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  Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sat May 13 2023   Day 1 Valid 16Z Sat May 13 2023 - 12Z Sun May 14 2023 


...16Z Update...

...Coastal Texas...

The evolution of the storms ongoing along coastal Texas has  continued to move offshore into the Gulf with the strongest  convection in Deep South Texas, and while there is some training  and slow moving storms along the middle Texas coast, this area of  convection is rather isolated. The storms north and east of there  approaching Houston are nearly stationary, but the rainfall rates  are either not there or very isolated. To summarize the above, the  flash flooding risk is far from widespread across south Texas.  Thus, the Moderate Risk has been downgraded to a Slight with this  update. Further, as for guidance for the rest of today into  tonight, few CAMs show showers and thunderstorms redeveloping at  all, and for those that do, that convection moves quickly, and  therefore even if the storms are able to produce heavy rain, the  rates will be short-lived over any one area. Thus, the Slight Risk  was also shifted eastward closer to where the ongoing convection  is, with little convection noted along the Rio Grande except well  to the north close to Rocksprings. This rather isolated convection  is training, but rates for now are 1.5 inches/ hour or less.  Isolated flash flooding may be possible, consistent with the  Marginal Risk over the area.

...Texas Panhandle to Oklahoma...

The Slight Risk over much of central and eastern Oklahoma was  downgraded to a Marginal with this update. The convection that was  over southeastern OK is dissipating. This afternoon, isolated to  widely scattered convection is likely to develop to the east of a  mesolow tracking over west TX and the TX/OK border. The storms may  be strong enough to produce heavy rainfall, but there's good  agreement that such storms will be moving quickly, and will  therefore be unlikely to produce anything more than isolated flash  flooding. Further west towards the TX Panhandle, stratiform rain is likely  to continue for much of the day into tonight, and while rates are  unlikely to ever get too high, the long period of steady rain  across this region west of the aforementioned mesolow may cause  isolated to widely scattered flooding.

...Iowa/Southern Minnesota...

Somewhat better agreement exists among the guidance that showers  and thunderstorms will redevelop in the Slight Risk area over IA  and southwestern MN this afternoon, which is likely separate from  the convection ongoing over eastern IA. The front will pivot  northwestward, which will allow the storms to train  north/northwestward with time. This area has also seen heavy  rainfall lately, particularly on the Minnesota side of the border.  The Slight Risk was expanded a bit to the north and east to cover  areas with lower FFGs that are likely to pick up additional heavy  rainfall late this afternoon through tonight.


...Previous Discussion... 

...Texas and Oklahoma... Tricky excessive rainfall forecast for today into tonight. The  inherited Moderate risk looks to be in some trouble, as convection  overnight into early this morning has been forward propagating at  a greater clip than anticipated. Thus the greatest flash flood  risk may be past by the time the new day 1 starts at 12z. It will  be close, but still think coastal areas of south TX should still  be getting heavy rainfall at 12z, and the eastern portion of the  line should still be onshore and producing some very heavy  rainfall rates over 2" in an hour. Although even this portion of  the line is forecast to begin a weakening trend by mid morning. So  while heavy rainfall rates will probably be ongoing at 12z, the  forward progression of convection and gradual weakening trend  should limit the magnitude/extent of the flash flood risk. With  that said, the heavy rates and short duration backbuilding could  still pose a locally significant flash flood risk into the morning  hours, especially over any urban areas that are impacted. 

Once this activity clears, the question becomes do we get another  round of convection tonight. Initially instability will be eroded,  so we'll have to see if the continued southeasterly inflow can  advect instability back into the area. Additional weak shortwave  energy may move into the area out of Mexico, and so could envision  convection forming in Mexico and moving back into the area  tonight. Several of the 00z HREF members depict a solution like  this, which results in 3"+ neighborhood probabilities of 40+  percent between 00z-12z Sunday. However it is not unanimous, and  the 06z HRRR notably does not develop additional convection. This  is a plausible outcome as well, and so will just need to monitor  observational and model trends through the day. Given some  potential lingering convection at 12z this morning, and at least  some potential for redevelopment overnight, did not want to  completely remove the MDT risk at this time (although certainly  some chance this is eventually dropped). Instead opted to just  reduce the size and trim back the western portion of the risk  area. 

A second area of convective focus should be across portions of  northern TX into OK. This is related to the retrograding  shortwave/MCV that is expected to lift north northwest across the  region today into tonight. Even here there remains uncertainty  with regards to the degree of instability and location of  boundaries helping focus convection. The overall setup does appear  to favor some swaths of training convection, so just a matter of  whether we get enough instability to drive rainfall rates high  enough, and exactly where this better convective concentration  will end up. For now left a pretty broad Slight risk over the  area. A bit broader on the southern extent to cover where some  overlap of additional convection and heavy antecedent rainfall may  occur, and a bit sharper with the TX Panhandle cutoff as this  areas has been drier and instability should generally be lower the  further north and west you go. Oklahoma is interesting and could  very well be the focus for heaviest rainfall today over the  Southern Plains within training banded features east of the  MCV/low. However ongoing convection this morning likely  complicates boundary locations and instability later today, so  even here confidence is low...but still within Slight risk levels.

...Northern Plains into Upper MS Valley... Training convection this morning into the afternoon should pose an  isolated to scattered flash flood risk over portions of eastern  SD, southern MN and central/eastern IA. Convection is focusing  along the 925/850mb boundary east of slow moving deep layered low,  and convergence along this boundary should remain pretty  persistent through the morning hours. Low level southeasterly flow  should maintain instability into the region as well...with  1000-2000 J/KG of CAPE. Expect that we will continue to see  periodic backbuilding/training convective segments along this  corridor, which should be enough to drive some flash flood risk.  HREF guidance supports a broad area of 1-2" and localized amounts  over 3". In fact trends support some 3-5" potential within swaths  across this corridor. Portions of southern MN are saturated from  recent heavy rainfall which would allow for a quicker development  of potential flash flooding. The heaviest rain today may end up  just south of this saturated area, but it will be close. Other  areas of the Slight risk have been drier and so it will likely  take longer training for issues to develop...but even here there  is enough of a signal to support at least an isolated to scattered  flash flood risk.

...OH/TN valley into the Mid Atlantic... Above average moisture and instability will cover portions of this  region on Saturday. A weak shortwave currently over OH will move  into the Mid Atlantic and help drive a convective threat.  Individual cells will be quick moving, but broad forcing is  supportive of multiple convective rounds. On top of that these  cells should be capable of dropping a quick 1-2" in localized  swaths. So do think at least an isolated, mainly urban, flash  flood risk could exist.

Chenard   Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt 

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