Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 16 2023 07:57:03 FOUS30 KWBC 160756 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 356 AM EDT Sat Sep 16 2023 Day 1 Valid 12Z Sat Sep 16 2023 - 12Z Sun Sep 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN MAINE... ....Eastern Maine... Hurricane Lee continues to move north over the northwest Atlantic with sights on western Nova Scotia and the Bay of Fundy. Heavy rainbands on the western flank of Lee are impacting Cape Cod and Nantucket with a general 0.5-1"/hr rates within the banding structures, keeping the heaviest precip focused off the coast. Heavier echoes will approach and bisect eastern ME by the morning and afternoon time frame allowing for a 3-6 hr period of 1-3"/hr rates at peak intensity as indicated by the probabilistic data within the 00z HREF/Global ensemble and deterministic guidance. We've seen a slight degradation in the western flank of the QPF field which translates to a more narrow heavy rain axis later today confined generally to the northeast of Gray/Portland, ME with the heaviest precip still located across Downeast ME from Bangor to the Canadian border. A general 2-4" of rain with max amounts around 6" are currently forecast across all major guidance including the 00z HREF blended mean and bias-corrected ensemble QPF over Downeast ME with a sharp decrease on the west side of the QPF field thanks to dry air advection expected on the western quadrant of the cyclone. Given the latest trends in guidance, only very minor adjustments to the SLGT risk were made to encompass the HREF blended mean and global ensemble probabilistic field of 40% or greater for 2" within the period of interest. ....Texas... A progressively moving disturbance originating from convective initiation over eastern NM and the TX Panhandle will continue on a southeast trajectory riding along the theta-E tongue situated from central TX up through the Caprock. Heavy rainfall within the core of the developing mesoscale complex will be in play through the disturbance's life cycle with 00z HREF neighborhood probabilities for 1-2"/hr ranging from 30-40% over the Permian Basin to 20-30% over Central TX by later this morning. The forward speed of the complex will limit the extent of flash flooding over west-central TX, but the axis of heaviest QPF within the hi-res deterministic and HREF mean bisect an area of lower FFG's thanks to previous days of precipitation. Local maximum around 3" will be possible where the rainfall is heaviest within the path of the impending MCS with the best location for higher totals likely oriented over the eastern Permian Basin and western rolling plains in both MAF/SJT CWA's.=20 Areas on the southeast side of the risk will have a bit less to do with the complex and more to do with convective development along the frontal boundary draped over south-central TX. There's a non-zero potential for local totals exceeding 3" over portions of the southeast TX coastal plain and areas just inland according to the latest HREF neighborhood probabilities where signals of ~30% for 2-3"/hr rates are forecast during the afternoon hrs when diurnal instability is at its maximum within a core of elevated PWATs running 1.5 standard deviations above normal. Despite higher FFG's located within the confines of Corpus Christi and surrounding locales, rainfall priming from this evening and potential rates add a slighter better than climo opportunity for localized flooding concerns within the synoptic and environmental setup across southeast TX. A Marginal Risk is in place across the areas mentioned above beginning in the Permian Basin down through central TX into the southeast TX coastal plain near CRP.=20=20=20 ....Tennessee Valley... A digging mid-level trough will enhance a difluent axis present over portions of the Tennessee and Ohio Valleys later today into early Sunday morning. Guidance continues to pinpoint locally heavy rainfall across much of Alabama up through eastern TN/KY before spreading into WV by the end of the D1 period. A prominent jet streak will be intensifying over Midwest, spreading eastward over time leading to large scale ascent downstream from the main trough axis. 00z hi-res deterministic and ensemble blend paints a stripe of heavy precipitation within the bounds of 0.75-1.5" with local maximum up to 4" within two zones; central and southern AL and eastern TN/KY mainly located where areal instability will be at its greatest. The first half of the day will be characterized by increased convective coverage due to diurnal destabilization within the zone of increasing difluence on the lead side of the shortwave trough. The second round occurring later tonight into tomorrow morning will originate from increasing large scale ascent and positive vorticity advection as strong mid-level vort max pivots over the Ohio Valley into WV overnight. A deep flux of moisture will stream northward ahead of the mean trough intersecting with the vorticity maxima generating a rapidly developing QPF field moving northeast out of the Tennessee Valley into the southern half of WV. Rainfall rates exceeding 1"/hr are increasingly likely for areas along and south of the I-64 prior to 12z Sunday as per the 00z HREF neighborhood probabilities which would spell a better signature for localized flooding given the low FFG values thanks to terrain influences and a primed ground from a busy stretch of convection within the past week to 10 days. The previous MRGL risk placement remains with only a few adjustments on the northern and western edges of the outlined risk area. ....Florida Atlantic Coast... A stationary front coupled with sea breeze interactions over eastern FL will create a localized flash flood risk later this afternoon extending from Daytona Beach down through Miami. Best chance for flooding will likely be over the coastal areas where urbanization provides lower FFG's compared to the inland locations, so the western extent of the MRGL risk in place will be fairly close to the coastal plain. The best indicator for the potential is the robust signature for enhanced rainfall rates in excess of 2-3"/hr located over the corridor above. In fact, 00z HREF neighborhood probability for 3"/hr or greater are breaching 50% for two areas in particular; Daytona down to Cocoa beach and West Palm to Fort Lauderdale. These two areas lie within a zone of elevated PWATs running 1.5-2 standard deviations above climo norm. Forecast bufr soundings at KPBI/KMLB indicated weak storm motion potential and prolific low-mid level moisture creating a favorable environment for heavy rains within a confined area. The MRGL risk remains in place with a slight northward extension based on HREF blended mean and probabilistic trends. Kleebauer Day 2 Valid 12Z Sun Sep 17 2023 - 12Z Mon Sep 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN NORTH CAROLINA INTO THE VIRGINIA TIDEWATER AND SOUTHEAST MARYLAND COASTAL PLAIN... ....Mid-Atlantic Coastal Plain... A robust shortwave trough over the Midwest will pivot eastward while taking on a neutral tilt as it crosses through the Ohio Valley. Surface cyclogenesis along the SC coast will initiate Sunday morning, tracking northeast up the Atlantic sea board leading to increasing precipitation coverage on the western side of the surface low as it works within the right-entrance region of a strengthening upper-level jet streak bisecting the Mid-Atlantic. Large scale ascent downstream of the mean trough will also generate scattered to widespread precipitation over the central Mid-Atlantic states Sunday morning and afternoon before shifting eastward, but the primary focus will be along the coastal plain(s) where general instability is highest and PWAT anomalies run upwards of 1 standard deviation above climatological norm. 00z GEFS ensemble probabilities of 1-2" of total rainfall are highest within a corridor from Charleston, SC up through the Maryland coastal plain with the best risk located over eastern NC where 50-60% probabilities are beginning to show up within the ensemble. This correlates well with the axis of best PWATs and instability which is what we want to see in regards to flash flooding concerns. It is possible that a secondary area over eastern WV could be included in the MRGL risk due to convective concerns on Sunday morning between 12-18z as thunderstorms over southern WV move off to the northeast given the mean flow. There's still some discrepancy in coverage and overall intensity during that time frame, but with low FFG's located along and west of the Blue Ridge, could see an expansion out that direction if later deterministic and HREF probabilistic fields support the addition. Otherwise, the Marginal risk remains over the eastern Mid-Atlantic in conjunction with convective development and eventual surface cyclogenesis.=20 Kleebauer Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4npm4pt8shDOsmIuY_IehtANz3tFlJILRSN0I52FNHR2= gZnbjMWYIc9Qzmh8IuFCl905FScULGw59r1S_jLab53Tg1g$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4npm4pt8shDOsmIuY_IehtANz3tFlJILRSN0I52FNHR2= gZnbjMWYIc9Qzmh8IuFCl905FScULGw59r1S_jLap8yIDy4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4npm4pt8shDOsmIuY_IehtANz3tFlJILRSN0I52FNHR2= gZnbjMWYIc9Qzmh8IuFCl905FScULGw59r1S_jLa9Xg08Kg$=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) .