Weather discussion

[WeatherWheeler35]

  Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Tue May 02 2023   Day 3 Valid 12Z Thu May 04 2023 - 12Z Fri May 05 2023 

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE  INTERMOUNTAIN WEST AND FROM PARTS OF THE LOWER MISSOURI AND MID  MISSISSIPPI VALLEYS TO THE OZARKS AND ARKANSAS RIVER VALLEY...

...Central and Northern Intermountain West... Stubborn, slow-moving mid-upper low/trough will take on more of a  negative tilt Thu-Thu night as shortwave energy pivots northward  through the Southwest and Central Great Basin. The uptick in upper  level forcing with the increased upper difluence/divergence/DPVA,  coinciding with areas of decent instability for this time of year  prior to monsoon season (MUCAPEs of 500-1500 J/Kg, owing in large  part to steepening H7-H5 lapse rates of 7-8+ C/KM), will allow for  more numerous, pulse-type convective clusters with the potential  of isolated 0.50+"/hr rainfall rates. This as PWs climb between  0.50-1.00" per the guidance, highest (>0.75") across eastern WA-OR  into northern ID, which would be 2-2.5 standard deviations above  normal for early May. By themselves, the dynamical and  thermodynamical parameters progged by the models would result in a  minimal excessive rainfall threat (i.e. below the 5% threshold for  a Marginal); however, when combined with the snowmelt in some  locales and burn scar areas in others, do anticipate a more  enhanced risk for localized runoff issues. As a result, the  Marginal Risk area inherited from yesterday's experimental Day 4  ERO was expanded northward to the Canadian border with WA-ID. 

 ...Eastern Kansas-Oklahoma into much of Missouri, Arkansas, and  Western Illinois... Downstream  of the aforementioned upper trough, flattening  mid/upper ridge will remain across the central CONUS. Strong upper  level confluence is expected to generate a 100-130 kt upper level  jet streak across the Midwest to Lower OH Valley Thu-Thu night.  The resultant uptick in low-level frontogenesis is noted from the  guidance, while SW/SSW 850 mb flow increases to 40-50 kts and  MUCAPEs between 1000-2000 J/Kg by Thursday evening and overnight  across the outlook area. The dynamical and thermodynamical  environment will foster the development and upscale growth of  organized convection Thursday night, potentially leading to an MCS  with an enhanced risk of cell training as the low-level jet (LLJ)  ramps up and 850-300 mb thicknesses become more divergent  downstream. Deep-layer moisture parameters are not overly  impressive/anomalous at this point however (maximum PW values of  1.5-1.7" within the outlook area). As a matter of fact, the 00Z  guidance did not indicate much of an overlap in terms of areas  with the best forcing (upper divergence/strongest low-level  frontogenesis across eastern KS-northeast OK into much of MO and  northern AR) and best deep-layer moisture/instability profiles  (south of the Red River and ArkLaTex). Given this, and the fact  that the models currently depict more forward (downwind)  propagation/Corfidi Vectors than otherwise, for now will maintain  the Marginal Risk that was inherited from yesterday's experimental  Day 4 ERO.

Hurley   Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt