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   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   759 PM EST Mon Nov 17 2025   
      
   Day 1   
   Valid 01Z Tue Nov 18 2025 - 12Z Tue Nov 18 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF   
   SOUTHERN CALIFORNIA...   
      
   ...Transverse Ranges of Southern California...   
      
   A Slight Risk remains, though the western end has been cut back=20   
   given the current placement of a Pacific cold front just west of   
   Los Angeles. The plume of max IVT (250 kg/m/s for inland locations   
   to near 300 kg/m/s at the coast) extended ahead of a cold front=20   
   into Los Angeles County at 00Z with a rainfall history of 0.25 to=20   
   0.50 inches in 15 minutes within the upslope regions of the=20   
   Transverse Ranges dating back to earlier this afternoon. The IVT=20   
   plume is forecast to weaken overnight as the cold front continues=20   
   to sink south and east, clearing San Diego County between 06-09Z,=20   
   with weakening and veering of low level flow. A narrow window of=20   
   time remains where hourly rainfall of 0.5 to 1.0 inches will be=20   
   possible (should brief training develop despite the overall=20   
   progressive nature of the front), but this threat looks to diminish   
   with time overnight. Localized/spotty additional totals up to 1.5=20   
   inches will be possible.   
      
   ...Central California...   
      
   As the mid-level closed low slowly parallels the central California   
   coast toward the south, steep mid-level lapse rates of about 7-8   
   C/km will support weak instability up to a couple of hundred J/kg.   
   Modest PWs of 0.6 to 0.8 inches and onshore flow may support some   
   brief bursts of heavy rain from showers/thunderstorms along the   
   coast. For inland locations, slow moving showers/thunderstorms will   
   linger for another few hours before the loss of surface heating   
   reduces instability and available fuel to support higher rainfall   
   rates.=20   
      
   The main concern for any isolated flash flooding across California   
   stems from very wet antecedent conditions given the past few days=20   
   of heavy rainfall and areas of flooding.   
      
   ...Lower Colorado River Valley into west-central Arizona...   
      
   As the closed low along the West Coast edges closer to the Desert   
   Southwest and height falls begin to overspread the region, low   
   level moisture transport from the south will increase and expand   
   weak instability from eastern California into southern Nevada and   
   western Arizona. The left-exit region of a 90-100 kt jet streak=20   
   may aid with ascent later in the period over the lower Colorado=20   
   River Valley. Showers and embedded thunderstorms are expected to   
   develop and expand in coverage through the 06-12Z period.=20   
   Unidirectional southerly winds should promote repeating and=20   
   possible training with peak hourly rainfall near 1 inch along with=20   
   the potential for isolated flash flooding with the threat extending   
   into the D2 ERO period / 12Z Tuesday.   
      
   Otto   
      
      
   Day 2   
   Valid 12Z Tue Nov 18 2025 - 12Z Wed Nov 19 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   WEST CENTRAL ARIZONA...   
      
   20z Update:   
      
   Upgraded to a Slight risk across portions of west central AZ with   
   this update. Strong forcing just ahead of the upper low will   
   support repeated rounds of showers and embedded heavier convective   
   elements within the deep layered southerly flow. Not a lot of   
   instability forecast, but values upwards of 500-1000 j/kg should   
   be enough to support locally heavier rainfall rates...over 0.5"/hr   
   but generally under 1". Nonetheless the repeated rounds and some   
   upslope component should result in rainfall locally exceeding   
   2-3" over the day. HREF neighborhood probabilities of exceeding 2"   
   are 50-70% and 3" are 40-50%. This rainfall should be enough to   
   result in isolated to scattered flash flood impacts, especially   
   over any more sensitive areas.   
      
   We also added a small Marginal risk centered over KY. A low moving   
   across the OH valley will place KY in the warm sector, with   
   instability forecast to increase over 1000 j/kg by Tuesday   
   afternoon. An initial batch of showers should move across during   
   the morning with the warm front, with another more robust   
   convective round expected during the afternoon and evening as   
   instability increases ahead of the cold front. Both the HREF and   
   REFS indicate 1"/hr neighborhood probabilities of 20-40% and the   
   HREF neighborhood probabilities of exceeding 3" during the period   
   are locally over 15%. It's a pretty marginal situation, but given   
   the degree of moisture transport and forecast uptick in   
   instability it does seem like an event that could locally exceed   
   model QPF forecasts...thus can not rule out some isolated, mainly   
   urban, flash flood concerns.   
      
   Chenard   
      
   ...Previous Discussion...   
      
   Maintained the previous;y issued Marginal risk area over portions   
   of Arizona with little need for more than cosmetic changes in   
   deference to the model guidance. A closed low within a broader   
   trough digs southward along the the west coast of the US and then   
   starts to move inland late. The overall synoptic pattern seems to   
   be handled comparably by the ECMWF/GFS and UK models for much of   
   Day 2 but some of the subtle differences in the details result in   
   non-negligible differences in QPF and the attendant risk of   
   excessive rainfall.   
      
   Precipitable water values get near 2 standard deviations above normal   
   for this time of year in southern-western AZ and most-unstable   
   CAPE values climb above 500 J/Kg south of the Mogollon Rim and into   
   the deserts and Phoenix Metro. This will maintain the risk of   
   elevated (0.50"+) hourly rainfall rates Tue-Tue night as noted from   
   the RRFS exceedance probabilities (through 00Z Wed), while also   
   implied from the 16/12Z RRFS 3-hourly QPFs. Much like the models   
   from Sunday afternoon...the 24hr QPF guidance from the 17/00Z model   
   cycle has come up. At this point there is still not enough   
   confidence from a duration perspective to upgrade to a Slight Risk   
   over the lower terrain south of the Mogollon Rim, which would   
   conceivably include the Phoenix area, but the potential to do so   
   will exist over the next day or so. The machine-learning first   
   guess fields also highlight part of the Marginal risk area with   
   some spotty 5 to 10 percent of exceeding flash flood guidance. At   
   this point...the machine-learning covers only the first half of Day   
   2...so the upcoming day shift will have a complete look at the   
   objective guidance for the Day 2 period.   
      
   Bann   
      
      
   Day 3   
   Valid 12Z Wed Nov 19 2025 - 12Z Thu Nov 20 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF   
   SOUTH CENTRAL TEXAS TO A PORTION OF THE ARKANSAS RIVER VALLEY AND   
   OVER THE SOUTHWEST...   
      
   20z Update:   
      
   Added a Marginal risk across portions of NM and AZ with this   
   update. With the closed low trending slower, scattered showers and   
   embedded thunderstorms are likely to continue into Wednesday   
   across these areas. Overall it looks like less of a focus than day   
   2, but nonetheless still enough of a convective signal to suggest   
   localized flash flooding could continue to be a concern.   
      
   Chenard   
      
   ...Previous Discussion...   
      
   a mid- and upper-level trough will be making its way eastward   
   across portions of the southern Great Basin on Wednesday and into   
   early Thursday. East of that system...low level moisture will start   
   to be drawn northward with dewpoints in the 60s by Wednesday afternoon   
   which should help feed showers and thunderstorms from the afternoon   
   into Wednesday night/early Thursday morning. Most unstable CAPE   
   pushes 1000 J per kg and precipitable water values in excess of   
   1.5 inches becomes established from parts of south-centrsl Texas   
   into eastern Oklahoma and western Arkansas. Based on the   
   GEFS...the precipitable water axis is roughly between 2 and 2.5   
   standardized anomalies above climatology for this time of year.   
   Based on that combination...suspect locally heavy rainfall rates   
   are possible. There has been a tendency for the upper trough to   
   slow its forward speed with a similar delay in convection   
   initiation. With the event spanning into Thursday with broader   
   areal coverage and an increasing risk of excessive rainfall (see   
   WPC Medium Range graphics and discussions for further   
   details)...opted to maintain the Marginal risk on the uncertainty   
   of timing.   
      
   Bann   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!8IC0VpfVV_gqbnE-GZ8-ihXwj8Fnb5jLTmAETMYJmdCw=   
   Y_r8X__LaGJHOhxv9j2T4wWixrrjcLVttKxVs8kTRSHJ3tM$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!8IC0VpfVV_gqbnE-GZ8-ihXwj8Fnb5jLTmAETMYJmdCw=   
   Y_r8X__LaGJHOhxv9j2T4wWixrrjcLVttKxVs8kTUCXaNFM$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!8IC0VpfVV_gqbnE-GZ8-ihXwj8Fnb5jLTmAETMYJmdCw=   
   Y_r8X__LaGJHOhxv9j2T4wWixrrjcLVttKxVs8kTRrizlTk$=20   
      
   =3D =3D =3D   
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