Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 05 2024 20:20:08 FOUS30 KWBC 052019 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 419 PM EDT Thu Sep 5 2024 Day 1 Valid 16Z Thu Sep 05 2024 - 12Z Fri Sep 06 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ALONG THE CENTRAL GULF COAST... 16Z Update... Introduced a Slight risk area over portions of coastal Florida into adjacent coastal Georgia where convection based on a combination of short term radar imagery showing convection lingering off-shore from an area where heavy to excessive rainfall fell on Wednesday...plus at least some hint that convection could still build westward during the day. With high precipitable water values, high freezing levels and weak steering level flow...felt a Slight Risk was warranted where the ingredients tended to overlap. On the other hand, deterministic QPF from any given global model and a number of the higher resolution CAMs are fairly light, the occurrence of excessive rainfall is far from certain. Along the Gulf coast...the previously issued Moderate and surrounding Slight risk areas appear to be in pretty good shape and only minor adjustments were made. Satellite/radar imagery during the morning continue to keep a majority of the coldest cloud tops/greatest rainfall rates off-shore and along the immediate coast where 2 to 3 inch per hour rainfall rates appear to be most likely. However...signals persist for those kind of rates to spread northward in association with deep-layer moisture flux convergence as suggested by the HRRR. If that were to happen...concern is for impactful flooding in more populated/urbanized regions with more widespread impervious land coverage. No change made in New Mexico. Bann ....Previous Excessive Rainfall Discussion... ....Gulf Coast, Lower Mississippi Valley and Southeast... In coordination with local forecast offices near the Mississippi Delta region, a Moderate Risk area was hoisted for this period to include portions of southeast Louisiana and southern Mississippi. There is consensus for heavier rainfall along the central Gulf Coast given the bulk of PWs >99th climatological percentile residing along the Gulf Coast. The region is also closer to the surface-850mb southeasterly flow that will sustain a steadfast regime of low level theta-e advection into the nearby stationary frontal boundary. Additionally, the latest guidance shows a potential for locally higher amounts upwards of 10" per NBM 95th percentile to focus in the vicinity of the New Orleans metro and points east and south. A Slight Risk spans from the southeast Texas coast to southern Mississippi. Deep-layer instability across portions of the Southeast may be lacking, however PWs of 2.2-2.4" will make for highly-efficient rainfall rates as the quasi- stationary front shifts a bit northward by tonight as the upper shortwave trough dips across the western Gulf Coast. A Marginal Risk spans from extreme southern South Carolina to central Florida and westward to the central Texas coast. ....New Mexico and Colorado... Convection is expected to fire up ahead of an approaching cold front thanks to PWs topping 0.75" and up to 500 J/kg of MUCAPE. Soils across this part of the region are sensitive given recent moisture and burn scars. The Marginal Risk area was maintained for the localized flash flood potential for the afternoon and evening. Campbell/Hurley Day 2 Valid 12Z Fri Sep 06 2024 - 12Z Sat Sep 07 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL MAINLY ACROSS=20 SOUTHEAST LOUISIANA... ....Gulf states and Southeast... Introduced a Moderate risk that was largely confined to southeast=20 Louisiana where low pressure riding along a quasi- stationary front will continue to focus and support areas of heavy to potentially=20 heavy rainfall on top of widespread moderate to heavy rainfall in=20 the Day 1 period. A mid and upper level trough will be digging=20 across the western Gulf Coast region which increases low- level=20 frontogenesis along the central-eastern Gulf Coast to the Georgia=20 Coast. Heaviest short-term rainfall rates should be confined to a=20 corridor where precipitable water values of 2.2 to 2.5 inches,=20 tall/skinny CAPE profile and greatest low-level FGEN along the=20 quasi- stationary surface boundary align.=20 While there continues to be some spread on exactly where the=20 highest QPF will focus...two camps of thought are developing with the broader/coarser guidance being slower to move the heaviest rainfall off shore while the higher resolution guidance favored a faster solution that brings more drying in from the north with correspondingly lower QPF. Given the probabilistic nature of the Excessive Rainfall Outlook, opted to introduce the Moderate risk area to highlight the potential for additional heavy rainfall with some overlap into the lower QPF shown by the higher res guidance favored by the WPC QPF desk. Either way...there is a general=20 consensus for 1 to 3 inch amounts in the rain area and several pieces of guidance that indicate isolated maxes of 4 to 8 inches,=20 especially along the coastline.=20 Bann Day 3 Valid 12Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE EASTERN GULF COAST AND SOUTHEAST ALONG WITH PORTIONS OF THE NORTHEAST... 2030 UTC Update... Trimmed away some areal coverage of the Marginal Risk area in the Northeast U.S. given an overall lack of instability to help support rainfall rates (an idea supported by pretty meager model QPF). Otherwise, few changes needed. Bann Previous Excessive Rainfall Discussion... ....Gulf Coast and Southeast... Slow moving convection will continue to focus along the draped frontal system along the Gulf Coast and Southeast U.S. Abundant Gulf moisture will continue to feed into the region leading to higher rainfall rates. Multi-day rains will have increased soil saturation and lowered flash flood guidance by this period thus there will be a continued threat for excessive rainfall and local flooding concerns. A Marginal Risk is in effect across northern Florida and southern Georgia. ....Northeast... An upper trough/low will pivot across the region during this period sending showers and thunderstorms across the Northeast where multiple recent rains have increased soil sensitivity. A Marginal Risk area is in effect for eastern New York, northeast Pennsylvania, northern New Jersey and northwest Vermont. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7aNHnrI3Bm0XSuoiueWPI2wBCh3dlAbfYhO2GwOXkaX8= l-zUrQw_TwjZcL99guA0JWgHeho9Vhzu2OPUq6zpiSovw0Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7aNHnrI3Bm0XSuoiueWPI2wBCh3dlAbfYhO2GwOXkaX8= l-zUrQw_TwjZcL99guA0JWgHeho9Vhzu2OPUq6zptqn_uUU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7aNHnrI3Bm0XSuoiueWPI2wBCh3dlAbfYhO2GwOXkaX8= l-zUrQw_TwjZcL99guA0JWgHeho9Vhzu2OPUq6zpOCa0JB4$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .