Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Sep 28 2023 08:54:00 FOUS30 KWBC 280829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Thu Sep 28 2023 Day 1 Valid 12Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC, NORTHEAST, FLORIDA AND THE OHIO VALLEY... ....Mid-Atlantic into Northeast... We added a Marginal risk across portions of eastern PA into NJ and southeast NY with this update. Lower confidence than normal for a day 1 forecast as an area of low pressure develops offshore and attempts to push an inverted trough/coastal front inland. The extent of the flash flood risk really comes down to the positioning of this front/convergence axis later tonight into Friday morning. If this axis stays offshore then any instability will also remain offshore, resulting in just some stratiform rainfall moving inland. However if the axis is along the coast or inland, then some weak instability should get ashore as well, and set the stage for the potential of shallow efficient convection. Most of the HREF members do bring some heavier rain onshore, with the highest probabilities focused over NJ. However each HREF member has a slightly different location, and most of the 00z global models keep the bulk of the heaviest rain offshore through 12z Friday. Given this spread and uncertainty opted to stick with a Marginal risk for now. The main threat for day 1 is the last 6 hours (06z-12z Fri), so will let the day shift take a look at the 12z HREF and make a call on whether to upgrade to a Slight or not at that time. ....Ohio Valley... A Marginal risk was maintained across portions of TN/KY into southern OH. Convection will be ongoing at 12z this morning, with some brief training/backbuilding remaining possible. We might end up with a downward trend in convective activity by mid to late morning...but with an elongated area of vorticity continuing to traverse the area through the day good synoptic ascent will remain in place. Thus once instability recovers from the morning activity we could see some additional convective development across the region this afternoon/evening. The magnitude/organization of this additional activity is conditional on the amount of recovery we get....but if we area able to ramp instability back up then some additional brief training could occur given the persistent low level inflow and convergence in place. Isolated flash flooding will be possible, both this morning and potentially again this afternoon/evening. ....Florida... Only minimal changes to the inherited Marginal risk area over FL. Anomalous moisture will remain in place, with a stationary front and stronger upper level flow helping trigger above average convective coverage again today. High rainfall rates will drive a localized flood risk over any more susceptible urban locations. Chenard Day 2 Valid 12Z Fri Sep 29 2023 - 12Z Sat Sep 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE THE MID-ATLANTIC REGION INTO SOUTHERN NEW ENGLAND... ....Mid-Atlantic and Northeast... The risk of flash flooding appears to be increasing across portions of the Northeast Friday into Friday night, although there remains quite a bit of uncertainty with the magnitude and location of the highest threat. The main feature to focus on will be an inverted trough/coastal front which will help focus a persistent area of convergence. We are also likely going to have enough instability advect in off the Atlantic along this convergence axis to support at least shallow convective elements and primarily efficient warm rain processes. The 00z HREF focuses the highest probabilities across NJ into southeast NY...with 5" exceedance probabilities ~50% and 8" over 20% from 00z Fri-00z Sat. The HREF also shows some low 100 year ARI exceedance probabilities over this same area. The HREF is likely depicting a plausible worst case scenario...where the inverted trough and persistent convergence focuses over land resulting in this excessive rainfall. In this scenario a significant flash flood risk may evolve. However I do think the uncertainty is probably greater than what the HREF is depicting. For one, the location is not locked in...with the 00z ECMWF further north and focusing the heaviest rainfall more over southern New England. On top of that it is not a given that the inverted trough sets up onshore...there is some chance this axis stays just offshore. If this were to happen then instability would stay offshore as well...and rainfall totals/rates would not be as high over land. For now we will carry a Slight risk, as the conditional flash flood risk is pretty significant, and higher end flash flooding is a possibility with this setup (although not a given at this point). We will focus the risk from northern NJ into southeast NY and into CT, which is where the best consensus amongst the high res and global models currently resides. However we will need to watch areas further northeast into southern new England...as while it appears to be a lower probability outcome at the moment, we can not rule out the further northeast 00z ECMWF solution either. ....Upper Midwest... Convection should be ongoing Friday morning across portions of SD/ND/MN, although do think this activity will be pretty quick moving. The better chance of isolated flash flood issues appears to be Friday night as the front slows and low level moisture transport sees a nocturnal uptick in intensity. This should be enough to result in another convective round, with propagation vectors supporting some backbuilding potential by this time. With PWs running near to above the climatological 90th percentile any training/backbuilding of cells could pose a localized flash flood risk ....The Florida Peninsula... Pretty much a persistence forecast across FL...with 2"+ PWs and a favorable environment for good convective coverage in the area of a slow moving frontal boundary. High rainfall rates over urban locations may again result in some localized flood concerns. Chenard Day 3 Valid 12Z Sat Sep 30 2023 - 12Z Sun Oct 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST AND FLORIDA... ....Southwest... A deep and anomalous closed low will move into the Southwest this weekend...with 500mb heights below the 5th percentile for the time of year. To the east of this deep low there should be an overlap of favorable PWs and instability across portions of far west TX into eastern NM. This corridor will also be within an axis of deep and fairly unidirectional southerly flow, which should favor some repeat convective activity. Thus overall the inherited Marginal risk looks in good shape, as would expect to see pockets of heavy and locally excessive rainfall. The ECMWF seems a bit dry given the ingredients in place...so prefer a wetter outcome, closer to the GFS. Of course as we get closer we'll have more high res guidance to look at and hopefully get a better idea of convective details. ....Florida... Pretty much a persistence forecast across FL...with 2"+ PWs and a favorable environment for good convective coverage in the area of a slow moving frontal boundary. Global deterministic and ensemble guidance appears a bit wetter Saturday compared to Friday...probably due to slightly better positioning of the upper jet and mid level trough. So possible we will have a bit better convective coverage, but still generally only looking at a localized urban flash flood threat driven by high rainfall rates. Chenard $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .