Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 29 2023 15:55:47 FOUS30 KWBC 291555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1155 AM EDT Fri Sep 29 2023 Day 1 Valid 16Z Fri Sep 29 2023 - 12Z Sat Sep 30 2023 ....A MODERATE RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER PORTIONS OF THE NORTHEAST... ....16Z Update... Only minor adjustments were made to the D1 package across the CONUS. Tightened up the eastern extent of the Moderate to align with current obs and short term trends where heaviest rainfall has occurred and continued higher end impacts are forecast. Also brought in the southern extent of the MRGL-SLGT to adjust for the trends in guidance. Main axis of each outlook was aligned with HREF blended mean and ensemble-bias corrected QPF footprint. No change was initiated over the MRGLs in the Midwest and Florida as the overall synoptic pattern is still on track. Flash flood threat will remain isolated within the respective convective environments this afternoon and evening within areas of anomalous PWATs.=20 ....Mid-Atlantic into Northeast... Heavy rainfall has impacted the western Long Island area into the NYC metro with area totals on the order of 4-6" with locally higher amounts located within the Brooklyn and Manhattan boroughs. 1-3" is very common in the surrounding boroughs and encompassing areas within the Lower Hudson, CT, and the northern third of NJ. Strong low-level frontogenesis located parallel to the NYC/NNJ/LI coast is analyzed on mesoanalysis and very well depicted within the banding structures persistent over the above locations. Cross-section analysis of both the 12z HRRR and 12z NAM nest show a rich moisture presence deep into the profile over northeast NJ up into western CT within a field of elevated Omega which aligns perfectly with the forecasted 1000-925mb frontogenesis fields. This is the main axis of concern through the rest of the D1 time frame where a north-south precip maximum will likely develop later this afternoon and evening. 12z HREF mean still shows a widespread area of 3-5" totals as far south as southern Monmouth county in NJ up through the Lower Hudson where terrain influences will aid in local heavy precip banding signatures as the strong U-vector wind anomalies provide upslope enhancement within the hills north of NYC across Orange and Putnam counties. Local totals exceeding 8" are increasingly likely within the Moderate coverage area, especially those areas that are already over 5-5.5" at this hour. 12z HREF neighborhood probability of >8" is up to 20-25% within NYC proper, a fairly robust signal in the means and the center of the MOD risk bounds. Our upper low to the west is slowly propagating through PA with center of circulation over the Laurel Highlands as depicted on WV satellite. The upper-level progression is important in the outcome of the downstream heavy rain threat as increased dynamics and textbook synoptic scale evolution will evolve over the course of the afternoon and evening, finally ending within the back end of the D1 period. Kleebauer ....Upper Midwest/Great Lakes... Shortwave energy propagating south-north into Canada will drive showers and scattered to isolated thunderstorm activity over the Upper Midwest today. Another inverted surface trough will direct and enhance anomalous moisture plume with PWATs of 1-2" into much of Minnesota and northwestern Wisconsin. Rainfall totals of 1-2 inches are possible over portions the Upper Midwest by tomorrow morning. ....Florida... The back end of a slow moving cold front will act as a focus for scattered to isolated showers and thunderstorms today. A Marginal Risk area was continued to account for any localized flash flooding concerns that may arise. Kebede Day 2 Valid 12Z Sat Sep 30 2023 - 12Z Sun Oct 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT BASIN AND FLORIDA... ....Great Basin/Nevada... A deep upper-trough will transport a modest plume of moisture into the West on Saturday. A cold front associated with a deepening low pressure system will act as a focus for convection over the Great Basin in particular. Some parts of central Nevada, southeastern Oregon and southwestern Idaho could see rainfall rates in excess of 0.25"/hr. Ample forcing over elevated terrain could lead to more efficient/enhanced rainfall. Between 0.5-1" total areal rainfall is forecast over 3-4 hour period therefore, localized flash flooding is our primary concern. ....New Mexico/west Texas... Convection supported by an upper trough to the West and shortwave energy downstream from it will support another localized flash flood threat over parts of eastern New Mexico and into west Texas. Decent instability, modest moisture flux and low level jet may produce bursts of heavy rainfall over our marginal risk area. Urban areas and dry washes are most vulnerable of flash flooding. ....Florida... A slow moving shortwave trough will allow for the continuation of scattered to isolated thunderstorms across the central/southern peninsula on Saturday. Some cells may produce localized heavy rainfall within the marginal risk area. Antecedent conditions will also contribute to the sensitivity to any isolated heavy rainfall threats. Kebede Day 3 Valid 12Z Sun Oct 01 2023 - 12Z Mon Oct 02 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF FLORIDA... ....Florida... Shortwave energy present across the state of Florida will support isolated flash flooding concerns on Sunday. Scattered to isolated thunderstorms will continue to develop along a quasi-stationary front draped across the southern peninsula. There'll be plenty of instability in place so there's always the risk of some cells over performing with efficient rainfall rates exceeding flash flood guidance. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7GcEMuL0MOPJy9rcFzeo6y4VQD4hnu1E5IqNIYCILHad= wjqpFuLHIAq3TwxnU2Ko9Smyw9b7h-Bgk9ZHMj82KG9mXXc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7GcEMuL0MOPJy9rcFzeo6y4VQD4hnu1E5IqNIYCILHad= wjqpFuLHIAq3TwxnU2Ko9Smyw9b7h-Bgk9ZHMj82RW1vkbo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7GcEMuL0MOPJy9rcFzeo6y4VQD4hnu1E5IqNIYCILHad= wjqpFuLHIAq3TwxnU2Ko9Smyw9b7h-Bgk9ZHMj826vYOq2k$=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. 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