Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 23 2023 01:05:38 FOUS30 KWBC 230105 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 905 PM EDT Tue Aug 22 2023 Day 1 Valid 01Z Wed Aug 23 2023 - 12Z Wed Aug 23 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE WESTERN U.S. AND SOUTH TEXAS... ....Northwest/Northern Intermountain West/Northern Rockies... A mid-level impulse over northwest NV this evening will continue tracking northeast to western MT tonight. Ample moisture with PW anomalies +2 to 3 standard deviations above normal with sufficient instability of 500-1000 J/kg will continue to allow heavy thunderstorms to develop this evening over eastern OH, ID and through western MT. These are areas that were impacted from Hilary remnants, making them more susceptible flash flooding. The Slight Risk was just trimmed based on radar trends and the Marginal was expanded southeast and east per HREF CAMs and radar trends. ....Northern Arizona to western Wyoming... An axis of deeper moisture with increasing mid level flow ahead of the remnant low moving over the northern the Intermountain West will continue drawing deeper moisture north across Utah into western Wyoming overnight. With moderate instability of 1000-2000 J/kg lingering through this evening, additional heavy rain the Slight Risk was maintained with some eastern expansion in UT into northern AZ given radar trends. ....South and Southwest Texas... Harold remnants have moved into Mexico with trailing inflow continuing to allow some heavy bands to be drawn in across south Texas from the Gulf of Mexico. Most overnight QPF is progged to be over northern Mexico with locally heavy activity moving through the Big Bend through Southwest Texas. The Marginal Risk is maintained, bumped north a bit and the Slight Risk is confined to these inflow bands. ....Upper Great Lakes... A slow-moving warm front and the re-intensification of the low level jet are expected to support another round of elevated storms tonight shifting southeast mainly from the U.P. to northern L.P. of MI. PWs of 1.5-2 inches on the nose of 40+ kt westerly inflow should allow training/redeveloping storms may produce an axis or two of locally heavy amounts. The Marginal Risk was expanded a bit given the remaining uncertainty of where these bands will develop and may overlap with the afternoon activity that tracked southeast from Traverse City. Jackson Day 2 Valid 12Z Wed Aug 23 2023 - 12Z Thu Aug 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF WEST TEXAS INTO SOUTHERN NEW MEXICO... 2000 UTC Update -- QPF guidance, particularly the high-res CAMs, came more in line at 12Z with the heavier rainfall footprint across West TX into south-central and southwest NM. This has afforded higher forecast confidence in introducing a Slight Risk over these areas, resulting from the remnants of T.S. Harold. PW anomalies of 2-2.5 standard deviations above normal are still noted per the 12Z SREF and GEFS, along with 850-700 mb moisture flux anomalies of +4 to +5. The latest 12Z HREF neighborhood probabilities of 24 hourly QPF exceeding the 2 year ARI peak between 30-60% within the Slight Risk area. Meanwhile, the 12Z GEFS-based, CSU first-guess ERO also shows an expanded area of Slight Risk probabilities in this area compared to the 00Z cycle.=20 Hurley Previous Discussion... ....Big Bend Region into the Southwest... Models continue to show a fairly well-defined mid to upper level shortwave associated with the remnants of Tropical Storm Harold moving around the southwestern periphery of the central U.S. high, lifting north from northwestern Mexico into the southwestern U.S. on Wednesday. Showers and storms fed by an axis of deep southeasterly flow and moisture will likely be ongoing across portions of the Big Bend and West Texas early in the period, producing locally heavy amounts and isolated flash flooding concerns. As the system continues farther north, its associated moisture and the more typical monsoonal moisture are expected to increase shower and thunderstorm coverage and a greater threat for isolated flash flooding across portions of the Southwest from southern New Mexico to southern Utah.=20 ....Great Lakes to the upper Ohio Valley... A warm front and the threat for overnight convection is expected to shift farther east, with areas from southeast lower Michigan to western New York and Pennsylvania most likely to be impacted late Wednesday into early Thursday. PWs are expected climb a little higher with this next round, with the general model consensus showing 1.75 to 2 inches along the leading edge of the low level jet. There is still plenty of uncertainly regarding how the resulting cloud cover from the previous night's convection may impact later convective development, and where and if heavy rains may develop late Wednesday into early Thursday. Therefore, the Marginal Risk was maintained for now. However, an upgrade may be required if the models begin to show increasing agreement for heavier amounts, especially across areas with lower FFGs within the Marginal Risk area. Pereira Day 3 Valid 12Z Thu Aug 24 2023 - 12Z Fri Aug 25 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF SOUTHERN AND CENTRAL UTAH.. 2000 UTC Update -- Minor modifications made to the Day 3 ERO. Expanded the Slight Risk in UT to capture more of the Slot Canyons across south-central UT. Across the OH Valley into the Mid Atlantic and Northeast, there continues to be quite a bit of spread in the model QPFs Thursday night-Friday morning with the MCS activity coming down the upper ridge. For now, we are casting a fairly wide Marginal Risk net, however given the spatial spread in the guidance, esp. across central-northern PA into southern NY where the current 1/3 hr FFG remains quite low, for now have refrained from hoisting a Slight Risk. Hurley Previous discussion... ....Southwestern U.S.... The mid-level remnants of Tropical Storm Harold and monsoonal moisture spreading north of the west side of the central U.S. ridge will support increasing shower and thunderstorm coverage across the Four Corners states and southern Nevada on Thursday, with PW anomalies of 1 to 3 standard deviations above normal forecast within much of the highlighted Marginal Risk area. Given the complex terrain and the center of some of the greater PW anomalies, a Slight Risk was maintained across portions of southern Utah. ....Lower Great Lakes and upper Ohio Valley to the Northeast... Models show plenty of potential, but also uncertainty, for heavy rainfall across the region. Showers and storms developing Wednesday night may remain ongoing into the early part of the period across New York/Pennsylvania before redeveloping farther to the east ahead of a wave moving through the eastern Great Lakes on Thursday. In its wake, a well-defined cold front will begin to drop south from the Great Lakes, with deep moisture pooling ahead of the front. While models are far from agreement, some do show a notable signal for heavy amounts in the vicinity of the eastern Great Lakes-upper Ohio Valley late Thursday into early Friday.=20 These amounts will be fed by the deep moisture and high instability ahead of the front and the potential for training/redeveloping storms. Similar to Day 2, model spread does not provide confidence for anything beyond a Marginal Risk at this point. However, should the models start to show better agreement, an embedded upgrade may be required in future issuances. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7vZJIYryNB3Om6NL7q-kvA04xIdvtcKtpQzZ9DYl7G5w= -lxC2METfcSwC0rAETNu9ixDOn-2fekJ09mIhmZmJHATTLE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7vZJIYryNB3Om6NL7q-kvA04xIdvtcKtpQzZ9DYl7G5w= -lxC2METfcSwC0rAETNu9ixDOn-2fekJ09mIhmZmCIFiJ0k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7vZJIYryNB3Om6NL7q-kvA04xIdvtcKtpQzZ9DYl7G5w= -lxC2METfcSwC0rAETNu9ixDOn-2fekJ09mIhmZmo8rG25M$=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) .