Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Oct 12 2023 19:20:28 FOUS30 KWBC 121920 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 319 PM EDT Thu Oct 12 2023 Day 1 Valid 1912Z Thu Oct 12 2023 - 12Z Fri Oct 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG EASTERN PORTIONS OF THE MID MISSOURI VALLEY... 19z Update: A southward expansion of the Slight risk was made with this update to account for ongoing training convection across portions of central NE...with hourly rainfall as high as 2-3". Some of this high rate rainfall will likely overlap areas that have already received heavy rainfall over the last 24 hours over north central NE...locally increasing a flood threat. The expectation is that this convection will begin to forward propagate off to the northeast by ~21 or 22z, which should result in a gradual decrease in the flash flood risk thereafter...although continued high rates will still pose a localized threat through the evening hours. Chenard ....16Z Excessive Rainfall Outlook Discussion... ....Southeast... Potential persists for storms to produce locally heavy rainfall amounts through late afternoon that results in additional inundation flooding. The potential will be aided by clearing skies that leads to an increase in instability along and south of a quasi-stationary front and the presence of pre-frontal convergence in the area. That could support additional spotty rainfall amounts of 1 to 3 inches and at least some chance of overlap with areas that received moderate to heavy amounts from late last evening into this morning. Additional information is available from the WPC Mesoscale Precipitation Discussion 1145 valid until 12/2100Z.=20 ....Northern Plains/Mid-Missouri Valley/Upper Midwest... Maintained the Slight and Marginal risks despite a relative lull in activity across the area in anticipation of additional rainfall as a deepening cyclone approaches from the Rockies. Moisture remains anomalous and the synoptic forcing should be sufficient to provide deep layer lift to yield an additional couple of inches...especially where the moisture overlaps with some instability. This was previously handled well and no significant changes were needed. ....0830Z Excessive Rainfall Outlook Discussion.., ....Northern Plains/Mid Missouri Valley/Upper Midwest... A deepening cyclone over the central Rockies and High Plains early today will drift east across the Plains and into the mid-Mississippi Valley by late Friday. Strong southerly winds ahead of the system will continue to transport moisture into a region of strong deep-layer ascent, afforded in part by favorable left-exit region upper jet forcing. 850-700 mb moisture flux anomalies are expected to range between 3 and 4 standard deviations above normal per the SREF and GEFS. For most areas the combination of ample moisture and strong forcing, but little instability, will produce an extended period of moderate rains, resulting in locally heavy accumulations but limited impacts. The placement of the Slight Risk area remains largely the same and centered near where greater instability and deeper moisture along the axis of strongest inflow may help to increase rainfall rates and the threat for short-term heavy accumulations and rapid runoff. Training storms may also raise the threat for heavy amounts and excessive runoff across this region. Adjustments to the Day 1 ERO, compared to yesterday's Day 2 outlook, included extending the Slight Risk area a bit farther south across eastern NE and western IA, to include much of the Omaha metro region. This is due to the better instability potential Thursday night per the guidance (MUCAPEs averaging between 1000-1500 J/Kg per the latest HREF, ECMWF, and GFS), thus a higher probability of heavier short-term rainfall rates. The latest CSU UFVS-Verified First Guess Field in fact supports this southern extension of the Slight. ....Southeast... Low pressure migrating slowly across the eastern Gulf and just off the west coast of the northern-central FL peninsula will begin to lift northeast and off the Southeast coast by late Friday. Favorable forcing and greatest low-mid layer moisture flux anomalies along and south of the associated quasi-stationary surface boundary early this morning will begin to shift off the Atlantic coast later today. However, continued rich moisture (PWs ahead of the front peaking between 2.3-2.6" or ~3 standard deviation above normal) along with ample deep-layer instability (mixed-layer CAPEs 1000-2000 J/Kg) will allow for peak hourly rainfall rates between 2.0-2.5" underneath the stronger cells (especially supercells) and/or where cell training sets up. However, supported the by the 00Z HREF exceedance probabilities, anticipate the coverage of these higher rates to be more isolated/localized than otherwise, and particularly prior to 18Z or early in the forecast period. Because of this, along with the relatively high FFG, at least for now have opted to not upgrade a portion of the Marginal Risk area inherited from yesterday's Day 2 ERO to a Slight Risk. Hurley Day 2 Valid 12Z Fri Oct 13 2023 - 12Z Sat Oct 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MIDWEST... ....Mid-Upper Mississippi Valley to the western Great Lakes... The overall setup on Day 2 is largely similar to the one expected on Day 1, except farther east as a closed mid-upper low tracks from the Plains into the Upper Midwest. Similar though perhaps not quite as robust to Day 1 (per the latest guidance), ample moisture and deep-layer forcing will support an extended period of moderate rains north of the low, but the lack of instability will limit the threat for heavy rainfall rates and flash flooding concerns. A relatively greater threat is expected to center near and ahead of the low, where deeper moisture and greater instability along the low-level jet may help to elevate rainfall rates and short-term runoff concerns. Areas that may possibly be impacted with heavy rain on Day 1, including much of Iowa, may see additional heavy rains this period, increasing the risk for flash flooding for those areas. Hurley/Pereira Day 3 Valid 12Z Sat Oct 14 2023 - 12Z Sun Oct 15 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Eastern Ohio Valley into the Northern Mid Atlantic Region... Closed mid-upper low at the start of the Day 3 period (Sat morning) will open during the period as the shortwave energy is absorbed within the amplifying longwave eastern CONUS trough. At the surface, the guidance shows a Miller Type B scenario as the Ohio Valley low on Saturday gives way a developing low off the mid Atlantic coast Sat night. Compared to the Day 2 and especially Day 1 forecast periods, the deep-layer forcing and (especially) moisture flux/transport aren't nearly as anomalous, as the setup would appear typical for a Miller B setup (transitional inland to offshore low), with DPVA and left-exit region upper jet forcing (100-120kt jet streak at 250 mb) leading to favorable deep-layer isentropic ascent over-top a more stable, CAD profile in the lower layers. Therefore expect the lack of event elevated instability to significantly retard the short-term rainfall rates, and despite the recent heavy rainfall (especially into southern NY), do not anticipate this long duration, mainly moderate rainfall (isolated totals of 2-2.5" in 24 hrs) to cause much of a concern with respect to short-term runoff issues, at least not up to the Marginal Risk threshold (i.e. a 5% or higher neighborhood probability). Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!73QR7AxLWoFNr3PDx27HNu4zX2DXwXIVrh0bYaGWdc-2= gp1pW9_utFO4ZgIAvTQI8eGowNzgppaORhR8-nlk6ekXvlQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!73QR7AxLWoFNr3PDx27HNu4zX2DXwXIVrh0bYaGWdc-2= gp1pW9_utFO4ZgIAvTQI8eGowNzgppaORhR8-nlky2dRnOw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!73QR7AxLWoFNr3PDx27HNu4zX2DXwXIVrh0bYaGWdc-2= gp1pW9_utFO4ZgIAvTQI8eGowNzgppaORhR8-nlk__nZaXQ$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .