Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Oct 13 2023 15:56:40 FOUS30 KWBC 131556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Fri Oct 13 2023 Day 1 Valid 16Z Fri Oct 13 2023 - 12Z Sat Oct 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF IOWA AND NORTHERN ILLINOIS... 16Z Update... Latest 12Z HREF shows a narrow swath of strong and efficient thunderstorms forming along the occluded front in central Iowa between 19Z Fri - 03Z Sat. Latest HRRR shows the nose of a 45-50 knot 700mb jet over central Iowa as well, delivering strong 850-700mb moisture flux along the front. Sampled model soundings also showed low-mid level RH values >90% and warm cloud layers as deep as 9,000ft in some cases, especially over southeast Iowa and northern Illinois. 12Z HREF probabilities of 3-hr QPF > 3-hr FFGs were as high as 10-20% just north and west of Des Moines between 21Z Fri - 00Z Fri. Given these reasons, did a minor expansion of the Slight Risk farther west to include more of western Iowa. Otherwise, the inherited ERO and its discussion remain on track. ....Coastal South Carolina & Coastal Georgia... Low pressure over northern Florida will slowly drift northeast along the Southeast coast this afternoon, while an inverted trough takes shape tho the north and west of the main circulation center. CAMs show an influx of 925-850mb theta-e directed towards parts of southern GA and coastal SC later this evening. Model soundings show an exceptional amount of low-mid level moisture originating from the Gulf Stream is being advected overhead with area-averaged RH values as high as 90% this evening. MUCAPE will generally range between 200-500 J/kg and PWs as high as 2.0", so while instability is not overly impressive, its overlap with near 2.0" PWs can still support locally excessive rainfall rates. Also of note, there is strong veering at low-mid levels this afternoon and continuing into the overnight hours, highlighted by sfc-6km wind shear that ranges between 30-40 kts and sfc-3km SRH values above 150 m2/s2. These parameters suggests any cells could contain weak mesoscale circulations and these features can enhance rainfall rates. No Marginal Risk was issued given the region has been drier of late and 12Z HREF probs for >2"/hr rainfall rates tonight were <10%, but localized downpours causing some ponding on roads cannot be fully ruled out tonight, particularly between 03-12Z Saturday. Mullinax ---Previous Discussion--- ....Mid-Upper Mississippi Valley to the western Great Lakes... The large scale synoptic system across the eastern Plains will track across the Midwest later today and into the Ohio Valley on Saturday. As the mid-upper low gradually opens, the degree of south-southeast low-level inflow will decrease with time today and tonight, as will the 850 and 850-700 mb moisture flux anomalies per the 00Z SREF and GEFS. Furthermore, MUCAPEs ~1000 J/Kg across eastern NE as of 0600Z 10/13 are expected to drop below 1000 J/Kg over IA later today, then average around or below 500 J/Kg across northern-central IL Friday evening and overnight. Compared to yesterday's Day 2 ERO, the Slight Risk was constricted a bit, while also shifted southward into the area where the most favorable coupling of forcing and instability is expected along/near the surface low track. Peak probabilities of >1"/hr rainfall rates per the 00Z HREF are between 40-80% across western portions of the Slight Risk area (i.e. Iowa) between 2100-0100Z, while probabilities of QPF exceeding 3" during the Day 1 period are 40-50% across the same area. Meanwhile farther east into northern IL, the latest HREF probabilities of >3" during Day 1 top between 30-40%. Hurley Day 2 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 Mid Atlantic Region... On Saturday-Saturday night, shortwave energy associated with the Midwest to OH Valley system will become 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 eventually gives way a developing low off the mid Atlantic coast Sat night. Compared to Friday and especially Thursday, the deep-layer forcing and (especially) moisture flux/transport aren't nearly as anomalous, which is typical during these transitional Miller B events. Across northern portions of the Mid Atlantic region, DPVA and left-exit region upper jet forcing (100-120kt jet streak at 250 mb) will lead to favorable deep-layer isentropic ascent over-top a more stable, CAD profile in the lower layers. Over these areas expect a steady moderate to occasional heavy rainfall, averaging between 0.50-1.5" in a 24hr period with little variation outside of that range. Over southern portions of the Mid Atlantic, particularly southeast VA and eastern NC, instability profiles may become a bit more favorable later Saturday afternoon into the evening as these areas may briefly sneak into the warm sector. However, given the fast progression of the main synoptic players at that point, expect any uptick in short-term rainfall rates to be short-lived per the simulated reflectivity forecast from both the NAM-CONUS Nest and FV3 Saturday afternoon and evening. This along with the relatively high FFG and low (<50%) soil moisture percentiles will likely result in a sub-Marginal flash flood risk, even if non-zero in some areas. Hurley Day 3 Valid 12Z Sun Oct 15 2023 - 12Z Mon Oct 16 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7RaMQIyFp5gIbpebfMpOmSNZmCb9MAkxw3vI8sMEra0Y= CSSL4U4JymaqPybFhD5lnrDgR9vuC4L0wXy3W7nf19TJWAc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7RaMQIyFp5gIbpebfMpOmSNZmCb9MAkxw3vI8sMEra0Y= CSSL4U4JymaqPybFhD5lnrDgR9vuC4L0wXy3W7nfXSgCsVA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7RaMQIyFp5gIbpebfMpOmSNZmCb9MAkxw3vI8sMEra0Y= CSSL4U4JymaqPybFhD5lnrDgR9vuC4L0wXy3W7nf-361MjE$=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) .