone of lower FFG's is still very much probable as modest theta-E advection into the northern plains will lay the ground work for a zone of potential, mainly along and south of a stalled frontal boundary positioned over the northern plains. Increasing difluence aloft with small vorticity perturbations will provide enough large scale ascent for a round of thunderstorms to develop across central ND over to the MN border. Ensemble means are fairly low with regards to top-end potential for the setup, but some deterministic are fairly robust for localized heavy rainfall within a corridor of low FFG's thanks to a prolonged drought in place. PWATS will also be steadily increasing with GEFS standardized anomalies approaching 1.5-2 standard deviations above normal by the end of the D2 period. Instability in-of the stalled boundary is tame compared to other periods of interest, but increased forcing ahead of a deepening upper trough to the west could provide enough destabilization to generate a period of thunderstorms. --- * SLMR 2.1a * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .