Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 04 2023 19:48:11 FOUS30 KWBC 041948 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 345 PM EDT Thu May 04 2023 Day 1 Valid 16Z Thu May 04 2023 - 12Z Fri May 05 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE INTERMOUNTAIN WEST AND FROM THE LOWER MISSOURI VALLEY AND EASTERN PORTIONS OF THE SOUTHERN PLAINS TO PARTS OF THE MID-SOUTH... ....Central and Northern Intermountain West... 16Z update... The environment remains favorable for isolated areas of moderate to heavy rainfall across much of the Intermountain West during this period. The latest hi-res guidance continues to show rainfall intensity increasing after 00Z with isolated hourly rates of 0.50 to 1.50 inches/hr. No changes were made to the Marginal Risk area. Campbell Stubborn, slow-moving mid-upper low/trough will take on more of a negative tilt today and Thursday night as shortwave energy pivots northward through the Southwest and Central Great Basin. The uptick in upper level forcing with the increased upper difluence/divergence/DPVA, coinciding with areas of decent instability for this time of year prior to monsoon season (MUCAPEs of 500-1500 J/Kg, owing in large part to steepening H7-H5 lapse rates of 7.5-8.5 C/KM), will allow for more numerous, pulse-type convective clusters, with isolated stronger cells having the potential of 0.50-1.00"/hr rainfall rates per. This as PWs climb between 0.50-1.00" per the guidance, highest (>0.75") across eastern WA and northeast OR into northern ID, which would be 2-2.5 standard deviations above normal for early May. Subtle changes were made to the inherited Marginal Outlook from yesterday's Day 2 ERO based on the guidance trends. This included a slight expansion of the outlook across central and north-central WA to the Canadian border. Have also pulled the Marginal a bit farther east to capture more of eastern ID. As has been noted in prior discussions, areas of heavy rainfall combined with the snowmelt, and/or occurring over burn scars, will likely lead to isolated or localized runoff issues. ....Central/Southern Plains into the Mid-Mississippi Valley and Mid-South... 16Z update... Convection across Texas is expected to increase its intensity during the afternoon hours. The latest hi-res guidance consensus show rainfall intensity ramping to 0.50 to 1 inch/hr by 20Z and increasing up to 2-2.5+ inches/hr by 00Z as the storms track across eastern Texas and Oklahoma. Given the nature of progression, still anticipate any runoff/flooding issues across the southern part of the Marginal Risk area to be fairly localized. A small eastward expansion was made into western Louisiana and across southeast Missouri. Campbell Downstream of the aforementioned upper trough, an upper level jet streak and associated mid-level shortwave across eastern NM this morning will lift northeast to along the KS-OK border by 00Z Fri. This wave will flatten the mid/upper level ridge across the central CONUS, traversing slowly eastward into the mid MS Valley by 12Z Fri. Strong upper level confluence is expected to generate a 100-130 kt upper level jet streak across the Lower OH Valley and central Appalachians Thu night. The 00Z guidance continues to show an uptick in low-level frontogenesis by 00Z Fri across the MO/KS/AR/OK 4 state area, while SW/SSW 850 mb flow increases to 35-45 kts. Elevated instability will see a modest rise; however, not to the degree as across and south of the Red River, where models show robust MUCAPEs between 1500-3000 J/Kg. Because of the lack of overlap with the best forcing and thermodynamics, the 00Z guidance continues to show a multi-modal distribution with the areas of heavy rainfall across the outlook region, with some of the CAMs also indicating another area of heavy rainfall farther north of the outlook area (NE-IA) associated with a weaker vort lobe. At this point, given the lack of instability noted in the models late Thu-Thu night, will keep areas from eastern NE into IA and northern MO out of the Day 1 ERO. Meanwhile, while certainly not as robust compared to the warm sector, the degree of (elevated) MUCAPE north of the warm front across the MO/KS/AR/OK 4 state area north of the Red River Valley is expected to range between 500-1500 J/Kg Thu night. This should be sufficient given the favorable dynamical forcing for at least some upscale growth of elevated convection, perhaps even an MCS per some of the high-res CAM trends (including the 06Z HRRR). Farther south (along and south of the Red River Valley), within the warm sector ahead of the dry line, the combination of more robust deep-layer CAPE (1500-3000+ J/Kg) along with PWs between 1.5-2.0" will favor hourly and sub-hourly rainfall rates of 2-3" underneath the strongest cells. Stronger low-mid level shear however, along with the higher downdraft CAPE potential and outflows, will also lead to faster downstream cell motions (better forward propagation), therefore would anticipate any runoff issues to be localized. Hurley Day 2 Valid 12Z Fri May 05 2023 - 12Z Sat May 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE MID-SOUTH AND TENNESSEE VALLEY AND THE CENTRAL/NORTHERN INTERMOUNTAIN WEST... ....Mid-south and Tennessee Valley into the Southern Appalachians and Parts of the Southeast... 21Z update... While the exact track of where the highest QPF will be remains a bit uncertain, the overall trend the past 3 cycles have been further south and with lighter amounts. WPC QPF for this issuance followed suite. In addition, the current area of convection tracking across the eastern Texas/Oklahoma/Kansas, southern Missouri and northwest Arkansas has been more east than the expected northeast path. This may support the southern trend that we're seeing with the day 2 period. Especially if it quickly dives south/southeast as hinted by the ECWMF/UKMET/HREF by having 1+ inches from eastern Arkansas to southwest Alabama. Given the consist lowering of QPF and southward shift, the Marginal Risk area was shrunk down and orientated more WNW to ESE and the Slight Risk area was removed. Campbell Organized elevated convection north of the surface warm front at the start of Day 2 (12Z Friday), again possibly an MCS, will track east-southeast along with WNW-ENE oriented vort lobe, while individual cell motions will be southeasterly per the 850-300 mb thickness orientation and layer Corfidi vectors. Once again, the models show latitudinal differences with respect to the stripe of heaviest rainfall; however, the ECMWF remains rather consistent over the past 3-4 runs. Given the pool of strong deep-layer elevated instability (MUCAPEs 2000-3000+ J/Kg) upstream and into the western portions of the outlook area, along with the favorable forcing ahead of the convectively-enhanced shortwave, WPC continued to side with a QPF closer to the operational ECMWF and NAM CONUS-Nest. There will be an enhanced potential for cell training Friday night with the uptick in low-level flow, which in this thermodynamic environment would foster hourly rainfall rates up of 1.5-2.5". Therefore we opted to continue the Slight Risk inherited from yesterday's Day 3 ERO. ....Central and Northern Intermountain West... 21z update... no changes made at this time. Campbell Upper level pattern changes very little across these areas Fri-Fri night, with broadly difluent upper level flow east of the H5-H3 trough axis (though the favorable upper level forcing does shift a bit northeastward into western-central MT). As was noted in the Day 1 ERD, the favorable forcing and weak deep-layer instability (relatively steep mid-level lapse rates) along a ribbon of above-average PW will favor isolated hourly rainfall rates of 0.50-1.00". These rates, along with the recent snowmelt and possibility of occurring over burn scar areas, could foster localized short-term runoff issues. As a result, the Marginal Risk that was inherited from yesterday's Day 3 ERO continues in today's Day 2 ERO. Hurley Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5i_5bsvVRrEcf_A2gUBZJSWR3oh8AumX1EfqIKo0KPpN= JTkvJm8Mga3UIgqSLxZFb4hKIwXMZDfQ4Ce9Goeu1_cbORE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5i_5bsvVRrEcf_A2gUBZJSWR3oh8AumX1EfqIKo0KPpN= JTkvJm8Mga3UIgqSLxZFb4hKIwXMZDfQ4Ce9Goeu8bUX9Yg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5i_5bsvVRrEcf_A2gUBZJSWR3oh8AumX1EfqIKo0KPpN= JTkvJm8Mga3UIgqSLxZFb4hKIwXMZDfQ4Ce9GoeuggXnGio$=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) .