Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 21 2023 19:54:11 FOUS30 KWBC 211954 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 253 PM EST Tue Nov 21 2023 Day 1 Valid 16Z Tue Nov 21 2023 - 12Z Wed Nov 22 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE SOUTHEAST TO MID-ATLANTIC... ....16Z Update... Current radar/sat composite shows a broad axis of rainfall across the eastern CONUS with embedded moderate to heavy rainfall within the overall precip field. Latest guidance is on track with regards to the timing and placement of the main precip shield with local convective enhancement relegated over the Deep South with the northern extension perhaps into southern VA where just enough instability presence could trigger some thunder within the confines of the VA/NC border. Majority of the precip will be of the stratiform variety, but a strong mid-level forcing regime will take place over the southern and central Mid-Atlantic later this afternoon and evening allowing for an enhanced precip field with a line of low-topped convection possible from the southern Blue Ridge to points east as a cold front propagates out of the Ohio and Tennessee Valleys. 7H VV forecast is fairly robust across central VA up into MD later this evening (00-06z) thanks to a strengthening mid-level jet max approaching 70kts later today. PWAT anomalies approaching 1.5-2 standard deviations above normal will be present ahead of the main trough axis from southern PA into the Southeast U.S, plenty moist enough to garner an environment favorable for heavier rainfall and totals approaching the 3hr and/or 6hr FFG intervals in pockets of the Mid-Atlantic and Southeast. 12 HREF EAS neighborhood probabilities for at least 2"/24-hrs is significant (>50%) from western NC up through central VA into MD which correlates to the 24hr, 10-year ARI exceedance probability between 20-30% within the aforementioned corridor. There are two characteristics that will keep the setup relegated to a MRGL risk and that's antecedent dry conditions still present east of the Appalachians, as well as lower-end rates with 1"/hr probabilities still fairly tame with the peak rates centered over the narrow convective maximum across northern GA into the western half of NC. Highest rainfall forecast will remain away from this convectively enhanced environment with large scale forcing and modest surface convergence this evening being primarily responsible for much of the period's rainfall. After careful interrogation of the latest guidance and recent radar trends, have maintained the previous forecast MRGL risk with only some slight adjustments on the western edge of the risk area based off the latest observations, and a tick upwards of the probability fields in regards to ARI exceedance intervals. Main flash flood threat will likely be confined to the terrain from north GA up through western NC into central VA, as well as the urban centers within the Mid-Atlantic, including the DC/Balt metro up to Philadelphia. A secondary area of interest will also be the VA tidewater where signals for heavy rainfall have increased for the time frame between 06-12z tomorrow morning, so made sure to have the area outlined for the period. Kleebauer ....Previous Discussion... Only very minor changes were made to the Marginal Risk area on the Day 1 outlook. Approaching longwave upper trough and attendant 130-150 kt jet streak on the lee side will favor a robust, elongated warm conveyor belt (WCB) with upper level moisture origins from the eastern Pacific south of Baja. Favorable along-stream upper level forcing (deep-layer Qs convergence and ascent) will get a boost from shortwave energy pivoting through the trough base, which will allow the trough to amplify further through Wed and Thu. As noted in the previous discussions, instability will be very limited, especially across the northern half of the outlook area. However, PWs averaging 2 standard deviations above normal (1.0-1.5" northern portion of the outlook area...1.5-1.75"+ south), along with 850-700 mb moisture flux anomalies between +3 and +4, will foster widespread rainfall totals between 1-3+", highest along the eastern slopes of the southern-central Appalachians give the terrain enhancement. If any area has slightly elevated chances for 4"+ amounts and increased isolated flash flood/rapid runoff concerns, it's along the Blue Ridge Mountains of central/northern VA. Here, the 00Z HREF depicts high probs (60-80%) for 24-hr QPF to exceed 5 year ARI values. Much of the region is experiencing very dry ground conditions and drought, with relative soil moisture within the top 40cm layer currently below the 20th percentile, and in many areas below the 10th percentile. Therefore much of this rainfall is expected to be beneficial, while the lack of even elevated instability should limit the short-term rainfall rates. However, isolated areas of flash flooding will be possible where drainage is impaired, especially with the fallen foliage or urban influence, and/or where the soils have become more hydrophobic given the recent dry spell (30 day rainfall <25% of normal across most of the outlook area per AHPS). Hurley Day 2 Valid 12Z Wed Nov 22 2023 - 12Z Thu Nov 23 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley/Kleebauer Day 3 Valid 12Z Thu Nov 23 2023 - 12Z Fri Nov 24 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!48Jj4-KsZtZQKVZ3IpgN_0SChFfokD505wK_YBIYc6iM= aNhRUxQy_iY0P7rs789wZEiJ74x8vAyqWgesbJeyeHk3VcU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!48Jj4-KsZtZQKVZ3IpgN_0SChFfokD505wK_YBIYc6iM= aNhRUxQy_iY0P7rs789wZEiJ74x8vAyqWgesbJeymvPIoQk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!48Jj4-KsZtZQKVZ3IpgN_0SChFfokD505wK_YBIYc6iM= aNhRUxQy_iY0P7rs789wZEiJ74x8vAyqWgesbJey0UHZ-ME$=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) .